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UC2856 minimum duty cycle and pulse skipping

Jerome Ronne
Prodigy 50 points
Other Parts Discussed in Thread: UC2856

Hello, I am trying to get a UC2856 Ic to work properly at no load and light load conditions. The problem I am having is that once the on time wants to drop below around 1us it starts to skip gate drive  pulses randomly. I have tried a number of different configurations and I can't seem to get a linear response on the lower end of my slope compensation even without running power at all. The ramp looks clean and smooth (at least without load).

Any suggestions?

 

Configurations tried:

Changing EA- voltage from 1.7V to 2.5V to 3.92 V and down to 1.2V.

Changing slope seen on CS- to CS+  1st  (0 - to 100mV) 2nd (0-200mV) and (100mv to 500mv slope)

  • 0
    TI__Guru 54515 points

    What is your test circuit look like? What is on EA+? It is better to provice the circuit in order to understand your observation.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    After a week I have made some progress, but it still seems to be a finicky chip around the low duty cycle area. EA+ has a resistor tied to +5.1 ref from the IC and a feadback resistor to the Comp pin as well as the collector of the feadback optocoupler. I have played with all these values to ensure the comp pin is only sinking current, and not sourcing it.

    Instead of me telling you what I have in my circuit, I would like to hear what you have in your circuit and what minimum on time you are able to achieve before the gate pulses start to skip.

  • 0 in reply to Jerome Ronne
    TI__Guru 54515 points

    Our circuit to test Pulse by Pulse Current Limiting is shown on page 6.

    xOUT min pulse width usually in the range of 100ns to 300ns in open-loop testing before the pulses disappeared completely.

    In close-loop systems, we need to know particular design details of circuit to comment/analyze the potential causes to the issues. There have to be some reasons why the conditions are established to aligh with the IC specs then cause the pulses disappear.

    Usually a circuit drawing can describe details in a much easier way than using a text for us to understand the issues and propose our ideas how to solve the issues.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    I ended up using the UCC2808A instead. This one I am able to get down to 50ns without pulse skipping.