H-Bridge Problem

Just peek at the circuit for a second, without knowing much of the rest. My suggestion is to try using a different MOSFET for the high side. Or better yet, try another proven design.

Not a great design, the mosfets would be slower to turn off than on.the upper left mosfet allways gets a current feed from the 33k base resistor connected to A.

I don't think there's any way that circuit can work. Something is missing, e.g., it's been copied incorrectly.

If the signal in is between 0v and 1v, then you're right, the upper-left BJT will always be turned on. Its base would have to get close to Vcp for it to turn off.

Or badly designed. :-) E.g., how can one expect a 1V square wave to switch an NPN at ground *and* a PNP at up to +18V???

It also suffers from basic architectural problems. The "signal" should first be squared up and prepared as a valid logic (on/off) signal, with one circuit. Then it can be inverted, level-shifted, buffered, etc., and prepared for driving the MOSFETs. But this needs to be done without delays in the wrong places, to prevent high rail-to-rail shoot-through currents through the FETs.

Next the MOSFET gates need high drive currents to turn them on and off. James' IRF3205 has of 3247pF gate capacitance, forming a disastrous 32us time constant with a 10k pulldown resistor. This big MOSFET hungers for amps of gate drive.

The easiest way to solve all these tough problems is to use an inexpensive H-bridge driver. I like to use the Intersil HIP4080A and 4081A for high-frequency bridges (e.g. 1MHz). There are many other good choices for use below say 200kHz.

--
 Thanks,
    - Win

Hi

I'm Eugene Blanchard and I first designed and posted the H-bridge schematics in 1994 to the newsgroups. There are several versions with errors floating around. Since then I've refined them until the latest version is located at

I've pretty well left them alone for the last couple of years until I run across someone having problems...

I would check the wiring, the only reason the upper left BJT would always be on is if the base resistor was grounded. A low turns on the upper BJT. There's some problems with this design as the motor will provide a large inductance to the circuit and cause a slow transition when switching. This puts the mosfets into the linear mode where they shouldn't be. They'll heat up and fry themselves.

Go to the link mentioned earlier and it discusses a better design. Good luck

Eugene, your design needs work. I'd like you to respond to the post I made in this thread (the most recent post before yours).

--
 Thanks,
    - Win

less bad might be a better description.

Rg_on = 100R Rg_off = 10k = 100*Rg_on

cross-conduction galore.

Cheers Terry

I guess not....

Cheers Terry

Perhaps my comment wasn't strong enough. The design should be immediately removed from circulation, before it hurts anything.

--
 Thanks,
    - Win

And worse still, teaches a whole new generation of electronics enthusiasts to design circuits that dont work.

Cheers Terry