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Can anyone tell me how to protect parallel MOSFETs and the driver?

2357 Views 8 Replies 4 Participants Last post by  Tony Bogs, 
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Overview: I have an electric quad that I have heavily modified. The motor will need to be replaced every now and then because I am driving a 48 volt motor with a 72 volt supply that can give extremely high currents (6 12V motorcycle batteries) . This is no problem as I dont mind getting a new motor periodically. I am looking to exploit the limits of this small vehicle!

Gate driving Problem: I am using a microcontroller to drive a Gate Driver to drive a MOSFET that powers a DC motor (with Flyback Diode IDW75E60 of course). There are two power supplies; one to power the motor (72V) and the other one powers the control circuitry (including Gate Driver) (12V). MOSFET Gate is pulled to ground with a 5W 220ohm resistor (probably too low/unnecessary resistance). Everything shares a common "star" ground.

Every time I try to test this circuit I notice 3 things happen: (1) The circuit seems fine initially at very low throttle before the motor even start to turn (I can hear the 500HZ PWM signal hum). (2) When throttle is increased slightly more to rotate the motor, the Gate Driver]1 BLOWS UP! (3) The MOSFETs are then ruined as Gate and Drain are shorted. As well as Source and Drain.

I have a large toggle switch to shut off the circuit in an emergency. The switch connects the Source on the MOSFET to Ground (turning it off breaks the motor circuit mechanically and reliably in an emergency event).

Instinctive Solution?: Add a Diode between Source and Drain on the MOSFET? Increase PWM frequency? Reduce the Gate-Source resistor to 10K. Add a 220ohm resistor between the Gate Driver output and MOSFET? All the above?

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S
got a schematic? I don't know if paralleling is your problem, does it work with just one mosfet?

There's a lot of issues going on there. like what is the 220 ohm pulldown (gate?) Are you feeding 72 volts to the gate driver supply?!?

Why are you putting it right under the cpu?

also you don't want long lines between the gate driver and the actual gates.

plus equalizing them thermally helps.
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T
Paralleling mosfets at high amps is not trivial.
But you've got the high amp driver, so you may want to give these parts a try :

MOSFET IXYS IXFN360N10T 360A 100V

IXYS DSS2x160-01A, 160A 100V, two per package, so in parallel 320A.

Mount the parts on a piece of aluminum that's at least 15 mm thick, 10cm wide.
Screw the aluminum onto a big cooler (10 x 20cm) with fins.

And a gate resistor is a good idea to prevent oscillations. METAL FILM 4R7 1W

PWM FREQUENCY AT LEAST 5 kHZ.

Good luck.

P.S. Keep the wiring short, especially in the gate drive circuit. Drive voltage about 12V (>10V)
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T
Phases? My assumption was DC motor, based on the description of the circuit.
Midpoint, if there is one, could be the common for the two windings of a DC motor without permanent magnets.
If it is, the setup should work if connected correctly with adequate cooling for suitable parts. Only diodes in parallel of course.;)
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T
Some pointers of you want to experiment with paralleling the IXYS mosfets:
1. Keep the high amp circuit as symmetrical as possible
2. Go to your local roofer or plumber and ask for soft copper sheet, at least 0.8 mm thick
3. Use the copper sheet in the high amp circuit to connect the IXYS diodes and mosfets
4. Use a 4R7 metal film resistor between each gate and the driver output
5. 220 is a bit low for a gate to source resistor. 4K7 to 22K is a good range.
6. Never use wire wound resistors.
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kennybobby
Without knowing the motor current and without a schematic it's hard to tell for sure, but it appears that you are trying to switch an inductive load and have not provided a return path for the off-state current, plus you may be exceeding the safe operating region of the FET.

As a result you are getting a huge voltage spike that is punching thru the fet, the driver and the resistor as the current tries to find a way back home.

You have the right to a return path--if you can not afford one, then a path will be provided for you.

https://en.wikipedia.org/wiki/Flyback_diode

With a 72 volt system and for a 2 msec pulse that FET SOA looks to be limited to a D-S current of about 10 Amps?

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T
500 Hz is way too low. Even for the 360A IXYS mosfet I suggested, 5 kHz is the absolute minimum (about 100A SOA @ 70V).

It is possible to guess the amps. An electric quad, light vehicle, not too exciting (the TS experiments with higher voltage to get more power), let's say 5kW nominal, 10kW peak.
DC 5kW, 48V, easy math results in 100A nominal.

And there you have the number that the 5kHz is based on.

Those IXYS parts are minibloc size, very low thermal resistance, mosfet has insulation.
Mounted on a thick aluminum plate it can take a hard power punch.
And not very expensive to blow up occasionally. Screw terminals, very handy in this loosely wired setup.
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T
Yeah, forgot the diode in the flyback path.
The Infineon diode has to go. No mosfet is going to survive the horrific reverse recovery of this diode at 100A.
The dual IXYS schottky diode I suggested does not have reverse recovery. It does have quite a bit of capacitance, but hey, the 4R7 gate resistor is there for a reason.
Again minibloc diode with screw terminals. Really handy.

Bypass caps are also a good idea. Maybe ten cheap 100V elcaps in parallel close to the source of the mosfet (-) and the cathode of the diode (pair) (+)
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T
Your question concerned mosfets in parallel, but I think you might want to read some background info on the safe operation of mosfets in general.

So I've traced a couple of good and compact Fairchildsemi notes on the Onsemi site.

This note clears up the specs in the datasheet in relation to the Safe Operation Area of a power mosfet.
http://www.onsemi.com/pub/Collateral/AN-4163.pdf.pdf

Maximum continuous drain current is a good example for your setup.
Without some sort of feedback the motor current can very easily exceed the SOA limit of the mosfet.
A magnetic current sensor is often used for current feedback. The inductance it introduces is a big bonus.
It will limit the dI/dt during huge load transients, giving the feedback circuit a short window of time in which the mosfet can be turned off safely.
So picking a bigger sensor and looping the wire a couple of times through the sensor can be part of a carefully designed and built motor controller.

This note adresses issues with reverse recovery and power mosfets:
http://www.onsemi.com/pub/Collateral/AN-9067.pdf.pdf
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