The semiconductor industry has always forced
the power-supply industry to follow its trendsetting
lead. For the last decade, that trend has
been to cram more transistors into a single package,
particularly microprocessors. This led to
microprocessors with smaller feature sizes and
tighter spacing between internal components.
To be operational, smaller feature sizes forced the processors
to operate at a lower voltage. This, in turn, required lowervoltage
power supplies with greater design challenges than
their predecessors of five to 10 years ago.
Designers can deliver low-voltage, high-current microprocessor
supplies. But when you add the requirement for high
efficiency (90% or better), the technology falls a bit short. It’s
unlikely that the high-efficiency requirements can be met
using present-day components and technologies, but it can
reach about 70% to 80%.
To understand processor power-source requirements, check
out the 2006 International Technology Roadmap for Semiconductors
(ITRS). It projects operation at 1 V and currents in
the 100-A region for processors in the year 2010. By 2020, the
expected supply voltage will be 0.7 V at higher currents.
A voltage regulator-down (VRD) configuration with all of
its components mounted directly on a computer’s motherboard
now powers most processors. Most VRDs have an 8-bit voltage
identification (VID) code whose eight input lines connect
to the corresponding eight VID pins of the processor.
By sensing the microprocessor’s VID code, the voltage regulator
sets the required operating voltage for the processor. The
processor also can employ dynamic voltage identification that
allows it to vary clock frequency and operating voltage “on the
fly,” in response to the processor’s workload and the thermal
environment.
Intel’s November 2006 Voltage Regulator-Down (VRD)
11.0 Processor Power Delivery Design Guidelines (www.intel.com/design/processor/applnots/313214.htm) is an example
of present-day processor power management. These powersource
design guidelines are for five different processors:
- Maximum supply voltage: 1.4 V to 1.425 V
- Maximum current: 75 A to 125 A
- Tight output voltage regulation (±5%) under all line, load,
and environmental conditions
- Very low ripple, typically less than 10 mV rms p-p
- Efficiency of 75% to 80%
- Fast transient response, consistent with microprocessor clock
frequency
- Overvoltage protection
- Overcurrent (short circuit) protection
- Overtemperature protection
- Thermal management of power-dissipating components
- Relatively small package size so that the supply can be located
close to its microprocessor load.
MULTIPHASE CONVERTER ICS
The only topology that can meet today’s processor power needs
is the multiphase switch-mode converter. It employs two or
more identical, interleaved cells connected so that their output is a summation of the outputs of
all cells (Fig. 1).
To understand the advantages
of the multiphase converter, look
first at the shortcomings of singlephase
converters relative to supplying
high current and low voltage.
With a conventional single-phase
converter, the output ripple and
dynamic response improve with
increased operating frequency.
In addition, the physical size and
value of the output inductor and
capacitor shrink at higher frequencies.
Unfortunately, after the frequency
reaches a certain limit, the
converter’s switching losses increase
and its efficiency declines. This
forces a design tradeoff between
operating frequency and efficiency.
To overcome these single-phase frequency limitations, the multiphase
cells operate at a common frequency, but are phase-shifted
so that conversion switching occurs at regular intervals controlled
by a common control chip. The control chip staggers the switching
time of each converter; therefore, the phase angle between
each converter switching is 360°/n, where n is the number of converter
cells. Because cell outputs are in parallel, the effective output
ripple frequency is n × f, where f is the operating frequency of each
cell. This provides better dynamic performance and significantly
less decoupling capacitance than a single-phase system.
Current sharing among the cells is necessary so that one cell does
not “hog” too much current. Ideally, each multiphase cell should consume
the same amount of current. To achieve equal current sharing,
the output current for each cell must be monitored and controlled.
Continue to page 2