11 Replies Latest reply on Oct 12, 2017 5:33 PM by jc2048

    ucc28c43 shorted output transistor

    harman

      Hello everybody,

      in my pwm controller I use UCC28C43. In many cases, after some time running, low side output transistor in ucc destroys itself (it is shorted). At first I did not D9 in the circuit, I supposed that it could solve the probelm, but not. With D9 problem persists. Has anybody any idea why?

      Thanks a lot.

      Jan

        • Re: ucc28c43 shorted output transistor
          jw0752

          Hi Jan,

           

          You have probably looked at the  Data Sheet already but here is a link just in case you haven't.

           

          http://www.ti.com/lit/ds/symlink/ucc28c43.pdf

           

          TI has provided rather extensive info on this chip. I looked it over and couldn't see anything obvious but I do not have all the details of your circuit and how you are powering it. What is your supply voltage and is there any chance spikes are being produced? Do you have a scope to look at the signals. Is it the on chip outputs that are being shorted or is it the external MOSFET?

           

          I am curious too what is going on so keep me posted what you find out.

           

          John

            • Re: ucc28c43 shorted output transistor
              harman

              Thank you for reply John,

               

              The shorted transistor is this on the chip low-side. The high-side transistor seems to be ok. Chip is powered by 12VDC, not stabilized, but there is protection zener diode. Unfortunatelly now I don't have any scope. Gate resistor R7 earlier was 10 Ohm, then 15 Ohm and now I tried 17 Ohm, but without success.

              Thanks

              Jan

                • Re: ucc28c43 shorted output transistor
                  jw0752

                  Hi Jan,

                  I am not the best person to ask and I am hoping that jc2048  will hop in here and give us his insights. AS far as I can see the only load on the output of the chip comes from the 10K R-13 resistor and the capacitance load of the gate of T1. The UCC28C43 can operate at quite high frequencies but it doesn't look like your application is high enough to be see an excessive load . I would probably try a 1K resistor for R7 as this should still be low enough to trigger the T1 and it would limit the current better than the 15 R you are currently using. As I said hopefully one of the other guys with more experience than I will lend us so other ideas.

                   

                  John

                    • Re: ucc28c43 shorted output transistor
                      jc2048

                      I am hoping that jc2048  will hop in here

                      Fortunately, you got Michael's insights instead (far more useful than my meanderings).

                       

                      The setup here is the same as for the falltime test that the semiconductor manufacturer does. That test allows the inductor to ramp to 7.5A and then turns the MOSFET off and measures the 10% to 90% time as the drain leaps up to a supply of 400V. That time is around 6nS. As an edge rate, that's something like 53kV/uS, so I can see why Michael says this kind of thing can be a problem.

                       

                      That will work back through the intrinsic capacitances of the MOSFET to the chip output. How much of the 400V arrives there is down to the various dividers - there's the capacitative divider with the internal MOSFET capacitance and the combined output FETs' capacitance in the chip, and there's also the resistance divider from the gate resistor and whatever resistance the low FET is down to by then (it won't necessarily have turned fully on by that stage), and there are the additional paths that come from parasitic capacitance in the pcb layout. So it might be that catching the chip output (or trying to catch it!) at the VCC level might help. I would have thought that that was more important than catching transitions below ground. I'd also look to protecting the current sense input too.

                      2 of 2 people found this helpful
                    • Re: ucc28c43 shorted output transistor
                      michaelkellett

                      You are trying to get a mains voltage converter working, using  a fairly state of the art SiC MOSFET (devices used for highest switching speed at high voltages) without a scope to look at signals.

                       

                      There is no way this will be successful.

                       

                      It is quite common in such circuits for parasitic inductance/capacitance to cause spikes that can destroy devices and without a scope, with high voltage probes, and suitable lab technique you are not going to be able to locate the issues.

                       

                      What is your overall objective in this work (knowing that we may be able to suggest  a workaround).

                       

                      @JW, increasing R7 to 1k will prevent the MOSFET from switching properly - from the Cree data sheet the useful range of external gate resistance for this device is 2.5 -> 20R, the driver chip range is 5.5 -> 25R so it's already too high with no series R at all. I suspect that the SiC MOSFET is not  a good pick to go with this chip. Using  a high gate resistance will at best make the expensive SiC MOSFET work no better than a cheap silicon one and at worst will cause it to get very hot, very quickly.

                       

                      MK

                      2 of 2 people found this helpful
                  • Re: ucc28c43 shorted output transistor
                    harman

                    Hi everybody,

                     

                    I catched some scopes.

                    C1: switched current (1V=2.5A)

                    C2: ucc output. Pin OUT(6).

                    C3: clock. Pin RT/CT(4).

                     

                    1st scope: at mid output power

                    2nd scope: zoomed

                    3rd scope: at very low output power.

                     

                    Problem could be at very low output power, because of higher voltage at VDD and consequently at the output (cca 15V).

                     

                    Thanks a lot for your ideas.

                    Jan

                    With mid output load

                    zoomedVery low output power.