Power integrations InnoSwitch3 Application Note page 17

AN-72
FEEDBACK Pin Divider Network (RFB
A suitable resistor voltage divider should be connected from the
output of the power supply to the FEEDBACK pin of the InnoSwitch3
IC such that for the desired output voltage, the voltage on the FEEDBACK
pin is 1.265 V. It is recommended that a decoupling capacitor (C
330 pF be connected from the FEEDBACK pin to the GROUND pin.
This will serve as a decoupling capacitor for the FEEDBACK pin to
prevent switching noise from affecting operation of the IC.
Primary Clamp Network Across Primary Winding
(D , R , R , and C )
SN S SN SN
See Figure 13. An R2CD clamp is the most commonly used clamp in
low power supplies. For higher power designs, a Zener clamp or the
R2CD + Zener clamp can be used to increase efficiency. It is
advisable to limit the peak drain voltage to 90% of BV
worst-case conditions (maximum input voltage, maximum overload
power or output short- circuit). In Figure 13, the clamp diode, D
must be a standard recovery glass-passivated type or a fast recovery
diode with a reverse recovery time of less than 500 ns. The use of
standard recovery glass passivated diodes allows recovery of some of
the clamp energy in each switching cycle and helps improve average
efficiency. The diode conducts momentarily each time the MOSFET
inside InnoSwitch3 turns off and energy from the leakage reactance
is transferred to the clamp capacitor C
series path, offers damping preventing excessive ringing due to
resonance between the leakage reactance and the clamp capacitor
C . Resistor R bleeds-off energy stored inside the capacitor C
SN SN
Power supplies using different InnoSwitch3 devices in the family will
have different peak primary current, leakage inductances and
therefore leakage energy. Capacitor C
must therefore be optimized for each design. As a general rule it is
advisable to minimize the value of capacitor C
value of resistors R and R , while still meeting the 90% BV
SN S
highest input voltage and full load. The value of R
R2CD
R
C
SN
SN
R D
S
PI-8502-041818
Figure 14. Recommended Primary Clamp Components.
Benefits
Component Cost
No-Load Input Power
Light-Load Efficiency
EMI Suppression
Table 9. Benefits of Primary Clamp Circuits.
www.power.com
, RFB )
UPPER LOWER
under
DSS
. Resistor R , which is in the
SN S
, and resistors R and R
SN SN
and maximize the
SN
DSS
should be large
S
Common Primary Clamp Configurations
VR
CLAMP
CLAMP
D
Primary Clamp Circuit
R2CD
Low
High
Low
High
enough to damp the ringing in the required time, but must not be so
large as to cause the drain voltage to exceed 90% of BV
ceramic capacitor that uses a dielectric such as Z5U when used in
clamp circuit for C
SN
) of type should be used.
FB
As a guide the following equations can be used to calculate R2CD
component values;
>
R =
SN
C
S
SN
Where;
V : Voltage across clamp circuit
C
I : Peak switching current
PK
F : Switching frequency
S
L : Leakage inductance
IK
V : Reflected output voltage
OR
dV : The maximum ripple voltage across clamp capacitor (10%)
.
CSN
SN
For example;
If V = 205 V, F = 100 kHz, I
C S
S
dV = 20 V
SN
Applying the equations above,
limit at
R = 92.4 kΩ, C = 1.08 nF and R
RSN S
Zener
D
CLAMP
R
S
D
PI-8504-041818
Zener
Medium
Low
High
Low
Application Note
may generate audible noise, so a polyester film
V 2
H
C
1 V # F
C S
L I 2
# #
^ h
2 I K PK
V - V
C OR
V
= C
RSN 1 F dV
# #
S CSN
1
L
2
= b l
I K
R
C
S
SN
= 1 A, V = 100, L
PK OR IK
= 68 Ω
S
R2CD + Zener
R
VR
SN
CLAMP
C
SN
R
S
PI-8503-041818
R2CD + Zener
High
Medium
Medium
Medium
. A
DSS
; Eq. (1)
; Eq. (2)
; Eq. (3)
= 5 µH and
D
CLAMP
D
17
Rev. A 10/18