What is a snubber? How does an RCD snubber work?
In flyback adapters you'll see a diode, a capacitor and a warm resistor on the MOSFET's drain, right next to the transformer. This trio protects the MOSFET from the voltage spike its own transformer makes.
What does it do? The snubber catches the spike energy from the transformer's leakage inductance the moment the MOSFET turns off, stores it in the capacitor and turns it into heat in the resistor. That keeps the drain voltage within what the MOSFET can withstand.
⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)
The moment the gate falls, the drain voltage shoots up; Dsn only conducts during that short peak. Then Csn's energy is spent in Rsn.
Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.
Phase 1/3: Q on
Current flows in the primary and energy is stored in the core. The snubber is idle.
PWM output: HIGH (≈12 V, switch on)
1. Drain (with snubber)
A short peak appears when Q turns off, but it's clamped by Csn's voltage (≈480–550 V). The MOSFET is safe.
On the scope
🔍 How to spot it on a board
- •A fast diode (FR107, UF4007, RS1M...) between the transformer's primary end and the MOSFET drain
- •A capacitor (1–10 nF, 1 kV) and resistor (47–220 kΩ, 1–2 W) in parallel from that diode's cathode to the 310 V rail
- •Heat-darkened board around the resistor
🔧 How to measure it
- •Unplugged: measure Dsn both ways on diode mode.
- •Measure Rsn on resistance (compare with its marked value).
- •Measure Csn with a capacitance meter.
- •Running, scope the drain: the turn-off peak should stay below 80% of the MOSFET's voltage rating.
⚠ Common faults
- •Dsn open: the snubber stops working and the MOSFET soon blows.
- •Rsn open or drifted high: Csn keeps charging and the spike isn't clamped.
- •Rule: if the MOSFET blew, don't fit a new one before measuring all three snubber parts.