I am trying to use an H-Bridge motor driver to control (heating and cooling) a Peltier TEC. Specifically, I am using the ST VNH2SP30 motor driver from a Pololu MD01B carrier board which can do PWM up to 20 KHz. An Atmega 328 based Arduino controls the H-bridge motor driver and provides the PWM signal. The Peltier I have (CUI CP40336) is rated for Vmax = 15.4 V, Imax = 4 A. My power supply is 0-18 V, 0-5 A.
At PWM frequency 250 Hz and Peltier supply voltage Vcc = 15 V, the output current is proportional to the PWM duty cycle. However, I read that it is bad to drive Peltiers at low frequencies (is it?). I am not sure what the optimal frequency is, but since the VNH2SP30 can do up to 20 KHz PWM, I am trying both 10 KHz and 20 KHz. I've checked the PWM signal on an oscilloscope.
Here is my problem: at high PWM frequencies (e.g. 10 KHz), I see this weird effect. When the Vcc is relatively "low" (below 12 V), the output current is proportional to the duty cycle. But as I slowly raise Vcc, there is a sudden transition where the current suddenly drops by a factor of 4, and the scope shows that there is now only a voltage pulse across the Peltier on every fourth PWM cycle! At 20 KHz, the drop is even worse (a factor of 7-8).
I am not experienced in electronics, so i don't understand what is happening. And since I don't understand what is happening, I don't know how to solve the problem. I could do PWM at 250 Hz, but supposedly that is bad for the Peltier. Or I could keep Vcc below the threshold voltage where the weird effect starts, but then I am not getting the most out of the Peltier.
From some Google searching, I think what I may need is an LC filter between the motor driver output and the Peltier. However, I am not sure if the effect I am seeing is due to the lack of an LC filter or due to some other unrelated problem. And if I do need an LC filter, what component values should I try?
If anyone could provide some guidance, I would be most grateful.
At PWM frequency 250 Hz and Peltier supply voltage Vcc = 15 V, the output current is proportional to the PWM duty cycle. However, I read that it is bad to drive Peltiers at low frequencies (is it?). I am not sure what the optimal frequency is, but since the VNH2SP30 can do up to 20 KHz PWM, I am trying both 10 KHz and 20 KHz. I've checked the PWM signal on an oscilloscope.
Here is my problem: at high PWM frequencies (e.g. 10 KHz), I see this weird effect. When the Vcc is relatively "low" (below 12 V), the output current is proportional to the duty cycle. But as I slowly raise Vcc, there is a sudden transition where the current suddenly drops by a factor of 4, and the scope shows that there is now only a voltage pulse across the Peltier on every fourth PWM cycle! At 20 KHz, the drop is even worse (a factor of 7-8).
I am not experienced in electronics, so i don't understand what is happening. And since I don't understand what is happening, I don't know how to solve the problem. I could do PWM at 250 Hz, but supposedly that is bad for the Peltier. Or I could keep Vcc below the threshold voltage where the weird effect starts, but then I am not getting the most out of the Peltier.
From some Google searching, I think what I may need is an LC filter between the motor driver output and the Peltier. However, I am not sure if the effect I am seeing is due to the lack of an LC filter or due to some other unrelated problem. And if I do need an LC filter, what component values should I try?
If anyone could provide some guidance, I would be most grateful.