How do I produce a steady DC voltage from a PWM supply?

Question

I am trying to control a 12V Peltier (6A maximum current, but I am unlikely to need to use anywhere near that) element with a 5V PWM supply (from an Arduino at the moment).

I am using a logic level MOSFET (IRLZ44N) to allow the 5V (low current) to control the 12V (high current) load. That seems to work ok and the MOSFET stays cool even without a heatsink.

One side of the Peltier is getting hot as expected. However, the other side is not getting very cold. I am given to understand that powering a Peltier element by PWM is not ideal, and a steady DC voltage would be much better.

My first thought is to use a low pass filter, however, I am having difficulty choosing the correct type and components to use.

As standard the Arduino produces PWM frequencies of 490Hz and 980Hz, although it can produce up to 65KHz with a bit of additional work.

Am I correct in thinking that a higher frequency would be better and make it easier for a low pass filter to smooth out the pulses? As long as the MOSFET can handle the higher frequency (I believe the IRLZ44N is for switching below 100kHz).

I have been told an inductive low pass filter would be best for this application, but I don't know how to choose the values for the resistor or inductor. I know they are dependant upon my desired cut-off frequency, but I don't understand how I should be determining that. I'd assume that I'd want as low a cut-off frequency as possible for a steady DC voltage? 1Hz? And then, I would also assume that I want a high value resistor, but that results in needing a huge inductor (several Henrys).

Please can someone tell me how I should be choosing these variables? Or should I be approaching this from a different direction entirely?

I have considered just turning the Peltier fully on/off for set time intervals but that doesn't provide a stable temperature and there is suggestion that it even damages the Peltier element.

Thanks.

Answer 1

If you add an inductor and a diode to your circuit you have a buck converter. The inductor value can be smaller at higher PWM frequencies, the relation is linear.
10 mH at 3A is a very big inductor, so try to produce at least 10 kHz. Even higher frequencies need a proper gate driver between the MCU and the MOSFET which adds complexity.
Play with the simulator and change the values of L1, C1 and V1 to see the effects on the output waveform.

^{simulate this circuit – Schematic created using CircuitLab}