EM5303 datasheet(10/11 Pages) EXCELLIANCE

10 / 11 page

2012/10/16

Rev.A.7

EM5303/A

The capacitor voltage rating should be at least 1.25

times greater than the maximum input voltage and

a voltage rating of 1.5 times is a conservative

guideline. The RMS current rating requirement can

be expressed as the following equation:

D(1

OUT

RMS

For a through hole design, several electrolytic

capacitors may be needed. For surface mount

designs, solid tantalum capacitors can also be used

but caution must be exercised with regard to the

capacitor surge current rating. These capacitors

must be capable of handling the surge current at

power-up. Some capacitor series available from

reputable manufacturers are surge current tested.

Power MOSFET Selection

The EM5303/A requires two N-Channel power

MOSFETs. These should be selected based upon

on-resistance, breakdown voltage, gate supply

requirement,

and

thermal

management

requirements.

In high current applications, the MOSFET power

dissipation, package selection and heat sink are the

dominate design factor. The power dissipation

includes two loss components: conduction loss and

switching loss. The conduction losses are the

largest component of power dissipation for both

the upper and lower MOSFETs. These losses are

distributed between the two MOSFETs according

to duty factor.

The power dissipations in the two MOSFETs are

approximately the following equation:

OUT

DS(ON)

OUT

UPPER

DS(ON)

OUT

LOWER

Where D is the duty cycle, tSW is the combined

switch ON and OFF time.

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