Skip to main content
D
DavidJohns2026
Visitor
July 9, 2026
Solved

Viper27HD chip shutting down for ~22ms, 250 ms after startup

  • July 9, 2026
  • 1 reply
  • 22 views

I’ve designed a flyback w/ Viper27HD.  Turn on voltage is 120Vdc.  Max design input volutage is 500Vdc.  Output voltage is 12V.  I designed the VCC tertiary winding for 16V, but due to spike rectification the actual votlage is 20.2V for Viper VCC.  The prime supply is a MagnaPower SLx-1.5-500-3-xxxxx power supply with voltage, current, power, control and + and - slew rate control. The Viper “wakes up” when the input voltage is at ~110V.  I have an internal POR circuit that holds off my controls until 250 ms later.  When the POR ends, the 12V housekeeping power is used to energize the gate drive of a FET from the input power rail to a large capacitor bank through a 40 Ohm “Soft Start” resistor.  When the gate drive initiates, the Viper chip shuts down for 22 ms which temporarily glitches the housekeeping power.  I’m using a FOD2711 isolation regultor for FB control.

 

I observe:

  1.  VCC of Viper is STABLE through the shutdown.
  2. VBR never approaches the brown out threshold
  3. FB switches hard to GND during shutdown then recovers
  4. My FOD device is not driving the shutdown as observed by measuring the bias current into the LED.
  5. CONT pin never exceeds 0.5V only, with very short noise spikes to 0.75V

The Viper chip is shunting the FB pin - why?  Its not OVP ( I don’t use the OVP branch), It’s not OCP, It’s not Over Temp, and it’s not Brown Out.

 

Please help.  I’m under critical project schedule pressure.

Me: 15+ years experience in custom high voltage power supply design for medical & semiconductor capital equipment markets, and 12+ years experience in power electronics engineering team management.

Best answer by DavidJohns2026

Update: I found it. I had only 450nf decoupling capacitance near the flyback input bus. The actual input voltage was drooping more than I thought it was - my 100:1 telemtry that I was relying on was too low BW to resolve the dip in the input voltage that the Viper was actually seeing. After adding 100+ uF for ride through support of the input bus the problem is resolved.

1 reply

D
DavidJohns2026AuthorBest answer
Visitor
July 9, 2026

Update: I found it. I had only 450nf decoupling capacitance near the flyback input bus. The actual input voltage was drooping more than I thought it was - my 100:1 telemtry that I was relying on was too low BW to resolve the dip in the input voltage that the Viper was actually seeing. After adding 100+ uF for ride through support of the input bus the problem is resolved.

Terms & ConditionsAccessibility statement