Hi.
First of all, apologies if I have posted in the wrong forum. I wasn't really sure where to put this.
I have built a simple H-Bridge for motor control and the schematic is below:
It works but all is not functioning as well as I would like.
When running in the forward direction input A is modulated, whilst input D is held 'closed' with the N channel low side driver conducting.
The inputs are driven with logic level signals, with their state inverted to give correct H-bridge operation (i.e. to 'right' the inversion of the driver stage)
In operation the high side PMOSFET is cool, however the lowside NMOSFET is getting very hot.
I am beginning to suspect that the low side MOSFET is seeing the motor voltage WRT to ground, and the gate driver voltage (10-15V) and is not remaining fully switched on, OR the pulsed voltage from the high side switching is interfering in some way with the operation of the low side driver.
I am looking for some advice/critique/improvement ideas from the knowledgeable folk here.
Any help or advice would be most welcome.
First of all, apologies if I have posted in the wrong forum. I wasn't really sure where to put this.
I have built a simple H-Bridge for motor control and the schematic is below:
It works but all is not functioning as well as I would like.
When running in the forward direction input A is modulated, whilst input D is held 'closed' with the N channel low side driver conducting.
The inputs are driven with logic level signals, with their state inverted to give correct H-bridge operation (i.e. to 'right' the inversion of the driver stage)
In operation the high side PMOSFET is cool, however the lowside NMOSFET is getting very hot.
I am beginning to suspect that the low side MOSFET is seeing the motor voltage WRT to ground, and the gate driver voltage (10-15V) and is not remaining fully switched on, OR the pulsed voltage from the high side switching is interfering in some way with the operation of the low side driver.
I am looking for some advice/critique/improvement ideas from the knowledgeable folk here.
Any help or advice would be most welcome.
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Quick thought, connect R2 to 24V and get rid of R4 - this will allow you to turn off the high side mosfet which currently will be on all the time. R3 needs to be close to the gate. The gate voltage probably needs to be less than 20V to keep from frying the fet - you could use a 15V zener clamp if this does not need to be fast. (Remember to limit the zener current to a safe value when the fet is turned on.)
There are high side driver ICs just for this application.
If you need speed consider totem pole NPN/PNP driver for the fet gate..
There are high side driver ICs just for this application.
If you need speed consider totem pole NPN/PNP driver for the fet gate..
The high side driver logic 0 input, gives predriver output 1, which is intended to drop bolts from Vcc, and modulate high side Pmos. This works.
Clue may be in that if I instead attempt to modulate the lowside and keep the highside closed, the whole thing doesn't work....
Thanks ill try that Kevin.
Interestingly, the previous design had no R2, but used R4. This didn't work at all.
Clue may be in that if I instead attempt to modulate the lowside and keep the highside closed, the whole thing doesn't work....
Thanks ill try that Kevin.
Interestingly, the previous design had no R2, but used R4. This didn't work at all.
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The thermal problem in the lower devices is because they are not fully saturating with a low voltage drop,
and are dissipating higher power than they should. The minimum Vgs on spec for this device is 10V.
When the lower logic input is low, the driver transistors Q3/7 are off, and the gate voltage is half of 15V, or only 7.5V.
Remove the resistors R8/16 to fix this. The devices will now be fully saturated with 15V from gate to source.
The upper driver circuit needs to be redesigned, to control the gates in reference to the +24V rail
(where the sources are connected), and to make the Q2/6 gates 15V below their sources
when Q2/6 are to be on. (The Vgs is 20V max rating, so do not switch the gates to ground.)
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hi i used this schematics which worked well but with only 2 inputs not 4
An externally hosted image should be here but it was not working when we last tested it.
In forward direction PWM in accord with the OP:
Each time Q2 turns off, inductance of the motor will force continuing current flow.
D2 or else Q4's body diode (slow but free) must turn on, or something will break.
Toni's circuit will need freewheeling diodes as his BJT's include no freebies.
He could use 1n400x since his transistor is horribly slow to turn off anyway.
You need some Schottky barrier, or super barrier, or trench barrier diodes to
insure fast recovery from conducting to blocking. And they must have lower
forward drop than your MOSFET's body diode to insure the slow recovering
body diode never turns on.
If you are switching so infrequently that recovery time (and several amps of
repetitive near dead short from both Q2 and D2 turned on ~500nS each ON)
doesn't matter, then body diodes alone are enough. 1n400x serves no useful
purpose in your schematic.
Q4 could be synchronously driven to short its own body diode and recover it.
I wouldn't bother going that far, but synchronus rectification is the next step.
Toni's Q3 (same position as your Q4) is driven like synchronus rectification,
but a BJT can't conduct in reverse, so can't assist a freewheeling diode even
if it the transistor were fast enough (which it surely isn't).
Your gate drive signal levels are out of whack, but you already knew that...
Each time Q2 turns off, inductance of the motor will force continuing current flow.
D2 or else Q4's body diode (slow but free) must turn on, or something will break.
Toni's circuit will need freewheeling diodes as his BJT's include no freebies.
He could use 1n400x since his transistor is horribly slow to turn off anyway.
You need some Schottky barrier, or super barrier, or trench barrier diodes to
insure fast recovery from conducting to blocking. And they must have lower
forward drop than your MOSFET's body diode to insure the slow recovering
body diode never turns on.
If you are switching so infrequently that recovery time (and several amps of
repetitive near dead short from both Q2 and D2 turned on ~500nS each ON)
doesn't matter, then body diodes alone are enough. 1n400x serves no useful
purpose in your schematic.
Q4 could be synchronously driven to short its own body diode and recover it.
I wouldn't bother going that far, but synchronus rectification is the next step.
Toni's Q3 (same position as your Q4) is driven like synchronus rectification,
but a BJT can't conduct in reverse, so can't assist a freewheeling diode even
if it the transistor were fast enough (which it surely isn't).
Your gate drive signal levels are out of whack, but you already knew that...
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