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ISL6566 Fiches technique(PDF) 9 Page - Intersil Corporation

No de pièce ISL6566
Description  Three-Phase Buck PWM Controller with Integrated MOSFET Drivers for VRM9, VRM10, and AMD Hammer Applications
Download  28 Pages
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Fabricant  INTERSIL [Intersil Corporation]
Site Internet  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL6566 Fiches technique(HTML) 9 Page - Intersil Corporation

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9
FN9178.3
July 25, 2005
Operation
Multi-Phase Power Conversion
Microprocessor load current profiles have changed to the
point that the advantages of multi-phase power conversion
are impossible to ignore. The technical challenges
associated with producing a single-phase converter that is
both cost-effective and thermally viable have forced a
change to the cost-saving approach of multi-phase. The
ISL6566 controller helps simplify implementation by
integrating vital functions and requiring minimal external
components. The block diagram on page 2 provides a top
level view of multi-phase power conversion using the
ISL6566 controller.
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 1 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 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 peak-to-peak inductor current.
In Equation 1, VIN and VOUT are the input and output
voltages respectively, L is the single-channel inductor value,
and fS is 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 2 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
The converter depicted in Figure 2 delivers 1.5V to a 36A load
from a 12V input. The RMS input capacitor current is 5.9A.
Compare this to a single-phase converter also stepping down
12V to 1.5V at 36A. The single-phase converter has 11.9A
RMS input capacitor current. The single-phase converter
must use an input capacitor bank with twice the RMS current
capacity as the equivalent three-phase converter.
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
1
µs/DIV
PWM2, 5V/DIV
PWM1, 5V/DIV
IL2, 7A/DIV
IL1, 7A/DIV
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IPP
VIN VOUT
() V
OUT
LfS VIN
------------------------------------------------------
=
(EQ. 1)
ICPP
,
VIN NVOUT
() V
OUT
LfS VIN
------------------------------------------------------------
=
(EQ. 2)
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT-
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
CHANNEL 1
INPUT CURRENT
10A/DIV
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 3
INPUT CURRENT
10A/DIV
INPUT-CAPACITOR CURRENT, 10A/DIV
1
µs/DIV
ISL6566
ISL6566


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