ISL6553

FN4931 Rev 1.00

Page 14 of 15

August 2004

Input Capacitor Selection

The important parameters for the bulk input capacitors are

the voltage rating and the RMS current rating. For reliable

operation, select bulk input capacitors with voltage and

current ratings above the maximum input voltage and largest

RMS current required by the circuit. 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 required for a multi-

phase converter can be approximated with the aid of Figure

13.

First determine the operating duty ratio as the ratio of the

output voltage divided by the input voltage. Find the Current

Multiplier from the curve with the appropriate power channels.

Multiply the current multiplier by the full load output current.

The resulting value is the RMS current rating required by the

input capacitor.

Use a mix of input bypass capacitors to control the voltage

overshoot across the MOSFETs. Use ceramic capacitance

for the high frequency decoupling and bulk capacitors to

supply the RMS current. Small ceramic capacitors should be

placed very close to the drain of the upper MOSFET to

suppress the voltage induced in the parasitic circuit

impedances.

For bulk capacitance, several electrolytic capacitors (Panasonic

HFQ series or Nichicon PL series or Sanyo MV-GX or equivalent)

may be needed. For surface mount designs, solid tantalum

capacitors can 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. The

TPS series available from AVX, and the 593D series from

Sprague are both surge current tested.

MOSFET Selection and Considerations

In high-current PWM applications, the MOSFET power

dissipation, package selection and heatsink are the dominant

design factors. The power dissipation includes two loss

components; conduction loss and switching loss. These losses

are distributed between the upper and lower MOSFETs

according to duty factor (see the following equations). The

conduction losses are the main component of power

dissipation for the lower MOSFETs, Q2 and Q4 of Figure 1.

Only the upper MOSFETs, Q1 and Q3 have significant

switching losses, since the lower device turns on and off into

near zero voltage.

The equations assume linear voltage-current transitions and

do not model power loss due to the reverse-recovery of the

lower MOSFETs body diode. The gate-charge losses are

dissipated by the Driver IC and don’t heat the MOSFETs.

which increases the upper MOSFET switching losses. Ensure

that both MOSFETs are within their maximum junction

temperature at high ambient temperature by calculating the

temperature rise according to package thermal-resistance

specifications. A separate heatsink may be necessary

depending upon MOSFET power, package type, ambient

temperature and air flow.

A diode, anode to ground, may be placed across Q2 and Q4 of

Figure 1. These diodes function as a clamp that catches the

negative inductor swing during the dead time between the turn

off of the lower MOSFETs and the turn on of the upper

MOSFETs. The diodes must be a Schottky type to prevent the

lossy parasitic MOSFET body diode from conducting. It is

usually acceptable to omit the diodes and let the body diodes

of the lower MOSFETs clamp the negative inductor swing, but

efficiency could drop one or two percent as a result. The

diode's rated reverse breakdown voltage must be greater than

the maximum input voltage.

References

Intersil documents are available on the web at

www.intersil.com/

[1] HIP6601/HIP6603 Data Sheet, Intersil Corporation,

File No. 4819

[2] HIP6602 Data Sheet, Intersil Corporation, File No. 4838

1.0

0.8

0.6

0.4

0.2

0

0

0.1

0.2

0.3

0.4

0.5

SINGLE

CHANNEL

2 CHANNEL

3 CHANNEL

4 CHANNEL

FIGURE 12. RIPPLE CURRENT vs DUTY CYCLE

0.5

0.4

0.3

0.2

0.1

0

0

0.1

0.2

0.3

0.4

0.5

SINGLE

CHANNEL

3 CHANNEL

4 CHANNEL

2 CHANNEL

FIGURE 13. CURRENT MULTIPLIER vs DUTY CYCLE

2







------------------------------------------------------------







2

----------------------------------------------------------

+

=

2

r



–





---------------------------------------------------------------------------------

=