20 W flyback with 800 V avalanche margin
The VIPER27HN is a monolithic offline flyback switcher from ST's VIPer™ plus series, integrating the PWM controller and an 800 V avalanche-rated power MOSFET in a single 7-lead DIP package.
Switching frequency and duty cycle limits
The internal oscillator runs at 115 kHz, a fixed frequency that keeps the transformer core size manageable while staying below the EMI quasi-peak limit at 150 kHz. The maximum duty cycle is 80 %, which sets the primary-to-secondary turns ratio for a given input range — at universal mains the 800 V breakdown gives enough headroom to avoid avalanche stress during line transients.
Thermal and protection envelope
On-chip fault protection covers current limiting, over-temperature shutdown, and over-voltage clamping — the supply rail itself is monitored between 8.5 V and 23.5 V, with a 14 V startup threshold that ensures clean turn-on into a discharged bulk cap.
The through-hole 7-DIP package is a standard, widely stocked form factor, so sourcing risk is low for both prototype and production volumes.