AN901 Datasheet, PDF (23/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
C19 = -₂---_----_f--_r--_i-_n-¹--_g---_R-----₁---₆-- = -₂---_-----_-----₃---₀----_¹---1---0---₆----_-----₈---₆-- = 62pF
Equation 68.
Closest standard component values of R16 = 82 Ω and C19 = 68 pF were selected and installed. Q1 Vds was
measured again to gage effectiveness of the RC snubber. Voltage spike was reduced to 74 V as shown in
Figure 11.
Figure 11. Damped Vds Ringing
R8 and C8 on the secondary side can be selected using the same methodology. Without a secondary side
snubber, the voltage spike across D1 at the instant that S1 closes was measured to be 35 V with a ringing
frequency of 59 MHz. T1 was removed from the board and its primary inductance leakage was measured to be
74 nH.
R8 = 2f_ringL_lkg = 2  5910⁶  7410^–9 = 27.4
Equation 69.
C8 = ₂----_----_f--_r--_i¹-_n---_g---_R-----₈-- = ₂----_-----_-----₅---₉----_---¹_1---0---6----_-----₂---₇----_.-₄-- = 98.4pF
Equation 70.
R8 is a 1% resistor value and C8 of 100 pF was chosen. The voltage spike was reduced to 23 V and the ringing
damped.
Rev. 0.1
23