AN901 Datasheet, PDF (9/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
2.8. Diode and Output Capacitor
In CCM, current flows through D1 only during the (1-D)T_sw portion of the steady state cycle. During the DT_sw
portion of the cycle, I_LOAD is sourced solely by the output capacitor C10. Output voltage ripple on C10 can be
calculated by:
Applying the charge balance of C10,
V_{OUT RIPPLE} = ^I--^L---^O----^A-_C---^D--₁--^D-₀---^T----^s---^w--
Equation 23.
–I_LOADDT_sw + I_{D1 AVE1 – D}1 – DT_sw – I_LOAD1 – DT_sw = 0
Equation 24.
I_{D1 AVE1 – D} = -^I-₁^L---^O-–---^A--_D--^D--
Equation 25.
When D1 is reversed biased, it must withstand:
V_{D1 REVD} = ^V---_n--^I-^N-- + V_OUT
Equation 26.
2.9. Optional Secondary Snubber
At the instant S1 closes, this reverse voltage applied to D1 can overshoot and ring before settling to V_D1,REV(D) as
given by Equation 26. A RC snubber can be used to limit the voltage stress across D1. Like the design of the
optional primary snubber, a first order approximation for determining R8 and C8 is to set them to the characteristic
impedance of the ringing caused by secondary side L_lkg of T1 and parasitic capacitance of D1.
R8  Z_C8  ^L----^l-^k--_C-^g---^_D--^s--₁-^e--^c----
Equation 27.
R8 can be determined by measuring L_lkg and ringing frequency:
R8  2f_ringL_{lkg sec}
C8 can be set to the same impedance using:
Equation 28.
C8  -₂---_----_f--_r--_i¹-_n---_g---_R-----₈--
Equation 29.
Rev. 0.1
9