Rev. A 10/18
17
Application Note
AN-72
www.power.com
FEEDBACK Pin Divider Network (RFB
UPPER
, RFB
LOWER
)
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
FB
) of
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
SN
, R
S
, R
SN
, and C
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
DSS
under
worst-case conditions (maximum input voltage, maximum overload
power or output short- circuit). In Figure 13, the clamp diode, D
SN
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
SN
. Resistor R
S
, which is in the
series path, offers damping preventing excessive ringing due to
resonance between the leakage reactance and the clamp capacitor
C
SN
. Resistor R
SN
bleeds-off energy stored inside the capacitor C
SN
.
Power supplies using different InnoSwitch3 devices in the family will
have different peak primary current, leakage inductances and
therefore leakage energy. Capacitor C
SN
, and resistors R
SN
and R
S
must therefore be optimized for each design. As a general rule it is
advisable to minimize the value of capacitor C
SN
and maximize the
value of resistors R
SN
and R
S
, while still meeting the 90% BV
DSS
limit at
highest input voltage and full load. The value of R
S
should be large
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
DSS
. A
ceramic capacitor that uses a dielectric such as Z5U when used in
clamp circuit for C
SN
may generate audible noise, so a polyester film
type should be used.
As a guide t
he following equations can be used to calculate R2CD
component values;
; Eq. (1)
R
L
I
V
V
V
F
V
2
1
SN
K
PK
C
OR
C
S
C
I
2
2
#
#
#
=
-
^
h
>
H
C
RSN F
dV
V
S
1
S
CSN
C
#
#
=
; Eq. (2)
; Eq. (3)
R
C
L
S
SN
K
I
2
1
=
b
l
Where;
V
C
: Voltage across clamp circuit
I
PK
: Peak switching current
F
S
: Switching frequency
L
IK
: Leakage inductance
V
OR
: Reflected output voltage
dV
CSN
: The maximum ripple voltage across clamp capacitor (10%)
For example;
If V
C
= 205 V, F
S
= 100 kHz, I
PK
= 1 A, V
OR
= 100, L
IK
= 5
µ
H and
dV
SN
= 20 V
Applying the equations above,
R
RSN
= 92.4 k
Ω
, C
S
= 1.08 nF and R
S
= 68
Ω
Primary Clamp Circuit
Benefits
R2CD
Zener
R2CD + Zener
Component Cost
Low
Medium
High
No-Load Input Power
High
Low
Medium
Light-Load Efficiency
Low
High
Medium
EMI Suppression
High
Low
Medium
Common Primary Clamp Configurations
R2CD
Zener
R2CD + Zener
Figure 14. Recommended Primary Clamp Components.
Table 9. Benefits of Primary Clamp Circuits.
C
SN
R
S
R
SN
D
CLAMP
D
PI-8502-041818
VR
CLAMP
R
S
R
SN
C
SN
D
CLAMP
D
PI-8503-041818
VR
CLAMP
R
S
D
CLAMP
D
PI-8504-041818