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HA17431HLP Datasheet(PDF) 9 Page - Hitachi Semiconductor |
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HA17431HLP Datasheet(HTML) 9 Page - Hitachi Semiconductor |
9 / 15 page 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. R1 R2 R3 R4 Transformer Photocoupler Light emitting diode Phototransistor Output C1 R5 REF GND A K VK IB IF HA17431H VF SBD PWM IC HA17384 HA17385 Vref V0 (+) (–) Figure 2 Typical Shunt Regulator/Error Amplifier Determination of External Constants for the Shunt Regulator DC characteristic determination: In figure 2, R 1 and R2 are protection resistor for the light emitting diode in the photocoupler, and R 2 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 = VF IB V K is the HA17431H operating voltage, and is set at around 3 V, taking into account a margin for fluctuation. R 2 is the current shunt resistance for the light emitting diode, in which a bias current IB of around 1/5 I F flows. Next, the output voltage can be determined by R3 and R4, and the following formula is obtained: V0 = R3 + R4 R4 × Vref, Vref = 2.5 V Typ The absolute values of R 3 and R4 are determined by the HA17431H reference input current Iref and the AC characteristics described in the next section. The Iref value is around 0.6 µA Typ. |
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