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| HA17431H Datasheet, PDF (9/15 Pages) Hitachi Semiconductor – Shunt Regulator | |||
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HA17431H Series
Design Guide for AC-DC SMPS (Switching Mode Power Supply)
Use of Shunt Regulator in Transformer Secondary Side Control
This example is applicable to both forward transformers and flyback transformers. A shunt regulator is
used on the secondary side as an error amplifier, and feedback to the primary side is provided via a
photocoupler.
Transformer
PWM IC
HA17384
HA17385
SBD
R1
IF
R2
R3
IB
Phototransistor
Photocoupler
VF
Light
emitting diode
VK
C1 R5
K
REF
HA17431H
A
Vref
R4
GND
Figure 2 Typical Shunt Regulator/Error Amplifier
(+)
Output
V0
(â)
Determination of External Constants for the Shunt Regulator
DC characteristic determination: In figure 2, R1 and R2 are protection resistor for the light emitting diode
in the photocoupler, and R2 is a bypass resistor to feed IK minimum, and these are determined as shown
below. The photocoupler specification should be obtained separately from the manufacturer. Using the
parameters in figure 2, the following formulas are obtained:
R1 =
V0 â VF â VK
IF + IB
, R2 =
VF
IB
VK is the HA17431H operating voltage, and is set at around 3 V, taking into account a margin for
fluctuation. R2 is the current shunt resistance for the light emitting diode, in which a bias current IB of
around 1/5 IF flows.
Next, the output voltage can be determined by R3 and R4, and the following formula is obtained:
V0 =
R3 + R4
R4
à Vref, Vref = 2.5 V Typ
The absolute values of R3 and R4 are determined by the HA17431H reference input current Iref and the AC
characteristics described in the next section. The Iref value is around 0.6 µA Typ.
Rev.0, Sep. 2001, page 9 of 15
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