Rev. A 10/18
16
Application Note
AN-72
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Step 9 – Critical External Components Selection
The schematic in Figure 13 shows the key external components
required for a practical single output InnoSwitch3 design. Component
selection criteria is as follows:
Primary Bypass Pin Capacitor (C
BPP
)
This capacitor works as a supply decoupling capacitor for the internal
primary-side controller and determines current limit for the internal
MOSFET. For a 4.7
µ
F or 0.47
µ
F capacitor select either increased or
standard current limit respectively. All though electrolytic capacitors
can be used, surface mount multi-layer ceramic capacitors are often
preferred for use with double sided boards as they enable the
capacitor to be placed close to the IC. A surface mount multi-layer
ceramic X7R capacitor rated for 25 V is recommended.
To ensure correct current limit it is recommended that either only
0.47
µ
F / 4.7
µ
F capacitors be used. In addition, the BPP capacitor
tolerance should be equal or better than indicated below taking into
account the ambient temperature range of the target application.
The minimum and maximum acceptable capacitor tolerance values
are set by IC characterization (Table 8).
Figure 13. Typical InnoSwitch3 Flyback Power Supply.
PI-8465-041818
Secondary
Control IC
L
V
OUT
RTN
R
T
F
R
C
1
C
2
R
LS1
R
SN
C
SN
R
S
R
FB(UPPER)
C
FB
R
PH
C
PH
C
BPS
C
SR
R
SR
C
OUT
R
FB(LOWER)
D
SN
D
BIAS
R
BP
R
LS2
R
FWD
R
IS
C
BPP
C
BIAS
N
SR FET
Primary FET
and Controller
D
V
S
IS
VOUT
BPS
FB
GND
SR
BPP
FWD
InnoSwitch3
t
O
L
F
F
B
BRD
CY
Secondary Bypass Pin Capacitor (C
BPS
)
This capacitor works as a supply decoupling capacitor for the
secondary-side controller. A surface-mount, 2.2
µ
F, 25 V, multi-layer
ceramic capacitor is recommended for satisfactory operation of the
IC. The SECONDARY BYPASS Pin voltage needs to reach 4.4 V before
the output voltage reaches its target voltage. A significantly higher
BPS capacitor value could lead to output voltage overshoot during
start-up. Values lower than 1.5
µ
F will cause unpredictable operation.
The capacitor must be located adjacent to the IC pins. The 25 V
rating is necessary to guarantee sufficient capacitance in operation
(the capacitance of ceramic capacitors drops with applied voltage).
10 V rated capacitors are not recommended for this reason. For best
results capacitors with X5R or X7R dielectrics should be used.
FORWARD Pin Resistor (R
FWD
)
The FORWARD pin is connected to the Drain terminal of the
synchronous rectifier MOSFET (SR FET). This pin is used to sense
the drain voltage of the SR FET and allows precise turn-ON and
turn-OFF control. This pin is also used to charge the BPS
(SECONDARY BYPASS pin) capacitor when output voltage is lower
than the BPS voltage. A 47
Ω
, 5% resistor is recommended to ensure
sufficient IC supply current and works for wide range of output
voltages.
A higher or lower resistor value should not be used as it can affect
device operation and effect synchronous rectification timing.
Care should be taken to ensure that the voltage on the FORWARD pin
never exceeds its absolute maximum voltage. If in any design, the
FORWARD pin voltage exceeds the FORWARD pin absolute maximum
voltage, the IC will be damaged.
Table 8. BYPASS Pin Capacitor Tolerance Values.
Nominal PRIMARY
BYPASS Pin Capacitor
Value
Tolerance Relative to Nominal
Capacitor Value
Minimum
Maximum
0.47
µ
F
-60%
+100%
4.7
µ
F
-50%
+100%