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UCC28C43 Current Limit Issue

Daniel Sonnenmark1
Prodigy 80 points
Other Parts Discussed in Thread: UCC28C43

Hello,

My circuit is a 10-15VIN, 24VOUT @ 300mA, isolated DC/DC power supply based on the UCC28C43 controller. 

I have come across an issue while testing my circuit. Using R469 and R470, I set the current limit at 320mA. What I mean by this is that at 320mA, the voltage starts to drop in proportion to the load. The issue I have is that although the voltage is dropping, the current keeps increasing! For instance, I can increase the load so that it is only putting out 5V, but at 1.3A, over 4 times my target current limit. This is obviously not good for short circuit protection. I tested the short circuit protection and promptly blew up my rectifying diode. 

I started digging deeper into the function of the current and voltage feedback circuits, and am having a very hard time understanding the operation. For instance, the voltage at the COMP pin increases with current draw...why? I see a higher voltage at that node (>6V when current limiting) than I do from VREF, how is this possible?

Is this normal operation? I haven't seen anything like this, so If someone could explain I would be greatful. 

  • 0
    TI__Expert 4015 points

    Hi Daniel,

    The UCC28c43 does not have constant output current limit capability. The setting the peak current limit on the primary side sets a maximum power that the converter can deliver. This corresponds to a max current at the regulated voltage. As you increase the load beyond this point, the output voltage drops because the power supply cannot supply enough power to keep it in regulation. But it supplies the same amount of power, which corresponds to a higher current at the lower voltage. Becasue the output is now out of regulation, the loop drives the comp voltage to its max to get as much power out as possible. You can see from the datasheet that the max comp voltage is 6.3V ( I assume the amp is fed from Vdd not Vref).

    To implement a constant current limit as you desire would require a separate current loop on the secondary side, such as shown in the attached schematic, although this uses an op amp rather than a 431 as the error amp. The current is sensed and when it is over the current limit threshold it pulls down on the referance of the error amp to reduce the votlage and hold the current constant. An open collector pull down is required.

    Hope this helps.

    Billy