Design and implementation of an RCD absorption circuit for a flyback converter

1. Introduction

The flyback converter is one of the simplest circuit topologies. It is formed by directly placing an inductor and coupling it from a Buck-Boost converter, that is, a transformer with an air gap. When the main power switch is turned on, the energy is stored in the transformer, and when the switch is turned off, the energy is sent to the output stage. Since the transformer needs to store energy when the main power switch is turned on, an air gap is added to the magnetic core. Since the flyback type requires very few components, it is a commonly used circuit topology for small and medium power supplies. For example: chargers, adapters, and DVD players.

Figure 1 Flyback converter circuit

(a) Flyback converter with parasitic components; (b) CCM operating waveform; (c) DCM operating waveform

Figure 1 shows several parasitic components of the flyback converter in continuous conduction mode (CCM) and discontinuous conduction mode (DCM). Such as the primary interstage leakage inductance, the output capacitance of the MOSFET, the junction capacitance of the secondary diode, etc. When the MOSFET is turned off, the primary current Id charges the Coss of the MOSFET. This voltage pressure is applied to Coss, Vds exceeds the input voltage, and the folded output voltage VIN + Nv is added. The secondary diode is turned on. The voltage on the inductor Lm is clamped at Nvo, which is the high-frequency resonance and high surge between LIK1 and Coss. In the CCM working mode, the secondary diode is always turned on until the MOSFET is turned on again. Therefore, when the MOSFET is turned on, the reverse recovery current of the secondary diode is superimposed on the primary current. Therefore, there is a large surge at the time of turn-on, which means that for DCM working conditions, the secondary current has dried up before the end of a switching cycle. Therefore, there is a resonance between Lm and Coss.

2. Absorption circuit design

The excessive high voltage caused by the resonance between LIK1 and Coss must be at a level that the circuit components can accept, so a circuit must be added to protect the main switch MOSFET. The RCD absorption circuit and key waveforms are shown in Figures 2 and 3. When Vds exceeds VIN+nV, the RCD absorption circuit turns on the absorption diode VDsn to absorb the current of the leakage inductance. Assuming that the absorption circuit capacitance is large enough, its voltage will not exceed.

When the MOSFET is turned off, Vds is charged to VIN+nV. The primary current passes through the diode VDsn to the capacitor Csn of the absorption loop, and the rectifier on the secondary side is turned on at the same time. Therefore, the voltage on it is Vsn-nV, and the slope of Isn is as follows:

Figure 2 RCD absorption circuit of flyback converter

Figure 3 Key waveforms of DCM with absorption circuit added

Where: isn is the current flowing into the absorption loop; Vsn is the voltage on the absorption loop capacitor; n is the number of turns of the main transformer; LIK1 is the leakage inductance of the main transformer. Therefore, TS can be calculated from the following formula:

Where: Ipeak is the peak value of the primary current.

The voltage of the absorption circuit capacitor, Vsn, is determined under the condition of minimum input voltage and full load. Once Vsn is determined, the power consumption of the absorption circuit under the condition of minimum input voltage and full load is:

In the formula: fs is the switching frequency; Vsn is 2 to 2.5 times nVo. From the formula, it can be seen that a very small Vsn also reduces the absorption circuit loss.

On the other hand, since the power consumption of the absorption circuit Rsn is, we can find the resistance:

Then the resistance of the absorption circuit is selected with appropriate power to consume this energy, and the maximum ripple voltage on the capacitor is calculated using the following formula:

Usually 5% - 10% ripple is allowable, so the capacitance of the absorption circuit can also be calculated using the above formula.

When the converter is designed to operate in CCM mode, the peak leakage current and the absorption circuit capacitor voltage decrease as the input voltage increases. The absorption circuit capacitor voltage at the highest input voltage and full load can be calculated by the following formula:

Where: fs is the switching frequency; LIK1 is the primary leakage inductance; n is the transformer turns ratio; Rsn is the absorption loop resistance; Ipeak2 is the primary peak current at the highest input voltage and full load. When the converter works in CCM state, the highest input voltage and full load conditions Ipeak2 is expressed as follows:

Where: PIN is the input power; Im is the transformer magnetizing inductance. VDCmax is the maximum input voltage value Vdc of the rectifier.

If the maximum value of Vds during instantaneous transition and steady state is lower than 90% and 80% of the BVdss voltage of MOSFET, the withstand voltage of the absorption loop diode must be higher than BVdss. An ultra-fast recovery diode with a current of 1A and a withstand voltage of 120% BVdss can be selected.