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HBridge motor control

Ok, I just thought that I do not need to control the high side of an H-Bridge?

Essentially the high side needs to be on all the time(obviously the opposite side needs to be off).

i.e., if its

H1 H2

L1 L2

Then H1 is on, H2 and L1 are off, L2 is PWM.

Obviously to reverse its the opposite(H2 on, H1 and L2 off, L1 PWM).

Is this correct and good practice? For some reason I was thinking I should PWM H1 and L2 so that both would conduct at the same time but this seems to be redundant because turning off L2 should be good enough?

The problem I'm having is that if I do it discretely then its a lot of fets(or BJT's and resistors + wasted power) and I can't seem to find a driver that works with high side Pch(they all seem to want to drive Nch).

Even if it takes a few ms to charge the gate capacitance of H1 and H2 it shouldn't matter because these only occur at the start and end of powering the motor(the motor will not be reversed often). So I should be able to drive them directly from logic which will save a few transistors. The low side will just be done using cmos.

Is this a good idea or not? If not are there any mosfet drivers that will do Pch high side instead of Nch? (or is it ok to use the ones that drive Nch with Pch(I'm afraid it will pull the Pch source to far down(Although its like +-20V so I guess its not that bad but doesn't leave a lot of room))

Thanks, Jon

Reply to
Jon Slaughter
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te

o

----Of course you only have to turn one of them off to interrupt the circuit.

do

-----You might use a gate resistor and a zener to clamp the gate voltage.

Reply to
gearhead

----Of course you only have to turn one of them off to interrupt the circuit.

Yes but is it a problem? Is it better to turn off both or does it really matter? (ultimately its probably better but probably doesn't matter 99.999% of the time?)

Reply to
Jon Slaughter

ld

%

I just remembered motors are inductive so you might want to provide a path for the current to continue to flow after you turn off the n- fet. That would mean a freewheel diode from the n-fet drain to the positive rail: anode to drain, cathode to +rail. And you have to keep the p-fet turned on to complete this circuit. So by all means save yourself the trouble of turning off the p-fet; not only is it easier but the motor will run better (and stick that diode in there). The diode will also keep the n-fet from getting hit with a voltage spike every time the PWM controller switches it off.

Reply to
gearhead

ms

y

The body diode of the low-side fet will be reverse-biased when you turn the fet off. It will not conduct. The body diodes in the upper pfets are oriented the same as a freewheel diode and would conduct as such, but you really should check the specs of those pfet's body diodes to see whether they are fast enough for the job. I suspect they are not. You may decide you want to put a fast diode in parallel with each pfet. Now, about the benefits of freewheeling the current from the motor's inductance: If you turn off both the high and low switches of the h bridge, one or the other will get a jolt from the inductance of the motor. It just depends on which one turns off faster. You can get rid of this effect entirely by providing a return path for the current, it's standard practice, why do it any other way? By leaving the upper fet on your job is easier -- you only have to PWM the low side, you eliminate troublesome voltage spikes, and the motor will run better. And the current doesn't run back into the battery, it just decays.

Reply to
gearhead

I just remembered motors are inductive so you might want to provide a path for the current to continue to flow after you turn off the n- fet. That would mean a freewheel diode from the n-fet drain to the positive rail: anode to drain, cathode to +rail. And you have to keep the p-fet turned on to complete this circuit. So by all means save yourself the trouble of turning off the p-fet; not only is it easier but the motor will run better (and stick that diode in there). The diode will also keep the n-fet from getting hit with a voltage spike every time the PWM controller switches it off.

-----

The FETS's have catch diodes built in(as almost all power mosfets do?) but of course I can add more.

So I agree with you but the issue is will it stress the n ch fet more because it will be doing all the "work"?

I'm curious now as the back emf brings up an issue ;/ Does the current go back into the battery or does the motor act as both a generator and motor exactly at the same time to balance out any flow into the motor(essentially trying to charge up the motor)? (obviously if you remove the fets and have just one path then it will try and charge so the only difference is the diodes)

Reply to
Jon Slaughter

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