Motor Driver 18v15 and Higher PWM Frequencies

Hi, I’m looking at purchasing your 18v15 High Power motor driver, and using it to drive a Peltier (TEC) with pulse width modulation. I filter the output to produce a ~DC current to the Peltier. The higher frequency the better, since it allows smaller filter components.

Your documentation describes that 40 kHz is a practical limit for PWM because the ability to control at high duty cycles is starts to be lost (the example given is that there is no ability to set duty cycles between 88% and 100%).

I’m more concerned about the low duty cycle end because I will often be performing light duty cooling, but trying to control the current output with some precision. My question is whether the same problem applies at the low end? Using the same example, would there be no control between 0% and 12%?

EDIT 2013-06-21: corrected to say 18v15 High Power motor driver instead of 18v7

Hi, Craig.

The only product we carry with the 18v7 designator is the Simple Motor Controller 18v7, but from your description, it sounds like you might be asking about our High-Power Motor Drivers. Could you please clarify which product you are referring to?


Do you really need a filter on the Peltier device? If so, why?

Hi, Craig.

The answer to your question is no, the low end control is not lost in the same way. There is a 3 microsecond dead time which you can think of as a delay you tack on whenever PWM transitions from high to low. If that delay is smaller than the off cycle time, it has no effect because it is effectively masked by the off time. If the desired off time is smaller than the dead time, the dead time dominates and the off time is fixed at 3 microseconds.

- Ryan

@Derrill, you’re right, I fixed the headline to read 18v15.

@Jim, good question. This is the first time I’m designing a Peltier application. Partly it’s to see if I can do it!

Also, as I said, a lot of the time I will be doing light-duty cooling. At full power supply voltage for the TEC, I calculate I would need to control the PWM duty cycle with quite fine precision (better than 0.2%) to control the TEC to 1C, so it’s the duty cycle problem all over again.

The control situation is a bit better if one can filter the motor driver output, because at lower input voltages to the TEC, one can get proportionally less cooling.

Heat diffuses through TEC modules pretty slowly, which would act to average the pulses due to PWM. I’m reasonably certain that a filter capacitor would not improve the performance. Indeed, this application note from Maxim doesn’t show one across the Peltier device: … vp/id/3318

@Ryan, awesome, thanks!

@Jim, the filter I’m talking about is an LC filter, a la a buck regulator. The Maxim 1968 datasheet shows an example of this.