13

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.

However, large gate-charge increases the switching time,

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.

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

P

UPPER

I

O

2

r

DS ON

()

×

V

OUT

×

V

IN

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

I

O

V

IN

×

t

SW

×

F

SW

×

2

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

+

=

P

LOWER

I

O

2

r

×

DS ON

()

V

IN

V

OUT

–

()

×

V

IN

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

=

HIP6304