Electromagnetic interference issues of power, electronics systems with ...

Electromagnetic Interference Issues of Power Electronics Systems with Wide Band Gap Semiconductor Devices Firuz Zare, Dinesh Kumar, Marian Lungeanu and Aupke Andreas Global Research & Development Center Danfoss Drives A/S Gråsten, Denmark [email protected] Abstract— In this paper electromagnetic interference issues of power electronics systems with Wide Band Gap devices have been studied. The main advantages of these power switches are fast switching transient and high switching frequency capabilities which can improve efficiency of power electronics systems and reduce sizes of passive elements such as inductors and capacitors. Several conducted and radiated emission tests and efficiency measurement have been carried out to analyze the switching frequency and transient effects on harmonic emission and switching losses. The results have been compared with the same drive configuration, layout and enclosure but with conventional power switches (Si switches).

beginning stage of a design in order to optimize the power electronics system. In the next sections, Pros and Cons of WBG devices used in the following different power electronics applications will be discussed with respect to size, efficiency and EMI emissions. • DC-DC converters and AC-DC converters such as switch mode power supplies and grid connected renewable energy systems with passive filters • DC-AC converters used in motor drive systems.

Keywords—electromagnetic interference, wide band semiconuctor devices, conducted emission, radiated emission

In power electronics systems there are two switching factors which should be considered for EMI noise prediction. The first one is the effect of switching frequency which mainly corresponds to current and voltage ripples and the second one is the effect of switching transient which can generate significant leakage current and over voltages due to (stray capacitance and dv/dt) and (stray inductance & di/dt), respectively. When the switching frequency is constant, fast switching transients (ton and toff) of WBG devices can reduce switching losses but on the other hand can increase EMI noise emission. In order to analyze this issue and factorize these parameters with respect to the frequency ranges, the following analysis has been considered for an inductor used in a Boost converter. This concept can be applied to different power electronics converters including a motor drive with an AC motor and a cable. As shown in Fig.1 (a), a high frequency model of an inductor can be considered as a combination of inductors, resistors and capacitors. The test and simulation results of the inductor magnitude and its phase angle over a frequency range of 1kHz to 30 MHz is shown in Fig.1 (b) and (c). A generic analysis of power converters shows that increasing the switching frequency can decrease the passive filter size for a same ripple current or voltage magnitude. When a pulse width modulated voltage is applied across an inductor in boost converter as shown in Fig. 2(a), a ripple current and a leakage current are generated. The current magnitudes and shapes depend on all capacitive and magnetic couplings, the voltage transient behavior during turn-on and turn-off times

gap

I. INTRODUCTION For several decades, Silicon (Si) based power electronics switches have been used in many different low and high power electronics systems. New developments in semiconductors have allowed Si semiconductor technology to approach the theoretical limits of the Si material but these switches cannot fulfil new requirements such as operating at higher switching frequencies with fast switching transients. To overcome these limitations, new semiconductor materials for power device applications are needed. Over the past two decades, significant efforts and development have been performed on new semiconductor materials called Wide Band Gap (WBG) semiconductors, such as Silicon Carbide (SiC), Gallium Nitride (GaN) due to their superior electrical and thermal performances compared to silicon power switching devices [1]-[4]. Conducted and radiated Electromagnetic Interference (EMI) emissions are major problems in power electronics systems which produce undesirable effects on electronic devices such as radio receivers, medical equipment, communication systems and cause malfunctions and nonoperations in control systems [5, 6]. A main drawback of these fast switching devices is EMI issue due to stray capacitances and inductances in interconnections, cables and AC motors. In modern power electronics systems, increasing power density and decreasing cost and size of a system are market requirements [7]-[10]. Switching losses, harmonics and EMI are the key design factors which should be considered at a

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II. ANALYSIS OF HARMONIC EMISSIONS

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(a)

(a)

Inductance (mH)

105

(b) Fig.2. (a) A boost converter with a high frequency inductor, (b) current waveforms

104

leakage current due to the capacitive coupling of the windings and the switching transients (dv/dt). If the ripple current magnitude is kept constant, its spectral content is shifted to higher or lower frequency depending on the switching frequency. Thus high frequency conducted or radiated emission may be a challenging issue for modern power converters with WBG devices due to their high switching frequency operation. The second issue is the leakage current. This current is generated whenever a switch is turned on or off. As shown in Fig.2 (b), the number of these spikes depends of the switching frequency. The magnitude of the leakage current depends on the switching transient times. To simplify this analysis, two different leakage currents based on a rectangular waveform have been considered. The peak of the leakage current through a capacitor is calculated based on ic=Cstray dv/dt. Assuming the switching transients of SiC power switches are four times faster than Si power switches, the leakage currents can be calculated as follows: Si Power Switches:

103

102

10 6

10 7

Frequency (Hz) (b)

dv=Vdc and dt=t1 then, iSi=C Vdc/t1 SiC Power Switches: 105

dv=Vdc and dt=t1/4 (4 times faster than Si) then, iSiC = C Vdc/(t1/4) = 4iSi

107

(c) Fig.1. (a) A high frequency model of an inductor; test and simulation results: (b) magnitude, (c) phase

Where Vdc is the switched DC-link voltage.

and also the switching frequency. These two current waveforms are shown in Fig.2 (b). There are two main currents which need to be analyzed. The first one is the ripple current due to the switching frequency and its inductance value and the second one is the

Considering a constant switching frequency for both cases, the shapes of the leakage currents are shown in Fig. 3(a) and it is clear that the pulse width of the iSiC is 4 times shorter than iSi (t1=4t2) but its peak is 4 time higher in magnitude. The rms valuesof these two signals are isi = I1 t1 and

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(a) I Ga

(a)

Rs

Ls

R

irect

L

+

I Gb

Cdc

I Gc

−

Cf

S a Sb S c

Rf

(b)

SiC Si

Frequency (0Ͳ30MHz)

(b) Fig.3. Modeling of leakage currents as two pulse patterns (a) time domain, (b) frequency domain

(c)

t isic = 4 I1 1 = 2 I1 t1 = 2isi . The spectral contents of these 4 two signals show (Fig. 3(b)) that the high frequency energy of the leakage current generated by the SiC devices is higher than the Si switches. It is also clear that WBG devices (SiC) can operate at higher switching frequency and with fast switching transient which can generate more high frequency noise than the Si switches. III. IMPACT OF WBG DEVICES ON PASSIVE COMPONENTS Using WBG devices operating at high switching frequencies can reduce the sizes of passive components inductors and capacitors - used in power electronics systems and make it possible to have a very compact power electronics system. The main advantages of WBG devices in these systems are improving the efficiency and reducing the size of the passive filters. A main drawback can be conducted and radiated emission issues which need to be investigated. In some topologies such as DC-DC converters and Active Front End systems (Fig.4 (a)&(b)), the stray capacitance of the inductor can generate significant leakage current due to dv/dt across the inductor. WBG devices used in these applications can increase switching frequency and decrease switching

Fig.4. Power electronics systems- (a) a single phase PFC system, (b) an Active Front End and (c) a motor drive system with stay capacitances connected to common ground (Protective Earth, PE)

transient times which have a big influence on leakage current and overvoltage noise levels. On the other hand the size of these passive components such as an inductor can be reduced which means the size reduction of the core and the windings, consequently its stray capacitance (Cstray) can also be decreased as well. This parameter and factor depends on the geometry of the passive component and the inductor is optimized with respect to high frequency performances. Therefore it is expected that the peak of the leakage current (i=Cstray dv/dt) is not increased proportionally to the dv/dt change. The other factor which can affect the high frequency noise emission of the converter with WBG devices is the high number of leakage current pulses. This will increase the energy level of the noise compare to Si switches operating at lower switching frequency (keeping the same current ripple). As shown in Fig.4(c), a voltage source converter used in motor drive applications consists of a DC-link capacitor, an inverter connected to an AC motor through a cable. Normally it does not have an output filter at the output side of the inverter except in some applications to suppress common mode voltage and/or over-voltages at the motor terminals. Trend in increasing switching frequency improves the quality of current waveforms in motor drive systems but due to fast switching

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transient, a high dv/dt is produced across a motor terminal and a cable - between the inverter and the motor - which can generate significant leakage current. A common mode voltage generated by pulse width modulated voltage creates shaft voltage through electrostatic couplings between the rotor and the stator windings and between the rotor and the frame. This can cause bearing currents when the shaft voltage exceeds a breakdown voltage level of the bearing grease. One of the main advantages of WBG devices is to decrease the size of passive components which cannot be generalized for motor drive applications as the size of the AC motor cannot be decreased when the switching frequency is increased. However this advantage can be utilized in motor drive applications if an output filter is used between the inverter and an AC motor. Increasing the switching frequency can reduce the size of the output filter and consequently improving the shaft voltage and conducted emission noise effects.

(a)

IV. TESTS RESULTS Based on the above analysis it is possible to perform several different tests for those different applications. Therefore to analyze the effects of fast switching devices on EMI noise emission, an 18.5 kW motor drive has been selected and the drive has been prepared with two different power inverter modules (Si and SiC). The rest of the system including PCB layout, interconnection, modulation pattern, control systems, load and cables are kept the same. The two different power inverter modules and the drive under the tests are shown in Fig.5. The switching transients of the SiC switches are almost 4 times faster than the Si power switches. The complete setup is shown in Fig. 5(a) when both conducted emission and also common mode radiated emission have been captured and measured. The system under the test has the following parameters and conditions: • • • •

(b)

Motor data: 18.5kW, 50Hz, 1470rpm, 400V, 35A, Cosij=0.84 Motor cable: 100m; no dv/dt filter at the output of the drive Efficiency measured with variation of motor speed, load and switching frequency (fsw) Yokogawa WT1800 power analyzer used to perform the power measurement

The conducted emission tests have been performed and the quasi peak values of conducted emission noises generated by these two drives and with different power inverter modules (Si and SiC) are compared as shown in Fig.6. The both drives have been tested at different switching frequencies (4 kHz, 8 kHz and 16 kHz) and the test results of these power drives are compared for the same switching frequency as shown in Fig.6.(a, b and c). As discussed in the above section, the low order harmonics are not changed significantly while the high frequency noise is increased for the drive with SiC power switches. As the switching frequency and the motor parameters are the same for the drives, therefore the ripple currents and their spectral contents are almost the same. However the significant leakage current generation of the drive with WBG device can be observed in the test results compare to the drive

(c) Fig.5. (a) A setup for the EMI measurement, (b) motor drive, (c) power inverter modules SiC and Si

with Si power switches. In order to evaluated noise level at higher frequency ranges (above 30 MHz), a common mode probe has been place at the input side of the drive to measure the magnetic field and the results are shown in Fig. 6(d) and the results show that the common mode noise generated by the SiC switches are much higher than the Si switches.

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different power levels and at three different switching frequencies (4 kHz, 8 kHz and 16 kHz). As can be seen from the graph, at fsw=16 kHz, the efficiency of the drive with SiC is increased around 3% for a broad range of power (92% to 95.2% at low power and 93.5% to 96.2% at high power). As the fast switching transient of WBG devices can reduce switching losses, therefore the efficiency improvement can be significant at higher frequency (fsw=16 kHz compare to fsw=4 kHz). 99,0% 98,0%

(a)

97,0% ] 96,0% [% cy n95,0% e ic fif E 94,0% 93,0%

SiC MOSFET inv. @ fsw=4kHz, 1125rpm SiC MOSFET inv. @ fsw=8kHz, 1125rpm

92,0%

SiC MOSFET inv. @ fsw=16kHz, 1125rpm 91,0% 6600

8600

10600 Power [W]

12600

14600

16600

(a) (b)

98,0% 97,0%

Efficiency

96,0% ] % [ 95,0% cy n e i icf 94,0% fE 93,0% Si IGBT inv. @ fsw=4kHz, 1125rpm Si IGBT inv. @ fsw=8kHz, 1125rpm

92,0%

Si IGBT inv. @ fsw=16kHz, 1125rpm 91,0% 6600

8600

10600 Power [W]

12600

14600

16600

(b) Fig.7. Motor drive efficiency at different power and switching frequencies (a) Si, (b) SiC

(c)

V. CONCLUSION

(d) Fig.6. Black graph: SiC (dv/dt = 9-11 V/ns), Light blue graph: Si (dv/dt = 2-4 V/ns): (a) fsw = 4 kHz, (b) fsw = 8 kHz, (c) fsw = 16 kHz, (d) fsw =16 kHz, (d) radiated emission noise (Blue: SiC & Yellow: Si)

Another advantage of the WBG devices is to improve efficiency of power electronics systems. Fig.7 shows the efficiency of these drives with Si and SiC power switches at

The main advantages of the WBG power switches are fast switching transient and high switching frequency capabilities which can improve efficiency of power electronics systems (less switching loss) and reduce the size of passive filters. As the switching frequency can be increased by WBG devices, the spectral contents of the noise are shifted to higher frequency. The noise magnitude depends on the topology and applications. In Active Front End and DC-DC converters, the size of passive filters can be reduced if the switching frequency is increased. This can keep the current or voltage ripple constant. Therefore the impact of switching transient (di/dt and dv/dt) on overvoltage and leakage current generation is reduced as the stray inductance and capacitance of the layout and passive components are reduced due to small passive components and compactness of the layout and the system.

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This conclusion is not valid for a motor drive system with WBG devices (SiC) as the size of the motor cannot be decreased as the switching frequency is increased. The main advantages of WBG devices cannot be fully utilized for motor drive applications as the high dv/dt can affect motor terminals and winding insulators. If an output filter is used in motor drive application, then the filter size can be reduced with WBG devices. Conducted and radiated emission tests have been carried out for an 18.5 kW motor drive system with two different power inverter modules (SiC and Si switches) and at different switching frequencies. The analysis and test results show that the WBG devices used in the 18.5 kW motor drive can improve efficiency of the system up to 3% for a broad range of load power and at fsw=16 kHz compare to the same drive but with Si power switches. On the other hand, conducted and radiated emission noises generated by the drive with WBG devices is increased up to 15 dBμV for the frequency range of 10 MHz and above. VI. ACKNOWLEDGMENT The research work presented in this paper is part of the Intelligent Efficient Power Electronics (IEPE) research project. REFERENCES [1] [2]

Millan, J. “A review of WBG power semiconductor devices”, Semiconductor International Conference (CAS), Oct 2012, pp 57-66. L.G. Rodriguez, et al, “Design of a GaN-based micro-inverter for photovoltaic systems,” IEEE power electronics for distributed

generation systems conference (PEDG), pp. 1-6, June 2014, Galway, Ireland. [3] H. Zhang, and L. M. Tolbert, “Efficiency impact of silicon carbide power electronics for modern wind turbine full scale frequency converter,” IEEE Trans. on industrial. electronics, vol. 58, no. 1, Jan. 2011. [4] P. G. Neudeck, R. S. Okojie and L-YU Chen, “High-Temperature Electronics-A Role for Wide Bandgap Semiconductors”, in Proc. IEEE, 2002, Vol. 90, pp. 1065-1076. [5] Jafar Adabi, Firuz Zare, Gerard Ledwich, Arindam Ghosh, “Leakage current and common mode voltage issues in modern AC drive systems”, Australasian Universities Power Engineering Conference, AUPEC 2007. [6] Scott, M.J., Brockman, J. , Boxue Hu, Lixing Fu, “Reflected wave phenomenon in motor drive systems using wide bandgap devices”, IEEE Workshop on Wide Bandgap Power Devices and Applications, 2014, pp 164-168. [7] N. Oswald, P. Anthony, N. McNeill and B.H. Stark, “An Experimental Investigation of the Tradeoff between Switching Losses and EMI Generation with Hard-Switched All-Si, Si-SiC, and All-SiC Device Combinations”, IEEE Transactions on Power Electronics, 2014, Vol. 29, pp.2393-2407. [8] J. He, T. Zhao, X. Jing and N.A.O. Demerdash, “Application of Wide Bandgap Devices in Renewable Energy Systems.Benefits and Challenges”, in Proc. Of IEEE International Conference on Renewable Energy Research and Applications (ICRERA), 2014, Milwaukee, WI. [9] X. Gong, J.A.Ferreira, “Comparison and Reduction of Conducted EMI in SiC JFET and Si IGBT-Based Moto Drives”, IEEE Transactions on Power Electronics, 2014, Vol. 29, pp. 1757-1767. [10] Mantooth, H.A.; Glover, M.D.; Shepherd, P. "Wide Bandgap Technologies and Their Implications on Miniaturizing Power Electronic Systems", IEEE Journal on Emerging and Selected Topics in Power Electronics, 2014, Vol. 2, pp. 374 - 385

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