UC3842 Working Principles

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.

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🔌 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:

1. Start-up phase – VCC is charged from high-value resistor from mains until ~16V, UC3842 starts switching.

2. PWM generation – UC3842 sends pulses to MOSFET gate to transfer energy through transformer.

3. Feedback control – Output voltage is sensed via optocoupler → adjusts duty cycle.

4. Stable operation – Keeps voltage steady under varying loads.

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⚠️ 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").

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💥 3. Why the Main FET Blows

The main switching MOSFET (or FET transistor) blows when:

1. No feedback / optocoupler fault → output rises uncontrollably → FET overstressed.

2. Short on secondary side → primary current spikes → FET overheats.

3. Snubber network failure (damaged resistor, diode, or capacitor) → high voltage spikes destroy FET.

4. Dry or leaky primary capacitor → unstable switching → MOSFET avalanche breakdown.

5. Gate drive issues → UC3842 output weak → MOSFET runs in linear region → heat buildup.

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🛠 4. How to Cure & Prevent This Problem

Step-by-step repair checklist

1. Replace blown MOSFET — use same or higher-rated part.

2. Check UC3842 — often damaged when FET shorts.

3. Test snubber network (across primary winding or FET drain-source) — replace burnt resistors/diodes/caps.

4. Check startup resistor — if open, IC may not start correctly.

5. Replace dried electrolytic capacitors — especially primary filter & VCC capacitor (47µF–100µF, 50V).

6. Check optocoupler & TL431 — ensure proper feedback.

7. Test load — shorted diodes or load on secondary can cause immediate FET failure.

8. 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.

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