SG3525 Full Bridge Inverter using Bootstrapped BJT/Mosfet Network

In this post we try to investigate how to design a SG3525 full bridge inverter circuit by applying an external bootstrap circuit in the design. The idea was requested by Mr. Mr. Abdul, and many other avid readers of this website.

Why Full-Bridge Inverter Circuit is not Easy

Whenever we think of a full bridge or an H-bridge inverter circuit, we are able to identify circuits having specialized driver ICs which makes us wonder, isn’t it really possible to design a full bridge inverter using ordinary components?

Although this may look daunting, a little understanding of the concept helps us realize that after all the process may not be that complex.

The crucial hurdle in a full bridge or a H-bridge design is the incorporation of 4 N-channel mosfet full bridge topology, which in turn demands the incorporation of a bootstrap mechanism for the high side mosfets.

What's Bootstrapping

So what’s exactly a Bootstrapping Network and how does this become so crucial while developing a Full bridge inverter circuit?

In one of my earlier posts I comprehensively explained how emitter follower transistor works, which can be exactly applicable for a mosfet source follower circuit as well.

In this configuration we learned that the base voltage for the transistor must be always 0.6V higher than the emitter voltage at the collector side of the transistor, in order to enable the transistor to conduct across collector to emitter.

If we interpret the above for a mosfet, we find that the gate voltage of an source follower mosfet must be at least 5V, or ideally 10V higher than the supply voltage connected at the drain side of the device.

If you inspect the high side mosfet in a full bridge network, you will find that the high side mosfets are actually arranged as source followers, and therefore demand a gate triggering voltage that needs to be a minimum 10V over the supply volts.

Once this is accomplished we can expect an optimal conduction from the high side mosfets via the low side mosfets to complete the one side cycle of the push pull frequency.

Normally this is implemented using a fast recovery diode in conjunction with a high voltage capacitor.

This crucial parameter wherein a capacitor is used for raising the gate voltage of a high-side mosfet to 10V higher than its drain supply voltage is called bootstrapping, and the circuit for accomplishing this is termed as bootstrapping network.

The low side mosfet do not require this critical configuration simply because the source of the low side mosets are directly grounded and therefore are able to operate using the supply voltage itself and without any enhancements.

How to Make a SG3525 Full Bridge Inverter Circuit

Now since we know how to implement a full bridge network using bootstrapping, let’s try to understand how this could be applied for achieving a full bridge SG3525 inverter circuit, which is by far one of the the most popular and the most sought after ICs for making an inverter.

The following design shows the standard module which may be integrated to any ordinary SG3525 inverter across the output pins of the IC for accomplishing a highly efficient SG3525 full bridge or H-bridge inverter circuit.

Circuit Diagram

Referring to the above diagram, we can identify the four mosfets rigged as an H-bridge or a full bridge network, however the additional BC547 transistor and the associated diode capacitor looks a bit unfamiliar.

To be precise the BC547 stage is positioned for enforcing the bootstrapping condition, and this can be understood with the help of the following explanation:

We know that in any H-bridge the mosfets are configured to conduct diagonally for implementing the intended push pull conduction across the transformer or the connected load.

Therefore let’s assume an instance where the pin#14 of the SG3525 is low, which enables the top right, and the low left mosfets to conduct.

This implies that pin#11 of the IC is high during this instance, which keeps the left side BC547 switch ON. In this situation the following things happen withing the left side BC547 stage:

1) The 10uF capacitor charges up via the 1N4148 diode and the low side mosfet connected with its negative terminal.

2) This charge is temporarily stored inside the capacitor and may be assumed to be equal to the supply voltage.

3) Now as soon as the logic across the SG3525 reverts with the subsequent oscillating cycle, the pin#11 goes low, which instantly switches OFF the associated BC547.

4) With BC547 switched OFF, the supply voltage at the cathode of the 1N4148 now reaches the gate of the connected mosfet, however this voltage is now reinforced with the stored voltage inside capacitor which is also almost equal to the supply level.

5) This results in a doubling effect and enables a raised 2X voltage at the gate of the relevant mosfet.

6) This condition instantly hard triggers the mosfet into conduction, which pushes the voltage across the corresponding opposite low side mosfet.

7) During this situation the capacitor is forced to discharge quickly and the mosfet is able to conduct only for so long the stored charge of this capacitor is able to sustain.

Therefore it becomes mandatory to ensure that the value of the capacitor is selected such that the capacitor is able to adequately hold the charge for each ON/OFF period of the push pull oscillations.

Otherwise the mosfet will abandon the conduction prematurely causing a relatively lower RMS output.

Well, the above explanation comprehensively explains how a bootstrapping functions in full bridge inverters and how this crucial feature may be implemented for making an efficient SG3525 full bridge inverter circuit.

Now if you have understood how an ordinary SG3525 could be transformed into a full fledged H-bridge inverter, you might also want to investigate how the same can be implemented for other ordinary options such as in IC 4047, or IC 555 based inverter circuits, …..think about it and let us know!

SG3525 Inverter Circuit which can be Configured with the the above Discussed Full Bridge Network

The following image shows an example inverter circuit using the IC SG3525, you can observe that the output mosfet stage is missing in the diagram, and only the output open pinouts can be seen in the form of pin#11 and pin#14 terminations.

The ends of these output pinouts simply needs to be connected across the indicated sections of the above explained full bridge network for effectively converting this simple SG3525 design into a full fledged SG3525 full bridge inverter circuit or an 4 N channel mosfet H-bridge circuit.

Feedback from Mr. Robin, (who is one of the avid readers of this blog, and a passionate electronic enthusiast):

Hi Swagatum
Ok,just to check everything is working I separated the two high side fets from the two low side fets and used the same circuitry as:
connecting the cap negative to the mosfet source then connecting that junction to a 1k resistor and an led to ground on each high side fet.Pin 11 pulsed the one high side fet and pin 14 the other high side fet.
When I switched the SG3525 on both fets lit up momentarily and the oscillated normally thereafter.I think that could be a problem if I connected this situation to the trafo and low side fets?
Then I tested the two low side fets,connecting a 12v supply to a (1k resistor and an led) to the drain of each low side fet and connecting the source's to ground.Pin 11 and 14 was connected to each low side fets gate.
When I switched the SG3525 on the low side fet's would not oscillate until I put a 1k resistor between the pin (11, 14) and the gate.(not sure why that happens).

Circuit diagram attatched below.

My Reply:

Thanks Robin,

I appreciate your efforts, however that doesn't seem to be the best way of checking the IC 's output response...

alternatively you can try a simple method by connecting individual LEDs from pin#11 and pin#14 of the IC to ground with each LED having its own 1K resistor.

This will quickly allow you to understand the IC output response....this could be done either by keeping the full bridge stage isolated from the two IC outputs or without isolating it.

Furthermore you could try attaching a 3V zeners in series between the IC output pins and the respective full bridge inputs...this will ensure that false triggering across the mosfets are avoided as far as possible...

Hope this helps

Best Regards...

From Robin:

Could you please explain how{ 3V zeners in series between the IC output pins and the respective full bridge inputs...this will ensure that false triggering across the mosfets are avoided as far as possible...

Cheers Robin


When a zener diode is in series it will pass the full voltage once its specified value is exceeded, therefore a 3V zener diode will not conduct only as long as the 3V mark is not crossed, once this is exceeded, it will allow the entire level of voltage that's been applied across it
So in our case also, since the voltage from the SG 3525 can be assumed to be at the supply level and higher than 3V, nothing would be blocked or restricted and the whole supply level would be able to reach the full bridge stage.

Let me know how it goes with your circuit.

Adding a "Dead Time" to the Low Side Mosfet

Thee following diagram shows how a dead time could be introduced at the low side mosfet such that whenever the BC547 transistor switches causing the upper mosfet to turn ON, the relevant low side mosfet is turned ON after a slight delay (a couple of ms), thus preventing any sort of possible shoot through.

Need Help? Please send your queries through Comments for quick replies! And please Bookmark my site :)


Ugoeze Akuekwe said…
Hello sir in this post the pulse from the ic is not configured in a diagonal form as your other full bridge configurations , is this oversite or the correct configuration for this sequence?
Swagatam said…
Hello Ugoeze, it's because of the presence of the BC547 transistors which invert the signal for the upper mosfet making sure that the only the diagonal mosfets are triggered at any instant during the osculations.
Silverman said…
Sir I think your high side and low side GATEs are incorrect position or connection. Am I right?
Hello Sir,
I am working on Full Bridge Inverter Circuit using IGBTs where the DC Voltage is not constant at the input, I wanted to know about the protection circuits which must be designed to avoid any damage.
Swagatam said…
Hello Prathamesh, which cirucit are using using? is it using the IRS based full bridge driver? please provide the schematic details....if it's an IRS based driver, then yo can use its SD (shut down pinout for the protection feature...for other info you can study the following article:

Swagatam said…
Nope, I have already explained the reason in the above comment....
Thank you Sir, the link was very helpful. Also needed some help with Overvoltage and Overcurrent Protection Circuits. Will you please tell me the devices which must be used to protect the inverter from sudden fluctuations where the output of the inverter is 1.4kW with a nominal input voltage of 200V and maximum input voltage 250V?
I'd appreciate any information you could give me.
Swagatam said…
Prathamesh, since the inverter is being operated using a battery there cannot be any fluctuations, but an overload (overcurrent) could cause problems, for which you can use the shut down pinout of your full bridge IC to protect the system.
shahid siddique said…
Where is the sechmatic
Swagatam said…
you can use any standard IC SG3525 circuit and just replace its mosfet stage with the above shown full bridge mosfet stage...for example you can try the the circuit which is show here and combine its output with the above explained full bridge circuit:
Abduleng Aminu said…
sir please is this full bridge circuit work and tested.100% working??
Swagatam said…
Abduleng, I have presented the concept after doing some intense research on the net, now it's upto the readers to decide whether it will work or not
Silverman said…
Thank you sir. I got now the idea of the bootstrapped implemented in your diagram. I already made your previous 1kw full bridge inverter and its fully functional. I'm now using it in my house to drive my appliances.

I will test this new inverter diagram and I will let you know sir.

Thank you very much sir for being helpful to us as a hobbyist.
Swagatam said…
You are most welcome Silverman, all the best to you!
Silverman said…
Hello sir.. good news! I finally build your design, and it's fully working. The output from the MOSFETs is 13.5vac using 12vdc/100ah battery. The output from the transformer is 240vac. My question is why 13.5vac output but my supply is 12vdc.?

I'm using IRF3205, my transformer build is 220v-12v with 60hz.
Swagatam said…
That's great silverman...I appreciate it a lot.

where exactly did you find 13.5V?

check it across drain/source terminals of the mosfet, you wlll find it to be around 6V,,,,that's the average value due to the ON/OFF cycles of a 12V supply
Silverman said…
Thanks for the reply sir. My bad.. I forgot my basic voltage check. I put my test probe across to the connector of 12v windings of transformer and I got AC readings at 13.5vac.

It is now partially completed inside the wooden box. And I also made a changeover circuit and I Incorporate it to your design.

Silverman said…
I checked again the drain/source as instructed, the drain/source voltage is 6.75V. I double checked the schematics and it's properly oriented. And it's all components are soldered in PCB.
Swagatam said…
that's wonderful, thanks for posting this valuable info!!
Robin said…
Hi Swagatam
Thanks for getting back to me in the pure sinewave inverter post.
I would like to know in this full bridge circuit above,if i have a 290v dc supply voltage from a high frequency source if the 10uf 50v capacitor must be changed to a higher voltage and if the irf540 must be changed as well to handle the higher voltage.I want to switch this high voltage DC to 220vac at 50Hz
regards Robin
Swagatam said…
Hi robin, as far as I think, the mosfet drains will need to be connected with the 290V DC, while the diode anode separately connected to the 12V supply...yes the mosfet will need to replaced with a 400V type for example IRF804 etc...
chikez okechuks said…
Hello swag, can this circuit be driven directly with a 24vdc supply?secondly , can the same transformer be used to charge the batterywithout any damage to the MOSFETs? Then thirdly why are the MOSFETs not having gate resistor like 10 ohms or any other thank .Karlas is my name and a follower of this blog .
chikez okechuks said…
Hello questions are simple, first. Can this h-bridge circuit be driven directly with a 24v battery system, secondly. Can the transformer be used to charge the batteries on utility mode. Then thirdly why are the MOSFETs not having gate resistor like other circuit s do. Thanks that all for now.
Swagatam said…
Hello Karlas, yes you can use 24V for the above shown H-bridge configuration.

The transformer can be used only in the inverter mode, it cannot be used for charging the battery unless some special changeover circuitry is incorporated.

the upper mosfets do not need gate resistor since the BJT's 1K itself forms the gate resistor for the mosfets...the lower mosfet could be added with a some low value gate resistor....although it's never too important.
Robin said…
Hi Swagatum
Ok on first attempt i burnt one of the IRF840's,so I'm just making sure that the resistor between pin 5 and 7 on the SG2535 causes a delay which it does.Also is'nt it a good idea to put a 1k resistor between the gate and source of each mosfet to make sure that it is not accidently on when you switch on,or would it affect the functioning of your circuit
Swagatam said…
Hi Robin, yes adding a slight delay or dead time will help to control mosfet burning..

you can add a resistor between gate and source if the mosfets are relatively away from the IC or the trigger source...

no it won't affect the above explained circuit...and please note that the BC547 emitters are only connected to the ground and not to the mosfet sources
Robin said…
Ok ,Thanks,I understand,will give it another go and let you know how it goes
Swagatam said…
Robin said…
Hi Swagatum
I've been wrestling with this H-bridge circuit above and it worked the first time i made it,I slowed down the hz so i could see it oscillating,the moment i put a load on (bang).
Ok,I've now disected it and first got the two high side Irf840's blinking,(source,1k,led,ground),pin 11 to the one IRF gate and pin 14 to the other but as soon as i switch the sg3525 on both led's come on at the same time thereafter it oscillates normally,would'nt this be a catastrophic occurance?.By the way the cap seems to have charged up via the load because i was getting 18v on the gates.
Then i got the low side IRF's blinking(+12v,1k,led,drain,source,ground)pin 11 to 1k resistor then to gate,pin 14 same to other IRF but they would not oscillate without the 1k to the gate?
Oh! I saw your comments with Bob the Blogger(interesting)
Regards Robin
Swagatam said…
Thanks for the update Robin,

sorry I could not get this: (source,1k,led,ground)
LED can be connect in series wit the BC547 base and across gate/source of the low side mosfet any other position could create problems wit the functioning or misleading indications.

Please let us know, if possible through images if the deign works for you correctly as proposed i the article...

wish you all the best..
Aondofa Injoh said…
Great work Swagatam the design is working well. Keep it on such a nice ideal. Aondofa Injoh from Nigeria.
Swagatam said…
Thank you Aondofa, i am glad it worked for you! keep up the good work!
Louis said…
Hello swagatan, please can ic sg3524 be used for this design? I can't find sg3525 here in my location.
Swagatam said…
Hi Louis, yes it can be used
Deogratia said…
how many watts and is it sine wave?
Swag said…
no it is not a sinewave.
berbs said…

Will it be possible to use a portable battery bank for phones to charge a laptop? I'm trying to use the USB output (5V/2.1A) as my source to the circuit. However, since SG3525 requires 8V and above, I'm thinking of boosting this 5V to 12V instead. I do not need it to supply enough capacity to charge a laptop fully but I just need it to work and be able to charge.

So far, I have made a SG3525 DC-DC Converter (push-pull) running at 180V. Didn't focus much on the inverter part yet because I believe that most recent laptop chargers are SMPS-based and rectification should be the first stage so DC might be possible. I tested the output with a dummy load of 680ohms at 180V and managed to get 0.1A so I assume the DC/DC converter is working fine. Load is assumed to be about 70W for laptop. Issue is with the input side.

The issue is that my input source is not able to supply sufficient current. If my input voltage is 12V so for a load of 70W, my input current will need at least 5.83A which is too big for my application. I'm only able to supply 2.1A at most using the USB output of my port battery bank. Are there any ways to amplify the current using transistors or other methods at the source side? I have heard of methods to keep the average current constant but able to have certain pulses of current to be much higher (e.g. 5A) and with certain pulses to be lower but still average it out to be the current the source supply (e.g. 2.1A in my case).

Swag said…
Hi, Amplifying current would result in dropping the voltage, and vice versa, in other words the output wattage can never exceed the input wattage. In your case the input wattage us 12 x 2 = 24W, if you divide 24 with 180 it comes to around 0.13 amps, and this figure cannot be increased without decreasing the 180V level. Whether it is in the pulsed form or continuous the o/p IxV will be always less than the i/p VxI. For getting 70 watt output you will need an input higher than 70 watts
ayam said…
Hi Mr. Swagatam...
can u design full bridge inverter using HCPL 3120 optocoupler to drive the MOSFET?
Swag said…
Hi Ayam, can you please specify why do you wish to use optos when the mosfet can be directly driven from the relevant sources?
ayam said…
I want to drive the modulation and frequency of the inverter. PWM source from Arduino and go to the HCPL 3120 and then drive the full bridge MOSFET with 310VDC. Its like to control the load using scalar method (V/F method). thanks!!!!
Swag said…
I could not correctly understand the principle of operation that you are trying to implement. Controlling an induction motor speed with V/F is quite complex therefore I must fist completely understand how your Arduino is adjusting the parameters through the optocoupler, only then I would be able to design it. If possible please provide more details
ayam said…
I'm done with the bipolar PWM for the full bridge with V/F control method using MATLAB/Simulink. the PWM signal I was deployed to the hardware (Arduino). So I want to send the PWM signal to the HCPL3120 and drive the MOSFET. I realize that the HCPL3120 can reduce my component if I'm using IR2110 to drive the MOSFET. any suggestion for me Mr. Swagatam?
ayam said…
I’m done with the bipolar PWM for the full bridge with V/F control method using MATLAB/Simulink. the PWM signal (4 PWM signal) I was deployed to the hardware (Arduino). So I want to send the PWM signal to the HCPL3120 and drive the MOSFET. I realize that the HCPL3120 can reduce my component than I’m using IR2110 to drive the MOSFET.

Here the output PWM of the simulation for 0.02sec (
Here the PWM with LC filter for 0.02sec (
here the output waveform if I variable the modulation and the frequency (

any suggestion for me Mr. Swagatam?
Swag said…
OK so you want to know how the opto coupler can be integrated with a full bridge network in order to implement the full-bridge operation correctly? right?

and what is the specifications of the full bridge, should it be using 4 N channel mofets, because using 4 channel moefets will require a special driver IC. Although I have designed circuits without using driver ICs, I haven't tested them practically.

Please clarify the above
ayam said…
yes. I would like to know how to implement the opto to control the MOSFET apart from using the component of discrete electronics. I intend to use four (IXFH22N60P3 - Power MOSFET, N Channel, 22 A, 600 V, 0.39 ohm, 10 V, 5 V) to create a full bridge network.

Maybe you can help me in terms of connecting and using other passive components to build this full bridge network such as the choice of appropriate value of capacitor, resistor, and diode by using HCPL3120 to drive the MOSFET
Swag said…
I won't recommend using discrete components as that wouldn't guarantee accurate results, I'll try to design it with a standard full bridge IC.

However I cannot understand why the high voltage opto is selected by you, any other low voltage opto can be also used because we are not connecting the opto anywhere along the BUS line of the mosfet, as far as I understand.

here the idea is to only ensure total isolation of the Arduino from the Full bridge network in every possible manner..
ayam said…
Thank you for your suggestion. I will wait if you are willing to design with full bridge IC standard as you said. for the selection of high voltage opto by me is because I have referred to another website stating that HCPL3120 is Robustness, Real-estate, and Simplicity instead of using IR2110. If you have other ideas, which are relevant to the requirements of my project, I would appreciate it if you can help me create a circuit that works well based on the needs of this project.
Swag said…
OK I'll try to do it soon, and let you know....
Swag said…
I have tried to do it here, you can check it:

here VCC1 = 5V
VCC = 12V

one side is shown, you will have to repeat the same layout for the other side of the bridge
ayam said…
Thanks sir. I very appreciate it. for the resistor, how about the power rating should I use? 1/4 W or bigger than that?
Swag said…
you can use 1/4 watt resistors since no high current is involved here.
ayam said…
hi sir. can I know what type of diode that u use in this circuit?
Swag said…
D1 can be 1N4148. other across the mosfets can be 1n5408. usually these are already present inside the mosfets...
thatcher said…
Hello sir!

I once constructed a circuit with boot strap and worked fine but I really don't know how exactly the bootstrap work.

How possible the n fets on high side are not burnt with high Vgs? For example, I have 370vds on the drain and Vg must be 5v minimum above supply voltage which means it must be 375v or more but the fet maximum Vgs is 30v.

Please could you explain this in detail.

Thank you.
Swag said…
Hello Thatcher,

since the hi-side mosfets are in a source-follower mode the high voltage on the gate is not considered by the mosfet, and it gets nullified by the equivalent drain high voltage, only the remaining 5V or 10V is considered for triggering the mosfet, this is due to the specific characteristic of all semiconductor devices, in which the conduction of voltage from one end to the other end becomes possible only when the required minimum specified forward voltage drop of the device is satisfied, otherwise the device will stay unresponsive and locked.

Therefore in mosfet or BJTs the full conduction is able to happen only when the gate or base voltage is pulled above the emitter voltage by a value which may be equal to the fwd drop voltage.
In normal configuration the drain voltage is ignored because the source is at the ground level that is at zero level, theefore only 10V becomes sufficient

The formula that needs to be satisfied for mosfets is Gate voltage = source voltage + 10V
since the load is attached with the source, the source has to first reach the drain level, then pass through the load to ground.

Therefore the source has to first reach at the drain voltage level, say 310V, but this can happen only when the gate voltage reaches 310 + 10 = 320V value

and that's exactly why we need to reinforce the gate with that extra 10V over the drain voltage level
thatcher said…
Thank you so much for this info I think I now know the concept.
Swag said…
I am glad it helped!
thatcher said…
Sir, I got something here. Now I want to contruct something different but almost similar, the last time I built an h- bridge with 12 volts as supply voltage band used bootstrap cap 10uf /50v. This time the supply voltage for the h-bridge is 300volts, do I have to change the voltage rating of cap to 400v? If no ,how could the cap charge to about 300v to meet gate voltage required at high side. Again on driver stage can I use any transistor with specs almost to thebc547 and bc 557 or I need absolutely with same?
Swag said…
Thatcher, 300V rating will not be required, the capacitor can be a 25V rated one, because the capacitor will store only the extra charge which is required to be raised over the drain level. The voltage stored inside the capacitor will be dumped in series with the drain level for a fraction of a second during the high side ON time switching, forcing the gate level to go up by the factor supplied by the capacitor charge value over the drain supply level.
thatcher said…
Hie, I want you to help me on another circuit but don't know how to post a picture on comments.please help.
Swag said…
Hi, you can upload it to any free image hosting site online and provide the link here...I'll check it out.
thatcher said…
I tried this circuit, with rail supply 12vdc and the output was 5.6 to 7vac. Then changed rail voltage to 315vdc and there was no output at all. What could be wrong with it? I intend to use this full bridge on an inverter which has 315vdc as bus voltage.
Swag said…
sorry, I am not able to troubleshoot ideas.
watson said…
Hi Swag,

I plan to make a similar project with 24V VDD. Do you have any experience/recommendations how do configure the bootstrap cap (double the value?) and the high side resistor to prevent overvoltage of the gate? Or would you see general problems with 24V VDD?

Thanks and best regards,

Swag said…
Hi Watson,

I have no idea about it, in fact I am myself struggling to understand the calculations, that's why it is better to use a specialized IC for the driver as explained below:
watson said…
Hi Swag,

thank you. I even had tested a 24V project with the IRC2453 two times , but had bad experiences (the circuit became instable after some time in use). The oscillator frequency was hard to set in lower ranges (~ 100 Hz for iron core transformer) ans changed permanently. Although I used a voltage regulator chip for the fullbridge IC. Maybe a design error, but I am now looking for another option than the IRC2453. Will make further test and maybe report what I found out.

Best regards,
Swag said…
OK, wish you all the best Watson, in the meantime you could also explore the following concepts
Aiqbal said…
Hi Swag,
I applied this circuit and it works nicely for me with irf3205 FETs, I have used this circuit for high voltage capacitor charging application,
I have one problem : at start FETs draws very high inrush current which have destroyed my fets some times, I don't want to use current limiting resistors due to power loss, plz suggest any ideal circuit using sg3525 and irf3205 for inrush current limiting ,
Second problem is that this circuit doesn't work with igbt instead of fet, what drive circuit u suggest for ixgx60n120,
Swag said…
Hi Aiqbal, I am glad it is working for you.

If you refer to the following article regarding the pinout details of the IC

You will find that pin#8 decides the slow or oft start for the IC by generating narrow pwms during initial power switch, so may be you can try increasing the capacitor value connected at this pin to some higher value for enabling higher level of soft start and subsequently reducing the initial surge.

Alternatively you can also try increasing the dead time parameter of the IC by increasing the resistor value at pin#7 of the IC
Aiqbal said…
Thanx Swag,
One more thing I want to ask you, have u worked on resonant power supplies, I want to design a constant current mode power supply,
plz suggest design using igbt
Swag said…
Hi Aiqbal, I have not yet worked with resonant power supply designs, whatever SMPS designs I have posted all are referred from other sites...
Mehedi Leon said…
Sir I have some questions
1. if I use 4 IRFZ44N( each 100w) and a 500w transformer then what will be the output watt?
2. here u described H-bridge circuit with 4 Mosfet, if I want to add more mosfet to increase wattage then how can I add more mosfet?
3. in the above circuit you drive the lower tow mosfet directly from pin 11 n 14, can I add gate resistance with each 4 mosfet? if I can then what resistance?
4. one another thing, if I want add safty diode (IN14007) then what will the diode connection?
Sir plz make me understand
Thank u.......
Swag said…
The switching devices must be rated above the transformer rating, if the trafo is 500 watt then the mosfets must be rated at minimum 600 watts.

you can more mosfets in parallel, just make sure to add a resistor separately with each mosfet gate.

yes you can add a gate resistors for the low side mosfet, and also add 1K resistors across gate/source

for diodes the cathode will go towards the drains of the mosfets...use 1N5408, and not 1N4007
Mehedi Leon said…
1.Sir if I use 500w trafo then 4 nos 600w mosfet( each 600w)? please make it clear
2.with each mosfet gate what will the resistance value?(gate resistance)? low side mosfet what will the gate resistance value?
4.sir u said "for diodes the cathode will go towards the drains of the mosfets" bt anode will go where?
Sir plz help me
Swag said…
1) The combined wattage of the mosfets on each channel can be 600 watt

2) low gate resistance for each mosfet can be 22 ohms 1/4 watt, high side mosfet will not require any gate resistor.

4) diode must be connected across drain/source of each mosfets
Mehedi Leon said…
Sir Thank u for your valuable reply
I want to know about switching wattage : that means in the above article you used IRF540 (4, each 150w)
here I want to know that 4nos IRF560(each 150w)
so what will be switching wattage?
Sir kindly say me in ur above circuit what is Switching wattage?
Swag said…
As I have already mentioned, the combined wattage of the mosfet gives its total power handling your case for one channel it will 300 watts and for the other channel it will 300 watts...

the output power will be decided by the transformer, not by the mosfets.
Mehedi Leon said…
Ok Sir I understand but I have a little misunderstand that is-
in ur above 300+300 (w) driving wattage
1.if I use 500w transformer then what will the out put watt from transformer?(4*150)
2.if I use 1000w transformer then what will the out put watt from transformer?(4*150)
that's the matter I want to know
Please sir make me clear the thinking.
Thank u
Swag said…
Mehedi, mosfets are like switches, you can compare them to a relay contact or any form of switch that we we use for operating an electrical device...they are designed only to carry the current which the transformer pulls from the the mosfet only needs to be rated at the transformer wattage level so that they can carry the current safely to the transformer and don't get burnt....the output from the transformer depends solely on the transformer wattage and the battery capacity.

1) for 500w trafo the output will be 500 watt provided the mosfets are rated to carry 500 watts and the battery is also rated to supply current for getting the 500 will be same for the 1000 watt also
Mehedi Leon said…
Sir can I use 10uf/25V cap? 50V is not available now.
and can I use 10ohms or 33ohms gate resistance instance of 22ohms?
Swag said…
Mehedi, what is drain side voltage you have used? The capacitor voltage rating must be 20V higher than the drain side voltage of the upper mosfets
Mehedi Leon said…
What is Drain side volt of upper mosfer? ( IRF540)
How can I read drain side volt of any mosfet from it datasheet? plz say me
In IRF540 datasheet we know that :-

Type Designator: IRF540
Type of Transistor: MOSFET
Type of Control Channel: N -Channel
Maximum Power Dissipation (Pd): 150 W
Maximum Drain-Source Volt |Vds|: 100 V
Maximum Gate-Source Volt |Vgs|: 20 V
Maximum Gate-Threshold Volt|Vgs(th)| 4 V
Maximum Drain Current |Id|: 30 A

my question is here what is the drain side volt here?
Swag said…
Here by Drain side voltage I meant to say the voltage which you prefer to apply across the drains of the upper mosfets.
NICK said…
hello SWAG. thanks for your idea. i tried your circuit using sg3524 ic.
the high side drain voltage is 48v, i replaced bc547 with tip41c.
my bootstrap cap. is 160v 10uF. my mosfets are irfp260n.
the 1N4148 is connected to a 12v supply. when i power on, the circuit works but the high side FETs gets hot after a few minutes without load. what i'm i getting wrong?
should i replace the 12v supply wih 48v?
Swag said…
Hi Nick, The upper mosfet could get hot if the gate voltage reaches higher than its rated gate value, you can try measuring this value by keeping the meter probes across the 10uF capacitor, and momentarily switching ON the trigger across the base of the relevant TIP141 transistor...the reading should not be above 60V, if it is above this value then we may have to think of a different configuration in which the drain voltage could be applied separately and the 12V applied independently yet make sure these two become a part of the 10uF cap.
NICK said…
Hello Swag; thanks for getting back to me so soon. what if i change the 1n4148 to 1N4007 and connect a resistor (say 22k) in parallel with the bootstrap cap? what do you think? i'll try it and let you know the outcome. thanks again
Mehedi Leon said…
ok sir I got it,
I am using 12V(13.5)V at upper mosfet drain side, so that the equation goes 12+20=32v/10uf
or 13.5+20=33.5/10uf
but they are not available
that's why 10uf/50v, am I right sir?
sir one more question :-
in lower mosfet gate, can I use 100R or 33R or 10R resistance? please say which is best
Swag said…
you are right Mehedi, the capacitor voltage rating should be ideally twice the supply level applied across its terminals, higher rating than this will do, but lower than this may not be advisable.
Swag said…
for lower mosfet gates you can use any value below 100 ohm
Swag said…
Sorry Nick, I am unable to simulate the configuration in mind so it would be better to get it checked practically and learn how it behaves...wish you all the best
Mehedi Leon said…
Thank you sir for your valuable Reply.
Sir I am designing an inverter for 400watt full load.
I am using your above Full bridge circuit with SG3524 ic circuit, almost everything done and I am getting out put from transformer.
But I am facing a problem that is, I am unable to chose right mosfet for actual 400Watt out put.
so I need your help to chose right mosfet.

I m using 650VA UPS transformer and
100AH battery
now say me what mosfet is needed for this inverter? and how much mosfet in total?
I have available mosfet in my palce :-

please u help me to use right mosfer for accurate Wattage
Thank u sir
Swag said…
Mehedi, assuming your battery voltage is below 48V, you can use the first one which IRF44Z, just one on each channel will be enough, with adequate heatsinks attached
Mehedi Leon said…
I m using 12V 100AH Battery
sir u said one on each Chanel will be enough. that's mean I need to use total 2 IRF44N.
but my IRFZ44N(Power Dissipation 94 W)
94+94= 188W
188W switching wattage will be 400W by 650VA(390W) Transformer????
sir I m confused how it possible?

in your previous reply u said that :-
Swag says
"The switching devices must be rated above the transformer rating, if the trafo is 500 watt then the mosfets must be rated at minimum 600 watts."
in this case now transformer is 390W so switching device (Mosfet) must be rated at minimum 490W,
sir I m so much confused!!!!!!!!!
Mho said…
Please send me your email sir. I have several questions I would love to ask you, and wouldn't want to do it here, as it may be different from the topic. I follow your tutorials, and I have been really enlightened.
Swag said…
You are right but then what about the drain/source rating of the mosfet, and the continuous drain current rating of the mosfet which are shown as 55V and 49 amps respectively.....please let me know your opinion about this??
Swag said…
Mho, I want our discussions to be visible to all the visitors here so that all can benefit through this, therefore it would be a better to present our discussions here itself, in emails the discussions will remain hidden and will be wasted.
Mho said…
Ok...good point. Charge pumps, bootstrapping, isolated drives and other means of high side N channel mosfet drive techniques. Could you talk a little about them? I have discovered that the H bridge for many ferrite inverters hardly use tlp250, but almost all the iron core transformer h bridge does. Why is that so? Please explain. Thanks.
Mehedi Leon said…
Sir I hv no perfect decision about it.
that's why I asked u
I m using 650VA (390W) UPS transformer and 100AH battery(12V)
I want 400Wattage output
now my questions are:-
1. if I use your above Full bridge circuit then how many MOSFET will need to use on each Chanel? and which MOSFET?
2.if I use center tape transformer then how many MOSFET will need to use on each chanel? and which MOSFET?

sir plz make me understand correctly about this
Swag said…
TLP250 is just an enhanced opto-coupler for ensuring safe switching for the connected mosfets, because mosfets can be highly sensitive and unpredictable devices, especially when the triggering frequency is high.

At lower frequencies the vulnerability is reduced to a great extent and therefore no such special arrangement is required for their switching.

However these cannot be used for the high-side mosfets because the bootstrapping needs to be applied directly to the mosfet gate, so intermediate buffer cannot work here.
Swag said…
Mehedi, I mean to say that if you have asked regarding the dissipation factor then you should have also asked regarding the VDss, and Id factors of the mosfets because these are more prominent factors than power dissipation..

anyway, power dissipation is the value which shows at what power the device may reach its maximum core temperature without a heatsink. So at 94 watts the device may dissipate temperature which may eb dangerous for the device, and therefore adequate heatsinking must be applied to it to ensure it is able to sustain the specified max level of 55 x 49 = 2695 watts

I hope you have understood it now...
Mho said…
ok....I get more enlightened each day by your blog ...Thanks. I want to implement a H bridge switching , not unlike the one above. Only I want to use a Microcontroller for the SPWM generation. What kind of drive circuitry can I use? Mostly for the High Side Mosfets? I already successfully generated the high voltage VDc from the dc dc converter. I need to understand more the modalities involved. I got a bunch of IR2110, but unfortunately, they are mostly inferior, and act rather unpredictible. I need an alternative drive pattern. Thanks.
Mehedi Leon said…
SIR THANK YOU, I hv understood it bt
IRFZ44N( just one on each chanel) total 2.
just 2 IRFZ44N will give out put of 400Wattage!? I m so surprised, I hv no batter idea about MOSFET, thank u sir, I have anoter question, if I use your above Full bridge circuit for 400watage out put then which MOSFET should I use?
Swag said…
unfortunately there is no better driver alternative than the specialized full-bridge driver ICs, therefore you will have to test and optimize your circuit using only these driver devices...make sure to add a 1K resistor across gate/source leads of the mosfets for better stability
Swag said…
Mehedi, since The mosfets are rated to handle that much wattage so they will be able to do it comfortably...pleas note that this is as per my analysis.

you can use the same mosfets for the above inverter also
Mho said…
I have taken apart several high frequency inverters, and most of them did not use any special drivers, some did. Others used totem pole for switching the low side, and a sort of charge pump for the high side. I didnt completely understand the full rationale for the charge pump. What is your take?
Swag said…
the charge pump is used for bootstrapping and will be required if no special driver IC is used, you can read regrading bootstrapping in this article:

bootstrapping raises the hi-side mosfet gate level typically to 12V over its drain level, to enable optimal conduction.
Mehedi Leon said…
Sir I have a 12V 95 Amp Sealed led acid Battery,
now what will be minimum and maximum volt and charging current? for safer charge for long lasting the battery life,
Swag said…
Mehedi, it is standard for all 12V batteries, 14.3V full, and 11V low. Charging current will be around 10 amps for 12 hours
Mehedi Leon said…
Sir I asked about charging volt. what is the minimum and maximum charging volt for 12V-95A battery.

sir u said charging current 10A and charging time 12Hr
*Total battery Amp / Charging Amp = charging time*
so that 95/10=9.5Hr, m I right?
Swag said…
Mehedi, I have provided the charging details in the earlier comment, you can refer to it for the details. Your charging period calculation is approximately correct...
Mehedi Leon said…
Sir can I charge 12V battery with 9V or 11V or 15V transformer? I aaked u what is minimum and maximum volt to charge a 12V battery? Sir I hope now u understand
plz make me understand
Thank u
Swag said…
Mehedi, you are repeating the same question which I have already answered....
Dylan said…
Hello Sir, Thank you for the awesome website. Sorry for the silly question but does the connection between the MOSFETs connect to the 12V side of the transformer as well as the 10uF caps or do they just pass over them?

Thanks Again!!
Swag said…
Thanks Dylan, Only the negative side of the capacitor is overlapping a few of the other lines...but it is connected only with the source of the respective mosfets, and nothing else.

I cannot see any other overlapping lines other than the above
Rudi said…
hi sir,

S1 S3
| |
-------- TR ---------
| |
S2 S4

is there any deadtime between high (S1) and low side (S2) mosfet? since there are came from 1 source (Pin11 SSG3525)
i think would be better if add some deadtime at the moment they are switching to prevent 1 side are not fully off but another side is aready on.

Swag said…
Hi Rudi, yes dead time can be set by adjusting the resistor at pin#7
Yusuf abdulazeez said…
Hi Engr Swagatam. Pls, where do I connect pin #10 of IC 2535 for shut down against overloading etc.
Swag said…
Hi Yusuf, you can connect pin#10 with the output of the following opamp circuit.

eliminate the transistor stage completely and connect pin#6 of the opamp directly with the pin#10 of the SG3525.
Make sure to use 3 or 4 1N4148 diodes in series so that the opamp output leakage offset voltage is blocked.
Swag said…
sorry, the said arrangement will prevent over voltage not overloading, to control overloading you may have to swap the input pin numbers of the opamp, meaning pin#2 and pin#3 must be swapped, and the recommended output diodes reversed
Rudi said…
hi sir,

sorry sir, i think you get me wrong. i know pin7 is to adjust deadtime between outpin11 and outpin14. but what i means is between low and high side of fet. since they are opposite, for example, if pin11 is high then the low side would be high too, but at the same time high side will turn low., if we operate at some high freq, is it possible the low side is go high while the high side of fet are not fully turn off?

i think would be better to have some deadtime between them for keep it safe. i know low and high fet (at the same side) cannot be turn on together, this will blow up the fet.

Swag said…
Hi Rudi, you mean to say during dead time since the BC547 BJTs would be OFF, the high side mosfets would be turned ON, you want to have all the mosfets turned OFF during the dead time, right? I am sorry that looks difficult for this particular design.
To satisfy this condition you may have to try a full bridge topology using p and n channel mosfets as shown in the following article:
Rudi said…
Hi sir,

Yes sir. During the bc547 turn off the high side gate to gnd is cut off and make it on by a cap (bootstrap). My idea is at right before that happening, it is better to let the low side off for a short of time to keep the things smooth.

I have read a lot of example on internet, there is one of many ways is to introduce a rcd (resistor,cap,diode) can achive that. But i dont know how to implement it into ur circuit.

Swag said…
Hi Rudi,

I have updated the required design at the bottom of the article please check it out and let me know if you have any doubts.
Rudi said…
Ok thanks sir
Rudi said…
Hi sir,

I have tested your circuit. When operatr at low freq says 50hz, the waveform look great, prefect square wave. But if i ran at some khz speed, let say 33khz my waveform is very distorted. Its more look like a banana shape.

Can you point me what cause this problem? How can i make this just look as great as 50hz?

Swag said…
Thanks Rudi, Glad you could test it successfully.
When you use high frequency, inductive spikes starts getting more active and these spikes could start developing across connecting wires, tracks, solder fluxes, or any other possible unwanted source creating harmonics and distortions.

I would advise you to use a well designed PCB and make the tracks as short as possible across the mosfets and the transformer. The transformer here will be obviously a ferrite based transformer.

also use suppressing capacitors across various nodes as suggested in the following article
Rudi said…
Ok thks for the advices. I will try it again when i am got free.

Rohan chinni said…
Hi, can this bootstrap circuit work at high frequency like 100khz, I require it to operate ferrite core transformer??
Swag said…
yes that's possible, just make sure the ferrite trafo is correctly matched with the frequency and the voltage
Will the transistor BC547 respond at that high frequency of 100khz??
Swag said…
yes it should, BC547 is rated at 1MHz
Rohan chinni said…
Thank you so much for helping, one last question, how many MOSFETs can I connect in parallel? for bc547 to be able to drive them? Or can u suggest me more powerful transistor for bc547, or can I use IGBT's
Swag said…
Rohan, mosfets and IGBTs require minimal current at their gate/base to operate, therefore no special BJTs are required, BC547 can be easily used
Rohan chinni said…
Thank you so much again
Swag said…
You are welcome!!
Bugeza said…
I made that full bridge circuit and I used atransformer which had 12v at its at its primary and 220v at its secondary and it inverted igot the out put voltage 220v but when I load my inverter the output drops to 170v but may inverter need feedback at the ic can you help me with that feed back circuit sweetable for that sg 3525 ic circuit
Bugeza bendict said…
Hallo Mr I did the circuit and it is succefull with atransformer of 220v output and 12v input but the output it was 250 and I set it to 220 but when I give it load of 100w bulb voltage dropes to 180v but I think it need feedback but I don't have a feedback circuit can you please help me with the feedback circuit to make my voltage stable
Swag said…
You can add a feedback to pin#1 of the IC as shown in this diagram

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