What caused this 400W H-bridge to burn!

[xavier60]

Even if the high side MOSFETs are being statically driven, any ripple or oscillation on the 24v rail can affect the Gate voltage.
I see there are large capacitors on the PCB, so maybe not a problem.
Also, every time a low side MOSFET turns off, the body diode in a high side MOSFET has to carry the motor's inductive current, causing some dissipation depending on motor load.

[MattHollands]

Side question that I'm hoping won't derail the thread: What purpose do D2 and D3 play? As far as I can tell they prevent current from flowing through Q3 and Q4, basically making them useless, but obviously this cannot be the case I assume.

[Phoenix]

Side question that I'm hoping won't derail the thread: What purpose do D2 and D3 play? As far as I can tell they prevent current from flowing through Q3 and Q4, basically making them useless, but obviously this cannot be the case I assume.

I think they are badly drawn zeners to limit the gate voltage.

[MattHollands]

Side question that I'm hoping won't derail the thread: What purpose do D2 and D3 play? As far as I can tell they prevent current from flowing through Q3 and Q4, basically making them useless, but obviously this cannot be the case I assume.

I think they are badly drawn zeners to limit the gate voltage.

Make sense. Cheers.

[xavier60]

Those zenners are subtracting from the 24v. If the 24v rail voltage drops, the Gate voltage will drop by the same amount.
 It would be better if they were used to limit Gate voltage by clamping it.

[ali_asadzadeh]

They are Zeners as others say

[Yansi]

The circuit as such is fine. The upper side drivers are slow, yes, but that's not really important, they are being driven statically for direction control, only the lower side switches are PWMing.

On a related note: which motor will accept 64 kHz switching? My experience with normal PMDC motors is, that at switching frequencies above 3..4 kHz you'll have enormous losses in the motor, to the extent that it won't even turn.

Well, you should see what others have done for full-bridge servo drives.  I make a line of brush and brushless servo drives.  We normally run them at 50 KHz.  But, the brush drives keep the motor shorted between PWM on pulses, unless there is excessive current.  That allows the current to recirculate in the motor winding.  I have dead time designed into the drive circuits, to prevent a high and low-side transistor from being on at the same time.  One of the features of this is it prevents the body diode in the FETs from ever conducting.  Those diodes are hideously slow, taking microseconds to turn on with large forward voltages.

There are two common schemes of driving brush motors with full bridges.  One is called synchronous antiphase, where having the motor stand still is done by having the two bridge halves at opposite 50% duty cycle square waves.  This forces a triangle-wave current in the motor, and can lead to substantial heating.  The other scheme is a sign-magnitude PWM, which it sounds like you may be using.  If you allow the current to recirculate between the PWM on-times, the motor will not heat up much.

Jon

These are not the only PWM modulation schemes. There is a metric shit ton of them.

You run motor controllers at 50kHz? You should definitely not do that. EMI, extreme losses in the motor, cable capacitances are the three strictly against your crazy design ideas. There in no way is any legit reason to do 50kHz, unless you have low pass filters on every PWM output which, as you might have noticed is NOT the case of the OP's converter topology.

[Yansi]

As I have already pointed out.  Throw away this discrete gate drive. It is not worth the trouble! *

Use a proper bootstrapped gate driver.  For example IRS2001 is what I would go for:
https://www.infineon.com/dgdl/Infineon-IRS2001-DS-v02_00-EN.pdf?fileId=5546d462533600a401535675a760277e

*Note: Fully working discrete bootstrap driver for the high side gate can be made from 3 transistors, but really, why? 
//EDIT: 2 transistors would be sufficient too.

[T3sl4co1l]

You run motor controllers at 50kHz? You should definitely not do that. EMI, extreme losses in the motor, cable capacitances are the three strictly against your crazy design ideas. There in no way is any legit reason to do 50kHz, unless you have low pass filters on every PWM output which, as you might have noticed is NOT the case of the OP's converter topology.

Gosh, you feel awfully strongly about this.  You must have a truly well reasoned, generally applicable, quantitative proof!  Please tell!

Tim

[Yansi]

Just have told you. Now it's your turn to tell me how you will cope with those listed above.

And yes, I am rock solid that 50kHz let alone 64kHz is pure foolishness for such drive topology the OP presented.

[ali_asadzadeh]

I can change everything, I also have lowered the PWM frequency to 10KHz and again after changing the Motor PWM from 50% to 10% one side would blow!   can the Back emf have higher voltages like more than 100V, so they would kill the mosfets!? what about the body diodes in mosfets? can't they coupe with the Motor current and back emf?

[mirbagheri1122]

Add a parallel 100nf cap with the R16 and R15, the EMI causes the P-MOSFET to turn on and a shoot trough would burn the Bridges, this would hopefully make  your design work!  

[xavier60]

I can change everything, I also have lowered the PWM frequency to 10KHz and again after changing the Motor PWM from 50% to 10% one side would blow!   can the Back emf have higher voltages like more than 100V, so they would kill the mosfets!? what about the body diodes in mosfets? can't they coupe with the Motor current and back emf?

Do you change the duty cycle gradually or abruptly?
How long does it take to fail?
Do you monitor rail voltage and any waveforms?
Do you understand the shortcomings that have been mentioned about the high side drive? The problem that the zenners can cause if the rail voltage dips.

[ali_asadzadeh]

Add a parallel 100nf cap with the R16 and R15, the EMI causes the P-MOSFET to turn on and a shoot trough would burn the Bridges, this would hopefully make  your design work!

Dude it's working properly now! How did you figure it out, that it was EMI, fucking EMI

[sibeen]

Add a parallel 100nf cap with the R16 and R15, the EMI causes the P-MOSFET to turn on and a shoot trough would burn the Bridges, this would hopefully make  your design work!

...and 22 minutes later the circuit is working. Now that is fast work

I also found it amusing that on an Australian website one Iranian helped another Iranian out

What a world.

[xavier60]

The High side MOSFET must be getting turned on through the Drain to Gate capacitance. Needs lower resistance in the Gate circuit.

[ali_asadzadeh]

This world is so big and so small at the same time!

[jmelson]

You run motor controllers at 50kHz? You should definitely not do that. EMI, extreme losses in the motor, cable capacitances are the three strictly against your crazy design ideas. There in no way is any legit reason to do 50kHz, unless you have low pass filters on every PWM output which, as you might have noticed is NOT the case of the OP's converter topology.

No extreme losses.  50 KHz with synchronous antiphase would indeed be really bad!  But, with sign-magnitude PWM and shorting the motor during the PWM off-period, there is no problem.
We do have a simple LC filter at the output terminals, so the motor sees only a very slight current ripple.  I have sold over 500 of these servo amps so far.

The reason for the 50 KHz is to keep the motor and filter inductors in continuous conduction.

Jon

[jmelson]

Add a parallel 100nf cap with the R16 and R15, the EMI causes the P-MOSFET to turn on and a shoot trough would burn the Bridges, this would hopefully make  your design work!

Dude it's working properly now! How did you figure it out, that it was EMI, fucking EMI

Do you have any idea how much EMI is produced by power transistors switching several amps in 100 ns or so?  You have to work very hard to reduce the loop area of the power traces.  You can easily see several Volts produced across a several inch-long power trace.  Totally amazing to see on a scope, you can't believe what is displayed is real!  I'm still amazed my servo amps work, they run up to 20 A at 120 V or so, I even have one guy running them on 168 V DC.  And, the CPLD that controls it is only an inch away and runs off 3.3 V.

Jon

[T3sl4co1l]

Even better, I designed an induction heater system, up to 400kHz at 650VDC.  The switching edge was only 50ns.  In an early prototype, we had the control board (at its heart, FPGA with 1.2V core, 2.5 and 3.3V IO) less than an inch away from the switching nodes of the inverter boards (a stack of boards, bolted together on standoffs, to share the current -- on the order of 100A total).  The inverter boards were also an early prototype that suffered from 80% overshoot, ringing at 60MHz.

All the graybeards (the guys who designed the previous generations of controls, using 15V CMOS usually) thought it wouldn't work, because 1.2V is too small, even inside a chip.  As is usually the case, this was a mistaken judgement not grounded in real measurements.  (They didn't even have ground pours / planes on their boards.  What were they thinking?)  The control board worked perfectly fine (given our code inside it, which was constantly being debugged..).  It was the gate drive cables that gave us trouble.  (But alas, that signalling was one of many suboptimal technical decisions made by management that troubled our development.)

Tim

[ali_asadzadeh]

Thanks guys for sharing this info, it would be nice if jmelson and T3sl4co1l could share us some photos or some circuits!

[T3sl4co1l]

Nothing technical like boards or circuits, unfortunately, but it looks cool at least:

https://www.seventransistorlabs.com/Images/Tallboy_HotStuff.jpg

Tim

[Yansi]

You run motor controllers at 50kHz? You should definitely not do that. EMI, extreme losses in the motor, cable capacitances are the three strictly against your crazy design ideas. There in no way is any legit reason to do 50kHz, unless you have low pass filters on every PWM output which, as you might have noticed is NOT the case of the OP's converter topology.

No extreme losses.  50 KHz with synchronous antiphase would indeed be really bad!  But, with sign-magnitude PWM and shorting the motor during the PWM off-period, there is no problem.
We do have a simple LC filter at the output terminals, so the motor sees only a very slight current ripple.  I have sold over 500 of these servo amps so far.

The reason for the 50 KHz is to keep the motor and filter inductors in continuous conduction.

Jon

And what did I say?

Still waiting for Kony and T3sl4co1l to explain what you need 50kHz switching for, without lowpass filters and how for example the cable capacitance magically disappears.

[jmelson]

Thanks guys for sharing this info, it would be nice if jmelson and T3sl4co1l could share us some photos or some circuits!

Here's a photo of my brush servo amp, which uses 4000-series CMOS running off 12 V for the control logic:
pico-systems.com/osc2.5/catalog/product_info.php?cPath=3&products_id=26

And, here's a pic of an older version of my brushless servo amp, which uses a Xilinx 9672XL CPLD.

Since these are commercial products, I'm not going to give out schematics.
The brush version uses two of International Rectifier's IR2113S, the brushless version uses 3 of the IR2181S for the FET drive circuits.
These deliver at least 600 mA both for charge and discharge of the FET gates.  That turns them on and off quickly with predictable timing.

Jon

[Yansi]

Not sure if I am allowed to post a photo of my own motor controller, so...  here's one. Unfortunately not a DC brushed motor. Induction 3~ instead.