let’s break this down step-by-step so it’s easy to follow.
We’ll go over UC3842’s working principle, how protection mode works, why the main FET blows, and how to fix/cure the problem.
🔌 1. UC3842 Working Principle
The UC3842 is a PWM (Pulse Width Modulation) current-mode controller IC used in SMPS power supplies — common in LCD/LED TVs, adapters, and other electronics.
Its main jobs are:
Generate PWM signals to drive the main MOSFET/FET.
Monitor voltage & current to keep the output stable.
Provide protection against overcurrent and under-voltage.
Key Pins & Roles
| Pin | Name | Function |
|---|---|---|
| 1 | COMP | Error amplifier output — adjusts duty cycle. |
| 2 | VFB | Voltage feedback from output via optocoupler. |
| 3 | ISENSE | Current sense from shunt resistor — protects MOSFET. |
| 4 | RT/CT | Timing capacitor & resistor — sets switching frequency. |
| 5 | GND | Ground reference. |
| 6 | OUTPUT | Drives MOSFET gate. |
| 7 | VCC | Power supply for UC3842 (~15V start-up). |
| 8 | VREF | Reference voltage (5V) for internal circuits. |
How it works in normal mode:
Start-up phase – VCC is charged from high-value resistor from mains until ~16V, UC3842 starts switching.
PWM generation – UC3842 sends pulses to MOSFET gate to transfer energy through transformer.
Feedback control – Output voltage is sensed via optocoupler → adjusts duty cycle.
Stable operation – Keeps voltage steady under varying loads.
⚠️ 2. How It Goes into Protection Mode
Protection mode triggers mainly through Pin 3 (ISENSE) or low VCC:
Overcurrent Protection (OCP) – If the current sense voltage exceeds ~1V, UC3842 shuts off the MOSFET for that cycle.
Under-voltage Lockout (UVLO) – If VCC falls below ~10V, IC stops switching.
Feedback failure – If optocoupler feedback is missing, output rises → OVP can trigger.
In many SMPS designs, protection mode means:
IC stops sending PWM pulses.
Power LED may blink (in TVs).
Supply keeps trying to restart ("hiccup mode").
💥 3. Why the Main FET Blows
The main switching MOSFET (or FET transistor) blows when:
No feedback / optocoupler fault → output rises uncontrollably → FET overstressed.
Short on secondary side → primary current spikes → FET overheats.
Snubber network failure (damaged resistor, diode, or capacitor) → high voltage spikes destroy FET.
Dry or leaky primary capacitor → unstable switching → MOSFET avalanche breakdown.
Gate drive issues → UC3842 output weak → MOSFET runs in linear region → heat buildup.
🛠4. How to Cure & Prevent This Problem
Step-by-step repair checklist
Replace blown MOSFET — use same or higher-rated part.
Check UC3842 — often damaged when FET shorts.
Test snubber network (across primary winding or FET drain-source) — replace burnt resistors/diodes/caps.
Check startup resistor — if open, IC may not start correctly.
Replace dried electrolytic capacitors — especially primary filter & VCC capacitor (47µF–100µF, 50V).
Check optocoupler & TL431 — ensure proper feedback.
Test load — shorted diodes or load on secondary can cause immediate FET failure.
Power up with series bulb tester — to avoid instant re-damage during testing.
Preventive design tips
Use proper snubber circuit to absorb voltage spikes.
Ensure adequate gate drive from UC3842.
Keep good quality capacitors in primary side.
Check transformer insulation — no shorts between windings.
