Smoothing PWM Output

For my application I am using the AX3500 to drive peltier (thermoelectric) elements to heat an cool. For this reason I need to smooth out the PWM output- ideally to <10% ripple. I have tried to add an inductor in series with the peltier elements and it helps to smooth the output.

The load is around 30A on each channel of the AX3500. Can someone please advise what values for the inductor I need to use? Also, what protection should I add to circuit?

Thanks in advance.

You should be able to determine this using modeling. You can download a very good free simulator from linear technologies ltspice.linear.com/software/LTspiceIV.exe

We have a model that simulates the PWM output of our controller at dev.roboteq.com/dev1/files/apnotes/MOSFETout-simul.zip. You would need to replace the motor model with a model of your peltier cell.

There are a few write-ups on the net about modeling peltier cells but these are quite complex. If you know approximately how such a cell behaves, you may be able to approximate it with simple elements.

Then, running the simulation you can see the current shape for various inductor values.

Thanks for the very detailed answer. I have used the simulations to choose some inductor values which give the desired smoothing of the voltage.

In order for the peltier elements to see the “smoothed” voltage during both heating and cooling (forward and reserve) can this only be achieved using two inductors connected in series either side of the peltier elements?

I don\'t expect that it matters wether the inductor is before or after the peltier cell but you would need to look at the simulation for answer

Hey cosma,

my master thesis is about simulating the roboteq AX3500. I downloaded the LTspice model a year ago, but no it is not available anymore. I need to reference the model in my thesis, so is it somehow possible to upload it in the download area?

How did you know that this model simulates is close to reality? (How did you know, that the resistors are right, for instance ?)

You would help me a lot! Thanks!