A Modular Power Electronics Instructional Laboratory

Size: px

Start display at page:

Download "A Modular Power Electronics Instructional Laboratory"

Estella Morrison
5 years ago
Views:

1 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 dual- MOSFET 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. 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. A. Basic Design II. FET CONTROL BOX 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 /03/$17.00c2003

2 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 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. PWM circuit Deadtime circuit Alternating circuit Fig. 1. Control Circuit q'(t) q A (t) q B (t) Isolated gate drive Isolated gate drive FET 1 FET 2 Power FET 1 D S High Low int ext Frequency Duty Ratio D q q' alternate Fig. 2. Front panel of the FET Control box FET Control Box University of Illinois Urbana - Champaign FET 2 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 R ds (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. D S

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.

A diode is mounted near each FET in the box for use as the freewheeling diode in test circuits.

3 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. 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. efficiency Fig. 5. Waveforms of the buck converter 48V to 12V FET Control Box converter 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

4 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 = fswitch ± 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 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. 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 V ac 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. 8. Waveform of Universal Frequency Changer 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.

gate drive signals, the bridge output voltage, and the output voltage of the on-board low pass filter. In Fig.

5 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 40V pk-pk bridge square wave with constant 50% duty ratio and associated current ripple of the output filter is shown for a 8ohm resistive load. 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 2V pk-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 (V gs (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 H- bridge 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 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. 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. Fig. 11. PWM process waveforms Fig. 12. Dead time in H-bridge FET gate drives

6 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. REFERENCES [1] P. T. Krein, "A broad-based laboratory for power electronics and electric machines," Rec., IEEE Power Electronics Specialists Conference, 1993, pp [2] P. T. Krein and P. W. Sauer, "An Integrated laboratory for electric machines, power systems, and power electronics," IEEE Trans. Power Systems, vol. 7, pp , [3] U. Drofenik and J. W. Kolar, "Survey of modern approaches of education in power electronics," Proc. IEEE Applied Power Electronics Conference, 2002, pp [4] F. A. S. C. Gonvalves, C.A., "JAVA applets for a WWW-HTML-based course in power electronics," Rec., IEEE Power Electronics Specialists Conference, 2001, pp vol. 1. [5] U. Drofenik and J. W. Kolar, "Interactive Power Electronics Seminar (ipes)-a web-based introductory power electronics course employing Java-applets," Rec., IEEE Power Electronics Specialists Conference, 2002, pp [6] W. Robbins, N. Mohan, C. Henze, and T. Undeland, "A building-blockbased power electronics instructional laboratory," Rec., IEEE Power Electronics Specialists Conference, 2002, pp [7] K. W. E. Cheng, C. L. Chan, N. C. Cheung, and D. Sutanto, "Virtual laboratory development for teaching power electronics," Rec., IEEE Power Electronics Specialists Conference, 2002, pp [8] A. O. Etxebarria, I.J.; Sanchez, M., "Power electronics and basic electronics real experiments through the World Wide Web," Proc. IEEE Frontiers in Education Conference, 2001, pp. F4D-F11 vol.3. [9] L. de V.B. Machado Neto, F. M. de Souza, R. C. Figueiredo, and R. C. Ivo, "Power Electronics Laboratory at PUC Minas/Brazil: simulation and experiment's tools," Proc. IEEE Frontiers in Education Conference, 2000, pp. S1E/1-S1E/6 vol.2. [10] C. Fernandez, O. Garcia, J. A. Cobos, and J. Uceda, "Self-learning laboratory set-up for teaching power electronics combining simulations and measurements," Rec., IEEE Power Electronics Specialists Conference, 2002, pp

Design Document. Analog PWM Amplifier. Reference: DD00004

Design Document. Analog PWM Amplifier. Reference: DD00004 Grainger Center for Electric Machinery and Electromechanics Department of Electrical and Computer Engineering University of Illinois at Urbana-Champaign 1406 W. Green St. Urbana, IL 61801 Design Document