HA17431FP datasheet(15/20 Pages) RENESAS

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HA17431 Series

REJ03D0678-0300 Rev.3.00 Apr 03, 2007

Page 15 of 19

Design Guide for AC-DC SMPS (Switching Mode Power Supply)

1. 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

HA17431

SBD

PWM IC

HA17384

HA17385

Vref

(+)

(–)

Figure 2 Typical Shunt Regulator/Error Amplifier

2. Determination of External Constants for the Shunt Regulator

A. 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 – VF – VK

IF + IB

, R2 =

VK is the HA17431 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

× Vref, Vref = 2.5 V Typ

The absolute values of R3 and R4 are determined by the HA17431 reference input current Iref and the AC

characteristics described in the next section. The Iref value is around 3.8

µA Typ. (V version: 2 µA Typ)

B. AC characteristic determination

This refers to the determination of the gain frequency characteristic of the shunt regulator as an error amplifier.

Taking the configuration in figure 2, the error amplifier characteristic is as shown in figure 3.

When R5 = 0

When R5

≠ 0

fAC

fOSC Frequency f (Hz)

* fOSC : PWM switching frequency

Figure 3 HA17431 Error Amplification Characteristic

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