GM431 查看數據表（PDF） - Gamma Microelectronics Inc.

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GM431

2.5V ADJUSTABLE SHUNT REGULATOR

Gamma Microelectronics Inc. 

AA

MICROELECTRONICS

Power Management

GM431

2.5V ADJUSTABLE SHUNT REGULATOR

Design Guide for AC-DCSMPS (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

GM384X

GM38C4X

SBD

R1

R3

IF

VF

Phototransistor Photocoupler

Light

emitting diode

GM431

IB

R2

VK

C1

R5

REF

Vref

R4

GND

(+)

Output

VO

(-)

Figure 16. Typical Shunt Regulator/ Error Amplifier

Determination of External Costants for the Shunt Regulator

Dc characteristic determination: In figure 16, 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 16, the

following formulas are obtained:

R1 = VO - VF - VK , R 2 = VF

IF +IB

IB

Next, the output voltage can be determined by R3

and R4, and the following formula is obtained:

R3 +R4

VO = R4 X Vref, Verf =2 .5 V Typ

The absolute values of R3 and R4 are determined by

the GM431 reference input current Iref and the AC

characteristics described in the next section. The Iref

value is around 0.7µA Typ.

VK Is the GM431 operating voltage, and is set at

around 3V, 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.

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