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UCC38084 blows up when I use resistive current sense.

Darius V
Prodigy 10 points
Other Parts Discussed in Thread: UCC38084

I am using UCC38084P PMIC for a 48V to 160V- 1.5KW Isolated Full Bridge DC-DC Converter (circuit diagram is attached).

This IC drives 4 MOSFETs (Full-Bridge) via 2 IR2110 Drivers. There is 0.01 ohm shunt for current sensing which connects to pin 3 via a 1K and 560pF filter. I also have a 47K slope comp res at pin 2.

All my signal grounds go to pin 5 (GND) of UCC38084. Also, signal grounds of IR2110s and the  low side of the shunt and large input capacitor negative are connected to pin 5.. The Power GNDs (Sources of MOSFETs and high side of shunt ) go to COM pin of 2110s. Power GND and Signal GND are connect via the shunt only.  

The control power is 12VDC which is taken from 48V input by a Simple Switcher locally.

The circuit works beautifully up to 1KW, as long as I increase the 48V input power, and 12V control power, together slowly up or down.  However, when I keep input 48V power ON and switch the 12V control power ON or OFF, UCC38084 IC blows up instantly the first switch (pins 8 and 5 becomes short internally).

My load is several parallel 220V light bulbs up to 1000W.  The problem happens at all load levels (even with a low 50W bulb load or no load at all).

If I short the 0.01 ohm shunt, and short pin 3 to GND, the problem goes away and the converter works perfectly on switch ON or OFF.

My questions are:

1. What causes this UCC38084 IC blow up? and what can I do about it? UCC38084 is the only component in the circuit which blows up. MOSFETS and IR2110s stay alive.   

I do have good bypass caps (1uF parallel with 10uF across pins 8 and 5). I even put a 12V-1W Zener across 8 and 5. Nothing seems to help

2. Can I ever use this IC with resistive shunt at all? Or, do I need isolated current sense (CT or Hall)? 

3. According to specs, pin 3 can go up to +6V.  To exceed this level with 0.01 shunt, I need to have 600A load inrush which is impossible.   I event put a 5V zener  from pin 3  to GND . But it does not help.

Thanks for your help.  Darius

   

 

 

  • 0
    TI__Mastermind 25570 points

    Hello Darius

    The UCC38084 should work well with a resistive current sense so I wouldn't change that.

    There may be a timing issue as VCC comes up on both the UCC38084 and the gate driver ICs at the same time. The UCC38084 should start a soft-start ramp when Vcc comes up but if the drivers don't become active until the ss ramp has expired then you will get a hard start.

    I'd agree that inrush current should not be a problem but you could get shoot through currents through the top and bottom MOSFETs that could put significant voltages across the sensing resistor

    I would suggest you try the following.

    Run the drivers continuously from a separate DC source which is on all the time. Then turn the Vcc to the controller on - see if the problem occurs or not

    look at the current sense signal to get a feeling for what is happening in the power train.

    Try clamping the CS pin at 600mV _ a BAS16 diode or similar would work well here - the linear operating range of this pin is to 570mV max so the diode won't affect normal operation