Highside mosfet gate spike

[MrHeckles]

Hi guys. A while back I ask for, and received, a whole lot of help in regards to creating a motor controller. Happily it all worked out well in the end.

I have recently decided to redesign the controller. Now I'm using pSpice to cut down on blowing up components.

The circuit I have uses an SG3524 PWM chip to provide a pulse train to a discrete highside gate driver, and the inverted pulse to a lowside gate driver. The idea being that I can get very quick speed responses to throttle changes on the motor, and can use it for regenerative braking. The fets are all N-channel.

In my pSpice simulations it works reasonably well, however there looks to be a small shoot-through occuring due to a voltage spike on the high side mosfet's gate when the lowside mosfet turns off.

I am working on getting pictures - but in the mean time, is there anything anyone can suggest that might cause such a thing?

 

[jgeidl]

Try putting a diode across the MOSFET to direct the bounce when the motor direction (polarity) changes.  Remember, the motor is just a big inductor and the change built up in it comes bouncing back when the current drops.


Jim

 

[MrHeckles]

Thanks Jim,

I tried that, but it didnt seem to make any difference to the spike. I have sorted out some pictures.

1st, the circuit setup

2nd, the voltages from the probes in the first picture

and a zoomed in view of the spike

finally, this is a trace of the amperage through the highside mosfet. You can see how the spike is causing the shoot-through.

 

[indulis]

MrHeckles

 

[MrHeckles]

I have used R13 and L2 as a basic simulation of a motor.

M7 is supposed to remain on all the time. The PWM chopping for the speed control is done via M6. The motor is only supposed to drive one direction at this stage.

M5 (I think this is other other lowside fet you are talking about) operates inversely to M6 which provides a very tight control on the motor speed. If the motor is driven faster (by the momentum of the unit it moves) than the current setting, a regen braking effect will cause it to slow down and maintain the selected speed.

Your right, I did forget the miller effect. Is there anyway I can stop the drain gate capacitance from causing the spike?

 

[AN920]

Introduce some dead-time in your switching

 

[MrHeckles]

Is there any good way to do that apart from sticking a larger resistor on the gate of the highside FET with an anti-parallel diode?

 

[indulis]

Hmmmm...

If M7 is on all the time and you turn on M5, you are shorting the motor... is this what you wanted? There also doesn't appear to be a "flywheel" device across the motor. You could put a clamp zener on the gates to "limit" the spike or you could try snubbers across M6 & M5. Have you ever looked at this site...

http://www.4qdtec.com/pwm-01.html

some interesting stuff there!!

 

[MrHeckles]

Thats exactly what should be happening. It's a fantastic braking method for the motor. It allows for very tight speed control. With the highside FET then turning back on, it gives very nice regenerative braking.

The highside gate goes up to 22v or so, the spike is only around 14v-16v, just enough to start it shooting through, though not enough for a full turn on. How would I go about clamping the spike when the required gate voltage is higher than it anyway?

I have seem the 4qdtec site. Thats where I got alot of information about the first motor controller. They dont think switching the highside FET is a good idea, but I cant get decent PNP Mosfets in New Zealand for a reasonable price.

 

[indulis]

In your simulation, the M6 gate signal is referenced to gnd. The only thing that matters is Vgs, so you have to use the M6 source as a reference when looking at whats going on at it's gate.

...I cant get decent PNP Mosfets...

PNP MOSFET's?? Do you mean P channel?

Electric golf carts use regenerative braking for "braking" and putting power back into the battery, not motor speed control. Shaft encoders are the general choice for speed control... you don't drive your car with one foot either on the gas or the brake at all times, why would you want to do it with a DC motor?

 

[MrHeckles]

I set the voltage difference pins on the gate and the source for the FET. Based on that I have moved the diode D6 to connect to the 12v supply, rather than the motor supply. That way it gives about +11 volts over the drain voltage to the highside gate (regardless of what the motor voltage happens to be).

On the gate of the highside, I can see that the spike on the gate is about 6v above the source. I am guessing that it is enough to start turning on the FET and allow the shoot through.

Slip of the tongue (so to speak). I did indeed mean P channel.

No, but you do drive a tank with two hands on the brakes. The whole unit has two motors (one for either side), and castor wheels at the front. The steering is provided by a joystick that sets differing motor speeds. The reason I need quick speed reduction for the motors via the braking action is so that the thing will respond quickly to steering inputs, rather than waiting for the inside motor to coast to a slower speed.

 

[steven2]

put an mov metal oxide varistor from drain to source on the mosfet , or you can try useing a transorb