Design of Integrated Shunt Current Sensors for IPEMs

Design of Integrated Shunt Current Sensors for IPEMs Yue Xu, R.D. Lorenz The Dept. of Electrical & Computer Engineering University of Wisconsin - Madison Madison, WI 53706 USA Abstract This project is investigating methods to integrate shunt current sensors and current controls into an integrated power electronic module (IPEM). This paper focuses on the different construction approaches for current shunts and the different isolation technologies that may prove suitable for IPEM integration. The application focus is for current loop and over-current protection of a motor drive IPEM although the technology is suitable for current sharing in a IPEM-based distributed power system. I. INTRODUCTION The challenge of designing a low-cost, reliable, efficient, variable speed motor drive forces drive designers to focus on high cost components and difficult-to-integrate components. One area of focus for low cost and high performance is motor current sensing. The cost and packaging compromises when using traditional current sensors, based on both open and closed loop forms of Hall effect field detectors, make them strong candidates for replacement. In investigating alternatives, the current shunt, and current sensor systems based on magneto-resistive or magneto-optical detectors all appear as potential candidates. This paper deals with the current shunt alternative. Other CPES effort deals with the other candidate technologies. Fig. 1 shows a typical threephase induction motor drive in which shunt current sensors have been integrated into the drive outputs, much as traditional current sensors are currently used. 3-Phase Input Power Supply Power Supply Control Interface Microcontroller Rshunt1 Rshunt2 Iso-Amp Current Sense Fig. 1. Typical three-phase AC motor drive with shunt-based phase current sensing 3-Phase AC Motor This work was funded primarily by the Wisconsin Electric Machines and Power Electronics Consortium in support of CPES and made use of CPES ERC Shared Facilities supported by National Science Foundation under Award Number EEC-9731677. For good drive performance and efficiency, two or more isolated precision current sensors are required to complete the necessary current feedback loops and device protection functions. The performance specifications for such sensors have been established as the CPES benchmarking effort [1]. For adequate closed loop motor control accuracy, the current sensor accuracy should be about ± 1%. For adequate closed loop motor control dynamics, the current sensor bandwidth should be over 20 khz. For adequate semiconductor device protection, the current sensor bandwidth should be over 100 khz. TheexistingclosedloopandopenloopHalleffect devices achieve this performance. The key advantages of the shunt current sensor are low cost of the sensing element, insensitivity to magnetic fields, compact package profiles, and inherent lack of measurement bias. The disadvantages include the inherent lack of galvanic isolation, and the inherent sensitivity to external inputs such as temperature and mechanical stress. This paper will focus on design considerations for IPEM integratable shunt current sensors. II. SENSE RESISTOR DESIGN The resistive shunt is conceptually a series resistor in which the voltage drop, based on Ohm's law, is proportional the current. In practice, the sensing element is a complex impedance with cross-coupled thermal and mechanical sensitivity. The first step in the design of this sensing element requires calculating the nominal resistance values appropriate in the circuit application, for example, resistance, power rating, etc. In addition, the tolerances on the value of the resistance need to be developed based on the circuit application. The second step is to evaluate the allowable effect of external inputs. This means quantifying the thermal, mechanical, etc. behavior. The resistor characteristics which follow are key to the design process. 2.1 Resistor Characteristics Resistor Tolerance Resistor tolerance is the deviation from the nominal value. It is expressed as a ±%, measured at 25 C with no load applied. Some resistor designs have extremely tight

tolerances. For example, precision wirewound resistors are made with tolerances as close as ±0.005%. Film resistors typically have tolerances of ±1% to ±5%. Temperature Coefficient of Resistance Temperature Coefficient of Resistance (TCR) is a measure of the rate at which a resistor varies with increasing or decreasing temperature. It is defined as follows. R 2 -R 1 TCR = R 1 (T 2 -T 1 ) x10-6 TCR = Temperature Coefficient of Resistance (ppm/ C: parts per million per degree Centigrade) R 1 = Resistance at room temperature (ohms) R 2 = Resistance at operating temperature (ohms) T 1 = Room Temperature ( C) T 2 = Operating Temperature ( C) TCR is usually treated as being linear unless very accurate measurements are needed, where a temperature correction chart is used. Typically, there are two contributors to temperature-related resistance changes; the resistor s temperature increases as it dissipates power and the ambient temperature. Power Rating For traditional resistors, the power rating is normally specified at +25 C and must be reduced as the resistor s temperature increases. A derating chart is often used (Fig. 2). For integration in power electronics, the power ratings will be based on the thermal properties of the integrated system. resistive element thus causing changes in resistance. The wider the temperature changes and the more rapid these changes in are, the greater the change in resistance Voltage Coefficient The voltage coefficient is the change in resistance with applied voltage and is associated with carbon composition resistors and carbon film resistors. It is a function of the resistance value and the composition of the carbon mixture used in the manufacture of the resistor. This effect is entirely different and is additive to the effects of self-heating. Noise Noise does not affect the value of the resistor but can have a devastating effect on low signals, digital amplifiers, high gain amplifiers, and other applications sensitive to noise. It is dependent on material conduction properties. Thermocouple Effect The thermocouple effect generates a thermal EMF at the junction of two dissimilar metals. In resistors, it is caused by the materials used in leads and the resistive element. It is normally insignificant, but may be important in low ohm resistors used for current sensing. Thermal EMF is minimized by keeping the resistor leads and body at the same temperature. Reliability Reliability is largely determined by thermomechanical stresses internal to the resistive element and on the interconnections. Fig. 2. Resistor power rating chart for traditional resistors Frequency Response and Rise Time Frequency response relates to the change in impedance with frequency, caused by reactive components from the resistor's parasitic inductance and capacitance. Rise time is an associated parameter, relating the resistors response to a step or pulse input. A typical fast rise time resistor has a rise time of 20 nsec or less. Stability Stability is the change in resistance with time at a specific load, humidity level, stress, and ambient temperature. The lower the load and the closer to +25 C the resistor is maintained, the better the stability. Humidity will cause the insulation of the resistor to swell applying pressure (stress) to the resistive element causing a change. Changes in temperature alternately apply and relieve stresses on the 2.2 Design of Current Sensing Resistors When considering the above resistor characteristics and specifications, the harsh environment and the critical requirement for sensing current in motor drives, resistive shunts are often designed to achieve: 1. Low resistance (to minimize power dissipation); 2. Low inductance (to minimize di/dt induced voltage spikes which could adversely affect operation); 3. Reasonable manufacturing tolerance (to maintain overall circuit accuracy); 4. Low temperature coefficient of resistance (TCR); 5. Four-terminal (Kelvin) terminations. 2.3 Kelvin Measurement A Kelvin connection avoids the error caused by voltage drops in the high current path. For a two-terminal current sensing resistor, as the value of resistance decreases, the resistance of the leads becomes a significant percentage of the total resistance. This has two primary effects on resistor accuracy. First, the effective resistance of the sense resistor can become dependent on factors such as how long the leads are, how they are bent, how far they are inserted into the

board, and how far solder wicks up the leads during assembly. Second, the leads are typically made from a material, such as copper, which has a much higher TCR than the material from which the resistive element itself is made, resulting in a higher overall TCR. Both of these effects are minimized when a four-terminal current sensing resistor is used. A four terminal resistor has two additional terminals that are Kelvin-connected directly across the resistive element itself as shown in Fig. 3. These two terminals are used to monitor the voltage across the resistive element while the other two terminals are used to carry the load current. Since these sense conductors carry negligible current (these are inputs to high impedance voltage measurements), there is negligible lead-dictated voltage drop to induce errors in the measurement. High Current Path Fig. 3. Four-terminal Kelvin connection Metering Circuit (High Impedance) In summary, good current-sensing resistor technology requires attention to power dissipation, ohmic value, low TCR, precision fabrication, and interconnection and mounting issues. Current-sensing resistors utilize several different construction technologies. The most common resistor technologies include wirewound, thick film, thin film and etched foil. In this section, these technologies and the feasibility of using them in integrated power modules are discussed. III. CURRENT-SENSING RESISTOR TECHNOLOGY 3.1 Wirewound Technology Wirewound resistors have some very important features not duplicated by newer material-oriented technology products. First, they can have very tight resistance tolerances, 0.005% is commonly achieved. Second, they are stable (15-50ppm/yr) maintaining their precision over time because they are made with stable materials. Third, their TCR is low (<10ppm/ C) and can be controlled by selecting special wire alloys. Special alloys with appropriately high electrical conductivity and low TCR are used as wire material for the wirewound current-sensing resistance elements. Most commonly used are: copper nickel alloy with small additions of manganese or nickel chromium alloy with additions of iron, which is oxidation proof and antimagnetic [2]. Traditional wirewound design is limited by the spiral geometry, which increases the inherent inductance and capacitance, therefore influencing the resistor s frequency response. Special winding techniques can be used to minimize these reactive components. In Aryton-Perry windings, a layer is first wound in one direction. After a layer of insulation, the next winding is wound in the opposite direction with the turns crossing every 180 degrees. 3.2 Thin Film and Thick Film Technology Thin film technology uses several thin layers of insulating, conducting, or semiconductor material that are deposited successively on a supporting substrate in precise patterns to collectively form all or part of an integrated circuit; the deposition can be performed by mechanical, chemical, or high-vacuum evaporation methods. The term thin film is derived from the fact that the deposited films are of the order of a few micrometers in thickness compared with the 10 to 50 micrometers for thick film. A thin film resistor is fabricated by depositing an alloy, metal, carbon, or other thin film. Some popular thin film material includes Nichrome, TAMELOX, Tantalum Nitride, and Silicon Chrome [3]. Thick film is an electronic interconnection technology in which conductive, resistive and insulating paste materials are deposited onto a ceramic substrate. The deposition is performed through an iterative process of screen printing, drying and high-temperature firing. This high-temperature process produces reliable, robust structures with excellent electrical and thermal performance. Thick and thin film technologies are well suited for low to medium volume custom circuits. Thick film has the advantages of lower cost, of being able to handle more power, and of being able to service a higher range of ohmic values. However, thick film resistors do not have tight resistance tolerances (>0.1%), have large TCR (50-100ppm/C) and change over time (500-1000ppm/yr.). Thick film technology is especially important in hybrid and integrated circuits because resistors can be "printed" onto the substrate, eliminating board loading and soldering steps. Thin film has the advantages of tighter absolute and relative tolerances, more environmentally stable components with lower noise, and lower TCR than thick film. Thin film technology is used when precision resistors are needed. 3.3 Etched Metal Foil Technology An etched metal foil precision resistor, unlike a precisionclass metal film resistor or wire wound resistor, is an ultraprecision resistor in which the primary resistance element is a special alloy foil several µm thick. Use of this metal foil as the resistance element gives superior performance not found

in other resistors. In particular, by strict quality control of alloy composition and foil stabilization treatment technology, the temperature coefficient of resistance can be reduced to an extremely low value, lower than 10 ppm/ C [4]. In addition, from the point of view of long-term stability which is an important property of a resistor, since the foil has a thickness of several µm instead of the extremely thin film of a metal film resistor, the natural stability of metal is preserved, resulting in very little resistance change over multiple years. The etched metal-foil resistors are also designed with an inductance canceling serpentine pattern that yields low inductance values, typically less than 8nH. By using fine photo etching technology, it is possible to form the complicated resistance pattern required for highly accurate resistance values. 3.4 Comparison for Current Sense Resistor Characteristics The evolution of current sense resistor technology is primarily application driven. For IPEM application, the general requirements for current shunts include low ohmic values, high power dissipation, low TCR, high bandwidth, intermediate precision, Kelvin connections, physical mounting provisions, and small size. Tables 1 and 2 compare the characteristics of the different types of current sense resistors. Table I. Current sensing resistor characteristics comparison [5] Technology Resistance Range Ω TCR (PPM/IIC) Maximum Power (W) Inductance (nh) Kelvin connection possible Wirewound 0.005~0.100-100~+100 7 Inductive Yes Thick Film 0.008~1-50~+100 1.5 N/A Yes Thin Film 0.022~4.7 200 0.25 N/A No Etched Metal 0.00005~100 < 10 1-1000 < 8 Yes Table II. Current Sensing Resistor Mounting and Packaging Comparison [5] Technology P. C. Board Surface Mount Wire Bondable Integral Heat Sink Wirewound Yes Yes No Yes Thick Film Yes Yes No Yes Thin Film No Yes No No Etched Metal Yes Yes Yes Yes IV. ISOLATION TECHNOLOGY One of the more difficult problems of current shunt sensing circuit design is trying to either galvanically isolate or dynamically level shift precision analog signals in an extremely noisy environment such as that found on the motor phase current sensing. The difficulty in galvanically isolating or level shifting precision analog current shunt signals arises from the large common mode voltage, the large variability of the common mode, and the transients that are generated by the switching of the inverter transistors (IGBTs). These very large transients (equal in amplitude to the dc supply voltage) can exhibit extremely fast rates of rise (greater than 10kV/µs), making it extremely difficult to sense the current flowing through each of the motor phases. The current shunt sensing circuits must maintain gain accuracy and minimize offsets. Current sensing should exhibit stability over time and temperature, as well as high common-mode rejection (CMR). So far, a number of commercial isolation technologies have become available [6]. For galvanic isolation, optical, inductive, capacitive, or piezoelectric methods exist. For level-shifting using junction isolation, creative circuits have been developed [10]. Most of these isolation techniques require modulation and demodulation technologies. Such methods include amplitude, voltage-to-frequency, duty cycle, pulse width, and so on. The demodulation voltage ripple is usually attenuated by an output filter. 4.1 Optical Isolation Optical isolation is one way of galvanically isolating the shunt resistor current sensing signal from the load current. The isolation barrier consists of a high bandwidth LED source made of gallium-arsenide-phosphide (GaAsP) or aluminum-gallium-arsenide (AlGaAs), and a photodiode. The input signal modulates the light output from the LED source and the receiving photodiode converts the light back into current. There are both analog and digital optical isolation techniques. If the signal is passed across the barrier in its analog form, a monolithic fabrication approach is required that uses a second matched photodiode to provide feedback compensation. This approach offers real-time continuous analog signal transmission and is relatively immune to interference. Another advantage is that no modulation or demodulation ripple voltage is generated in the output. One

example is the isolation amplifier ISO100 made by Burr- Brown Corporation, shown in Fig. 4 [7]. Fig. 4. A commercially available, analog, optical isolation amplifier [7] Alternatively, the analog signal can be digitized, passed across the barrier, then converted back to an analog signal. The isolation amplifier HCPL-7800 made by Hewlett- Packard is one such device. Fig. 5 shows how the voltage drop created by phase current flowing through a currentsensing resistor would be the input digitized by a sigma delta modulator into a high speed bit stream. This digital pulse train is fed through the optical coupler and demodulated and filtered to a useable analog signal. In operation, the sigma-delta modulator, Σ, is a pulse density modulated (PDM) digital wave train, whose time average is proportional to the sampled analog voltage. This PDM signal is then further encoded and the resulting pulse train drives the LED that converts the signal to light. The pre- LED temporal encoding scheme was chosen so that any variations in LED performance do not affect the digitized data. After the light signal crosses the galvanic barrier it is detected by a photodiode and decoded (reproducing the PDM signal). Then the digital PDM signal is converted to analog format, and low-pass filtered to obtain the final output signal. The modulation and high frequencies needed for encoding and decoding limit the bandwidth of this methodolgy. Fig. 5. Block diagram of commercially available, analog-digital-analog, optical isolation amplifier using Σ modulation [8] 4.2 Inductive Isolation High frequency transformers are well-established means of providing galvanic isolation. The current sensing resistor signal modulates a high-frequency carrier and is transformercoupled from input to output. Potential disadvantages for IPEM integration include lower transient immunity due to interwinding capacitance and electromagnetic susceptibility due to packaging the transformer inside the IPEM. One example of a commercial transformer-coupled isolation design is the hybrid isolation amplifier model 3656 made by Burr-Brown Corporation, and shown in Fig. 6. The innovation that was said to make it economically feasible to put this isolation amplifier into a ceramic hybrid package is the hybrid-compatible transformer design. Fig. 6, a photograph of an uncapped isolation amplifier, shows the location of the transformer, the rest of the components (the op amps, resistors, capacitors, and diodes) and the goldplated pins. The toroidal transformer assembly is the dominant feature in the center. Its windings are made of gold rather than magnet wire, a sharp departure in construction from the state-of-the-art until now.

Fig. 6. A commercially available, transformer-coupled hybrid isolation amplifier [9] 4.3 Capacitive Isolation The signal modulates a high-frequency carrier and is capacitively coupled from input to output (see Fig. 7). Either duty-cycle or frequency modulation techniques are used, and the signal is passed differentially across the barrier. The 25 pf capacitors can be formed from elements of the IC package leadframe, reducing the overall cost. Capacitive devices have lower transient immunity performance since some fast transient common-mode pulses pass across the coupling capacitor and are not filtered out. The internal circuit is mainly divided into two segments: the high side circuit and the low side circuit. High voltage level shifter devices isolate these circuits and allow two independent reference ground potentials within the monolithic integrated circuit. The high side circuit, floats up and down from 0 to 600V with respect to the low side ground potential (COM). This high side circuit contains most of the important functions such as the high precision operational amplifier, the analog-to-digital conversion unit, and the pulse generator. There are two P-channel level shifters, which transfer the digital PWM information in the high side circuit to the low side circuit by trailing edge pulses. The low side circuit contains simple circuits such as the PWM pulse reconstruction circuit, dv/dt filter and the output buffer. Although 600V and 1200V high voltage junction isolation technologies are available, precision measurement of small analog signal under harsh environment is a significant challenge. Monolithic integration, low power consumption on the secondary, floating power supply, wide range of the secondary voltage supply, and low temperature drift are the key design issues. Fig. 7. Capacitive isolation in which the signal and control information are passed differentially across the capacitive barrier 4.4 Dynamic, Active Level Shifting Junction Isolation Dynamic, active level shifting junction isolation is an alternative to full galvanic isolation. One example of this is embedded in a commercial product, IR2171 a linear current sensing device developed by International Rectifier [10]. Fig. 8 shows a functional block diagram of the device. The design is based on a monolithic integrated circuit, fabricated with high voltage junction isolation. Fig. 8. Level Shifter Isolation: Functional Block Diagram [10]

Fig. 9 shows an application with the IR2171. In order to sense the motor phase current, the location of sensing devices is ideally in series with the motor phase leads, since they provide the current waveform of the motor fundamental frequency. The motor current feedback control can be tightly integrated. from electrical to electrical via minute levels of high frequency mechanical motion. Fig. 9 shows the principle of this transformer. The electric power is first transformed into minute levels of high frequency mechanical motion in the primary armature (motor), then these high frequency mechanical motion are converted back to electric energy in the secondary armature (generator). (a) Electric Energy Primary Armature Analogy to Motor Mechanical Energy Secondary Armature Analogy to Generator Electric Energy Fig. 9. Commercial Level Shifting Current Sensing on a Three Phase Motor Drive [10] 4.4 Piezoelectric Transforme Isolation Another possible solution for galvanic isolation is through use of an integrated piezoelectric transformer (IPT). IPTs appear to be a promising technology for low power, high power density applications. IPTs can be an attractive alternative to integrated magnetic transformers to reduce size and weight and allow integration in IPEMS with decreased problems due to cross-coupled magnetic fields. In a magnetic transformer, the coupling between the primary and the secondary windings is achieved by an electromagnetic flux linkage. In a IPT, energy is transformed (b) Fig. 10. Piezoelectric Transformer (a) and Power Conversion Analogy (b) The integrated piezoelectric transformer has the advantages of high efficiency, high degree of insulation, small size and low price. It has previously been used for isolation in converter design [11, 12]. It is seen as an attractive potential alternative isolation barrier for integrated shunt current sensors. A summary comparison for the different isolation or level shift technologies is shown in Table 3. Table III. Comparison of Alternative Isolation Technologies for IPEM Integrated, Current Shunt Sensors Isolation Voltage Rating V rms Operating Temperature Range C Isolation Barrier Bandwidth Cost Package Slew Rate Settling time kv/µs µs Opto-coupler 1000-40~+100 100 KHz Low Yes 15 5~10 0.004 Transformer coupling 2000-55~+100 30 KHz High Yes N/A 500 0.05 Non-linearity % Capacitive Coupling 1500-25~+85 50 KHz Low Yes 2 350 0.02 Lever Shifter 1200-55~+150 40KHz Med Yes 50 1.5 0.005 Piezoelectric Transformer High N/A 50MHz Very Low Yes Under Investigation V. CONCLUSIONS AND FUTURE DIRECTION Shunt resistor design and isolation technology are both investigated in this research work. The result shows that several current shunt resistor types may be suitable for IPEM integration, including thick film and etched metal foil types. The isolation technology options that may be suitable for IPEM integration include opto-couplers, magnetic and piezoelectric transformers, capacitive couplers and active junction isolation. The differing modulation/demodulation control circuits and floating power supply needs also affect the IPEM integration potential. Future work includes expanded experimental evaluation of current shunt resistors and isolation methods and their fabrication methods. Their consideration for the next generation of IPEMS must be consistent with the CPES established benchmark goals for current sensors.

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