Download A Modular Power Electronics Instructional Laboratory

A Modular Power Electronics Instructional Laboratory
R. Balog and P.T. Krein
Grainger Center for Electric Machinery and Electromechanics
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
Urbana, Illinois 61801, USA
Abstract— The study of power electronics draws upon a
broad range of knowledge and often required a fair amount of
experience. This suggests that laboratory instruction should be
an integral component of a power electronics curriculum. However, before a single watt is delivered, the student must understand not only the operation of the converter topology but also
more advanced concepts such as control theory, gate drive isolation, and layout issues. Our approach is to use a “blue box”
module were these details are pre-built for convenience, but not
hidden inside a “black box.” Recent improvements to our “blue
box” modules are described in this paper and include a dualMOSFET control box with independently isolated FET devices
and a high quality PWM inverter built discretely.
I. INTRODUCTION
Power electronics is an appropriate subject for an advanced
junior or senior level undergraduate course. The subject is
largely application driven, but draws from a broad knowledge
base including basic circuits and electronics, control systems,
power systems, and semiconductor devices. For many students, a power electronics laboratory can provide an early
experience in synthesis, requiring them to use knowledge
across their full curriculum. Compounding the challenge is
the need for attention to detail. It is well known that issues
such as wiring configuration, circuit layout, and device selection can dominate the performance of a converter. This suggests that laboratory instruction has great value as a component of a power electronics curriculum.
This paper presents two recent improvements to the power
electronics laboratory that have greatly increased the value of
the experimental instruction. The new designs and details are
freely available to educators who wish to apply our approach.
A. Educational Challenge
A significant drawback to power electronics laboratory
work is that a considerable level of background effort is required before complete nontrivial working systems can be
built. Gate drives, supply isolation, layout, control functions,
and other issues must be addressed before a single watt is
delivered. A potential source of frustration is to spend several weeks in the laboratory preparing the first fully functional converter. One way to avoid this drawback is to take a
“black-box” approach: prepare a complete function module
in advance, and have the students assemble circuits integrating the function module. This approach is common among
commercial vendors of power electronics laboratory equipment. It can lead to rugged, sophisticated units that can perform high-level power conversion functions.
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The obvious drawback of a black-box approach is that students are not motivated (or often expected) to understand the
inner workings – the heart of any real power converter. An
alternative is the “blue box” approach introduced in [1]. A
blue box differs from a black box in at least two ways:
• The internal circuit is studied in some depth prior to use.
• The internal circuit should be simple enough that elements
do not have to be ignored. Students should be able to understand the box’s function completely as they perform
their work.
Our approach uses blue-box modules to allow students to
start immediately on converter circuits, but eventually replaces the box with complete student-built discrete equivalents. The blue box approach has been in successful use for
more than ten years, following from [1, 2].
B. Alternative Approaches
Web based instructional tools have become the center of
attention recently [3-5]. While Java applets can be a valuable
supplement to formal lecture allowing students to experiment
with “what-if” scenarios, they de-emphasize the hardware
aspect and avoid the complexities of converter fabrication.
Laboratory based education in the current literature generally falls into one of three categories: “virtual labs,” “system
modules,” or “component modules” similar to our “bluebox.” System oriented modules such as [6] typically include
the power semiconductors, in some cases passive devices,
and a switch matrix to realize a number of topologies. They
offer the advantages of pre-selected power components and a
proven circuit layout. Extending this concept, so called “virtual labs” [7-10] offer the possibility of combining simulation
with pre-configured hardware and allows opportunity for
direct comparison of simulation and measured data. However, in many cases the hardware is located remotely. While
both these approaches return the focus to hardware, a sealed
system limits flexibility in component selection and in some
designs may limit duty ratio and frequency.
II. FET CONTROL BOX
A. Basic Design
The key improvement is a new dual-MOSFET “blue box”
that can implement nearly any dc-dc or dc-ac converter, providing students with a means to quickly begin to explore various switching power supply topologies. The box contains a
simple PWM controller with frequency and duty ratio as di-
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rect front-panel commands. The main design attributes are an
internal flyback converter that supports gate-drive isolation,
and the introduction of two electrically independent FET devices. It is essential to the blue-box concept that there not be
any “hidden” functions or control action. Protection is deliberately kept to the absolute minimum – the box does not
“self-protect” or use additional circuitry to mitigate errors.
This simplicity of function, without the distraction of highside gate drives or circuit layout, provides the user with
enough details of the operation to understand how the module
works. In the instructional environment we have found that
the module helps to solidify and de-mystify fundamental concepts such as PWM generation, isolated gate drive, snubber
design, and control while at the same time alleviating the
student from the burden of implementing these on a breadboard. There is time for each student to move past these details (while not ignoring them) and focus on the fundamental
behavior of a wide range of converter topologies and methods.
The control section of the FET Control Box is based on the
3526 PWM IC. This particular IC has been valuable because
of its linear PWM action, wide frequency range, and ease of
synchronization. A block diagram of the new dual unit is
shown in Fig. 1. Frequency and duty ratio are controlled via
front panel knobs for typical implementation, illustrated in
Fig. 2. If modulation of the duty ratio is desired, as in a
PWM inverter or even to implement a control loop, a duty
ratio BNC style input “D” overrides the duty ratio knob.
Multiple boxes can be ganged together in a master/slave arrangement via external synchronization through the BNC
“q(t)” connector. For maximum flexibility, the unit has simple internal logic that can be switched so the FETs operates in
one of three modes:
• “Matching mode,” in which both devices operate with the
same switching function. This can be used for parallel operation or for full-bridge circuits.
• “Complementary mode,” in which the two switching functions are in complement, with a short dead-time. This is
the operating mode used in conventional designs of halfbridge and synchronous-rectifier converters, two-quadrant
dc-dc converters, and many other circuits.
• “Alternating mode,” in which the two switching functions
alternate which is appropriate for push-pull converters.
panel
mounted
frequency and
duty ratio
control knobs
external duty
ratio input
external
sync.
q(t)
q(t)
PWM
circuit
Deadtime
circuit
q(t)
q'(t)
Alternating q (t)
A
circuit
qB(t)
Isolated
gate drive FET 1
Isolated
gate drive FET 2
FET Control Box
Power
University of Illinois
High
Urbana - Champaign
Low
D
Frequency
Duty Ratio
D
S
FET 1
S
int
ext
q(t)
D
q
q'
alternate
FET 2
Fig. 2. Front panel of the FET Control box
These three choices allow the realization of almost any dcdc converter, including direct, indirect, flyback, and most
two-switch forward converters. Two boxes used in synchronous fashion can operate full-bridge forward converters. A
single box also supports dc-ac converters such as voltage
sourced and half-bridge PWM inverters. Two boxes together
support full-bridge inverters and even ac-ac converters.
The ratings of the power semiconductors were chosen to be
sufficient for power levels up to at least 100 W and provide
enough headroom to limit the possibility of over-voltage or
over-current damage. Since the switching devices are inside
the box and therefore not optimally located in proximity to
the rest of the converter, each FET has a simple lossy turn-on
turn-off snubber. The purpose of the snubber is primarily to
protect the FET from excessive over-voltage and since it is
located inside the box, it cannot be altered to optimize performance. Over-current protection is provided by a PCB
mounted fuse. Since the FET control box was designed as a
generic FET substitution module, the FET was “overdesigned” for voltage and current. The result of this design,
given current technology, is a Rds(on) of 0.2 Ω. Students are
encourages to examine the datasheets for the power devices
and include this information in power loss calculations.
Isolation is probably the most important aspect of the
module to allow each FET to be placed arbitrarily within any
topology: high-side, low-side, reversed, series-connected,
negative supply voltage, etc. Further, each of the two FETs
must be independently isolated. Safety mandates that the
control and the inputs be ground referenced. Therefore, the
internal power supply for the FET Control Box contains two
isolated gate drive supplies, and ground-referenced 12V and
5V supplies for the controls. Fig. 3 is a picture of the inside
of the box. The toroid in the upper right of the frame is the
flyback inductor. Power for the control circuitry is provided
via a rear panel mounted standard IEC style line cord and
stepped down through a transformer (top left of Fig. 3.) The
FET control box is protected from the ac line is with a fuse
for over current, a MOV for surges, and PCB isolation clearances from the transformer isolated secondary. The box can
also be supplied from a dc source via a barrel style power
connector on the rear panel.
Fig. 1. Control Circuit
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Fig. 4. Buck converter with FET Control Box
Fig. 3. Inside of the FET Control Box
B. Example Topology: “Buck Converter”
A straightforward dc-dc topology is the buck converter.
However, the high-side switch complicates controlling the
circuit. Fig. 4 shows the FET Control Box wired as a single
high-side switch. A diode is mounted near each FET in the
box for use as the freewheeling diode in test circuits. External bypass capacitors, the choke, load resistor, and smoothing
capacitor are required to complete the circuit. Typical waveforms are shown in Fig. 5. The top trace is the voltage across
the freewheeling diode, the middle is the inductor current,
and the bottom is the ripple on the output voltage. The performance of the system is sufficient to observe salient features such as the ESR jump of the smoothing capacitor.
Fig. 5. Waveforms of the buck converter
A potential drawback of any module designed for a broad
range of applications is limited performance. The requirement of high current and high voltage rated FET devices implies high Rds(on) given current technology. A single FET
circuit designed to process hundreds of watts will be inefficiency at tens of watts. In the FET “blue box” we used an
IRFP360 FET to achieve high current and voltage ratings
while maintaining sufficiently low losses. A 48V to 12V
converter was used to test the efficiency of the FET Control
Box over the range of a few watts to about 100W. The result,
obtained with standard components available to the students,
are shown in Fig. 6.
The functional quality and performance of the FET Control
Box has been improved considerable with the new version.
Major changes in addition to adding the second FET and
gate-drive control logic include improved gate drives and
better interfaces to the PWM IC. The result is a much more
linear PWM system. A student lab team recently used the
FET Control Box, configured as a half-bridge inverter, with
audio input as a class-D amplifier. While sound quality was
not perfect (qualitatively it was about equivalent to AM radio), this operation impressed the students with the broader
possibilities for PWM power conversion.
48V to 12V FET Control Box converter
0.9
efficiency
0.88
0.86
0.84
0.82
0.8
0.78
0
20
40
60
80
100
P
out
Fig. 6. Efficiency of buck converter
C. Example Topology: “Ac-Ac Conversion”
Ac to ac conversion is perhaps the most general energy
transfer in power electronics. Until recently, motor-generator
sets provided frequency conversion and line frequency transformers provided voltage conversion. Even today, with solid
state switching devices, direct ac to ac conversion is nontrivial. Fundamental to the problem is the requirement for
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bilateral switches – the switch is required to carry current or
block voltage or either polarity. Because of this and the control complexity, most first level courses do not discuses direct
ac – ac conversion. Even most commercial ac – ac converters
bypass these requirements by utilizing a dc link.
Direct ac – ac switching achieves frequency conversion by
connecting the source and load through a switching matrix
and operating the switches such that f out = f switch ± fin . The
half-bridge circuit shown in Fig. 7 is realized by using two
FET Control Boxes and a function generator. Both Control
Boxes are connected to the function generator via the q(t)
input selected to external. FET 2 on both boxes is selected to
operate in q’(t) mode to internally generate the complementary switching function (plus some dead-time.) Each auxiliary diode is wired anti-parallel to provide a freewheeling
current path.
Fig. 8. Waveform of “Universal Frequency Changer”
Typical waveforms for a “universal frequency changer” are
shown in Fig. 8. The external function generator provided a
5V TTL level 50% square wave set to 300 Hz shown at the
top of the figure. Phase voltages were obtained from a
120:25 Vac center tapped secondary transformer. The
“chopped” 60Hz results in an output with equal magnitude
frequency components at 240 Hz and 360 Hz and is shown
superimposed on one phase voltage for reference. Using an
external function generator, arbitrary frequency conversion
can be performed.
Choosing a switching frequency less that the phase frequency results in a “slow switching frequency converter.”
Fig. 9 shows the FFT of the output voltage when the switching frequency is set to 40Hz by the external function generator. The desired difference frequency is 20 Hz and the summation frequency is 100 Hz. The greater frequency separation of the two large magnitude components results in simpler
filtering of the desired frequency.
In this configuration, two FET control boxes allow students to easily experiment with frequency based speed control of small ac machines.
Fig 7. Direct ac – ac converter using two FET boxes
Fig. 9. FFT of “Slow-Switching Frequency Changer”
III. PULSE WIDTH MODULATION INVERTER
With the successful application of an FET-based halfbridge to an audio application, we set out to prepare an even
higher quality full-bridge inverter circuit that could be used to
shown the operation of pulse-width modulation in complete
detail. In this case, the objective is a blue-box circuit that
provides the elements of a PWM system with as few extraneous functions as possible. A full bridge PWM inverter is a
complicated circuit to build discretely in a typical undergraduate laboratory class, particularly when linearity and
other performance attributes are considered. While PWM
ICs are common enough, the essential elements of the PWM
process are often buried inside the chip and inaccessible. The
extra circuitry for soft start, overload protection, and control,
means that PWM ICs do not support emphasis on the fundamentals of the process as well as the salient features of implementing a full bridge inverter. In accordance with the
blue-box concept, we wanted a circuit that could be understood in complete detail prior to use.
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easy to motivate as an essential element of operation. Deadtime is realized in discrete logic with Schmitt inverters, AND
logic gates, and a single R-C circuit. The result, shown in
Fig. 12, is two switching functions (A and B) with 280ns
dead-time and their complements (A* and B*), all derived
from the original PWM comparator output shown as the top
waveform. Test-points on the PCB allow each gate drive
signal to be observed during operation.
Fig. 10. PWM inverter PCB
In the new “blue-box” PWM inverter, shown in Fig. 10, all
aspects of the PWM process are implemented using discrete
components. Test points identifiable by color and number are
provided so that students can examine the triangle carrier
waveform, the modulating waveform, the resulting PWM
waveform of the comparator, the four gate drive signals, the
bridge output voltage, and the output voltage of the on-board
low pass filter. In Fig. 11, an example of a 132 kHz triangle
carrier wave with a superimposed continuous dc modulating
function is shown. The resulting 40Vpk-pk bridge square wave
with constant 50% duty ratio and associated current ripple of
the output filter is shown for a 8ohm resistive load.
Key components were chosen to allow construction of high
quality, highly linear, PWM with respect to the modulating
function. The PWM generation process is based on a LM566
VCO and a LM311 comparator. Both devices are common
components with sufficient bandwidth and linearity. The low
pass output filter was designed with a cut-off frequency of
20kHz. The result is a PWM process with audio-level quality
– high enough to serve in place of a commercial audio amplifier. One experiment leads students into the discovery of
class D audio amplifiers with a CD player to supply music as
the modulating function and a moving coil speaker as the
load. The quality is much higher than that attainable with the
FET Control Box, and emphasizes all attributes of PWM as a
high-quality power conversion control process.
The triangle carrier frequency can be adjusted from tens of
kilohertz to over 200 kHz. The gain of the comparator is set
for a typical audio line level of about 2Vpk-pk while allowing
for modulation indexes greater than 100%. Analog input is
supplied through the 3.5mm stereo headphone jack, the left
and right channels are summed into a mono signal, and to
ensure correct dc biasing, the analog input is ac coupled.
The “volume knob” is simply an attenuator for the input signal. In order to keep the circuit as simple as possible, all gate
drive signals are ground referenced. While this limits the
bridge voltage to about 20V (Vgs(max) for the output FETs)
sufficient power can be processed to drive a small speaker.
In one experiment, students use the PWM Amplifier as an
inverter to drive a small ac motor through a step-up transformer.
Fundamental to the discussion of inverters is the issue of
dead-time. While any practical implementation of an Hbridge inverter must incorporate dead-time for FET switching, this is often treated as an advanced topic not explored in
an introductory level power electronics class. Specialty gate
drive ICs provide the functionality but obscure the operation.
In contrast, the blue-box inverter uses simple delay-based
dead time generation – easy for students to understand and
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Fig. 11. PWM process waveforms
Fig. 12. Dead time in H-bridge FET gate drives
IV. CONCLUSIONS
New blue-box module designs for an undergraduate power
electronics instructional laboratory have been described. A
dual MOSFET box supports an unlimited range of dc-dc, dcac, and ac-ac circuits with a clear and simple circuit that can
be fully understood by students as they use it. A full-bridge
PWM inverter allows students to get “deep inside” a real
PWM process to see how high-quality power conversion
works. Both modules have been deployed and used in our
laboratory. Feedback so far has been excellent. The dual
FET box has allowed expansion of experiments to address
many more topologies. Performance of the controls is sufficient to allow audio “amplification” in a half-bridge inverter
topology. The PWM inverter box has allowed us to help students see how power electronics applies in a wide range of
areas well beyond the conventional.
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