HA17431 Hitachi Semiconductor, HA17431 Datasheet - Page 19

• Manufacturer Part Number: HA17431
• Description: Regarding the change of names mentioned in the document/ such as Hitachi Electric and Hitachi XX/ to Renesas Technology Corp.
• Manufacturer: Hitachi Semiconductor

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.
Determination of External Constants for the Shunt Regulator
DC characteristic determination: In figure 2, R
in the photocoupler, and R
below. The photocoupler specification should be obtained separately from the manufacturer. Using the
parameters in figure 2, the following formulas are obtained:
V
R
flows.
Next, the output voltage can be determined by R3 and R4, and the following formula is obtained:
The absolute values of R
characteristics described in the next section. The Iref value is around 3.8 µA Typ. (V version: 2 µA Typ)
2
K
is the current shunt resistance for the light emitting diode, in which a bias current I
is the HA17431 operating voltage, and is set at around 3 V, taking into account a margin for fluctuation.
Phototransistor
HA17384
HA17385
PWM IC
R
V
0
1
=
=
R
V
Transformer
0
3
R
– V
+ R
I
F
4
+ I
F
4
Photocoupler
– V
B
× Vref, Vref = 2.5 V Typ
3
and R
K
Figure 2 Typical Shunt Regulator/Error Amplifier
2
is a bypass resistor to feed I
, R
2
4
are determined by the HA17431 reference input current Iref and the AC
=
SBD
V
emitting diode
I
B
F
V
Light
F
1
and R
HA17431
K
2
minimum, and these are determined as shown
are protection resistor for the light emitting diode
I
F
I
B
V
K
K
A
R
R
1
2
C
1
REF
R
Rev.1, Sep. 2002, page 17 of 24
5
Vref
GND
HA17431 Series
B
R
R
of around 1/5 I
3
4
Output
V
0
(+)
(–)
F