We all probably know regarding the H-bridge configuration which has some very important applications in electronic circuits. However this configuration is never easy to make and implement, especially when mosfets are are involved.

Fundamentally an H-bridge configuration consists of 4 transistors or four mosfets laid down such that the connected load (primarily inverters)becomes switchable in a push pull manner using a single supply.

Yes that's probably the main feature of an H-bridge which allows the load to be operated in forward and reverse direction without the need of dual supply voltages.

Using H-Bridge Configuration


An H-bridge design typically includes a couple of P-type devices positioned at the positive end and a couple of N-type devices positioned at the negative end of the supply rails and interconnected together and with the load in a manner that quite resembles the letter "H", and hence the name H-bridge.

The devices may be transistors or mosfets. However mosfets being extremely efficient with its characteristics, are preferred more than the transistors nowadays.

But when it comes to mosfets, things start becoming critical especially with its switching parameters.

When mosfets are used in an H-bridge, there's always a fear of an "shoot through", that is switching of the mosfets in a single line instead of the diagonal ones, which must be strictly avoided.

However conditioning the correct switching frequencies to the respective mosfets becomes a big headache, especially with ordinary gates and transistors.

Although there are mosfets driver for tackling this issue, making your own version using ordinary components can be even more interesting as ot allows the many newcomers to understand the issue more clearly.

Eliminating Mosfet Shoot-Through


I have faced this problem while designing an inverter and therefore was forced to design a simple clocking circuit for  mosfet H-bridges which would completely eliminate the danger of a "shoot through" and yet be easy to build.

A shoot through issue primarily happens due to the absence of a transition gap between clock polarity changeovers. Meaning when the input clock to the H-bridge changes from positive to negative or vice versa, there's always an intermediate period when it's neither fully positive nor fully negative, giving rise to a situation where all the mosfets tend to conduct for a fraction of a second.

Since a mosfet is a very sensitive, this situation instantly drives a huge current through all the mosfets, blowing them off instantly.

You try replacing them with new ones, you just keep blowing them.

So it becomes imperative to incorporate a switching device which inserts a momentary "dead zone" between the transition periods, such that none  of the mosfets conduct during the switching polarity thresholds.

The use of the IC 4017 and the transistors in the circuit effectively curbs this problem.

Simulation and Working


Looking at the figure we see that the outputs of the IC 4017 are connected to the transistors such that the transistors conduct in sequence, but the skipping of a pin out from the IC between the two outputs makes sure that the transistors operate only after passing through a dead zone.

For example suppose the pin#2 of the IC is high, the relevant transistor switches ON, switching the respective N and P channel mosfets and the load is switched ON in one direction.

Now when the pin#2 switches OFF, the sequence shifts to pin#4 which is not connected anywhere and becomes the dead zone.

During this time pin#2 goes low, switching off the respective mosfets.

When the sequence is at pin#4 all mosfets are off, until the time when the sequence reaches the pin#7 when the next transistor switches ON, making the load switch oppositely.

This inclusion of a "dead zone" at pin#4 of the IC keeps all the dangers of a shoot through perfectly at bay.

Circuit Diagram





Parts List


All resistors are 10K 1/4 watt

Mofets: IRF540, and IRF9540

BJT = BC547

IC = 4017

Clock Input: can be from any IC 555 astable circuit for other similar oscillator circuit

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