AN901 Datasheet, PDF (7/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
2.5. Optional Primary Snubber
Snubbers are used for two purposes in a flyback converter: to limit the peak voltage on the drain of the Q1, and to
attenuate high frequency ringing that leads to emissions. There are several methods to create a primary side
flyback snubber. The RC snubber is presented here. The energy stored in the leakage inductance L_lkg does not
transfer to the secondary and must be dissipated in the primary. The power dissipated in the leakage inductance is
given by:
P_lkg = ^L----^l-^k---₂^g---^I-_T-^m2--_s--^-_w-^P----^K----
Equation 14.
When S1 opens, the current flowing in the primary will charge the drain-source capacitance of Q1 causing the
voltage at the drain to increase rapidly. When this voltage exceeds V_IN + nV_OUT, a ringing occurs with frequency
dependent on the inductance leakage L_lkg and C_ds. The RC snubber presents a load for which to dissipate the
power stored in the inductance leakage. This load limits the switching speed of Q1, which limits the peak voltage
across the drain-source. A first order approximation for determining R16 and C19 is to set them to the
characteristic impedance of the ringing caused by L_lkg of T1 and C_ds of Q1.
R16  Z_C19  ^L-_C---^l-_dk---^gs-
Equation 15.
R16 can be determined by measuring L_lkg and ringing frequency:
C19 can be set to the same impedance using:
R16  2f_ringL_lkg
Equation 16.
C19  ₂----_----_f--_r--_i-_n-¹--_g---_R-----₁---₆--
Equation 17.
2.6. Input Capacitor
The purpose of C2 input capacitor is to provide filtering for V_IN during the switching cycle and reduce voltage ripple
at the converter input. Operating in CCM, during t_S1 portion of the cycle C2 current is given by:
I_C2 = I_IN – I_{m AVE} = D – 1_n----^I_--^L-₁--^O--–--^A---_D^D-----_ = –-^I-^L---^O--_n--^A----^D--
Equation 18.
The voltage ripple on C2 can be written as:
V_{IN RIPPLE} = _-^I_-^C_----²_--C-^D_----2-^T_----^s_--^w_-- = ^I--^L---^O--_n--^A---_-^D----^DC----^T2----^s---^w--
Equation 19.
Rev. 0.1
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