Buck Boost Using IC 555

The post explains an universal IC 555 based buck-boost circuit which can be used for various different applications involving efficient power processing requirements.

Using IC 555 for the Buck-Boost

This highly efficient and effective buck-boost circuit using the work horse IC 555 would allow you  to convert an input source voltage to any required degree, either bucked or boosted, as desired.

We have already learned the concept comprehensively through one of my previous articles where we discussed the versatility of this buck-boost type of topology.

As shown in the circuit diagram below (click to enlarge) the configuration is basically a combination of two distinct stages, viz the upper buck-boost converter stage and the lower IC 555 PWM controller stage.

The buck-boost stage consists of a mosfet which acts like a switch, the inductor which is the main power converting component, the diode which just like the mosfet forms a complementary switch, and the capacitor quite like the inductor forms a complementary power converter device.

The mosfet needs to operate through pulsed triggering so that it alternately switches the input voltage ON and OFF across the inductor in response to its gate voltage.

Therefore the gate voltage should also be in a pulsed form which is accomplished through a IC555 PWM generator stage.

Simulation and Working

The associated IC555 PWM generator is integrated to the mosfet for accomplishing the above discussed operation.

During the ON time of the mosfet, the input voltage is allowed to pass through the mosfet and is applied right across the inductor.

The inductor owing to its inherent property tries to counter this sudden infliction of current by absorbing and storing the power in it.

During the subsequent OFF period of the mosfet, the input voltage is shut off by the mosfet, the inductor now experiences a sudden change in current from peak to zero. In response, the inductor counters this by reversing its stored power across the output terminals via the diode which now acts in the forward biased condition.

The above power from the inductor appears with opposite polarity across the output where the intended load is connected.

The capacitor is positioned to store a portion of the power in it, so that it can be used by the load during the ON time of the mosfet when the diode is reverse biased and power cut off across the load.

This heps to maintain a steady and stable voltage across the load during both ON and OFF cycles of the mosfet.

Using PWM as the ontroller

The level of voltage, whether it's a boosted voltage or bucked voltage depends upon how the mosfet is controlled by the PWM generator.

If the mosfet is optimized with higher ON time than the OFF time then the output would generate boosted voltage and vice versa.

However there may be a limit to this, care must be taken not to exceed the ON time beyond the full saturation time of the inductor, and the OFF time must not be below the minimum saturation time of the inductor.

For instance suppose it takes 3ms for the inductor to become fully saturated, the ON time in this case can be set within 0 - 3ms, and not beyond that, This will result in a boost from minimum to maximum depending upon the value of the chosen inductor.

The associated pot wit the IC555 PWM generator can be effectively tweaked for acquiring any desired buck-boost voltage at the output.

The inductor value is a matter of trial and error, try to incorporate as many winding as possible for acquiring better and efficient results and diverse range.

 Circuit Diagram

buck-boost control circuit

The above design can be suitably upgraded for implementing an automatic output voltage correction with the help of the following modifications:

The 1K preset may be appropriately set initially for determining the desired control point.

IC 555 Pinouts

IC 555 pinout

Need Help? Please send your queries through Comments for quick replies!


Raja Sekhar said…
sir may i know the design considerations of this circuit sir?
Swagatam said…
Hi Raja,

No critical things are involved, just has to be built as per the given schematic.
kkaranjia said…
Dear Swagatam,
whats the minimum value of input voltage, how much power does the circuit itself consumes and whats the max ampere capacity of the circuit
Swagatam said…
Dear kkaranjia,

minimum voltage is 5V

the IC will consume 5mA, not sure about the other components, will depend on how the coil and capacitors are optimized.

Max current will depend on the mosfet and inductor ratings, there's no restriction to it.
kkaranjia said…
cant it be modified to run on li-ion battery and give op of 5volt
Swagatam said…
you will have replace the 555 oscillator with a transistor type oscillator, then it would be possible.
kkaranjia said…
can you help me design that transistor type oscillator
Swagatam said…
I think you should try the following circuit, it looks more suitable for your application:

kkaranjia said…
dear swagatam i want constant 5v o/p to charge the mobile battery so is this concept usable there ?/
Swagatam said…
You can use a 7805 at the output with a 1n4007 in series, that will hopefully do the job.
kkaranjia said…
wont the o/p be fluctuating and wont it harm the mobile...?
Swagatam said…
what is the input voltage?
kkaranjia said…
will be using 3.7 volt Li-ion battery so the input will vary from 4.2v to 3.6v
Swagatam said…
i did not quite understand, you want to use a 3.6V Li-ion battery at the input for charging another 3.6V Li-ion battery at the output???
kkaranjia said…
yes you are right i want to do that but some mobile phones like micromax wont get charged if it is supplied below 5volts.
Swagatam said…
OK, understood.

The linked circuit which you provided is fine, you can make it.
Ferry Effendy said…
Hi Bro, I'm newbie electronic hobbies, and become addictive due to the articles at your site. Kudos for you.

Right now I'm trying to built the above circuit, but do not have the right inductor in hand.

Could you guide me for diy to make an alternate inductor using ferrite rod dia. 10 mm x 80 mm long with isolated single core wire from regular power cord?
Btw, refer to your above design, are the 2 diodes arrangement across the 50K Pot correct?

Thank you for your help and regards.
Swagatam said…
Thanks Bro,
you can proceed with the listed inductor materials, however it will need to be done through trial and error, by altering the number of turns and by checking the output voltage correspondingly.
Yes the two diodes are connected across the two ends of the pot, you can click the diagram to enlarge.
Ferry Effendy said…
Thanks for your reply Bro,

Just fyi, I have tried with 682 inductor (that off-course far away from your suggested value), bypassing the Zener diode and the 10K resistor across it, and feed the circuit with 12 V lipo battery, max output was only about 3.6V, mosfet and inductor became hot. I will try it again later with suitable variable inductor if I get it in hand.

Thanks again and have nice week end days.
Swagatam said…
The above circuit can act both as a buck converter or a boost converter, will depend on how the 555 pwm output is adjusted, also the inductor will need to be dimensioned as per the load current (amps) for optimal functioning.
Hi want convert 24 to 12 dc to dc is possibe with ne 555 buck convertor
Swagatam said…
yes it's possible.
Vivek Samson said…
please give rating of all the compnents used
Swagatam said…
mosfet is IRF540, rest everything is given in the diagram, please click it to enlarge.
Swagatam said…
blocking diode is BA159
Mark Ledford said…
i love the design im kinda curious what the amp rating would be in the current configuration that way i can try to boost it or build 3 of these units in parrellel for a solar project to handle 100 Amps from solar
Swagatam said…
amp rating can be of any value as required by the user, for acquiring 100amp you could just use 3nos of IRF540 in parallel in the same circuit
Swagatam said…
try a 1K 2 watt resistor.
Swagatam said…
the maximum wattage of your panel is 14 x 3 = 42 watts, so you cannot get by any method an output more than 40 watts from your solar panel....so 100 watt is entirely beyond range.
Hi Swagatam, if I want to change my 555 timer to a duty cycle of 70% what values do I change on the circuit above
Sir Swagatam how do I change the duty cycle
Swagatam said…
Hi Vongani,

you can do it by adjusting the given pot and simultaneously checking the pin3 DC voltage with a meter, when the meter reads 70% of the supply then you can be sure of the required level being achieved, for example if the supply 10V then the meter should indicate 7V at pin3 of the IC 555
imanul sofian said…
Dear Mr Swagatam,
what components should be added to make the dutycycle automatically adjust itself according to the varying load? This means that the output voltage stays stable at a predetermined voltage regardless any changes in load. I'm thinking about IC LM2575 here as a comparison, which has an add-on voltage reference (voltage divider) on the output. Thank you.
Swagatam said…
Dear Imanul,

I'll update the diagram with the required feature and post it...soon.
khem cabuga said…
Sir, what is the maximum voltage input?
Swagatam said…
it can be anything, you just have to upgrade the fet, the capacitor and the 555 IC input resistor watt accordingly.
which resistor is the 555 input resistor?
Swagatam said…
which input? do you mean the supply input?
Yes, the supply input
And to be more exact what is the maximum supply input for the design above (with no changes)
Also, what the wattage of the resistors in the path for the 555.
How can I calculate the required resistors (in the supply path for the 555) values for higher or lower voltages?
if I wanted to use higher voltages (60+ Volt DC unregulated) should the L (in Henry) and C in Farad values stay the same or should I recalculate it based on of your other articles (https://homemade-circuits.com/2013/06/exploring-buck-boost-circuit-concept-in.html)
Thank you in advance
Swagatam said…
the 10k resistor connected with the pin4/8 of the IC 555 is the supply input resistors for the IC.

using a 1 watt for this resistor will be enough for upto 60V

rest all can be 1/4 watt

L will need to changed for different volt inputs.... by rule of thumb it could be 1 v per turn, that is 60 turns for 60V
Swagatam said…
the supply resistor can be calculated by using the following formula:

R = V/0.01
Swagatam said…
answer will be in ohms
Thank you very much for your detailed answers,
May I ask more questions?
in case of yes,
What is the operating frequency of 555 o/p ?(i think frequency here is independent of duty cycle percentage right?)
How can I change the operating frequncy? (which components decide it?
I assume the 100nF connected to the 50k pot is responsible for the frequency with the 50K am I right?
what if I changed the 100nF (because I do not have it for example ) to 220nF, what else I should change?
and how to tweak or tune the switching frequency?
Thank you in advance
Swagatam said…
the frequency should be above 50kHz for optimum response from the ferrite inductor.

frequency can be changed by changing the piin6/2 capacitor, 50k is for changing the duty cycle, not the frequency

the duty needs to be tweaked for adjusting the current of the output with respect to the load, frequency can remains constant.
VM said…
Hi Swagatam,
I am a newbie to electronics and an avid reader of your blogs.

I need a voltage booster (12 volt input - (variable) max 22 volts out) circuit to be connected in series with the load. i.e. the circuit will be connected between the battery (12 volts) and the load hence I need only 2 wires on the circuit (series connection).

Can this circuit be modified for series application? If possible could you please suggest how I should modify it?

Also the power draw would be 15Amps DC max. Could you please suggest the ferrite core inductor size and winding wire details to make it capable of 15 Amps max?

Appreciate your help.

VM said…
Hi Swagatam!

I am a newbie to electronics and an avid reader of your blogs.

I need to connect a pwm based variable voltage booster circuit in series between the battery and the load.
Input voltage : 12 Vdc
Output voltage : Variable up to 24 Vdc Max
Max amps : 10 - 15 A max

For this circuit, I need only 2 wires (one input and one output) to be able to connect in series with the load.

Is it possible to modify the 555 based buck boost circuit for this application?
Also could you please suggest the inductor type and the wire winding details for this circuit?

Appreciate your help.

Swagatam said…
Hi VM, yes you can try the first circuit from above for getting the required output.

the circuit has two wires only, there's no third wire here.

the inductor number of turns should be twice that of the supply voltage input wound over a ferrite rod or any suitable ferrite core, wire gauge can be 22SWG

VM said…
Hi Swagatam,

Thanks for your quick response.

I have made a drawing for your reference to exactly show how I intend to connect this boost circuit.


After checking the drawing, could you please update which points I should connect exactly?

Also for the ferrite rod or torroid core, is there a specific minimum diameter I should use for maximum efficiency?

Thanks for your help once again.
Swagatam said…
Hi VM, after you make the above circuit connect the 12V source to the left side "input voltage" points and the load across the right hand side "buck/boost" points.

The formulas for calculating the parameters are extremely complex...you can try it by trial and error....you can start with a ferrite rod which were used in AM radios long ago.

Number of turns can be around 30 to 40 using any magnet wire between 22 to 27 swg
VIJAY AJ said…
Sir.I like your blog so much most of the circuit u posted in this blogs works well.now I need a circuit to charge my 6v.1.3ah battery from 6v.300ma current rating solar panel.can I use this circuit to increase a voltage in range of 7v
Swagatam said…
Thanks Vijay, the above shown designs can be a little complex for your need, instead you can try the following circuit and optimize the coil to suit your application need

VIJAY AJ said…
Thank you very much sir for your reply.I need one clarification sir whether this circuit automatically change from buck mode to boost mode and boost mode to buck mode based upon the need????
Swagatam said…
as far as I have understood yes this circuit can be used for achieving both buck and boost outputs depending on the setting of the PWM pot.
VIJAY AJ said…
Thanks again for your reply sir. Please help me to solve this doubt sir. I am using 18v solar panel to charge my 12 v battery. I build your lead acid battery charging circuit successfully thanks for posting useful circuits. I noticed one think that by using lm317 i made a constant voltage to charge my battery at this case we are losing some amount of energy in the form of heat to avoid this I planned to use a buck boost circuit for following reason

1) If the solar panel output is to high for ex. If I get 18v from the output of solar panel to charge 12v battery I planned use this remaining 6v to increase the value of current to increase the efficiency of charging by using buck circuit
2)In the case if the solar panel output voltage decrease at evening period, at this time I need to step up the voltage for few period of time by using boost circuit
Shall I use this circuit to do both process automatically?? Whether this circuit works or not.please reply me sir
Swagatam said…
Vijay, a 12V battery will require minimum 14V to get optimally charged so there would be hardly 4V extra for you to use during peak sunshine.

Yes you can try out the above design, and try adjusting the PWM control to achieve the results, however you may have to adjust the pot every after 3 hours to regulate the output, I am not very sure abut the proceedings since I haven't yet tried the concept practically.
VIJAY AJ said…
OK sir. If there is any circuit with window comparator and rtd to make the pwm automatically without adjusting the pot manually. Similar to solar tracking system circuit u posted
Swagatam said…
Vijay, you can try the first circuit from this article:

VIJAY AJ said…
Sir can u please tell me how this 555ic produces pwm.I am 3rd year eee student in my books they used reference and carrier signal to generate pwm. I didnt see any reference and carrier signal on this circuit.please reply sir
Swagatam said…
Vijay, the reference is fixed at pin#5 of the IC internally...while the pot value determines (adjusts) the peak volatge of the triangle waves at pin7 ....which is compared with the potential at pin5 for dimensioning the PWMs.
Unknown said…
dear mr. Swagatam,
I want to boost a dc input that can vary from 1.2v to 4.3v to an output voltage between 5v-20v (that i adjust) and it should be regulated. can i use your circuit? if not, what do you recommend me to do ?
Swagatam said…
The above circuit won't work because the minimum voltage spec is 4.5V, I think you should go for a joule thief kind of circuit, you could probably try out the following:

Mark Ledford said…
Hello Swagatam, i love the work you do. I have been following your Simple Solar Mppt Part 2 and than I look at this unit over and over again thinking Wouldn't a buck boost type system be simpler using pwm. Is there something I am missing as to why this with a battery charger setup added wouldn't be better as a charge Controller?
Swagatam said…
Hello Mark,

thanks very much for liking my site.

managing and optimizing two features together could pose some complication that's the reason I included only the buck converter in my MPPT design, and if a boost converter is required in the design one can apply it separately through a different set of calculation.
emmantrop said…
Hello sir, please what is the different between this design and this https://homemade-circuits.com/2014/11/self-optimizing-solar-pwm-charger.html and which one is more effective and optimise? Thanks for the good work you are doing here.
Swagatam said…
Hello emmantrop, both are effective depending on the panel specs...if panel is low current high voltage type then the linked article idea is good, if the panel is low voltage high current type then the buck/boost concept can be better.
Owais Ahmed said…
Hello Sir. First time on the website. Glad to see intellectuals like you.. I am designing a buck boost circuit. For now my circuit differs in two ways.
1 I am giving the the PWM through a function generator.
2. I am using IRF840. The problem is it switches at 20 volts.Which is quite strange. what am i doing wrong? Should a resistor be attached before the pulses go to the gate?
3. What type of Load are you using ? Is it dynamic or static? to what limitations should i consider while using a load of DC motor of 24V and 2amperes?

Your help will be highly appreciated :)
Swagatam said…
Hello Owais,
the PWM can be fed from any source that's not critical.
the fet should start working from 9V onwards, so 20V is abnormal, may be the fet is faulty or duplicate...I would recommended a BJT first to check the response and then you can replace it with an FET later on.

The load power requirement should not exceed the FET or BJT power specs...that's the only thing we have to take care of, rest of the optimization will depend on the input power specs, the PWM duty cycle and the output capacitor.
salman blast said…
sir i am working on pedal power generation project i am using a car alternator whose rating is 12v 49amps when i am driving the alternator i am getting a constant output of 15v . i want to boost the voltage till 26v so i can charger 2 12 volts battery in series
and buck it to 6v and 14v so can i use this circuit if not what changes must be done
how can i vary the voltage which one acts as a potentiometer
Swagatam said…
Salman, you can use the available 15V itself and charge the batts directly, in parallel instead of putting them in series.

for buck conversion you can try the following design:

Ello Oku said…
Hello sir, What can be the Max input and output?
Swagatam said…
It can be configured at any desired level
Ello Oku said…
Sir, I will like to get 5v 2A as an output to power 4 devices of the same rating. Input will be 20v 7A. Please how do I do that?
Swagatam said…
Ello, your application is of a buck converter, so you can try the last circuit from this article


the details are provided in the article itself

Sir, Have you made the circuit ?
AJAY K.V said…
sir, how did you calculate the values of the inductor and capacitor??
I went through your article on "how buck boost converters work" and texas instrument page on buck boost design,but did not find any formulas for inductor and capacitor calculations.
I want to design a circuit with 12V input and 24V output,could you please give me the inductor and capacitor values and the formulas you used.
Swagatam said…
Ajay, I have tried to explain a simpler approach for the new hobbyists through this article


for a more technical approach you can read the following article


for the calculating capacitor, you can probably apply the theory explained in this article

olupot said…
hello sir need some help regarding the circuit i made it but output is giving me what i want thats 24v. input is 12.9v pin #3 of ic is 5.6v and output is giving me 5.5v.yet i have changed inductors from 40uH to 150uH thanks your help is apreciated
Swag said…
Olupot, do one thing, try the concept which is presented in the following article, it is the traditional concept and easier to adjust


ignore T2 stage initially
Mamman said…
Hi Engineer Swagatam, Please can I use any diode for the blocking diode thanks.
Mamman said…
Hello Engineer Swagatam, Thanks for you nice work. Please can I use any rectifier diode for the blocking diode?
Swag said…
Hi Mamman, any diode will not do, you will have to use a fast recovery type of diode...you can try a BA159
A.B.M. Zahirul Huq said…
Dear sir,
How much current I draw from this circuit? I know it's depend on mosfet and inductor but i want to know max. amount of current if I want.

Swag said…
Max current will depend on the max capacity range of the mosfet, coil and the diode rating
A.B.M. Zahirul Huq said…
I think so.

Thank for your reply.
matt said…
From the above circuit does it mean that the input GND voltage become the output POSITIVE voltage?
Swag said…
yes that's right, as per the concept it becomes positive for the load only during mosfet switch OFF periods
peter said…
hello, i am making a booster circuit with 5v output using your circuit but the voltage output isnt 5v. is it okay to use any mosfet? i used stk0825 but the output is below 1v. my input voltage is 3.7v li-ion battery, and the coil i used is toroid coil with 30 turns. i also used a 16v 1000uf. also, the preset i usedwas 103.please give me some advice
Swag said…
I have not yet tested the circuits explained in the above article, Please try one of these designs instead:


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