DC Converter

9 kW SiC Mosfet based DC/DC Converter

GRZESIAK Lech M.

9 kW SiC MOSFET based DC/DC Converter Ł. J. Niewiara1, T. Tarczewski1, M. Skiwski1, L. M. Grzesiak2 1 INSTITUTE OF PHYSICS, FACULTY OF PHYSICS, ASTRONOMY AND INFORMATICS, NICOLAUS COPERNICUS UNIVERSITY Grudziadzka 5 Torun, Poland 2 INSTITUTE OF CONTROL AND INDUSTRIAL ELECTRONICS, WARSAW UNIVERSITY OF TECHNOLOGY Koszykowa 75 Warsaw, Poland Tel.: +48 / (56) – 611 24 39 E-Mail: [email protected], [email protected]

Keywords «Silicon Carbide», «DC/DC converter», «high frequency power converter», «SiC MOSFET».

Abstract In this paper a 9 kW DC/DC converter based on SiC MOSFET devices is presented. Precise control of the output voltage is realized with the help of a state feedback controller with an additional integral action. Simulation and experimental test results as well as an efficiency of DC/DC converter are presented. The behavior of the converter was investigated during no-load and variable load operation. Device efficiency was also investigated and it was found that the efficiency exceed 98% in a wide operating range.

Introduction The development of Silicon Carbide (SiC) based power devices makes it possible to reach performance that was unreachable for Silicon (Si) semiconductors technology [1,2]. These transistors have low switching losses so it is possible to increase the switching frequency of converter. In such a case reduction of passive components size and mass used in a converter is possible. The efficiency of power electronics system increase since SiC power devices are applied [3-5]. In this paper the behavior of designed 9 kW (600V, 15A) DC/DC converter is investigated. The switching frequency of power devices was set to 36 kHz. Values of the passive components were calculated from the following formulas for a switching frequency depicted above [6]: L=

Vin 4 f s Δi L

(1)

C=

iload 4 f s ΔvC

(2)

where: L –calculated value of coil inductance, Vin – input voltage value, fs – switching frequency, ΔiL – accepted coil current ripple level, C – calculated value of capacitor capacitance, iload – average value of load current, ΔvC – accepted output voltage ripple level.

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© assigned jointly to the European Power Electronics and Drives Association & the Institute of Electrical and Electronics Engineers (IEEE)

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GRZESIAK Lech M.

From (1) and (2) it can be seen that increase of switching frequency, in case of the same work conditions, reduces the value of inductance and capacitance of the passive components. In such a case it is possible to minimize mass and dimensions of the converter. The increase of switching frequency would increase energy losses. In comparison to a system with silicon carbide transistors the power losses will be much larger using silicon for the same switching frequency. For that reason mass and dimensions of passive components used in Si based DC/DC converters will always be heavy with comparison to SiC based DC/DC converters. It was assumed, that for a given for the given switching frequency, current and voltage ripple should not exceed 1.5A and 4V respectively. A state feedback controller was used to precise control of output voltage. Thanks to use of the SiC MOSFET power devices it was possible to achieve higher switching frequency and higher efficiency of the converter in comparison to Si based solution. Simulation and experimental test results of the proposed control system are presented. Simulation tests were carried out in Matlab/Simulink/PLECS environment. High performance of the device makes it possible to use it as a Voltage Matching Circuit (VMC) in drive control applications. The use of VMC enables to control DC-link voltage in voltage source inverter (VSI). Proposed solution can be used for current ripple (and, as result torque) minimization of PMSM [7].

Proposed control system Proposed control system consist of an output LC filter fed by 2 SiC MOSFET transistors in half bridge configuration. Block diagram of the considered control system is shown in Fig. 1.

Fig. 1: Proposed control system The output voltage vC is regulated with the help of a linear-quadratic regulator. The chosen controller structure has many advantages (i.e. guaranteed robustness and non-linearity tolerance) and it ensures stable system operation with load [8]. In order to design state feedback controller, the mathematical model of the DC/DC converter should be formulated in a state-space representation [9]: dx = Ax + Bu + Fr + Ed dt

(3)

where: ⎡ iL ⎤ x = ⎢⎢vC ⎥⎥ , ⎢⎣ ev ⎥⎦

⎡ R ⎢− L ⎢ 1 A=⎢ ⎢ C ⎢ 0 ⎢⎣

−

1 L

0 1 u = uC ,

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⎤ 0⎥ ⎥ 0⎥, ⎥ 0⎥ ⎥⎦

⎡Kp ⎤ ⎢ ⎥ ⎢ L ⎥ B = ⎢ 0 ⎥, ⎢ 0 ⎥ ⎢⎣ ⎥⎦ r = vCref ,

⎡0⎤ F = ⎢⎢ 0 ⎥⎥ , ⎢⎣− 1⎥⎦

⎡ 0 ⎤ ⎢ 1⎥ E = ⎢− ⎥, ⎢ C⎥ ⎣ 0 ⎦

d = iload ,

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GRZESIAK Lech M.

iL – coil current, R – coil resistance, L – coil inductance, vC – output capacitor voltage, C – capacitor capacitance, Kp – gain of the converter, uc – input voltage, vCref – reference value of the output voltage, iload – load current . The third state variable in (3) corresponds to the integral of the output voltage error: t

[

]

ev (t ) = ∫ vC (τ ) − vCref (t ) dτ 0

(4)

The additional state variable is necessary to eliminate steady-state error for step changes of reference value and load. In order to simplify synthesis process of state feedback voltage controller, the dynamics of the DC/DC converter power stage was approximated by using proportional element Kp (i.e. linear area of converter operation and omitted dead time of the power transistors are assumed). For chosen control structure the control law is described by following formula: uc (t ) = − K x (t )

where:

K = [K I

Kv

(5) Ke ]

KI – gain of the coil current path, Kv – gain of the output voltage path, Ke – gain of integral path. Gain coefficients of discrete state feedback controller were calculated with the help of Matlab lqrd function. Values of penalty matrices needed to solve Riccati equation were chosen experimentally. As a result the following controller gain coefficients were obtained (Tab. I). During synthesis process our primary goal was to achieve good dynamic response of converter output voltage as well as proper disturbance (i.e. load current) compensation. It was assumed that reference voltage signal and disturbance can be step functions. Introduction of an additional state variable (4) should eliminate steady-state error of output voltage. The realized controller structure with gain coefficients is shown below. The controller input signals are: reference voltage, measured output voltage and measured coil current.

Fig. 2. Discrete state feedback controller structure.

Table I: Controller gain coefficients values Coefficient

Value

KI

0.3257

Kv

0.0634

Ke

54.7723

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GRZESIAK Lech M.

Simulation test results Simulation tests of the DC/DC converter with state feedback controller has been investigated in a Matlab/Simulink/PLECS environment. Designed discrete controller and measurement unit were implemented in triggered subsystems in order to ensure proper generation of the control signals (Fig. 3). The current and voltage measurements are triggered by a synchronization signal in order to realize accurate measurements in a midpoint of the PWM generation. The switching frequency fs was set to 36 kHz.

Fig. 3: Schematic diagram of the designed control structure - simulation model The behavior of designed DC/DC converter obtained in a simulation study environment is shown in Fig. 4-6. It can be seen that proposed control structure provide proper control of coil current and output capacitor voltage. Fig. 4 shows the input signal (duty ratio 0.6), output voltage and coil current in a steady-state (3.3A). Output voltage is stable and the current ripple does not exceed 300 mA.

Fig. 4. Simulation results - behavior of the DC\DC converter in a steady-state Fig. 5 shows step response of an output voltage and coil current waveform in a case of no load operation.

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GRZESIAK Lech M.

Fig. 5. Simulation results - DC\DC converter behavior for reference voltage (no load operation) Fig. 5 shows the output voltage behavior in a case of variable load operation (at t = 18ms load increases from 0 A to 1.7 A, at t = 28ms load increases from 1.7 A to 4.7 A, at t = 50ms load decreases from 3A to 1.6 A). It should be mentioned that the designed control system properly maintain the level of the DC voltage.

Fig. 6. Simulation results - output voltage behavior for variable load and reference voltage

Experimental test results In this section experimental test results obtained for designed and built DC/DC converter (Fig. 7) were presented. As a power devices SiC MOSFET transistors (C2M0080120D) [10] and SiC shottky diodes (C4D10120A) [11] were used. In order to realize coil current and capacitor voltage measurements LEM sensors (i.e. LEM LV25-P for voltage and LEM LTS15-NP for current) were used. Passive components of the output LC filter were chosen as a compromise between minimization of current and voltage ripple and minimization of mass and dimensions of the converter. The 3 mH coil with amorphous core and 30 µF foil (EPCOS B3292 X2 MKP/SH) capacitors were chosen. Input capacitance is 3,8 mF. The designed control structure was implemented in dSpace DS1104 R&D controller board. Experimental tests were performed for a input voltage equal to 120 V. Because of limitation of the DS1104 controller board, the maximum possible switching frequency was 36 kHz.

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GRZESIAK Lech M.

The power resistors (60Ω/200W and 33Ω/400W) connected in parallel were used as a load circuit with variable resistance. Step change of load value was realized with the help of electrically controlled relays connected to dSpace card.

Fig. 7: DC\DC converter Experimental results show that the output voltage in steady-state is stable (Fig. 8) and the coil current ripple pk-pk value is less than 500 mA (about 300mA).

Fig. 8. Experimental results – behavior of the DC\DC converter in steady state The converter behavior in a case of no load operation is shown in Fig. 9. It should be mentioned that experimental results match the simulation one pretty well. The output voltage reaches the reference value without steady-state error and the rise time of the system is about 5ms. Coil current waveform is also similar to simulation results. The system is able to increase and decrease the output voltage level.

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GRZESIAK Lech M.

Fig. 9. Experimental results – DC\DC converter behavior in no load operation It can be seen that in for load variations (at t = 18ms load increases from 0 A to 1.7 A, at t = 28ms load increases from 1.7 A to 4.7 A, at t = 50ms load decreases from 3A to 1.6 A) designed control system properly maintain output voltage of the converter. There is no steady-state error in case of load current changes and reference voltage changes.

Fig. 10. Experimental results – output voltage behavior with variable load and reference voltage

Fig. 11. Experimental results – output voltage behavior during load changes

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GRZESIAK Lech M.

Fig 11. shows output voltage waveform for reference voltage during step changes of load current. There is no error in steady – state. Proposed control algorithm maintains output voltage at desired level independently of load current changes. The converter efficiency was also investigated. Efficiency was measured by using Tektronix TPS 2024B oscilloscope with Tektronix A622 AC/DC current probe and Tektronix P5122 voltage probe. TPS2PWR1 Power Analysis Application software was also used. Obtained converter efficiency versus duty of PWM was presented in Fig. 12. It can be seen that, thanks to SiC MOSFET power devices, efficiency of the designed converter is high (> 98%) in a wide operating range (0.15 – 0.95 of PWM duty). The accuracy of efficiency measurement doesn’t exceed 0.2 % of measured value (relative error). Using of the SiC MOSFET power devices causes that achieving high efficiency and for a high switching frequency is possible.

Fig. 12: Converter efficiency vs. duty

Conclusion In this paper a DC/DC converter with a state feedback controller was presented. Simulation and experimental test results confirm proper operation of the designed control system. Similar simulation

and experimental results proves that, for controller synthesis, the dynamics of the DC/DC converter power stage can be approximated by using proportional element. Controlled voltage in steady-state is stable and equal to the reference value. Simulation test results correspond to the experimental results. Since SiC MOSFET power devices are used, high efficiency for a high switching frequency could be achieved. Size and mass of a DC/DC can be reduced. Designed DC/DC converter will be used as a voltage matching circuit for PMSM drive fed by VSI in the future.

References [1] Biela J. Schweizer M. Waffler S. Kolar J. W.: SiC versus Si – Evaluation of Potentials for Performance Improvement of Inverter and DC-DC Converter Systems by SiC Power Semiconductors , IEEE Trans. On Ind. Electronics, Vol. 58 no 7, pp. 2872- 2882 [2] Kondrath N. Kazimierczuk M. K.: Characteristics and Applications of Silicon Carbide Power Devices in Power Electronics, International Journal of Electronics and Telecommunications, Vol. 56 no 3, pp. 231 – 236 [3] Zdanowski M. Rąbkowski J. Barlik R.: Three-phase, two-level voltage source inverter with SiC Z-FETs (in Polish), Przegląd elektrotechniczny, vol. 11/2014, pp. 104-107 [4] Colmenares J. Peftitsis D. Tolstoy G. Sadik D. Nee H.P. Rabkowski J.: High-efficiency three-phase inverter with SiC MOSFET power modules for motor-drive applications, Energy Conversion Congress and Exposition (ECCE), pp. 468-474

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[5] Oswald N. Anthony P., McNeil N., Stark B. H.: 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 Trans. on Power Electronics, vol. 29, issue 5, pp. 2393 – 2407 [6] Niewiara L., Tarczewski T., Grzesiak L. M.: 3-phase bridge voltage source inverter with DC voltage control (in Polish), Przeglad elektrotechniczny, R. 90, no. 6/2014, pp. 109-114 [7] Tarczewski T. Niewiara L. Grzesiak L. M.: Torque ripple minimization for PMSM using voltage matching circuit and neural network based adaptive state feedback control, 16th European Conference on Power Electronics and Applications (EPE'14-ECCE Europe), pp. 1-10 [8] Safonov M.G., Athans M.: Gain and phase margin for multiloop LQG regulators, IEEE Trans. Autom. Control, vol. 22,no 2, pp. 173-179 [9] Forsyth A.J. Mollov S.V.: Modeling and control of DC\DC Converters, Power Engineering Journal, vol. 12,no.5, pp. 229–236. [10] CREE C2M0080120D data sheet available on the Internet at http: \\ www.cree.com \ [11] CREE C4D10120A data sheet available on the Internet at http: \\ www.cree.com \

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