PWM driven 24 V fan noise issue

Question

I have a weird issue with a 24 V blower fan. We are driving it with an N-channel MOSFET at a PWM freq of 12 kHz. It has a periodic artifact that appears about 230 Hz or so.

Observable behavior consists of the fan's performance going from 40% to 100% RPM instead of 0 to 100%.

I feel a snubber across the drain and source of the FET would work, or a ferrite bead on the supply, but I'm not entirely sure. It's tough to simulate considering we are unaware of the part number for the fan (want to know the winding inductance). Below is the circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Disclaimer: We were using the PMV20ENR for the 12 V version of this project, but wanted the ability to go to 48 V, hence the difference in MOSFETs - the circuit behaves the same with both FETs.

Here is the scope view of a clean MCU PWM signal:

Here is the nasty drain of the MOSFET:

Both together:

Both together zoomed:

Another thing we tested was another PWM output we have on the system, which is an LED driver(M2). Honestly not sure what the PWM freq of the LED driver is but I know it's below 12 kHz.

Drain of M2 with fan duty cycle to 0% (MCU probe wasn't connected):

Drain of M2 with fan duty cycle to >0%:

Answer 1

You should configure your oscilloscope with the correct probe attenuation and scale so that it displays correct and consistent voltages. This will make debugging easier and save a lot of confusion - I spent quite a while figuring out exactly what you were measuring.

I'm not seeing obvious signs of voltage spikes here so I'm guessing there's already a flyback diode built into your fan.

The "nasty" measurement you showed on the MOSFET drain would make sense if there's a capacitor in parallel with the motor. When you turn the MOSFET off, the drain voltage slowly ramps up from 0V to 12V, which means the voltage drop across the fan is slowly dropping from 12V to 0V.

Flyback spikes occur because an inductor induces a voltage drop across itself in order to impede changes in current. In a case where there's no flyback diode, turning the MOSFET off instantly reduces the current through the circuit to zero, causing the voltage across the motor windings to skyrocket. In a low-side switch configuration, this would exhibit itself as a high voltage spike on the MOSFET drain.

However, with a flyback diode installed, the voltage at the drain should jump up toward the supply voltage almost instantly because the voltage drop across the motor windings is essentially clamped by the flyback diode.

What's probably happening is that there's a small capacitor in the fan somewhere. This could be as part of some onboard motor control electronics (fans are often more than just a motor with some blades attached) or it could be part of an additional RC snubber network. If you simulate this added capacitance (e.g. a 10nF capacitor and 1Ω resistor in series, placed parallel to the flyback diode) you'll see that the voltage across the motor drops to 0V over a period of a few microseconds, instead of instantly dropping to 0V.

In addition, switching at 12kHz is likely causing the motor current to "average out" in a near-steady state, which is probably contributing to the 40-100% behaviour you observed. Below a certain duty cycle, the inductance of the motor doesn't allow the motor current to drop quickly enough to have an appreciable effect. Lower PWM frequencies might be required to run the motor at lower speeds. Drop the frequency down to 1kHz or so and see what happens.

On the SQ2362ES you're barely turning the MOSFET on with a 3.3V gate drive. Looking at the on-resistance vs gate-to-source voltage graph from the datasheet, you've probably got somewhere in the region of 0.2Ω of Rds(on) with that gate voltage. The gate charge graph shows that you're only just going past the Miller plateau. The output characteristics graph also doesn't look good for 3.3V drive. A rule of thumb is to avoid operating a MOSFET at a Vgs lower than the minimum Vgs that the datasheet specifies Rds(on) for, which in this case is 4.5V. I'd recommend using a MOSFET gate driver IC here, or alternatively you could swap out the MOSFET for one with a Vgs(max) greater than 24V and use a BJT as a logic level converter to drive the gate directly from your 24V supply.

It isn't fully clear why your drain voltage is all over the place for the LED drive side of things, but you might be seeing a parasitic self-turn-on effect here. Usually this occurs when there are fast changes in Vds resulting in a capacitive divider effect on the gate due to the MOSFET's parasitic Miller capacitance. Essentially what happens is that turning off the MOSFET causes high dVds/dt (e.g. due to an inductive spike), which is conducted through the parasitic Cgd/Cgs, causing a temporary increase in gate voltage, which turns the MOSFET back on. The key factor for susceptibility is the Cgs/Cgd ratio, which on the SQ2362ES is quite low, making it more likely to be a problem. Given that you're running the MOSFET with a really low Vgs, these kinds of parasitics start being a problem, particularly if you've got noisy supplies or other transients going on at the same time.

The 20Ω gate drive resistors are probably unnecessary, given how little gate capacitance these MOSFETs have and how low your switching speed is. I'd swap to using gate driver ICs (they're cheap!) and follow their design docs.

Answer 2

This is sort of derivative of polynomials answer, but I believe the blower containing a capacitor completely explains what you are seeing.

The throttle will only go as low as 40%, because whenever you turn on the mosfet, the a large inrush current charges up a cap in the blower. PWM does not "average out" to a proportional voltage when you have a capacitive load.

For the sake of argument, imagine you had a low power LED instead of a blower, and it had a large capacitance in parallel. It's clear here that you could not dim it with PWM, as the cap would simply top off during each ON pulse. That's whats happening with your blower, but to a lesser degree because the capacitance is not so high compared to the blowers current draw.

There are complicated ways you could solve this, but I would suggest first just reducing the PWM frequency a LOT, try something like 400hz. 12khz seems excessive for anything with natural inertia and inductance.