[Editor's Note: This tech bite was originally posted on the Fairchild Semiconductor website. Richard Chung, the applications manager at Fairchild, would like to bring these tech briefs from his team to the Planet Analog audience.]
Consisting of very few components, Flyback circuitry is one of the simplest topologies. It is a very popular choice for low- and medium-power applications such as battery chargers, adapters, DVD players, and LED drivers. However, regardless of its circuit simplicity, the design of a reliable Flyback converter still can be quite challenging. For example, the resistor-capacitor-diode (RCD) snubber, the key element of a Flyback converter, requires solid calculation and experiment verification to figure out the optimum parameters. Otherwise, the reliability, or the efficiency and cost of the converter, could be severely impacted.
Figure 1 shows a Flyback converter with some critical parasitic components, such as Llk — the primary leakage inductance of the transformer — and Coss — the parasitic junction capacitance of the MOSFET. The figure also presents the key waveforms of the Flyback Converter with RCD snubber in discontinuous conduction operation mode (DCM).
When the MOSFET turns off, the primary current id charges Coss . Once the voltage across Coss exceeds the input voltage plus the reflected output voltage Vin +nVo, the secondary diode turns on, and clamps the voltage across the magnetizing inductor Lm to n*Vo. Then, due to the presence of Llk & Coss , a high frequency resonance takes place and causes excessive voltage on the MOSFET. This excessive voltage should be suppressed to an acceptable level. Typically, the RCD snubber as shown in Figure 1 is used for this voltage suppression purpose.
At the transient when the diode turns on, the voltage across Llk is Vsn -nVo. Vsn is the voltage across the snubber capacitor Csn . The primary current isn flows into Csn as shown in Figure 1. The slope of isn is
where n is the turns ratio of the main transformer, and Llk is the leakage inductance of the main transformer.
Empirically, the RCD snubber should be configured in such a way that it clamps Vsn at about 2-2.5 times of nVo . Besides, Vsn plus the maximum input voltage should not be higher than 80 percent of the rated breakdown voltage of the MOSFET (BVdiss ). Too high a Vsn will require using a MOSFET with a much higher rated breakdown voltage. A very small Vsn will result in a severe loss on the snubber circuit. The power dissipation of the snubber circuit is obtained by:
Where ipeak is the peak current of the transformer primary current under the minimum input voltage and full-load operation condition. The time ts is:
The snubber resistor Rsn with proper rated power should be chosen based on the power loss. Therefore, the resistance Rsn can be calculated using the following equation.
The maximum ripple of the snubber capacitor voltage is obtained as:
In general, a 5% to 10% ripple is reasonable. Therefore, the snubber capacitance Csn can be calculated using the above equation.
The snubber capacitor should be a ceramic or film capacitor that offers low equivalent series resistance (ESR). Electrolytic or tantalum capacitors are not appropriate for this snubber application.
Want to design a Flyback LED Driver with optimum RCD snubber in minutes? Check out our Power Supply WebDesigner tool.
Want to learn more about design of snubbers for Flyback converter? Download our app note: Design Guidelines for RCD Snubber of Flyback Converters.