12V pump project

I have a pump project in the works using the attached schematic as core, pump motor is 12 (13.6)VDC, 1.5A continuous,
I want to use 20kHz PWM to get out of audible range, will the 10k R23 in the diagram be OK for that switching speed? If not, what would be a good compromise between switching speed and on state power dissipation in R23? Also I have a FQP27P06 MOSFET on hand, will I be able to get away without a heat sink pulling 1.5A through it?
PWM will be about 60 - 100%.
TIA

But why make it more complex by switching the high side? Can't you just switch the low side?

Can't find a logic level N-channel, guess I'll order a few.
Tnx anyway.

I want to use 20kHz PWM to get out of audible range, will the 10k R23 in the diagram be OK for that switching speed?

Well lets work that out:

From datasheet the worst-case gate charge at 10V is 43nC, but we use 12V, so lets call that 50nC

With an average of 6V across 10k (average as the gate voltage slews from -12V to 0V while switching
off), thats a current of 600uA.

50nC / 600uA = 83us. So it takes about 80us to switch off worst case.

At 20kHz PWM you switch on every 50us and off every 50us.

So you will have no chance at all of that circuit working.

For 20kHz PWM I'd recommend keeping switching times down to 500ns or less (1% of duty cycle),
which for that MOSFET is a gate current of 100mA minimum into/outof the gate.

So stick a low-side MOSFET driver chip in there (the clue is in the name), most such chips can source/sink
more than 100mA easy. And make sure the driver chip has copious ceramic decoupling (0.1uF + 10uF
is a good start).

Why make like harder by using a p-FET? n-FETs are about 3 times lower resistance all else being
equal.

My favorite high performance low-side MOSFET driver is the MIC4422, more than adequate here.

Hey, MarkT & TNX much, K++
Once knew a lot of the math but it's fading fast.

Why on earth use 20kHz PWM? The switching losses may be significant . And why make things hard?

The motor won't notice using the standard Arduino PWM at about 1kHz ....

regards

Allan

note, 1kHz only on pin 5 and 6 by default.

allanhurst:
Why on earth use 20kHz PWM? The switching losses may be significant . And why make things hard?

The motor won't notice using the standard Arduino PWM at about 1kHz ....

regards

Allan

As in the original post, to get the motor whine out of the audible range...

The pump project is for my niece who used to complain about the 15750 Hz "squeal" from the flyback transformer on old "picture tube" TV sets. >:(

Then add a smoothing capacitor across the motor - eg 1000uF

regards

Allan

allanhurst:
Then add a smoothing capacitor across the motor - eg 1000uF

If you are driving the motor with PWM then surely this will blow the driver FET ! Inductor in series maybe.

OK - a small resistor as well in series - couple of ohms?

Allan

allanhurst:
OK - a small resistor as well in series - couple of ohms?

Not really. All the PWM AC power will be dissipated as heat in the resistor. The worst case would be for 50:50 mark space when it would dissipate approx 6 volt * 6 volt / 2 ohms = 18 Watts!

A series inductor of about 1mH rated at 1.5 amp or better would have a reactive impedance of about 100 ohms at 15750 KHz and would attenuate significantly the vibration noise. A simple search of ebay indicates that one could be obtained for a couple of quid. Whether or not the vibration attenuation would be acceptable to the child is a different matter.

Put some acoustic shielding round the motor..

Allan