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