ADVERTISEMENT FEATURE
“Digital controllers
… enable new
features, providing
the ability to create
software
programmable and
flexible solutions
with precise and
predictable
behaviour.”
Overcomi
Libraries remove complexity from digital power design. By Olivier Monnier.
T
here is no doubt that future power supplies will be digitally controlled. It is now
a question of whether the market will
adopt digital power quickly or whether it will
take time to overcome the ‘fear factors’ of performance, reliability and complexity.
Digitally controlled power supplies are not
new; microcontrollers have been used for some
time, mainly for supervisory, monitoring and
diagnostic purposes. A true digital power supply,
however, features a digital control loop, rather
than the traditional analogue control loop.
Whilst analogue controllers have good bandwidth, resolution and ease of use, they suffer
from component drift and aging, tolerance
degradation and inflexibility. Digital controllers,
meanwhile, are insensitive to temperature drift
and enable new features, providing the ability to
create software programmable and flexible solutions with precise and predictable behaviour.
Silicon is now available that enables digital
power supplies, so performance should no
longer be seen as a factor. TI’s C20000 range of
DSP Controllers combines three key features:
cpu performance; command precision; and
Figur e 1 : software module classification
application independent/
peripheral independent
application configurable/
peripheral independent
INV
SQR
CNTL
2P2Z
in
Sponsored Tutorial
For more information, go to
www.ti.com/c2000appsw
in
incr
out
SLEW
LIMIT
in
SINGEN
T1
out
Precise regulation
Along with a PWM with a resolution of 150ps
and an on chip A/D converter capable of
12.5Msample/s, this family enables complex
power supply topologies. As an example, it is
now possible to achieve precise and tight regulation of an output DC buck converter with
a switching frequency of 1MHz, along with
current regulation and the start of A/D conversion synchronised with the PWM.
Event Managers carrying the PWM unit
(the command) have been designed in the heart
of the application target. Flexible timer units
support phase shifting, multiphase interleaving
and zero voltage full bridge power topologies.
So it is possible for one TMS320F2808 – part
of the TI C2000 family – to drive 14 DC channels at the same time. Communication tasks
are supported, using such integrated synchroapplication configurable/
peripheral independent
(peripheral driver)
ZVS
FB
DRV
out
FILT
BIQUAD
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gain
freq
offset
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ref
fdbk
feedback capability. The cpus range from
40MIPS to 150MIPS and support 32bit math
operations, solving the traditional numerical
problems that developers encounter when moving to the digital world.
phase
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riegdb
out
MPH3
IL
DRV
FILT
2P2Z
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out
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PWM1
PWM2
PWM7
PWM8
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W
M
H
W
EPWM1A
EPWM1B
EPWM2A
EPWM2B
EPWM3A
EPWM3C
New Electronics 23 May 2006
SPONSORED TUTORIAL
POWER
ng fear factors
Figur e 2 : modularity and software module connection
Vref
(Q15)
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DRV
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2P2Z
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(Q15)
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FILT
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Vout
(Q15)
CPU dependency only:
• math/ algorithms
• per unit math (0 to 100%)
• hardware independent
nous and asynchronous interfaces as SC, SPI,
I2C and CAN.
The Digital Controller enables features to be
expanded through software. This ranges from
power stage sequencing to a soft start profile that
can be adapted to various load profiles. Users can
define intelligent fault management strategy,
active in rush control and an intelligent current
share strategy.
Efficiency demands
Alongside cost and performance, two other
concerns are complexity and reliability. However, C20000 DSP Controllers have been used
for 15 years in power electronics applications,
where EMI and noise issues have been taken
into account early in the design.
But the software obstacle remains; power
supply designers need ‘jump start’ solutions and,
in order to get a feel for digital control’s modularity, reuse and efficiency, new users need to follow some rules to avoid the design cycle turning
from weeks into months.
Software is key in digital power and increasingly significant in AC/DC rectification. There
are two software approaches for digital power.
One, which eases development time, would be
to write all the firmware in a high level language (C or C++) using conventional function
calling and parameter passing, with the option
of using a real time operating system. This
New Electronics 23 May 2006
duty
ADC
SEQ1
Rslt0 DRV
Rslt1
Rslt2
Rslt3
E
P
W
M
H
W
A
D
C
H
W
depends on:
• PWM frequency
• system clock frequency
EPWM1A
ADC_A0
ADC_A1
ADC_A2
ADC_A3
depends on:
• the number of A/ D conversion bits
• unipolar or bipolar
• offset
approach would need a device offering 200 to
300MIPS in order to handle the tight regulation loops required.
The second, more realistic, method is based
on getting the most from existing devices that
fit, from a CPU and peripheral set point of
view, digital power applications. This needs a
combination of C and assembly language, ‘flat’
in line encoding, non conventional function
calling/parameter passing and a simple interrupt service routine structure.
Different approaches are required for every
type of software block and it important to
observe some rules in order to define good software. One rule is delineation between code and
peripherals, requiring peripheral drivers. Software modules must be written for efficiency and
high performance so code is executed in minimal time. Modules also need to be easy to use,
interpret, debug and modify in order to articulate the complete firmware in a friendly framework. As shown in figure 1, it is important to
organise and classify the software modules that
will be used in the application.
The first category is the peripheral dependent
module or peripheral driver that writes directly
to the digital controller’s peripherals register.
With a zero voltage full bridge driver, this module would initialise those hardware components,
including PWM polarity and timers, which
involve a write to the processor registers.
Such modules don’t deal with regulation
itself, but simply applies the command law computed by a peripheral independent, but application specific, module. These modules include
biquad filters, two pole two zero controllers, PID
regulators and filters which impact system performance. A third category of module is purely
software. These run as code on the CPU independently of the application and the peripherals:
for instance, calculating a root mean square
value of a voltage. Modularity is another rule
defining software blocks with standardised input
and output formats. Good software implies
making a choice of numerical representation,
allowing easy ‘variable connection’ between different modules.
Firmware needs to be organised inside a
simple and easily understood framework. This
framework should be efficient from an interrupt management point of view, deterministic
(all events synchronous and submultiples of
interrupt service routine frequencies) and
should allow a high degree of visibility during
debug and development.
Structuring code and defining good software strategy are major obstacles for analogue
designers and ease of use will determine how
successfully digital power is adopted.
In order to reduce frustration, to provide
‘jump start’ solutions and to reduce Digital
Power development time, TI has launched a
Digital Power Supply library – a collection of
peripheral drivers and software modules that
translates power supply topologies into the
digital domain.
With fully documented software and
source code, the Digital Power Supply library
is downloadable from TI’s web site and is fully
compatible with standard DSP tools, such as
the 2808 eZdsp. The modules are integrated
inside a friendly framework and delivered
within a Code Composer Studio IDE project,
allowing real time visualisation and debug of
key variables.
With dedicated application notes and the
Digital Power Supply library, TI is making the
digital power approach easy and is building a
bridge to digital innovation for analogue
designers by removing the ‘fear factors’.
Author profile:
Olivier Monnier is European C2000 business
development manager for Texas Instruments.
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