H-Bridge Design suggestions

audioguru · Dec 12, 2007

I think the driver circuit for the Mosfets should be like this:

ahmedragia21 · Dec 12, 2007

thanks dknguyen
i understood the problem i reversed the C of the PNP's to be E

audioguru , thanks
you choosed 22ohm , low resistacance to compensate the input capacitance of the mosfet ? to make it switch fast in fast PWM frequencies ? or what was the purpose ?

mneary · Dec 12, 2007

Is your supply voltage 12V or less?

ahmedragia21 · Dec 12, 2007

24V
also can 2n2222 stands this ? or not ?

dknguyen · Dec 12, 2007

Yeah, he chose the 22ohms so that the gate capacitance would charge faster so you could switch the FETs faster. You might not need them at actually- only if your FETs turn on too fast and make too much noise or other problems.

There is a difference between the voltage that a transistor can withstand across it's primary terminals and across it's control (base/gate) terminal. SO a 600V transistor can withstand 600V across C-E or D-S, but it can only take a maximum of 20V at B (or G). He's asking because you might be thinking of powering your gate driver form the same source as your motor and if the source is too large you will destroy your transistor bases/gates. Check your datasheets for all your NPNs, PNPs, NMOSs, and PMOSs.

audioguru · Dec 12, 2007

If the Mosfets don't switch very quickly then for a moment the top and bottom ones are both turned on and short the power supply. It is called the shoot-through current. Mosfet driver ICs have a small delay to prevent it.
The 22 ohm resistors should be very close to the gate of each Mosfet (4 are needed for your circuit) and stop the Mosfets from oscillating.

ahmedragia21 · Dec 12, 2007

thanks for the reply ,

so its a hardware delay then right ?

the input to the transistors will be 5/0V from a microcontroller and the supply VCC to all transistors is 24V

dknguyen · Dec 12, 2007

Yeah, an RC-type timer thing with the gate capacitance and your gate resistor.

THat 24V supply will destroy the gates and bases on your transistors. Use a 12V or 15V regulator (depending on your MOSFETs and BJTs) to provude the power to your gate drive so you aren't driving the bases and gates beyond their voltage limits.

audioguru · Dec 12, 2007

ahmedragia21 said:
the supply VCC to all transistors is 24V

Then the Mosfets will blow up because their max allowed gate voltage is 20V.

A Mosfet driver IC has a circuit that allows any output voltage and the gates get only 10V or 12V max.

ahmedragia21 · Dec 12, 2007

Thanks alot )
very helpfull
what do you suggest to read?
Art of electronics BJT , FET Chapters ?

ahmedragia21 · Dec 12, 2007

i saw this design and the designed used it and its worked and he used a 24V supply as you see , what's the difference between my circuit and this ? and which is better ..

audioguru · Dec 13, 2007

Look at all the links about H-Bridge Circuits in Google.

audioguru · Dec 13, 2007

ahmedragia21 said:
i saw this design and the designed used it and its worked and he used a 24V supply as you see , what's the difference between my circuit and this ? and which is better ..

That circuit has the PNP transistors the correct way around, yours are backwards.

The diodes and 470 ohm resistors reduce the max gate voltage to about 22.6V for the N-channel Mosfets. The gate voltage for the P-channel Mosfets is 24V.
But the absolute max allowed gate voltage for both Mosfets is only 20V. They are spec'd with 10V.

ahmedragia21 · Dec 13, 2007

What about putting a 12V Zener diode at the Gate of each mosfet ?

dknguyen · Dec 13, 2007

ahmedragia21 said:
What about putting a 12V Zener diode at the Gate of each mosfet ?

THe zener cannot clamp on and off fast enough to keep up with the PWM waveform you are sending to the gate. Doing that is better for protecting the gates against overvoltage spikes that may happen, not but not from frequent overvoltage spikes (like 20,000 times per second for every second that the H-bridge is operating).

And even if it was fast enough (which I don't think it is, but I may be wrong) it's pointless anyways since if you just use a zener regulator (or a linear regulator) for the gate drive power supply. THen you only need one zener instead of 6, one for every BJT and MOSFET.

ahmedragia21 · Dec 13, 2007

so what's the soultion then to this problem ?? i posted the updated schematic. .

thanks for your help really =)

audioguru · Dec 13, 2007

If you limit the gate voltage to 12V with a zener diode then the bottom N-channel Mosfet will be turned on, and the top P-channel Mosfet will also be turned on at the same time. They will create a dead short to the power supply's 24V.

The solution is to use a Mosfet driver IC.

ahmedragia21 · Dec 13, 2007

hmmm, what about choosing a POWER BJT instead of the mosfets ? would the circuit be changed ? i dont think so ?

audioguru · Dec 13, 2007

ahmedragia21 said:
hmmm, what about choosing a POWER BJT instead of the mosfets ? would the circuit be changed ? i dont think so ?

Then the power transistors need a high base current through big high power resistors. Maybe the complimentary driver transistors can't supply enough current.

Power transistors switch slower than Mosfets so there will be a very high shoot-through current .

ahmedragia21 · Dec 13, 2007

I used the same circuit and changed the P Mosfets to PNP TIP147 Transistors and N Channel Mosfets to NPN TIP142 transistor and the circuits doesnt work in the simulator ...

H-Bridge Design suggestions

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