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UCC28911 unknown fault

Other Parts Discussed in Thread: UCC28911

Hi,

I am getting a fault on start up using the UCC28911. I get two (2) cycles/pulses at Vdrain when VDD = VDDon ~ 9.5V. This continues the reset and restart, or hiccup, sequence described in section 9.3.6 and figure 30 on the UCC28911 data sheet. According to the Fault Protection section (9.3.7) there are five faults:

Output Over Voltage

Input Under Voltage

Primary Over Current

VDD Clamp Over Current

Thermal Shutdown

 In order to determine what fault, I disconnected the Auxiliary winding as shown n Figure 30 (Start Up and Auto Re-Start Operation). I still get two (2) cycles/pulses when VDD = VDDon ~ 9.5V. Figure 30 indicates I should continue getting pulses (with AuxWinding Disconnected) until VDD = VDDoff ~ 6.5V.

To further diagnosis the fault source, I removed the transformer and installed a 1K resistor in place of the primary. This should eliminate Primary Over Current as the fault. Now I get one (1) cycle/pulse when VDD = VDDon ~ 9.5V.

I do not think the fault is Output Over Voltage as VS = 0V.

I do not think the fault is Input Under Voltage as I am only getting one (1) or (2) cycles(2).

I do not think the fault is Primary Over Current as my current is ~ 150V / 1K = 150mA.

I do not think the fault is VDD Clamp Over Current as I have Aux disconnected.

I do not think the fault is Thermal Shutdown as I do NOT get the short diagnostic pulse on Ipk pin.

I am attaching the schematic. The input comes from a PFC circuit (hence the 385V dc rail). This flyback will provide the Vcc to the PCF chip. Therefore on startup, the input is 110Vac rectified and filtered (110Vac * 1.414 = 150Vdc).

The attached schematic shows +15V primary output (Vcc for the PFC) connected to VDD. I realize this is WRONG. I will be making a separate winding for the +15V primary output (Vcc for the PFC).

Please let me know what I can do to determine the fault.

THANKS!

Derek

 

 

  • Most likely the UCC28911 is going into UVLO on the VDD rail. This will happen if there is not enough capacitance on VDD or the output is overloaded at startup. Monitor VDD and check that VDD does not drop below VDD(off) or 6.5V
    Your schematic was not attached but if you are connecting VDD to a separate 15V rail then the UCC28911 will be damaged or if you are connecting VDD to the bias power pin of a pfc chip then the UCC28911 will be unable to start up because of the loading of the pfc controller.
  • Hi John,

    Thanks for the fast reply! Sorry for my delay, I was on vacation Fri & Mon.

    I do NOT believe this is a UVLO on VDD. As I indicated above, I get one (1) or two(2) pulses VDD = VDDon ~ 9.5V. This continues the reset and restart, or hiccup, sequence described in section 9.3.6. The voltage at VDD looks identical to the figure 30 on the UCC28911 data sheet.

    I will try to paste the sch below and attached as a file.

    I am NOT using J1-4 (+15V Primary). Also, I tried replacing C3 (cap on VDD) from 100uF to 10uF with the same results.

    I look forward to your assistance. Experience tells me I’m doing something stupid but I don’t know what?!?!

     

    Thanks,
    Derek

    SCH-091-229-101-A PLC12-40 HOUSEKEEPING.pdf2816.SCH-091-229-101-A PLC12-40 HOUSEKEEPING.pdf

  • Hi Derek,

    The controller of UCC2891x is designed to work always in discontinuous mode which means the controller waits the valley of the VS pin waveform before enabling the next switching cycle. If the VS pin doesn’t sense the first valley on VS pin, the Mosfet cannot be switched on.

    So replacing the transformer by a resistor won't work for debug purposes because of that.

    For the inital issue, can you post a scope plot / plots of the Drain, Vs, Ipk and Aux winding waveforms zoomed in to the two pulses that you see? This should help figure out the issue.

    One thing I would check initially is the inductances in your transformer to make sure there are no internal shorts which could be causing the problem.

    Thanks

    Billy