Shindengen Electric MFG.CO.,LTD

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6.6 Application circuit examples

6.6.1 Indirect control

If output voltage precision is not an issue, a constant voltage control can be provided at the primary side without

using a photocoupler. Figure 6.19 shows an example of 12 V output design.

(1)

Circuit configuration

The circuit consists of a transistor and a current control resistor that control the F/B pin (Pin 2) and a zener

diode that detects voltage. In cases where an increase in voltage under light load is an issue, add a dummy

resistor on the secondary side.

(2)

Circuit operation

The zener diode in the additional component for indirect control surrounded in a frame in the diagram detects

the output voltage of the control coil. The detection signal controls the F/B pin (Pin 2) directly via the transistor.

(3)

Problem

A ringing voltage attributable to transformer leakage inductance can result in significant variations in voltage

precision. This can also increase the output voltage under a light load.

Fig. 6.19 Indirect control

6. Supplementary design information