AVERAGE CURRENT MODE CONTROL IN POWER ELECTRONIC CONVERTERS – ANALOG
K. D. Purton * and R. P. Lisner**
*Department of Electrical and Computer System Engineering, Monash University, Australia,
and Switch Mode Power Conversion P/L, Melbourne, Australia
**Department of Electrical and Computer System Engineering,
Monash University, Australia
This paper presents Average Current Mode Control (ACMC) as a general purpose, high-
performance all-round control method for AC-DC conversion, DC-DC conversion, and DC-
AC conversion (including grid-feed inverters). A detailed examination of the typical analog
circuit implementation and waveforms, based on simulations and experimental results, is used
to explain how ACMC achieves superior performance. Several reported digital
implementations are critically examined. Finally a hybrid analog-digital control
implementation of ACMC is proposed.
1. INTRODUCTION “PWM Conductance Control” appears to be the first to
Average Current Mode Control (ACMC) is typically a use the output of an “integrator-zero” compensated
two loop control method (inner loop, current; outer CEA and a linear ramp as inputs to the PWM
loop, voltage) for power electronic converters. Many comparator. This approach was developed
of these applications have been in the higher switching commercially by Unitrode .
frequency, lower power segment (up to 10kW, at The usual implementation of ACMC relies on analog
20kHz and above), but this is changing. A 30kW three operational amplifiers as error amplifiers, and makes
phase inverter using analog ACMC has been reported use of wide-band sensing of the inductor current, to
. The main distinguishing feature of ACMC, as include both the AC and DC components. Figure 1
compared with peak current mode control, is that shows a basic buck converter with synchronous
ACMC uses a high gain, wide bandwidth Current rectification. The voltage waveform representing the
Error Amplifier (CEA) to force the average of one inductor current is connected to one input of the CEA
current within the converter, typically the inductor with large gain at DC and low frequencies to force the
current, to follow the demanded current reference with average value of the inductor current to follow the
very small error, as a controlled current source. current reference, which is connected to the other
Advantages of ACMC include large noise margin, no input.
requirement for additional slope compensation, easy
current limit implementation, excellent voltage and
current regulation, simple compensation, good
behaviour in both continuous and discontinuous
inductor current modes, and has inherent Vin and Vout
feed-forward properties. All this is achieved with only
a slight increase in complexity over earlier schemes.
2. PRINCIPLES OF OPERATION
An early paper on average current control was
published by Papathomas and Giacopelli of Bell Labs
in 1979 . This used digital hardware (no computer)
rather than analog hardware. A current controlled
oscillator clocked a counter and when this count
(proportional to the inductor average current) matched
a current reference count, the switch was turned off. A
more conventional analog approach was reported by Figure 1. Simplified schematic of buck converter
O’Sullivan et al of the European Space Centre in 1988 with synchronous rectifier and average current
, in which the authors claimed to have been using mode control regulating output voltage.
this scheme for the previous decade. Their so-called
Figure 2 shows real waveforms in a low power test
circuit. Fig. 2 (a) shows regulation of the average
inductor current (Iout) at 0.5A, Fig. 2 (b) at 1.0A, and
Fig. 2 (c) at 1.5A. The CEA output is an inverted and
amplified version of the difference between the
inductor current and the current reference signal, with
a positive DC offset. This CEA output is then
compared with a large amplitude ramp waveform at
the converter switching frequency, at the inputs to a
Pulse Width Modulation (PWM) comparator. These
sawtooth waveforms intersect at two points in each
cycle, defining the rise and fall instants of the PWM
pulse train to the switches. If the reference input to the
CEA is the output of a suitably compensated voltage
Error Amplifier (VEA), the average inductor current
will be controlled to force the converter output voltage Figure 2 (b). Oscilloscope printout of
to track the voltage reference. If the CEA reference is experimental buck converter with ACMC.
a half-sine waveform, the average inductor current Duty cycle is around 50%, and Iout is around
will track this, eg, to force a sinusoidal current into the 1A. Note that inductor current (bottom trace)
power grid via an unfolding bridge.
minimum is zero, not negative, under these
The comparison of the wide band inductor current conditions.
waveform with the PWM ramp waveform results in an
inherent fast feed-forward of input and output voltage
changes, without involving the feedback loops and
without direct monitoring. Since the up-slope and
down-slope of the inductor current waveform are
proportional to the input and output voltages, any
change in these slopes results in immediate adjustment
of PWM duty cycle. Clamping the CEA reference
input limits the converter inductor current. This is
often the output current and so an adjustable output
current limit is easily implemented.
Figure 2 (c). Oscilloscope printout of
experimental buck converter with ACMC.
Duty cycle is around 75%, and Iout is around
1.5A. Note that inductor current (bottom trace)
minimum is positive under these conditions.
Compensation of the CEA is based upon high gain at
Figure 2 (a). Oscilloscope printout of DC and low frequencies. This is what forces the
experimental buck converter with ACMC. average of the controlled current, typically the
Large sawtooth is switching frequency ramp inductor current, to track the current reference. This
input to PWM comparator. Smaller sawtooth integrator function is implemented by R1 and C1 in
is CEA output to other input of PWM Fig. 1. At the zero frequency, determined by C1 and
comparator. Middle trace is PWM output to R2, the CEA gain is levelled off and a phase boost
main switch. Duty cycle is around 25%, Iout is back towards zero degrees of lag, from the constant 90
around 0.5A. Bottom trace is the buck inductor degrees of phase lag from the integrator, results. The
current waveform. Note that under these actual flat gain above the zero frequency is determined by
running conditions with a synchronous R2/R1. A higher frequency pole (C2, R2) rolls off the
rectifier, inductor current is actually negative gain near the switching frequency. The Bode plot of
for part of the switching cycle. such a compensator is shown in Fig. 3.
The less commonly listed disadvantages of digital
• software development is tedious, error-prone, and
time consuming, and hence expensive
• microcomputers and DSPs suffer from noise
interference, and generate large amounts of
noise − they rely on good layout, bypassing,
shielding, ground-plane techniques, etc as do
• sampling and quantisation result in steps in time
Figure 3. Bode plot of a typical compensated and amplitude, which degrade accuracy and
analog ACMC current error amplifier. The performance
magnitude response (3 straight line segments)
• the hoped-for component reduction due to large-
shows the low frequency high gain roll-off of
scale integration is usually lost in the number of
the integrator component, the mid-band
constant gain after the zero component, and support ICs (many of which are analog) and
the high frequency roll-off due to the second discrete components
pole. The phase response (inverted bathtub • due to the clocked, serial nature of digital
shape) shows the phase boost improving the computers, everything they do takes time. Simply
phase margin. put, the more complex the task, the longer it
(Vertical: -20dB → +60db and 0° → +180°, takes. This time delay results in phase lag, which
horizontal: 1 Hz → 1 MHz) detracts from performance.
3. ANALOG CONTROL VERSUS Digital controllers are typically mixed-mode
DIGITAL CONTROL controllers. The real world is analog, and must be
processed by analog circuitry before being digitally
Analog control processed. Amplifiers, summers, buffers, level-
shifters, precision rectifiers, anti-aliasing filters,
The commonly listed advantages of analog control voltage references, sample and holds, analog to digital
include: converters, digital to analog converters, final filters,
• relative simplicity etc typically require op-amps. A digital
• lower cost implementation of ACMC may require more analog
• wider bandwidth circuitry than would an analog implementation, yet
• small delay between cause and effect can result in lesser performance with a much higher
cost. The component cost of an analog
• finer resolution of time and amplitude
implementation, involving a few op-amps, a
The commonly listed disadvantages include: comparator, a 555 timer, CMOS logic, and some
• a fixed and relatively simple functionality resistors and capacitors, may be no more than,
say, $US1.50 in quantity manufacture. The
• susceptibility to noise, ageing and drift development time may be a few days. Compare this to
• a large number of components. a digital implementation. The hardware cost will be
considerably more, coupled with the added software
cost. The overall cost will be many times more, and
The commonly listed advantages of digital control the performance worse.
Accuracy, reliability, repeatability, and freedom from
ageing and drift are not necessarily intrinsic to digital
• the possibility of intelligent, adaptive, linear or controllers, nor absent from analog controllers.
non-linear control Analog controllers depend on resistors and capacitors
• the possibility of self-calibration and self- in feedback networks to set gains and frequency
diagnosis etc response. Components of adequate specification with
• accuracy, reliability, repeatability tight tolerance, low drift, and a small temperature
• ability to communicate with other systems coefficient are available. Digital controllers generally
• no ageing or drift require a lot of analog support circuitry which, in turn,
must be adequate for the task (and typically is).
• large noise margins.
While typical analog circuitry cannot compete with switching period delays between parameter
the possible intelligence and adaptability of control measurement and control response by extrapolating to
using a digital computer, non-linear analog control a future response from past measurements. While this
can be very practical and cost effective. For example, predicting of the future based on past trends may be
consider the non-linear functions within a Unitrode helpful if things keep going the way they have been,
UC3854 analog power factor controller IC: like some weather forecasting approaches; such a
multiplication, squaring, and division. A very fast scheme cannot compete with a fast analog controller
fuzzy logic controller, with complex non-linear when the unexpected occurs.
properties, can be built from analog components.
4. INVERTER CONTROL INCORPORATING
There are many digital controllers doing good work
now in industry, for example in variable speed,
variable voltage AC induction motor drives. When a PWM inverter is synthesizing an AC
Computers, ranging from single chip microcontrollers waveform from a relatively constant DC input, the
to advanced DSP processors, are being used by the duty cycle is adjusted from near zero at the zero
thousands. Upon closer examination, it appears that crossings, to near maximum at the peaks. Depending
these devices are not primarily being used as fast, on the topology of the power converter chosen, this
precise compensators for good transient response, but variation will alter the controller compensation
rather as versatile, flexible, adaptive system requirements to a greater or lesser extent. One of the
supervisors. For example, generation of three-phase most difficult to optimise is the buck-boost, or flyback
PWM with variable amplitude, variable frequency, is DC-DC converter, used with an unfolding stage.
readily done with one powerful CPU. It can also be Schlecht  reported the use of time varying feedback
done well with analog ICs and discrete components. gains to counteract the 120Hz time dependant
However, using a microcontroller to do this is more in response of the flyback topology when used in an
the role of a modulator rather than a compensator. A inverter. The result was closed loop poles that
digital PID control function is usually tacked on, to remained stationary, making possible sharper cusps in
avoid using more external circuitry. the half-sine waveforms and so reduced distortion in
the current waveform.
Digital computer implementations of ACMC have
been reported. Some of these have relied on a brute
force sampled version of the analog approach. In , Buck derived converters are more common at higher
Holme and Manning reported a digital ACMC power levels than buck-boost types, and less
scheme. A powerful DSP processor, combined with a demanding to control. Even so, the control-to-output
very fast ADC and a digital hardware PWM module, transfer function varies with duty cycle  and
sampled the inductor current many times per improved inverter control can result from time varying
switching period to locate the peak value. The current compensation, as opposed to fixed parameters.
samples, along with samples of Vin and Vout were
processed to force the average inductor current to
track a reference. However, this takes time, and a two- 5. HYBRID ANALOG – DIGITAL ACMC
switching-period delay resulted between sampling and One approach to optimising performance and cost is to
PWM adjustment. This sampling/processing time combine both analog and digital techniques as
delay translates to a considerable phase lag, which appropriate. The authors are investigating a hybrid
greatly complicated stability issues. Of even more ACMC scheme for grid-feed inverters based on a fast
concern is the unmentioned orders of magnitude analog core for optimum tracking and regulation,
increase in cost and complexity of this digital copy combined with an economic micro-controller. The
over the analog approach, to achieve an inferior result. primary task of the latter is to generate the required
If the digital approach resulted in some worthwhile reference, and to adjust gains and corner frequencies
improvement, then a cost-benefit analysis may justify by digitally controlled resistances, according to
it. monitored conditions, such as the operating point
within the AC waveform at the time, and the nature of
Other forms of digital current mode control have been the load. Such adaptive-gain-scheduling non-linear
proposed, such as the predictive approach; for control is able to improve on both the pure analog and
example, those by Holmes and Martin , and Gow pure digital control schemes, with minimum cost.
and Manning . These aim to compensate for multi-
An example of time varying gains is shown in 7. REFERENCES
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