HBridge motor control

[Jon Slaughter]

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

 

[gearhead]

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?

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

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

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

 

[Jon Slaughter]

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?

----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?)

 

[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?)- Hide quoted text -

- Show quoted text -

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.

 

[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)- Hide quoted text -

- Show quoted text -

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.

 

[Jon Slaughter]

----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?)- Hide quoted text -

- Show quoted text -

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)