Tool/software:
We are utilizing the UCC28950 for a full bridge with synchronous rectification to drive fans and pumps. The output of the full bridge goes to a drive circuit where it is converted to a PWM to drive the fans and pumps. We are probing the PWM control signal from UCC28950 to the gate driver of one of the full bridge MOSFETs (Out A of UCC28950). When we are operating at a higher load (higher duty cycle to the fans), PWM is continuous (see screenshot.) When we drop down to a lower duty cycle to the fan and pump (= lower load on the full bridge) we get intermittent drop out of the PWM from the UCC28950. In the screenshot below I have highlighted 2 drop outs of the PWM and a zoomed in example.
We have the controller running in Current Mode with RSUM connected to ground via resistor.
We have reviewed the data sheet but we are not sure if this is the intended operation or if there is some external factor or some instability of the fan operation at that lower duty cycle that is causing a disruption in the PWM signal.
- Is this an expected function of the PWM controller where a lower load causes the PWM to intermittently go to "zero" duty cycle?
- Is this a protection mechanism (hiccup mode) of the PWM controller that may indicate instability of the fan current?
- Can you suggest other probe points to identify the exact cause of this behavior?
Higher load continuous Out A PWM:
Lower load Out A PWM interruption:
