Electronic – MOSFET Driver Stress

There are several factors that you must trade off to decide on a switching speed.

1. Motor vibrating. You want to switch fast enough so that the mechanical inertia of the motor low pass filters the individual switching pulses so that the motion and torque are the average, not the individual pulses.

2. Coils whining. Each bit of wire in the motor windings experience mechanical force. It is rare that the windings are potted, so they can usually move at least a little bit. If you hit the right frequency they can vibrate and make a annoying audible whine. It's also not good for the wires and things they are rubbing against.

3. Switching losses. Nothing switches off-on or on-off instantly. The switch transistor will take a fixed amount of time transitioning from one state to the other. During that time it is partially on and is dissipating much more power then when fully on or fully off. Since the switch transition takes a fixed time, it is a larger fraction of the total time as the switching frequency is increased.

4. Output capacitive transients. Motors are physically big and have some finite capacitance accross their leads. In some cases deliberate snubbers may be added. Each time the switch changes state a finite charge is transfered to or from this capacitance. The more often this happens, the higher the effective current drawn by the capacitance.

5. Gate drive current. As you mentioned, FETs can have substantial effective gate capacitance. This gets charged and discharged once per switching cycle, and therefore represents a average current. This current is directly proportional to switching frequency.

6. Diode switching losses. The inductive kickback catch diode does not instantly stop conducting when it is reverse biased. It leaks substantially for a little while until it goes high impedance. This is called the reverse recovery time. This backwards flow of current thru the diode can be substantial, although generally short. It represents a fixed amount of charge wasted once per switching cycle, so is effectively a current proportional to switching frequency. At low voltages (10s of Volts, up to maybe 100V) Schottky diodes are available. These have nearly instant reverse times for most purposes. However, if the motor is being driven by higher voltage or there are other reasons a Schottky can't be used, then this is a serious issue that needs to be taken into account.

Only you can look at your motor, how much you're willing to spend on the drive electronics, how efficient it needs to be, where this will be used, and a number of other factors to weigh the issues above.

A lot of motors get run around 30 kHz, just above the human audible limit. That may piss off all the dogs and bats in the neighborhood, but you'll just hear a smoothly running motor.

From the driver to the gates, the wires are ~15cm. Does this cause rining?

Almost certainly, and it's a fair bet that this is destroying your MOSFETs, by one or more of these mechanisms:

1. exceeding \$V_{G(max)}\$ even for the briefest instant
2. exceeding \$V_{DS(max)}\$
3. simple overheating due to slow switching and unintended conduction

#3 should be pretty obvious when it occurs, but the other two can be hard to see, since they are transient conditions that may be too brief to be visible on the scope.

C2 and C3 are not decreasing the ringing. You get ringing on the gates because the capacitance of the MOSFET gate (and C2, C3 which add to it) plus the inductance formed by the loop of wire through the driver and the MOSFET gate-source form an LC circuit. The ringing is caused by energy bouncing between this capacitance and inductance.

You should put the driver absolutely as close to the MOSFETS as possible. 1cm is already getting to be too long. Not only does the inductance created by the long trace to the gate cause ringing, but it limits your switching speed, which means more losses in the transistors. This is because the rate of change of current is limited by inductance:

$$ \frac{v}{L} = \frac{di}{dt} $$

Since \$v\$ is the voltage supplied by the gate driver and you can't make that any bigger, the time it takes to increase the current from nothing to something is limited by the inductance \$L\$. You want the current to be as much as possible, as soon as possible, so that you can switch that transistor fast.

In addition to putting the gate driver close to the MOSFETs, you want to minimize the loop area of the path the current through the gate must take:

^{simulate this circuit – Schematic created using CircuitLab}

The inductance is proportional to the area illustrated.

The inductance limits the switching speed, and it also limits how well the gate driver can hold the MOSFET off. As the drain voltage on the MOSFET that just turned off changes (due to the other MOSFET turning on, and the mutual inductance of the coils), the gate driver must source or sink current as the internal capacitances of the MOSFET charge or discharge. Here's an illustration from International Rectifier - Power MOSFET Basics:

In your case, if the gate traces are long, then \$R_G\$ is also an inductor. Since the inductor limits \$di/dt\$, the gate driver can only respond so quickly to these currents, and then there is significant ringing and overshoot in the resonance between the gate trace inductance and the MOSFET's capacitance. Your C2 and C3 just serve to change the frequency of this resonance.

As the gate voltage is ringing, it sometimes crosses over \$V_{th}\$ of your MOSFETS, and one begins to conduct a little when it should be off. This changes the current and voltage of the connected inductor, which is coupled to the other inductor, which introduces these capacitive currents in the other MOSFET, which can only exacerbate the problem. But, when the coils aren't powered, then the drain voltage is at 0V regardless of the transistor switching, and these capacitive currents (and consequently, the total gate charge that must be moved to switch the transistor) are much less, so you see much less ringing.

This inductance can also be coupled magnetically to other inductances, like your solenoid coils. As the magnetic flux through the loop changes, a voltage is induced (Faraday's law of induction). Minimize the inductance, and you will minimize this voltage.

Get rid of C2 and C3. If you still need to reduce ringing after improving your layout, do that by adding a resistor in series with the gate, between the gate and the gate driver. This will absorb the energy bouncing around which causes the ringing. Of course, it will also limit the gate current, and thus your switching speed, so you don't want this resistance to be any larger than absolutely necessary.

You can also bypass the added resistor with a diode, or with a transistor, to allow for turn-off to be faster than turn-on. So, one of these options (but only if necessary; it's much preferred to simply eliminate the source of the ringing):

^{simulate this circuit}

Especially in the last case with Q3, you have essentially implemented half of a gate driver, so the same concerns of keeping the trace short and the loop area small apply.