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Simple minded H-bridge driving

throbscottle

Well-Known Member
So, I finally went back to my ridiculously over-complicated automatic drill feed controller after abandoning it for many years. Took a few days to figure out how it works. I was obviously smarter back then!

Anyway, I re-visited the H-bridge design and added a couple of driver transistors to it. The gates of the P and N mosfets which form each leg of the bridge are tied together and have a pullup resistor, and are driven by a level shift transistor which can pull it to ground, or the resistor can pull it up. Well known simple design.

But the major flaw with the design is that there is plenty of opportunity for shoot-through, which I think is happening (drive is PWM from a 555 timer gated through a 4066 where it is the signal, not the control). It's ok if you're just changing direction with it, but it doesn't like PWM.

The PWM signal is gated through two channels of a 4066 quad analogue switch.

So I changed the design so that the four bridge transistors all have separate gate pullup resistors so the N mosfets are still always on, and the P mosfets are still always off, but now with separated control, and the inverse of the gating signal for the PWM turns off the N mosfets, and the gated PWM drives the P mosfets via the driver transistors, which is great for one leg, but the other leg doesn't have an inverse gating signal, so that's a small problem.

Rather than add yet another transistor to the design, I tried changing the input signal of the 4066 switch to 0v, and applying the gating signal to the emitter of the driver transistor, leaving PWM input to it's base.

It looks as if it will work fine but I wanted to get some more experienced eyes to take a look at my one-transistor "gate". The mosfet in the simulation represents one P-mos from the H-bridge, and the motor model is arbitrary, I have no idea what real values to use.

What do you think?
PWM-gating.jpg
 

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alec_t

Well-Known Member
Most Helpful Member
The gate looks to be working ok, but there are some nasty negative voltage spikes on M1 drain, which will require a catching diode.
 

throbscottle

Well-Known Member
Yeah I worked out eventually where that one was supposed to go! The real H-bridge has them built in :)
Thanks for the feedback - I was half expecting to be told this was a terrible idea!
 

dr pepper

Well-Known Member
Most Helpful Member
I in real life the fet gets warm you can rig a simple but effective pull up for the gate from a npn tranny, a diode and a resistor.
 

throbscottle

Well-Known Member
I doubt it will be problem as the motor is very small and I've already given the gate a 1k pullup anyway, which I can't seem myself improving on. But I'm now curious as to what you have in mind?
 

dr pepper

Well-Known Member
Most Helpful Member
Here is a circuit that doesnt even use the diode, but for P channel you'd have to mirror everything below the gate line, the collector & 10k would then go to B+, and of course an Npn tranny, its near the bottom of the page with the red box around it.
If you were using a fet with huge gate/miller capacitance then you might need an R inline with the base:
 

throbscottle

Well-Known Member
Very useful link dp, also the one following on about calculating gate resistance. I hadn't even thought about gate capacitance so I'll have to spend some time on this one. Thanks :)
 

dr pepper

Well-Known Member
Most Helpful Member
Gate capacitance or more appropriatley gate charge needs to be taken into account, also miller effect affects switching time and loads the gate driver.
 

throbscottle

Well-Known Member
TBH I've got an old board-pulled integrated bridge and driver (TA7291S) in my junk box for small motors, I'm increasingly inclined to just use that since my component count is creeping up. But I'll still do the reading ;)
 

dr pepper

Well-Known Member
Most Helpful Member
I got some 2 quid motor drivers off ebay, I was ordering some other stuff and just added them on, they are an smd and can handle 30a without any kind of 'sink, mental.
Switchers can get complicated, there are lots of little traps to get you, thats why I buy several sets of fets & things.
 

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