FPGA Based Control of Series Resonant Converter for ... - IEEE Xplore

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for Contactless Power Supply by Artur J. Moradewicz* and Marian P. Kazmierkowski, Fellow IEEE**. *Electrotechnical Institute IEl, ul. M. PoŜaryskiego 28 ...
FPGA Based Control of Series Resonant Converter for Contactless Power Supply *

by Artur J. Moradewicz* and Marian P. Kazmierkowski, Fellow IEEE** Electrotechnical Institute IEl, ul. M. PoŜaryskiego 28; 04-703 Warsaw, Poland, [email protected] ** Warsaw University of Technology, ul. Koszykowa 75, 00-662 Warsaw, Poland, [email protected]

Abstract—A novel inductive Contactless Energy Transmission (CET) system is presented in this work. The energy is transmitted using rotatable transformer with adjustable air gap and power converter. To minimize total losses of the system a series resonant circuit is applied. This ensures zero switching condition for MOSFET power transistors. The expression and analytical results of the transfer dc voltage gain are shown in this paper. The novelty of the system lies in application of FPGA based controller and protection system. The resonant frequency is adjusted by extreme regulator which follows instantaneous value of primary peak current. Some simulated and experimental results which illustrate operation of developed 3 kW laboratory model are presented. Keywords—contactless transmission of electrical energy, series resonance converter, FPGA controls.

I. INTRODUCTION The contactless energy transmission (CET) systems are recently developed and investigated widely in [2, 4-7]. This innovative technology creates new possibilities to supply mobile devices with electrical energy because elimination of cables and/or slip-rings increases reliability and maintenancefree operation of such critical systems as in aerospace, biomedical and robotics applications. The core of CET system is inductive or capacitive coupling and high switching frequency converter. The capacitive coupling is used in low power range (sensor supply systems) whereas inductive coupling allows transferring power from a few mW up to hundred kW [5]. The aim of this paper is to report on a new developed inductive CET system with a rotatable transformer and MOSFET based resonant converter. The novelty of the system lies in FPGA implementation of extreme regulator for resonant converter control and protection circuit. Simulation and experimental results of 3kW prototype system operated with 60 kHz switching frequency are presented. II. CONFIGURATION OF CONTACTLESS ENERGY TRANSMISSION (CET) SYSTEM.

rotating transformer with air gap which can be adjusted up to 2.8 cm large. At the energy feeding input end is a full bridge MOSFET converter and at the secondary side an electronic consumer module with bridge diode rectifier is connected. This solution has following advantages: secondary circuits can be movable relative to primary, control and power supply system is located on the primary side and is electrically separated from the secondary circuit. In conventional applications a transformer is use for galvanic insulation between source and load, and its operation is based on high magnetic coupling coefficient between primary and secondary windings. Because of used two halves cores and air gap, CET transformers operate under much lower magnetic coupling factor. As result the main inductance L12 is very small whereas leakage inductances (L11, L22) are large as compare with conventional transformers. Consequently, the magnetizing current increase causes higher conducting losses. Also, winding losses increase because of large leakage inductances. Another disadvantage of transformers with relatively large gap is EMC problem (strong radiation). To minimize the above disadvantages of CET transformers several power conversion topologies have been proposed which can be classified in following categories: the flyback, resonant, quasi-resonant and self-resonant [1]. The common for all these topologies is that they all utilize the energy stored in the transformer. In this work resonant soft switching technique has been used. To form resonant circuits two methods of leakage inductances compensation can be applied: S-series or P-parallel giving four basic topologies: SS, SP, PS, and PP (first letter denotes primary and second a secondary compensation). PS and PP require an additional series inductor to regulate the inverter current flowing into the parallel resonant tank. This additional inductor increase EMC distortion and total cost of CET system. Therefore, only SS and SP topology has been considered. If we assume the same numbers of primary and secondary winding n1 = n2, the inductances of presented transformer can be described as follows: L1 = L11 + L12 L1 = L11 + L12

The configuration of the presented experimental inductive CET system is shown in Fig. 1. The core of the system is a

978-1-4244-1666-0/08/$25.00 '2008 IEEE

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L2 = L22 + n 2 L12 ⇒ L2 = L22 + L12 M = nL12

M = L12

(1)

T3

T1

i 2 (t )

230 V 50 Hz

Ez

i1 (t )

C1

u1 (t )

L 11

C r1

L 22

im (t )

L 12

Voltage sensor

Ro

C r2 u 2 (t )

Co

i 2 (t ) Current sensor

T4

T2

I0

N2

N1

TR 1

Rotatable Transformer with air gap

CONTROL FPGA MOSFET DRIVER

Stratix II EP2S60F1020C3ES

Fig. 1. Configuration of Contactless Energy Transmission (CET) System.

where:

1− k M, k = M /L (2) k If the fundamental component of u2 (t) is in phase with i2 (t), the output rectifier with capacitive filter behaves as load resistance transformer. The value of this resistance is equal to:

Z R GV = B e Z A ZC

L1 = L2 = L, L11 = L22 = L − M =

Re =

8

π

2

Ro = 0.8106 ⋅ Ro

From equation (3) to (11), the resulting equation of the transfer gain GV express as:

(3)

 X1 ⋅ X 2   2  X1 + X 2 +  Xm X  GV = 1 + 1  +    Xm  Re    

Impedance of secondary side in case of chosen compensation topology is: -for series compensation: Z C = Re + jX 2 (4) -for parallel compensation:

 1 Z C =  jωL11 + 1 /( jω ⋅ C r 2 + ) (5) R e  The equations for component impedance and reactance, shown at various points in fig.1 can be written as: jX m ⋅Zγ ZB = (6) jX m + Zγ Z A = jX 1 + Z β

(7)

1 X 1 = ω s L11 − ω s Cr 1

(8)

1 X 2 = ω s L22 − ω s Cr 2

X m = ωs M where ωs = 2πfs – operation inverter frequency.

(9) (10)

The transfer gain of voltage CET system for SS compensation topology in Fig.1 is:

(11)

     

1 2 − 2

    

(12)

From equation (12) follows that GV is unity at compensated frequency, even though the leakage inductances of the rotating transformer in CET system are very large. Where ω0 = 2πf0 – resonance frequency – (compensated frequency), is derived for condition X1 = X2 = 0

ω o = 1 / Lr C r = 1 / L11C R1 = 1 / L22C R 2 (13) Based on equations (7, 8 and 13) the expression for X1 and X2 can be rewritten by:  1 X 1 = ω s L11  1−  ω2

   

 1 X 2 = ω s L 22  1−  ω2

(14)    

(15)

where: ω =ω s / ω o (16) Because of used two halves cores and air gap, CET transformers operate under lower and changing magnetic coupling factor k. Also, if the coupling factor varies during the work, rewriting the equation (12) as a function of k is desirable. From (12) to (16) the voltage gain can be express as

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2

 1−k  1  GV = 1+ 1−  k  ω2  

The above equations are illustrated in Fig. 3.

2

  1  1−k  1  +Qacω − 1+ 1− 2    ω  2k  ω 

(17)

Cr1 = result ⋅ Cr 2

10

where circuit quality factor for SS compensation topology is: ω (L11 + L22 ) ωLr = (18) Qac = Re Re

9 8 7 6

a)

5 4 3 2

example: for k = 0.7 C r1 ≈ 2Cr 2

SP

SS

1

Magnetic coupling coefficient - k

0

0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 Fig. 3. Primary capacitance for SS and SP compensation as function of coupling coefficient.

1

III. FPGA BASE CONTROL SCHEME The block diagram of CET control and protection system implemented in FPGA Stratix II is shown in Fig. 5. The FPGA’s clock frequency is 100 MHz and the resonant switching frequency is 60 kHz. The primary capacitor voltage Ucr1 and inverter output current i1 are measured and sent to A/D converters via operation amplifiers. A 12-b A/D converter, AD9433 is used in the designed system. The typical analogy input signal is 5V (VDD), hence maximum of amplitude correspond to 2.5V. The FPGA device is Stratix II EP2S60F1020C3ES.

i1'

i1

uCR1 Fig. 2. The analytical results of transfer dc voltage gain in CET system for SS topology compensation. a) k = 0.2, b) k = 0.6

MEASUREMENTS

b)

uc’

Main blocks of control algorithm in VHDL i1f

FILTER

ucf

(FR)

i1f

EXTREME REGULATOR (ER)

ucf

fref stop

PROTECTION

SIGNALS GENERATOR

(PR)

(SG)

T1 - T2 T3 - T4

FPGA Stratix II EP2S60F1020C3ES

The analytical results of the voltage gain based on equation (17) are illustrated in Fig. 2. The calculations were executed versus angular frequencyω, two values coupling coefficient and various Qac. The required resonant capacitors values for desired resonant frequency can be expressed as follows: - for series secondary compensation

Cr 1 =

L22 Cr 2 = Cr 2 L11 = L22 L11

(19)

- for parallel secondary compensation

Cr1 =

L222 ⋅ Cr 2 1 = = Cr 2 (20) 2 L11= L22 1− k 2 L11 ⋅ L22 − (k ⋅ L11 ⋅ L22 )

Fig. 4. Block diagram of the CET control and protection system.

To attenuate noise in input signals (i1’, uc’) from measurements block (Fig.5) a digital recursive filtering algorithm has been applied. These filtered signals i1f and ucf are used in extreme regulator (ER). The ER is based on reversible counter which determinates converter switching frequency fref. Signal fref is delivered to signal generator (SG) which generates gate pulses for MOSFET power transistors T1…T4. Also, dead time compensation is implemented in this block (SG). To guarantee stabile operations, the regulator ER in every N-period sequence searches the highest current amplitude i1fm and, after comparison with previous data, adjusts reference value of

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switching frequency fref. Additionally, the filtered input signals i1f and ucf are delivered to protection block (PR) which continuously watch up and in case when limit values i1f(lim) and ucf(lim) are achieved, blocks gate pulses T1 …T4. To limit current during the converter start-up, regulator (ER) sets the switching frequency higher as resonance frequency fref > fo and then in every N-period sequence reduces fref to follow the maximum of current amplitude. If the voltage and/or current amplitude reaches nominal value i1f (N) < i1f (lim), ucf (N) < ucf (lim), the regulator (ER) increases converter switching frequency. This results in increase of circuit impedance and as consequence current and capacitor voltage will be reduced. The period N of regulator operation has been selected experimentally N=7.

available in PSpice library International-Rectifier transistors model (IRGPC50S), whereas in the laboratory prototype Semikron MOSFET power transistors (SKM120B020) have been mounted).

IV. RESULTS To verify and test performance of developed CET system, PSpice simulation model as well as 3 kW laboratory set-up has been constructed (Fig. 5). The basic parameter of rotatable transformer and resonant circuit are given in Table 1 and Fig. 7. Figures 8 to 11 show simulated and experimental oscillograms of basic waveforms in the resonant converter for steady-state operation. Note that power MOSFET transistors operate at Zero Current Switching (ZCS) conditions. This reduces switching losses considerable and increase overall system efficiency.

Fig. 6. Total efficiency versus air gap width and load resistance for SS compensation method, (simulated results - dashed/dotted line).

Note that for higher load resistances higher efficiency can be achieved, but the circuit becomes more sensitive for changing of magnetic coupling coefficient.

Fig. 7. Measured self inductance primary and secondary winding of rotating transformer in laboratory model. Fig. 5. Laboratory setup of 3kW contactless power supply system with rotatable transformer and series resonant MOSFET inverter

In practical applications, however, converter switch near to zero current at NZCS conditions because of dead time needed to protect inverter leg against short circuit or in case of small quality circuit factor, look at the Fig. 8c. Figure 5 shows total input-output efficiency of the CET system versus transformer air gap width and the load for laboratory and simulation model. Figure 6 shows total input-output efficiency of experimental set-up versus transformer air gap width and system. The efficiency curves are identical in form (Fig.5), however, the slight difference between simulation and experimental values is result of different transistor models (in simulation we used

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TABLE I PARAMETERS OF ROTATING TRANSFORMER AND RESONANT CIRCUIT Parameter Value Unit N1 N2 L11 L12 L22 M k CR1 CR2 Adjustable air gap

32 32 166,5 203,5 166,5 203,5 0,55 63 63 10,5

coils coils µH µH µH µH nF nF mm

Simulated

Experimental u1 (t )

u (t ) 1

i1 ( t )

i1 ( t )

i2 (t )

i

u 2 (t )

(t )

2

u 2 (t ) Fig. 8. Steady-state waveforms of the voltages u1, u2, and currents i1, i2. Converter operation with resonance frequency. (5µs/div, 50V/div, 4A/div)

u1 (t )

u (t ) 1

i1 ( t )

i (t ) 1

i2 (t ) i2 (t )

u (t ) 2

u

2

(t )

Fig. 9. Steady-state waveforms of the voltages u1, u2, and currents i1, i2. Converter operation below resonance frequency. (5µs/div, 50V/div, 4A/div)

u1 ( t )

u1 (t )

i1 ( t )

i1 ( t )

i2 ( t )

i2 (t )

u 2 (t )

u

2

(t )

Fig. 10. Steady-state waveforms of the voltages u1, u2, and currents i1, i2. Converter operation above resonance frequency. (5µs/div, 50V/div, 4A/div)

V. SUMMARY AND CONCLUSION This paper presents a novel Contactless Power Supply (CPS) system. The core of the system is inductive Contactless Energy Transmission (CET) with rotatable transformer and series resonant inverter operating at 60 kHz switching frequency. The

control and the protection system have been implemented in FPGA Stratix II EP2S60F family. To compensate high leakage inductance of the rotatable transformer with large air gap and converter switching losses, the series resonant capacitive circuit has been used. The resonance frequency is adjusted by extreme

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regulator which follows the instantaneous value of primary peak current and guaranties zero current switching (ZVS) conditions for power MOSFET transistors of the inverter. Theoretically, there is no power transfer limit, even with low coupling coefficient, if the circuit operates with the resonance frequency of secondary current, with compensated primary winding conditions, and resonant frequency of primary current is equal to secondary. From (19) it can be concluded that in SS compensation topology both resonant capacitances are equal, if L11 = L22. The design procedure has been verified by simulation and experimental results measured on the 3 kW laboratory setup. The total efficiency achieves 0,93 – 0,85 for rotatable transformer air gap 0.1 up to 2.8 cm, respectively. REFERENCES [1] [2]

[3]

[4]

[5]

[6] [7]

[8]

W. Erickson: Fundamentals of Power Electronics, Kluwer Academic Publisher, 1999 A. Moradewicz: Study of Wireless Energy Transmission Systems Using Inductive Coupling”, In Proc. of International Conf. PELINCEC, 2005, Warsaw, Poland (CD). J. T. Matysik: “A New Method of Integration Control with Instantaneous Current Monitoring for Class D, IEEE Trans. on Industrial Electronics, vol. 53, No 5, 2006, pp. 1561-1576. J. Hirai, T. W. Kim, and A. Kawamura: Study on intelligent battery charging using inductive transmission of power and information. IEEE Trans. on Power Electronics, vol. 15, no. 2, 2000, pp. 335-344. Ch. Apneseth, D. Dzung, S. Kjesbu, G. Scheible, and W. Zimmermann: Introduction wireless proximity switches. ABB Review 4/2002, pp. 4249. K. O’Brien, G. Scheible, H. Gueldner: Analysis of Wireless Power Supplies for Industrial Automation Systems. In Proc. of IECON’03. J. Lastowiecki and P. Staszewski: Sliding Transformer With Long Magnetic Circuit For Contactless Electrical Energy Delivery To Mobile Receivers, IEEE Trans. on Industrial Electronics, vol. 53, No 6, 2006, pp. 1943-1948. Ch-S. Wang, O. H. Stielau, and G. A. Covic: Design considerations for contactless electric vehicle battery charger, IEEE Trans. on Industrial Electronics, vol. 52, No 5, 2005, pp. 1308-1313.

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