EM5305VT datasheet(11/12 Pages) EXCELLIANCE

11 / 12 page

2013/7/11

A.2

EM5305

Input Capacitor Selection

Use a mix of input bypass capacitors to control the

voltage overshoot across the MOSFET. Use small

ceramic capacitors for high frequency decoupling

and bulk capacitors to supply the current needed

each time the upper MOSFET turn on. Place the

small ceramic capacitors physically close to the

MOSFETs and between the drain of the upper

MOSFET and the source of the lower MOSFET. The

important parameters of the input capacitor are

the voltage rating and the RMS current rating.

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 EM5305 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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