HA17431H Series

Rev.0, Sep. 2001, page 9 of 15

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

Photocoupler

Light

emitting diode

Phototransistor

Output

REF

GND

A

K

HA17431H

SBD

PWM IC

HA17384

HA17385

Vref

(+)

(–)

Figure 2 Typical Shunt Regulator/Error Amplifier

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

fluctuation. R

around 1/5 I

Next, the output voltage can be determined by R3 and R4, and the following formula is obtained:

× Vref, Vref = 2.5 V Typ

The absolute values of R

characteristics described in the next section. The Iref value is around 0.6

µA Typ.