Flyback · topology: how it works
Almost every low-power isolated supply is a flyback. The transformer really works like an inductor: energy is stored in the core while Q is on, and transferred to the secondary when Q turns off.
⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)
Gate HIGH: energy builds up in the core and no secondary current flows. Gate LOW: the energy empties into the secondary. Never both at once; that's where the flyback gets its name.
Measuring points: tap the numbers on the diagram
Phase 1/2: Q on
Current flows in the primary and energy is stored as a magnetic field in the core. Because the windings are opposed, the secondary diode is off; the capacitor feeds the load.
1. Bulk capacitor
Rectified mains: ≈310 V DC.
On the scope
Real circuit: circuit tour and fault scenarios
The same topology with every part you'd find on a real board. Follow how it works step by step with the "Circuit tour", or pick a fault scenario below to see what to measure, in order, on the schematic.
⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)
Each period the IC drives OUT HIGH and the primary current ramps up. When the ramp on CS reaches the threshold set by FB, OUT goes LOW and the energy moves to the secondary. More load raises the threshold and lengthens the pulse.
Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.
Phase 1/2: Q on
The IC turns Q on from its OUT pin. Primary current flows to ground through Rs; the CS pin watches that current.
UC384x OUT pin: HIGH (≈12 V, switch on)
1. Q's drain
The flyback's signature: ≈0 V when on; a spike (limited by the snubber) and ringing when off.
On the scope
🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic
🔍 How to recognise it on a board
- ✓One MOSFET (or one IC with the MOSFET inside) and one small transformer
- ✓No output inductor on the secondary: the capacitor comes right after the diode
- ✓An optocoupler (usually a PC817) and a TL431 between the transformer and the secondary
- ✓An RCD snubber (resistor + capacitor + fast diode) on the MOSFET's drain
Where it's used
Phone chargers, laptop adapters, TV and monitor standby supplies, supplies up to ~100 W