[011] Flyback Converter RCD Clamp Design

Design equations and analysis of the most commonly-used RCD clamp circuit used for the popular flyback converter.

Introduction

All PWM converters have parasitic components that lead to ringing waveforms which must be properly suppressed. Without this, semiconductors can fail, and noise levels will be higher than necessary. This article describes the most commonly-used RCD clamp circuit used for the popular flyback converter, together with its design equations.

Flyback Converter with No Snubbers

Without a snubber, the leakage inductance of the flyback transformer rings with stray capacitances in the circuit, producing large amplitude high-frequency waveforms as shown in Figure 1.

Many application notes and designs ignore the ringing waveforms and operate the converter without addressing the issue. There are two problems with this: firstly, there is excessive voltage on the drain of the FET which can lead to avalanche breakdown and eventually failure of the device. Secondly, the ringing waveform will be radiated and conducted throughout the power supply, load, and electronic system, creating noise issues and even logic errors. The ringing frequency will also show up as a peak of the EMI spectrum in both radiated and conducted EMI.

Figure 1: Flyback converter drain voltage

In most designs, this is not acceptable, and it is necessary to add circuit elements to damp the ringing (using an RC snubber), or to clamp voltages (with RCD clamps), or both. The design of these networks is a combination of measurements and analysis to ensure a rugged and dependable result.

Primary RCD Clamp for the Flyback Converter

Figure 2 shows an RCD clamp circuit used to limit the peak voltage on the drain of the FET when an RC snubber is insufficient to prevent switch overvoltage. The RCD clamp works by absorbing the current in the leakage inductor once the drain voltage exceeds the clamp capacitor voltage. The use of a relatively large capacitor keeps the voltage constant over a switching cycle.

The resistor of the RCD clamp always dissipates power. Even with very little load on the converter, the capacitor will always be charged up to the voltage reflected from the secondary of the converter, v_f. As the load is increased, more energy will flow into the capacitor, and the voltage will rise by an additional amount, v_x, above the ideal square wave flyback voltage. The waveform defining these voltages is shown in Figure 2.

Figure 2: Flyback converter with primary RCD clamp