04-01-2013, 04:54 PM
A Practical Introduction to Digital Power Supply Control
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ABSTRACT
The quest for increased integration, more features, and added flexibility – all under constant cost
pressure – continually motivates the exploration of new avenues in power management. An area gaining
significant industry attention today is the application of digital technology to power supply control. This
topic attempts to clarify some of the mysteries of digital control for the practicing analog power supply
designer. The benefits, limitations, and performance of the digital control concept will be reviewed.
Special attention will be focused on the similarities and differences between analog and digital
implementations of basic control functions. Finally, several examples will highlight the contributions of
digital control to switched-mode power supplies.
INTRODUCTION
Power management is one of the most
interdisciplinary areas of modern electronics,
merging hard core analog circuit design with
expertise from mechanical and RF engineering,
safety and EMI, knowledge of materials,
semiconductors and magnetic components.
Understandably, power supply design is regarded
as a pure analog field. But from the very early
days, by the introduction of relays and later the
first rectifiers, power management is slowly
incorporating more and more ideas from the
digital world. Ones and zeros are translated to
on and offs but at the end a diode can be
viewed as a digital component. The
introduction of switched mode power conversion
required even more digital knowledge seeping
into the repertoire of the practicing power supply
designers. The know-how of the first discrete
implementations of the PWM logic using
comparators, gates and latches have faded away
long time ago. Integrated pulse width modulator
ICs have turned those simple digital circuits to
history and have introduced even more digital
content to power management.
GOING DIGITAL
Despite the tremendous amount of digital
circuitry used in power management integrated
circuits, it remained mainly hidden from the
users. Most externally accessible functions are
implemented by fundamentally analog circuit
blocks today. Thus PWM controllers and other
power management integrated circuits have
successfully upheld their analog feel to them,
making analog measurements and accepting
analog controls. Their interfaces to the outside
world are the various comparators and amplifiers
monitoring the operating conditions and
providing a choice of protection options for the
designer of the power supply. This elemental
principle prevails in existing controllers as
demonstrated in Fig. 1.
What Really is “Digital Power”?
Digital power is an inaccurate description
of a new direction in the controller design of the
power supply to replace the analog circuits by
digital implementations. Accordingly, digital
power really stands for digital control of the
power supply. Digital power supply control
attempts to move the barrier between the analog
and digital sections of the power supply right to
the pins of the control IC.
What is Changing?
The striking difference between analog and
digital control is the quality and the amount of
information available for the controller to make
decisions regarding the operation of the power
stage. For example, the output of a comparator
carries limited information about the monitored
parameter, i.e. only whether it is above or below
a threshold. When the border between analog and
digital is moved from the output of a comparator
to the input by converting the actual information
to digital form, the controller suddenly knows the
concrete value of the parameter. Now, in addition
to comparing it to a threshold, changes in the
parameters value can be detected, stored and
later reported back to a supervisory system
Advantages of Digital Control
Flexibility is definitely the most noteworthy
benefit of digital controllers. It is especially
remarkable considering the consolidation of all
necessary functions into one highly integrated,
sophisticated controller. As mentioned before, by
the introduction of a digital controller, the
hardwired, hardly customizable control flow of
analog controllers is exchanged for an open
structure, where the designer has the ultimate
freedom to decide the right course of action to a
given stimuli. This new opportunity can be rather
overwhelming for practicing power supply
designers, because most of these decisions were
made for them by the semiconductor
manufacturers of analog controllers. In addition,
this freedom comes with another new
complication, the digital controller must be told
what to do. Software must be written to program
the execution of all the functions assigned to the
µC or DSP. The software carries the knowledge
and intellectual property which was previously
realized in the controller hardware.
DIGITAL BASICS
To fully understand the potential benefits of
digital control, some basic operating principles
and terminology must be clarified. The two
fundamental building blocks to understand are
the time base and how the analog signals are
converted to digital form. These two circuits
interface with the surrounding analog world and
are critical to the digital controllers performance.
Digitizing Variables – Voltage Sense
In addition to the effects of discrete time
steps the various control variables are also
digitized in the digital controller. This introduces
another level of complexity in the design. First of
all it is important to note that the digital
controller measures everything as a voltage input
connected to an on-board analog-to-digital
converter, usually through a multiplexer. All
voltages monitored by the ADC must be scaled to
the input voltage range of the ADC, generally
between ground and the reference of the ADC.
Typical reference levels are either 1.25V or 2.5V
and these references can be internal or external to
the analog-to-digital converter. After the
conversion, the measured voltage level will be
represented by a digital value as shown in Fig. 9.
Digital Pulse Width Modulation
Now that some of the basic blocks and
characteristics of a digital controller have been
introduced, the operation of the analog and
digital pulse width modulator can be compared.
Fig. 10 illustrates the fundamental process to
obtain the operating duty ratio of a power supply
using an analog (top drawing) and a digital
(bottom drawing) approach.
SUMMARY
This paper aimed to introduce digital power
supply control to the practicing power supply
design community. Like most new technology,
digital control in power supplies is expected to
start its proliferation slowly. But this is definitely
a trend not to overlook in the years to come.
At the same time, it is important to remember
that the power supply is still a fundamentally
analog circuit. The knowledge of various power
supply topologies and related analog design
expertise can not be substituted even by the most
advanced software algorithm. On the other hand,
digital implementation of the converters control
offers new opportunities to develop advanced
features and make the power supply a more
visible, more integrated part of the system.