Budgeting Flyback RCD Clamp Stress and Dissipation

An RCD clamp limits the drain voltage excursion associated with a flyback transformer’s leakage inductance, but the chosen clamp voltage also affects dissipation. Selecting parts from a single room temperature waveform can leave inadequate margin at another input, load, or transformer condition.

Define the voltage references

Draw the clamp connection and label input voltage, reflected output voltage, clamp capacitor voltage, and MOSFET drain voltage. These are related quantities, not interchangeable names for the same number. Include diode behavior and wiring parasitics in the interpretation of the measured peak.

The TI article on snubbing a flyback converter explains the tradeoff between clamp voltage and dissipative loss. It notes that clamp loss approaches leakage inductance energy at high clamp voltage, while lower clamp voltage changes the loss relationship. Use the full design model rather than assuming every clamp resistor dissipates exactly the leakage energy estimate.

Estimate the energy scale

Stored energy in an inductance is one half of inductance times current squared. For a hypothetical 1 microhenry leakage inductance carrying 2 A, that energy is 2 microjoules. Repeating that energy event at 100 kHz corresponds to 0.2 W. This establishes an illustrative scale, not a final resistor rating.

Use the actual leakage measurement method and relevant switching current. Review tolerances and operating conditions that change either quantity. Because energy depends on current squared, a modest increase in peak current can have a larger effect than an intuitive linear estimate suggests.

Check each component’s job

Review clamp resistor continuous and pulse loading, capacitor voltage and ripple, and diode repetitive current, reverse voltage, and switching behavior. Account for nearby heating and the final enclosure. A resistor whose nominal wattage exceeds the calculated average can still require review of pulse energy and temperature derating.

Separate the main clamp action from residual high frequency ringing. Adding another network to reduce ringing can change loss and peak voltage, so evaluate the complete arrangement after each adjustment.

Verify across operating corners

Measure drain voltage and clamp behavior at relevant input and load conditions, including startup and specified fault recovery. Use a voltage measurement system with suitable range and high frequency common mode performance. Preserve probe attachment and bandwidth settings between comparisons.

Record steady temperature after the converter reaches the intended thermal condition. Compare multiple representative transformers if leakage variation is material. Release the clamp only when voltage stress and dissipation have both been evaluated; reducing one does not automatically improve the other.