9

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.

and R are protection resistor for the light emitting

2

diode in the photocoupler, and R

is a bypass

2

resistor to feed I

Minimum, and these are

K

determined as shown below. The photocoupler

specification should be obtained separately from the

manufacturer. Using the parameters in figure 16, the

following formulas are obtained:

V

Is the GM1431 operating voltage, and is set at

K

around 3V, taking into account a margin for

fluctuation. R is the current shunt resistance for the

2

light emitting diode, in which a bias current I

of

B

around 1/5 I flows.

F

and R , and the following formula is obtained:

4

The absolute values of R and R are determined by

34

the GM1431 reference input current I

and the AC

ref

value is around 1.3µA Typ.

Determination of External Costants for the Shunt Regulator

R1 =

, R 2 =

V

- V - V

OF

K

I +I

FB

V

=

X V

, V

=2 .5 V Typ

O

ref

erf

R3 +R4

R4

GM384X

GM38C4X

Transformer

Phototransistor

Photocoupler

Light

emitting diode

REF

GND

(+)

(-)

Output

Vref

Figure 16. Typical Shunt Regulator/ Error Amplifier

SBD

GM1431

A

A

MICROELECTRONICS

GM1431

ADJUSTABLE SHUNT REGULATOR

Power Management