ISSN 0005–1144 A TKAAF 47(1–2), 31– 37 (2006) M. Milanovi~, J. Koreli~, A. Hren, F . Mihali~, P . [libar Re duc tio n o f Rin ging Losses in F lyb ack Conver ter by Usi ng the RC-RCD Clamp Circuit UDK 621.376.5:621.314.6 IF AC 4. 3. 1 Original scientific paper Fl yba ck converter is on e of the mos t po pula r DC- DC con ver ters for low power sup ply . Du e to the tra nsf ormer leakage induct ance the co nverter suffers fro m the vo ltage spikes, which can b e »control led« by the dissipat ive RC D or non-dissipat ive LCD clamp cir cui ts. Both o f the clamp circuits cons ist o f the diod e. The diod e reve rse recover y charge cau ses the osci llation, w hich resul ts in additi onal dissip ation of the clamp ci rcuitry . This paper de- scribes t his ringin g phenomenon and t he use of an RC-RCD clamp ci rcuit fo r damping t he clamp- diode's oscilla- tion. This clamp circ uit is capable fo r improving a flyback con verter' s power ratio. Key words: clamp circuit, flyback converter, ringing losses 1 INTRODUCTION The large surge voltage arises during transistor tur n-o ff in the switching power conver ter s with transformer. The surge generation is due to trans- former lea kag e inductance, and an RC cla mp ci r- cui t is u sually ins erted to a bso rb th e ene rgy sto red in the inductance and to suppress the surge volta- ge. F rom the vi ewpoint of minimi zing th e dissip a- tive energ y in the clamp ci rcuit, th e optimi zatio n ofthe R C clamp has been re port ed in [1]. Due to con- verters efficie ncy the non-dissipativ e clamp circuit was proposed in [2] where the authors instead ofthe conven tional RC clamp intr oduce d the non-dis- sip ati ve LC cla mp ci rcui t. Bo th of these princ ipl es do not consider the diod e rev ers e recovery eff ect whic h causes the well kno wn oscill ation in surg e voltages during transistor turn-off. This paper exp lor es t he re ver se recover y diode effect on the clamp circuits. T he diode capacitance C rr(reverse recover y charge i s store d in th is capa - citor) and leakage inductanceL establish the se- ries resonant tank circuit when the diode is turned- -off. The high frequ ency oscillat ion appears . This undesi rable dio de power dissip ation in the reverse reg ion c an be su ppr esse d by the introducti on of an RC-RCD struc tured clamp circ uit. Conseq uentl y, the po wer rat io a nd the EMI o f the co nverter can be improved . This process was investig ated by simu- lation, and verified experimentally. 2 DISSIPATIV E RCD CLAMP CIRCUITS The ope ration of most transformer-iso lated co n- verters can be adequately understood by modeling the transformer with a simp le equiv alent circui t consi sting o f an ideal transformer, a leakage and magnetizing inductances (Figure 1). The magneti- zing inductance must then follows all the usual rules for inductors. This means that the induc tor AUTOMA TIKA 47(2006) 1–2, 31– 37 31 Fig. 1 Dissipative RCD clamp circuit in fly-back convertermust be designed according to the current ripple requ ire ment s. In o rde r to av oid t he ind uct or satu - ratio n, the tran sformer must be desi gned wi th air gap, consequently, the leakage inductance ( L ) is not negligible.
