AN1262 Datasheet, PDF (17/42 Pages) STMicroelectronics – APPLICATION NOTE

AN1262 APPLICATION NOTE
and its power rating has to be:
P_R
=
---^V----^R2-----
R_min
+
12_--
⋅
L_LK
⋅
I_l2im
⋅
f_sw
Usually the resistor value will be selected much higher than the minimum to reduce losses. The clamp capacitor
will then be quite larger than the minimum as well.
The blocking diode must be not only very fast-recovery but also very fast-turn-on type to avoid additional drain
overvoltage. A 1A rated diode with a breakdown voltage at least V_PKmax + V_R is needed. Table 12 shows the
suggested ST parts.
Table 12. Recommended ST parts for blocking diode.
110 V_AC
Diode
V_RRM
Package
Diode
BYT01-400
400
F126
STTA106
SMBYT01-400
400
SMB
STTA106U
220 V_AC or WRM
V_RRM
600
600
Zener clamp. The Transil (or zener) clamp voltage should be equal to:
V_CL = V_R + V_spike
Package
F126
SMB
(16)
Usually Transils are rated by their stand-off Voltage V_RM at 25°C temperature, which is defined at low current,
whereas the desired clamp voltage is to be considered at operating junction temperature and I_lim current.
To take this into consideration, as a rule of thumb the stand-off voltage can be selected as high as 70% of the
desired clamp level. Please refer to [2] and [3] to see how these problems are handled.
The Transil or zener must have an adequate power handling capability in steady state operation:
P_Z = ¹²-- ⋅ -_V----_C--^V--_L---^C-–--^L--_V----_R-- ⋅ L_LK ⋅ I_l2im ⋅ f_sw .
Table 13 lists some recommended devices available from ST.
The same recommendations as in the RCD clamp case apply to the blocking diode in series to the Transil. Only
the breakdown voltage could be derated to V_PKmax.
Table 13. Recommended ST parts for clamping.
VR
Pz ≤ 0.75 W
≤100 V
BZW04-154
BZW06-154
SMAJ154A-TR
130 V
BZW04-188
BZW06-188
SMAJ188A-TR
Pz = 1W
BZW04-154
BZW06-154
SMBJ154A-TR
BZW04-188
BZW06-188
SMBJ188A-TR
Pz = 1.5W
P6KE180A
1.5KE180A
SMCJ154A-TR
P6KE200A
1.5KE200A
SMCJ188A-TR
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