WEBENCH® Tools/UCC2813-3-Q1: IC not starting up

Prodigy 160 points

Replies: 27

Views: 226

Part Number: UCC2813-3-Q1

Tool/software: WEBENCH® Design Tools

Hi,

I'm attempting to use a UCC2813 chip to make a AC->24Vdc supply.  I used WEBENCH to start the design and basically copied its output directly:

However I don't get any output from the supply.  What I'm finding is that Vcc of the UCC2813 is very low - it hovers around 3.8-4V, which is right around the UVLO threshold of the chip and also right at the hairy edge of Vgs on the switching FET.

The circuit straight from WEBENCH (as shown in the schematic above) doesn't start up at all, because Vcc is around 3.6V.  I removed D13 (Zener), increased C13 from 15uF to 120uF, and decreased R2+R3 from 500k to 300k.  Doing those will get the UCC2813 to switch, but Vcc is still hovering around 4V, and is apparently not enough to switch the MOSFET:

In both screenshots here, the green trace is the gate of the MOSFET and the yellow trace is the source pin of the MOSFET.  It looks like the UCC2813 tries to switch through a normal startup - when you zoom in you can see the output PWM start at a very low duty cycle and ramp up - and it switches until Vcc falls below the UVLO, then it gives up; then C13 recharges and it tries again.  But nothing ever gets through the transformer.

One caveat on the scope screenshots is that I took those with 50Vdc across the input capacitor, rather than rectified AC.  What seems interesting is that Vcc is the same if the input is 50Vdc or rectified 120Vac and the behavior of the circuit doesn't change.  I was actually able to get a stable output from the supply in this configuration - I reduced R2+R3 to 82k, and with 50Vdc input I got a solid 5ish volts output.  When I tried to scale this back up, something smoked and the MOSFET shorted internally.  Oops!  But I think what this tells me is that something is dragging down Vcc, but I haven't figured out what it could be.  Any help would be greatly appreciated! 

Thanks

27 Replies

  • Hi Elliot! 

    Thanks for reaching out and for using our Webench tool! Nothing jumps out at me here, but let me retrace your footsteps and see if I encounter the same issues in simulation. I will get back to you with an update on Thursday. 

    Thanks again for getting in touch, I look forward to talking more!

    Aidan Davidson

  • In reply to Aidan Davidson1:

    Excellent, thanks!

  • In reply to Elliott Potter:

    Hi Elliot! 

    I took a look at your schematic, thanks again for sharing! It definitely seems like the bootstrap circuit is having issues starting up and keeping the UCC2813 IC running. 

    Can you try lifting up the VCC pin from the board, isolating it, and providing VCC with a bench DC supply? Try starting the power supply with different load conditions and observe the time it takes for voltage to appear on the aux winding. I suspect you may need an even larger capacitor than the 120uF you changed over. 

    Also, if you're concerned about current leaking back through R2 & R3 to the HVDC rail, you can add a diode in series to prevent leakage. 

    Be careful about reducing R2 & R3 too far, you may provide too much current to the device. However, from the datasheet, the recommended max current at the VCC pin is 25mA and with 50VDC and R2+R3=83kOhm I don't think you should be coming close to that. 

    Thanks again for reaching out, give that test a try and think about possibly increasing C13's value further. 

    Aidan Davidson

  • In reply to Aidan Davidson1:

    Hi!  OK I tried a couple of things.

    First, I replaced C13 from 120uF to 200uF; it didn't seem to have any impact.

    Next, I removed D5 and R3, and powered the UCC2813 from a benchtop supply (10V, current limited to 50mA).  I applied AC and then powered up the UCC2813 and got 24V, briefly (maybe 4-5 seconds), then the AC input fuse (F3, it says 1A on the schematic but it's actually 2A) opened.  I had no load on the output (I meant to plug in a 60 ohm resistor but forgot).  Nothing appears to be damaged, but I'm not sure why the fuse opened so I stopped there.  Any thoughts?  The current draw on the benchtop supply never hit that 50mA limit but I didn't see what it was doing.  If nothing leaps to mind, on Monday I'll try using the 50V benchtop supply instead of the AC input and another 10V supply for the UCC2813, that way I can safely probe it with the oscilloscope while it's starting up.

    Thanks!

  • In reply to Elliott Potter:

    Hi Elliot! 

    Just to confirm, your test with the benchtop supply was with the VCC pin isolated from the rest of the board, correct? Were you able to observe the timing of the aux winding kicking in? 

    Aidan Davidson

  • In reply to Aidan Davidson1:

    Hi!  I did not lift the VCC pin from the board, the idea was that by removing R3 and D5 from the board, and applying voltage across C12 (which was convenient), it would power the UCC2813 without interacting with the rest of the circuit.  Was this the right thing to do?

    Unfortunately I didn't catch any of the other timing but will try it tomorrow.

    Thanks!

  • In reply to Elliott Potter:

    Hi Elliot! 

    It is true that removing R3 and D5 and applying voltage across C12 accomplishes isolating the VCC pin, but it also disconnects the bootstrap capacitor from the AUX winding which maybe might have something to do with your AC fuse blowing. Can you try replacing R3 and D5 and carefully lifting and isolating the VCC pin? This way the bootstrap capacitor can stay connected to the AUX winding and you can get a good impression of the timings involved here. 

    Let me know how your testing goes with the DC supply also! 

    Aidan Davidson


  • In reply to Aidan Davidson1:

    Hello!

    OK, I replaced D5 and R3 and lifted the VCC pin to power it separately, and replaced the fuse.  And ... it works!  There's an audible squeal and the output (into a 20 ohm load) is a bit messy, but it was there and pretty solid (scope_6.png).

    I put a 56 ohm resistor across C13 to give it some load (otherwise, with no load, the voltage crept up >20V) and then checked the time it took to reach about 5V; that looked like 35-40ms (scope_7.png):

    ... though that was with the 50Vdc supply and R2+R3=82k.  Gate-Source captures, if they're helpful, are also attached, scope_4 and scope_5.

    I wonder about the audible noise and the less-than-perfectly stable output.  Is it possible that the transformer I have is not made correctly?

    Thanks!

  • In reply to Elliott Potter:

    Hi Elliot! 

    I'm very excited that we're getting output!

    First, after reading into this a bit more it might actually be better for you to use the UCC2813-2 instead of the UCC2813-3. This device has a wider gap between the UVLO on/off threshold and that may be just what you need to overcome your issue with the PWM IC startup. 

    Generally the 2 main sources of audible noise in designs like this one are the transformer and ceramic capacitors. Noise in the transformer can generally be resolved with varnish which prevents the core from vibrating. Audible noise from ceramic capacitors is most commonly found in caps that experience high dV/dT swings like output, snubber, and clamp capacitors. This type of noise can be resolved by replacing the ceramic cap with a metal film cap or maybe a ceramic cap with a different dielectric. However, this noise may be caused by the instability on our output. 

    As you vary the load (both decreasing and increasing), does the stability / audible noise get worse or better? Can you take a couple more scope shots with varying loads? 

    Have you tried with the AC supply? When you do, does the fuse still blow? 

    Aidan Davidson

  • In reply to Aidan Davidson1:

    Hello!  I was very happy to see output too, I feel like a solution is near!

    I am a bit concerned about Vth of the MOSFET too - even with a UCC2813-2 device that can switch with a lower voltage, if I cannot drive the MOSFET efficiently enough then I don't know if startup will work anyway.  Maybe, I can certainly get the chip and try it out.

    As for the noise, I am using mostly ceramic capacitors so I will try switching to a metal film cap for the snubber to see if it helps.

    I switched back to the AC input and tried three different loads.  The good news is that the fuse did not blow!  The scope captures below are from the 24V output.

    The first load is the same 20 ohm resistor, it looked about the same, though the audible noise was quite loud and constant:

    The ripple seemed basically the same as before.  Then I tried a 250 ohm load, which is closer to steady state current consumption of the device.  In this case, the audible noise was much lower but the output ripple was huge:

    Last, I tried a brushed DC motor on a linear actuator, drawing about 1A:

    In this case I could not hear the audible noise over the motor, so if it was loud it didn't matter!

    What could cause such large spikes on the output?  I can add additional capacitors but this seems like a lot.

    Thanks!