MS1003SH datasheet(34/41 Pages) SHINDENGEN

34 / 41 page

MS1003SH･MS1004SH

Application Note Ver.2.2

SHINDENGEN ELECTRIC MFG. CO. , LTD

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(5) Circuit protection

The Vcc pin may break down during a short circuit test.

If so, protect the circuit using a zener diode (DZ306), as

shown to the right. A zener diode for 30 V or greater

should have negligible effect on IC functions for normal

use.

5.5.6 Setting resonating capacitor

The capacitance set for the resonating capacitor should be between 100 pF and 3,300 pF for real-world

applications. No other restrictions apply.

(1) Conditions under which a relatively large capacitance is selected

 The partial resonance trough is close to 0 V because, for example, input voltage is low and switching

loss is expected to be very small.

 The conducted emissions are high.

 The surge voltage is large relative to the withstand voltage of the main switching device, and there is

no margin.

(2) Conditions under which a relatively small capacitance is selected

 The main switching device generates significant heat.

 Standby power must be minimized.

The following table lists the effects of changes in the capacitance of the resonating capacitor on power

supply performance.

Item

Reduce capacitance.

⇔

Increase capacitance.

Main SW device peak voltage

Rise

⇔

Fall

Droop point power

Increase

⇔

Decrease

Heat buildup in the main SW

Decrease

⇔

Increase

Main SW device current immediately after

powering on

Decrease

⇔

Increase

Main SW peak current under the same output

power conditions

Decrease

⇔

Increase

Regulation of output voltage

Decline

⇔

Improve

Regulation of Vcc voltage

Decline

⇔

Improve

Power supply efficiency

Improve

⇔

Decline

Noise

Rising tendency

⇔

Declining tendency

In efforts to optimize power supply performance, changes in the capacitance of the resonating capacitor

often involve trade-offs. Carefully examine the advantages and disadvantages of the change when

determining the constants. It may be possible to improve the trade-offs by redesigning the transformer.

Consider redesigning the transformer to optimize power supply performance.

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