ISL6219A

FN9093 Rev.1.00

Page 6 of 16

Mar 20, 2007

Operation

Multi-Phase Power Conversion

Multi-phase power conversion provides the most cost-effective

power solution when load currents are no longer easily

supported by single-phase converters. Although its greater

complexity presents additional technical challenges, the multi-

phase approach offers cost-saving advantages with improved

response time, superior ripple cancellation, and excellent

thermal distribution.

INTERLEAVING

The switching of each channel in a multi-phase converter is

timed to be symmetrically out of phase with each of the other

channels. In a 3-phase converter, each channel switches 1/3

cycle after the previous channel and 1/3 cycle before the

following channel. As a result, the three-phase converter has a

combined ripple frequency three times greater than the ripple

frequency of any one phase. In addition, the peak-to-peak

amplitude of the combined inductor currents is reduced in

proportion to the number of phases (Equations 1 and 2).

Increased ripple frequency and lower ripple amplitude mean

that the designer can use less per-channel inductance and

lower total output capacitance for any performance

specification.

Figure 2 illustrates the multiplicative effect on output ripple

frequency. The three channel currents (IL1, IL2, and IL3),

combine to form the AC ripple current and the DC load current.

The ripple component has three times the ripple frequency of

each individual channel current. Each PWM pulse is terminated

1/3 of a cycle after the PWM pulse of the previous phase. The

peak-to-peak current waveforms for each phase is about 7A,

and the dc components of the inductor currents combine to feed

the load.

To understand the reduction of ripple current amplitude in the

multi-phase circuit, examine the equation representing an

individual channel’s peak-to-peak inductor current.

the switching frequency.

The output capacitors conduct the ripple component of the

inductor current. In the case of multi-phase converters, the

capacitor current is the sum of the ripple currents from each of

the individual channels. Compare Equation 1 to the expression

for the peak-to-peak current after the summation of N

symmetrically phase-shifted inductor currents in Equation 2.

Peak-to-peak ripple current decreases by an amount

proportional to the number of channels. Output voltage ripple is

a function of capacitance, capacitor equivalent series

resistance (ESR), and inductor ripple current. Reducing the

inductor ripple current allows the designer to use fewer or less

costly output capacitors.

Another benefit of interleaving is to reduce input ripple current.

Input capacitance is determined in part by the maximum input

ripple current. Multi-phase topologies can improve overall

system cost and size by lowering input ripple current and

allowing the designer to reduce the cost of input capacitance.

The example in Figure 3 illustrates input currents from a three-

phase converter combining to reduce the total input ripple

current.

FIGURE 2. PWM AND INDUCTOR-CURRENT WAVEFORMS

FOR 3-PHASE CONVERTER

1

s/div

PWM2, 5V/DIV

PWM1, 5V/DIV

IL2, 7A/DIV

IL1, 7A/DIV

IL1 + IL2 + IL3, 7A/DIV

IL3, 7A/DIV

PWM3, 5V/DIV



–

 V

OUT

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

=

(EQ. 1)

–

 V

OUT

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

=

(EQ. 2)

FIGURE 3. CHANNEL INPUT CURRENTS AND INPUT-

CAPACITOR RMS CURRENT FOR 3-PHASE

Channel 1

input current

10A/DIV

Channel 2

input current

10A/DIV

Channel 3

input current

10A/DIV

Input-capacitor current, 10A/DIV

1

s/div