Archive for December 3rd, 2012

Peltier Module PWM Frequency

The tech reviewer for my Circuit Cellar columns on the MOSFET tester commented that the 32 kHz PWM frequency I used for the Peltier module temperature controller was much too high:

Peltier Noise - VDS - PWM Shutdown

Peltier Noise – VDS – PWM Shutdown

He thought something around 1 Hz would be more appropriate.

Turns out we were both off by a bit. That reference suggests a PWM frequency in the 300-to-3000 Hz range. The lower limit avoids thermal cycling effects (the module’s thermal time constant is much slower) and, I presume, the higher limit avoids major losses from un-snubbed transients (they still occur, but with a very low duty cycle).

Peltier Turn-Off Transient

Peltier Turn-Off Transient

The Peltier PWM drive comes from PWM 10, which uses Timer 1. The VDS and ID setpoints come from PWM 11 and PWM 3, respectively, which use Timer 2. So I can just¬†not tweak the Timer 1 PWM frequency, take the default 488 Hz, and it’s all good. That ever-popular post has the frequency-changing details.

I’d still use a snubber:

Peltier Drain - 82 ohm 3.9 nF snubber

Peltier Drain – 82 ohm 3.9 nF snubber



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