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VNH2SP30 H-Bridge Unexpected reaction to current control

Question asked by JDW on Mar 21, 2016
Latest reply on Sep 5, 2016 by JDW
For a project I'm working on I have to be able to switch the polarity of a peltier element (or other device) and be able to linear control the current through the peltier.
I didn't want to put a normal relay in this system do to the mechanical wear, so a H-bridge was the best option. The VNH2SP30 was the best choise in my opinion after reading the datasheet. So the design was made based on this H-bridge.


I have made a prototype PCB with this H-Bridge and saw a unexpected reaction to the lineair current control. (The lineair current control is a OPAMP, MOSFET based circuit I have used many times before and this circuits reacts/works as expected on the PCB)


When I enable INA, enable the PWM to a DC value so lets say a PWM of 100% and limit the max dc current to lets say 2Adc through the peltier. The Output of the H-bridge will pulse in a PWM-like dutycycle based on the current limit setting I use. The higher the current limit the higher the PWM duty-cycle. When the current reaches maximum that is possible to go through the peltier (at a 15Vdc PSU) the output of the H-Bridge is a stable 100% "PWM".


After reading the datasheet I was under the impression that it was possible to use this H-Bridge in a DC configuration (with a 0Hz "PWM"). So my question is, where does the PWM on the output come from? Is this normal behaviour for the VNH2SP30? 

The signals displayed in the images below are taken when the current limit was set to 2A
5.gif

4.gif

Circuit:
currentcontrol.gif

Outcomes