RCD Snubber Design Guidelines for Flyback Converters - Application Note AN4147

Application Note AN4147 Application Note AN4147 Application Note AN4147 Application Note AN4147 Design Guidelines for RCD Snubber of Flyback wwwfairchildsemicom REV 100 6305 Abstract When the MOSFET turns off a high voltage spike occurs on the drain pin because of a resonance between the leakage inductor Llk of the main transformer and the output capacitor COSS of the MOSFET The excessive voltage on the drain pin may lead to an avalanche breakdown and eventually damage of the MOSFET Therefore it is necessary to add an additional circuit to clamp the voltage In this article some design guidelines for RCD snubber of flyback converters are presented 1 Introduction One of the most simple topologies is a flyback converter It is derived from a buckboost converter by replacing filter inductors with coupled inductors ie gapped core transformers When the main switch turns on the energy is stored in the transformer as a flux form and the energy is transferred to output during the main switch offtime Since the transformer needs to store energy during the main switch ontime the core should be gapped Since flyback converters need very few components it is a very popular topology for low and medium power applications such as battery chargers adapters and DVD players Figure 1 shows that a flyback converter operates in continuous conduction mode CCM and discontinuous conduction mode DCM with several parasitic components such as primary and secondary leakage inductors an output capacitor of MOSFET and a junction capacitor of a secondary diode When the MOSFET turns off the primary current id charges COSS of the MOSFET in short time When the voltage across COSS Vds exceeds the input voltage plus reflected output voltage VinnVo the Figure 1 Flyback converter a configuration with parasitic components b CCM operation c DCM operation Vin Llk1 id Vds iD Vo n1 Lm im Llk2 Cj Coss VinnVo iD id Vds resonance between Llk1 and Coss diode reverse recovery current VinnVo iD id Vds resonance between Lm and Coss resonance between Llk1 and Coss id id Vin id iD id iD t t t t a flyback converter with parasitic components b CCM operation c DCM operation AN4147 APPLICATION NOTE 2 REV 100 6305 secondary diode turns on so that the voltage across the magnetizing inductor Lm is clamped to nVo There is therefore a resonance between Llk1 and COSS with high frequency and high voltage surge This excessive voltage on the MOSFET may be a main cause of its failure In the case of the CCM operation the secondary diode remains turned on until the MOSFET is gated on Therefore when the MOSFET turns on a reverse recovery current of the secondary diode is added to the primary current and thus there is a large current surge on the primary current at the turnon instance Meanwhile since the secondary current runs dry before the end of one switching period in the case of the DCM operation there is a resonance between Lm and COSS of the MOSFET 2 Snubber design The excessive voltage due to a resonance between Llk1 and COSS should be suppressed to an acceptable level by an additional circuit in order to protect the main switch The RCD snubber circuit and key waveforms are shown in figures 2 and 3 respectively The RCD snubber circuit absorbs the current in the leakage inductor by turning on the snubber diode Dsn when Vds exceeds VinnVo It is assumed that the snubber capacitance is large enough that its voltage does not change during one switching period When the MOSFET turns off and Vds is charged to VinnVo the primary current flows to Csn through the snubber diode Dsn The secondary diode turns on at the same time Therefore the voltage across Llk1 is VsnnVo The slope of isn is as follows Figure 2 Flyback converter with RCD snubber Figure 3 Key waveforms of the flyback converter with RCD snubber in DCM operation where isn is the current flows into the snubber circuit Vsn is the voltage across the snubber capacitor Csn n is the turns ratio of the main transformer and Llk1 is the leakage inductance of the main transformer Therefore the time ts is obtained as follows where ipeak is the peak current of the primary current The snubber capacitor voltage Vsn should be determined at the minimum input voltage and full load condition Once Vsn is determined the power dissipated in the snubber circuit at the minimum input voltage and full load condition is obtained as follows where fs is the switching frequency of the flyback converter Vsn should be 225 times of nVo Very small Vsn results in a severe loss in the snubber circuit as shown in the above equation On the other hand since the power consumed in the snubber resistor Rsn is Vsn 2Rsn we can obtain the resistance as 1 lk o sn sn L nV V dt di Vin Vsn Rsn Dsn Llk id Vds Csn isn iD Vo n1 ipeak ts Vin Vsn nVo iD id isn Vds peak o sn lk s i nV V L t 1 s o sn sn lk peak s s peak sn sn nV f V V L i t f i V P 2 2 1 2 APPLICATION NOTE AN4147 REV 100 6305 3 follows Then the snubber resistor with proper rated power should be chosen based on the power loss The maximum ripple of the snubber capacitor voltage is obtained as follows In general 510 ripple is reasonable Therefore the snubber capacitance is calculated using the above equation When the converter is designed to operate in CCM the peak drain current together with the snubber capacitor voltage decreases as the input voltage increases The snubber capacitor voltage under maximum input voltage and full load condition is obtained as follows where fs is the switching frequency of the flyback converter Llk1 is the primary side leakage inductance n is the turns ratio of the transformer Rsn is the snubber resistance and Ipeak2 is the primary peak current at the maximum input voltage and full load condition When the converter operates in CCM at the maximum input voltage and full load condition the Ipeak2 is obtained as follows When the converter operates in DCM at the maximum input voltage and full load condition the Ipeak2 is obtained as follows where Pin is the input power Lm is the magnetizing inductance of the transformer and VDC max is the rectified maximum input voltage in dc value Check if the maximum value of Vds is below 90 and 80 of the rated voltage of the MOSFET BVdss at the transient period and steady state period respectively The voltage rating of the snubber diode should be higher than BVdss Usually an ultrafast diode with a 1A current rating is used for the snubber circuit 3 Example In this section an example will be shown An adapter using FSDM311 has the following specifications 85 Vac to 265 Vac input voltage range 10 W output power 5 V output voltage and 67 kHz switching frequency In the case where the RCD snubber uses a 1nF snubber capacitor and a 480kΩ snubber resistor Figure 4 shows several waveforms with 265 Vac at the instance of the AC switch turnon Figure 4 Startup waveforms with 1 nF snubber capacitor and 480 kΩ snubber resistor There are the drain voltage Vds 200 Vdiv the supply voltage Vcc 5 Vdiv the feedback voltage Vfb 1 Vdiv and the drain current Id 02 Adiv in the following order The maximum voltage stress on the internal SenseFET is around 675 V as shown in Figure 4 However the voltage rating of FSDM311 is 650V according to the datasheet There are two reasons for exceeding the voltage ratings One is the wrong transformer design the other is the wrong snubber design Figure 5 gives an example of incorrect snubber design Figure 5 Steady state waveforms with 1 nF snubber capacitor and 480 kΩ snubber resistor s o sn sn peak lk sn sn nV f V V i L V R 2 1 2 2 1 s sn sn sn sn C R f V V 2 2 2 2 1 2 2 peak s lk sn o o sn I f R L nV nV V 2 max max max max 2 o DC s m o DC o DC o DC in peak nV f V L nV V nV V nV P V I m s in peak f L P I 2 2 574V 451V AN4147 APPLICATION NOTE 4 REV 100 6305 As mentioned above for reliability the maximum voltage stress at the steady state should be equal to 80 of the rated voltage 650 V 08 520 V Figure 5 shows the voltage stress on the internal SenseFET is above 570 V with Vin 265 Vac at steady state However the fact that VinnVo is around 450 V 375 V 15 5 V implies the turns ratio of the transformer is 15 which is a reasonable value Therefore the snubber circuit should be redesigned Let Vsn be twice that of nVo 150 V And Llk1 and ipeak is 150 µH and 400 mA by measuring respectively Then we obtain the snubber resistance as follows And the power emission from Rsn is calculated as follows Let the maximum ripple of the snubber capacitor voltage be 10 then the snubber capacitance is obtained as follows The results with 14 kΩ 3 W and 10 nF are shown in Figures 6 and 7 Figure 6 Startup waveforms with 10 nF snubber capacitor and 14 kΩ snubber resistor Figure 7 Steady state waveforms with 10 nF snubber capacitor and 14 kΩ snubber resistor The voltage stresses on the internal SenseFET are 593 V and 524 V at the startup and steady state respectively These are around 912 and 806 of the rated voltage of FSDM311 respectively k k u nV f V V i L V R s o sn sn peak lk sn sn 14 67 75 150 150 40 2 150 1 150 2 1 2 2 2 1 2 W k R V P sn sn 61 14 1502 2 nF k k V R f V C s sn sn sn sn 10 67 12 14 120 AN4147 APPLICATION NOTE 6305 00m 001 StockAN30000010 2005 Fairchild Semiconductor Corporation DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PROD UCTS HEREIN TO IMPROVE RELIABILITY FUNCTION OR DESIGN FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS NOR THE RIGHTS OF OTHERS LIFE SUPPORT POLICY FAIRCHILDS PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPO RATION As used herein 1 Life support devices or systems are devices or systems which a are intended for surgical implant into the body or b support or sustain life or c whose failure to perform when properly used in accordance with instructions for use provided in the la beling can be reasonably expected to result in significant injury to the user 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support de vice or system or to affect its safety or effectiveness wwwfairchildsemicom by GwanBon Koo Ph D FPS Application Group Fairchild Semiconductor Phone 82326801327 Fax 82326801317 Email koogbfairchildsemicokr