Hailed as the next generation of digitally controlled power-supply ICs, Microchip
Technology's Digital Signal Controller (DSC) ICs give power supplies internal
digital control. While they aren't the first ICs intended for this purpose,
their price tag starts at $2.99 each in lots of 10,000, which is far below any
of their predecessors.
These devices are configured within the supply s feedback loop, so speed is
critical. That's because the analog-to-digital converter (ADC) and processor
must sample the output and react as quickly as possible to any changes. The
Digital Signal Controller ICs meet all these requirements.
First, consider the basic internal digital control IC configuration for a switch-mode
power supply (Fig. 1). Compared with analog
supplies, Digital Signal Controller ICs provide extensive fault monitoring,
better transient response, and lower-cost redundancy options.
Also, the digital approach eliminates a power supply's drift and need for temperature
compensation. This digital technique can program operational settings, which
eliminates the manual tweaking of power-supply adjustments.
External digital control used with the PMBus and Power-One's Z-One system also
can set output voltages and monitor power-supply performance. But they require
an external power manager or processor and the appropriate power-supply interface.
In contrast, Digital Signal Controller ICs can perform the same task primarily
using firmware and in some cases sensors.
Digital signal control requires fewer hardware platforms because firmware can
control power-supply performance while using the same internal hardware. Plus,
this type of control can change topologies and configurations on-the-fly: from
buck to boost or boost to buck, as well as from continuous to discontinuous.
About the only necessary change might be to use different external power output
stages. External digital control can change a power supply's operating parameters,
but it cannot change topologies on-the-fly.
A unique aspect of these DSCs is that their analog comparators can terminate
the pulse-width modulation (PWM) signal early. This allows cycle-by-cycle current
limiting, which is required for current-mode power supplies.
Additionally, digital signal control enables the implementation of power factor
correction (PFC) by adding appropriate firmware and some external hardware.
An analog-based supply would require considerably more hardware. And, external
digital control cannot provide this capability.
Digital signal control opens the door to extensive creativity, limited only
by the available features, the designer s imagination, and the amount of available
program memory. For example, this controller can monitor the performance of
an individual component within the power supply.
Furthermore, the same basic circuit could be used for uninterruptible power
sources (UPSs), power inverters, and digital lighting. This degree of flexibility
isn't available with an analog-based supply or external digital control supply.
ECONOMIC VIABILITY
To be economically feasible, the DSC IC must cost-effectively provide the necessary
high-speed power-supply functions. Besides the IC s price, there are other cost-related
issues, such as learning the digital control design philosophy and developing
the necessary firmware. But once this learning curve is mastered, a similar
design approach can be used for all supplies.
Designers should ask themselves two necessary questions. First, what applications
fit DSC-based power supplies? The obvious answer is ac-dc and dc-dc converters.
The second question is what power levels make the most economic sense. It all
depends on the price-performance characteristics of the DSC IC.
Today, the cost of this controller IC could become a small percentage of the
overall power-supply cost at 100 W and above. This could include front-end power
supplies, high-power dc-dc converters, and bus converters. In the future, economies
of scale could permit use of the DSC IC for power supplies below 100 W.
It now appears that Microchip Technology has the solution with a DSC family
that meets the requirements for speed, price, and features. This family includes
the dsPIC30F1010, dsPIC30F-2020, and dsPIC30F2023. Virtually, their only external
components are in the power output stage (see Fig.
2 and 3 and the
table).
These devices are able to coordinate the analog-to-digital sampling point relative
to the PWM cycle. Configurable control also allows the analog comparators to
react to random events and prevent their effect on power-supply performance.
On-chip multiple PWM circuits enable these devices to function independently
without adding to processor overhead. Also, multiple PWM circuits permit their
parallel use for different functions, such as voltage regulation and PFC control.
The devices analog comparator can be used to terminate the PWM pulse early,
enabling cycle-by-cycle current limiting. The multiple-channel ADC permits the
parallel sampling of voltages and current throughout a power supply.
The MPLAB integrated development environment supports these ICs to meet the
need for simplified firmware development. This tool includes an assembler, compilers,
a visual device initializer, and graphical user interface plug-in options. An
evaluation board familiarizes designers with the hardware and firmware in a
typical buck converter.
AVAILABILITY
Selected devices are available for early adopter sampling. General sampling
is expected in July at http://sample.microchip.com. Volume production is expected
in August.
Microchip Technology
www.microchip.com/POWER