UCC25661: Output Power Limiting

Christopher Moses

Part Number: UCC25661

Tool/software:

I am using the UCC256612 chip for an LLC circuit that is designed to output 1260 W at full load at 34 V output. The circuit works fine until I reach ~1050 W and then it starts entering some fault state where the chip will repeatedly hiccup and restart. I've designed the circuit using the Excel design tool and according to that, I should be able to get full power out of my circuit. I have already messed around changing the TSET resistors to change the TSET programming options but that hasn't had any impact.

1) Just to confirm, should I be able to get 1260 W using this chip?

2) What other pins on the chips or external parts will affect the maximum power I am able to get out of the circuit before the chip starts limiting it?

Thanks,

Christopher

4 days ago

0 SIRIPURAM YAMINI 4 days ago

TI__Intellectual 1370 points

Hi Christopher,

1.The limit is not on the UCC256612 controller, it is on the converter topology, and half bridge LLC can work for the chosen output power.

2. The power limit can be adjusted through TSET pin and also check the FETs that you are using and the gate driver circuit.

To see which fault you are getting, can you please attach the respective waveforms (time taken to restart, hiccup behavior along with LO).

Thanks & regards,

Yamini.

0 Christopher Moses 3 days ago in reply to SIRIPURAM YAMINI

Prodigy 10 points

Thanks for responding Yamini. Here are some screenshots of the waveforms I am seeing. Channel 1 (Dark Blue) is LO and Channel 2 (Light Blue) is my output voltage.

Once I hit the power limit at about 1050 W LO stops switching for about 5 ms, making Vout drop to 0 V before restarting.

This cycle repeats itself about every 200 ms. In each cycle you can see that there is another period of time where LO stops switching but Vout does not drop which I am also confused about why that occurs.

Channel 3 (purple) shows the input current from my power supply. Once I hit the output power limit (shown on the left at cursor a) my output voltage remains at 34 V, but the input current starts ramping up until it hits the current limit of my power supply (5 A in this screenshot), where it then starts the cycle of the output turning on and off I showed before. This happens no matter what I set the current limit to, but the duration between shutoffs once the current limit is hit changes.

This shows that during the input current ramp up I showed in the previous image, LO stops switching periodically, even before the output voltage starts dropping.

Hopefully these pictures can help you figure out what is happening and how I might be able to fix it. Thank you.