Author name used in this publication: K. W. E. ChengVersion of RecordPublishe

Cheng, KWE

Author name used in this publication: K. W. E. ChengVersion of RecordPublishedVoR allowe

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Frequency and pulse width modulation for zero-voltage -switching resonant pole inverter K.W.E.Cheng Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong Fax: +852 23301544 Abstract - The resonant pole inverter has the advantages of less component count and simple operation but the disadvantage is that the zero voltage switching can only be achieved for a very small load range. A new modulation technique is proposed to ensure the zero-voltage-switching to be maintained. Comparison of the proposed method to other modulation and inverter is presented. Theoretical and experimental results are shown. I. INTRODUCTION Inverter is an important circuit in Power Electronics and Drives System. Its application includes induction motor drives, AC power supplies and actuation system. For higher output power, typically greater 2kVA, three phase system will be more appropriate. The conventional hard switched inverter, as shown in Fig 1, suffers from switching losses. This type of inverter can usually switch at 12kHz as maximum. Higher switching fiequency is possible but at the expense of higher loss. The recent development of IGBT and other switching devices such as MCT provides a possible higher switching frequencies, but it only limits to 20-3OkHz. Output frequency of the inverter in many cases requires higher than 50Hz. For examples high speed drives and aerospace inverters which require 400Hz output fiequency. A very high switching frequency is therefore necessary. Fig. 1 Conventional hard-switched inverter The conventional hard-switched inverters suffer fiom switching losses and hence cannot work at very high switching frequency. Recently some resonant topologies have been reported to overcome this problem. The resonant DC link inverter [ 11 requires high voltage rating transistors and the auxiliary resonant commutated pole inverter (ARCPI) 0-7803-5769-8/99/$10.000 1999IEEE [2] is an improved version of the resonant pole inverter (RPI)[3], however it has a high component count and the auxiliary transistors are only zero-current-switching rather than zero-voltage-switching transitions. Clamped Quasi- resonant inverters [4] is very similar to conventional resonant link inverter [ 11 and they both have problem of zero-voltage detection. The resonant snubber inverter [5] is using auxiliary device to assist resonance, its principle is similar to ARCPI [2] and also requires additional switching devices. Anyway the RPI although has a limited load range but also has a low component count and its operation is simple. The modulation strategy can either be natural or regular sampling and hence it is very easy to be implemented. The aim of this paper is to present a modulation method which can enable the RPI to maintain zero-voltage-switching for very wide load ranges. 0 5 . .  - Fig.. 2 Schematic diagram of 3-phase Resonant Pole Inverter 11. PRINCIPLES OF OPERATION Fig. 2 shows a typical circuit of RPI. It can be seen that this circuit is the same as that of a conventional hard-switched inverter except that resonant capacitors C,, to C, are added in parallel with the transistors. The added capacitors are usually small and stray capacitors of the switching devices (QI-Qs) form part of the capacitance. The operation of the RPI is explained here by using the inverter leg A (Q, and Q4): When Q4 is turned off, C, is gradually charged to vd, by current in LFa. Hence Q4 is turned off under zero-voltage switching. When voltage across C, rises to V,,, D, conducts. Q, is then turned on and hence Q, is turned on under zero- voltage switching. The duration between the turn-off of Q4 1073 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 15, 2009 at 00:00 from IEEE Xplore.  Restrictions apply.and the ttun-on of Q, is called the dead-time Tdead. The principles of operation of zero-voltage-turn-on of Q, and turn-off of Ql, and the zero-voltage-switchings of the other inverter legs are similar. .,I. However, when instantaneous magnitude of i, is small or iLF is still pmitive, there is insufficient current in or the current flow direction is incorrect for charging the voltage of C, to V,, before the next --on of QI, hence zero-voltage switching cannot be achieved in Q,. The conventional regular or natural sampling sinusoidal modulation defmes the pulse width as a function of modulating signal. Generally near the crest or trough of a sine wave, the pulse width of the gate signal to Q, or Q1 is very small respectively. There is insufficient time for the current to build up in & so as to charge C,, or C,, respectively. because regular sampling modulation can still be fulfilled. 'B M 1(Sl -1- used because it will seriously decrease the pulse width near the crest or trough of a sine wave. The inductor current i,, is different from a sinusoidal waveform and is a characteristic of this inverter. The fcat that it cannot follow a sinusoidal waveform makes it unable to charge or discharge the resonant capacitor near the crest or trough of a sine wave. It can be seen that i, cannot switch to sufficient magnitude opposite polarity when the sinsoidal wave (output voltage Va) has already reached the creast or trough. The magnified waveform is shown in Fig. 4. It canbe seen that the rising current of i, has a sufficient magnitude to discharge the resonant capacitor to zero voltage before the next turn-on of the switching signal Gate-4 (Gate signal of Q4). However when Q4 is on, there is insufficient time for the current to change to sufficient magnitude in the opposite direction. Hence the resonant capacitor C, cannot charge to VdC because Q1 is turned on (Gate-1 is on). 111. ZERO-VOLTAGE SWITCHING MODULATION TECHNIQUE The above problem can be improved by decreasing the carrier fi-equency hence the pulse width can be extended whereas the lovl - Fig. 3 Simulated waveforms of the resonant pole inverter using cIassicaI be the capacitance of C, and & be the inductance of La. The mark-space ratio, d, can be given by regular symmetrical sampling modulation: P W M  techniques ( 1": VG4; P VGl; P: V,; 4": iLF; 5": V,,) d=0.5( 1 +Msin(2nfmt)) (1) Figure 3 shows a simulated results of the resonant pole inverter. Classical regular sampling sinusoidal modulation is used. The circuit parameters used were: Cr=22nF, LF=75pH, CF=4.7pF, V,, = 270V, fm=400Hz and M=0.85. Switching frequency=l5kHz. Higher switching frequency cannot be where M is the modulation index and f, is the modulating frequency. The inductor current i, can be given by: di dt 2 L F 3  =Vdc- Vdcd OI) : W) ............................... - 2. iLF is to charge CI4 to V,, before the lapse of Tdead, ie. Q, is turned on: ............................... ............................... 0.0 Fig. 4 Simulated waveforms of the transistor voltage Vsw near the crest Eqn (2) and (3) give f;.=4(1-4<. (4) ( 1": Vci3; P Vtil; 3*: V,; 4Ih: iLF) 1074 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 15, 2009 at 00:00 from IEEE Xplore.  Restrictions apply.where m 20.0 F ]  o a  f, is the maximum value of the carrier frequency f,. f, is proportional to (I-d)d. It can also be seen that f, is maximum when d is 0.5 and approaches to 0 when d =O or 1. In practice, f, must be limited to a minimum value. - 7 .............................. : .............................. : ................................ .i ......................... : ..................... .......................... IV. SIMULATION OF THE INVERTER The above variable frequency regular sampling symmetrical modulation has been programmed to simulate the RPI using simulation package SABER. The parameters used were: C,=22nF, L=75pH, CF=4.7pF, V,, = 270V, fm=400Hz and M=0.85. Range off, = 12.5kHz - 40kHz. It can be seen that the carrier frequency varies as the mark-space ratio is varied. At crest or trough of the modulating (sine wave) signal, the carrier frequency is low. is is characterized by a sinusoidal wave shape. The magnified waveforms are shown in Fig. 6 .  J ....... m "I, (*.) - I U  -,::I ma 0- O O D I  .- 0- .U 0-7 amra ..OD. - - - v.3 Fig. 5 Simulated waveforms of the resonant pole inverter using zero- voltage switching modulation technique ( 1"': VG,; 2* VG,; 3': V,; 4": i,; 5". VJ 1 -)/7f2200.0 ............................. 1 ............................ 1 ................................ L 0.0 ........................... I ..................................................... -201  I 0.0(1804 0110806 O o 0 8 0 8  omm W) Fig. 6 Simulated waveforms of the resonant pole inverter using zero- Voltage switching modulation technique ( 1": VG4; P VG,; 3"': Vu3; 4Ih: i,) It can be seen that the pulse width of Gate-4 is extended because of the carrier frequency is also modulated. I, is now switched to sufficient magnitude in the opposite polarity. The transistor 4 4  is now switched under zero-voltage switching. V. EXPERIMENTAL RESULTS A prototype RPI has been built and tested. Same component values as Section IV were used. The modulation signal is programmed by a DSP TMS320C3 1. The transistors used were third generation 600V, 50A IGBT. Three half-bridge modules are used. The on-state voltage is low. A small thermal subsystem is sufficient. Sandwiched DC bus bar are used to minimise the stray inductance and increase the decouple filtering effect.. Fig. 7 and 8 shows the experimental results at 1.8kW. Fig. 7 shows the waveforms of the i, and VQ4. It can be seen that V, decreased to zero before gate signal V, was applied. It is confirmed that zero-voltage-switching has been achieved. Fig. 8 shows the experimental waveforms of the inductor current and the output phase voltage V, at output frequency of 400Hz. The Total Harmonic Distortion measured was 8.8% which met the international standards MIL-704E and BSI 3G 100: Part 3. The output voltage has an observable distortion near the crest and trough. This is because the instantaneous switching frequency is modulated to 15kHz. The instantaneous switching fiequency near the zero-crossing is 40kHz, hence the ripple voltage is small. Nevertheless, the overall THD of the waveform is still low. It should be noted that the small filter size (LF, C,) is used. Its THD can be improved further if it is needed. The measured efficiency is 95%. Usually for medium or high power system, the efficiency is higher than 92%. The Fig. 7 Experimental waveforms of the inverter (I?  V,,: 2OV/div; 2"? i,,,: 2ONdiv; 3"': V,, 2OV/div; 4": V, 200Vldiv; Spsldiv) 1075 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 15, 2009 at 00:00 from IEEE Xplore.  Restrictions apply.2+ 4- Fig. 8 Experimental waveforms of the inductor current and output voltage (upper: i,, 2OAldiv; lower: output phase voltage V, lOOV/div; 500ps/div) prototype inverter, because of small component count and using zero-voltage switching, have achieved above average efficiency. VI. COMPARISON WITH OTHER INVERTERS Soft-switching is one of the future technologies of the power electroni'cs development. Because the switching devices can now switched at much high frequency, it is common concept to increase the switching frequency to as high as possible. Using the conventional modulation applied to RPI causes problem in the non-zero voltage switching. It therefore generate switching noise and lost. The switching frequency cannot be very high, typical 12.5kHz, but still suffer from switching loss during the crest or trough of the modulating signal. The problem becomes serious when the output frequency (modulating frequency is high) such as 400Hz. High oiitput frequency is very common in industrial applicati'on such as aerospace power inverters and high speed drives. Low switching frequency causes problems of output voltage distortion. Auxiliary resonant commutated pole inverter (ARCPI) [2] is a very common inverter and is popular for its zero-voltage switching for a wide load range. This converter has a very high components and hence it is usually used in the very high power application. Its reliability is rather poor. The proposed RPI is therefore is better than the ARCPI especially when it is operated in medium power level, typically less than 1OkVA. VII. HARMONICS CONSIDERATION The output harmonics are important factor of consideration Fig. 9 Harmonic spectrum of the output voltage of RPI using conventional modulation method M a g o  : 1 W )  Va . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  . . . . . . . .  f(W Fig. 10 Harmonic spectrum of output phase voltage the RPI using zero- voltage switching modulation method because it may produce distortion in the output voltage and torque ripple when it is driving a machine. The spectrum of the RPI using conventional modulation techniques has been simulation and is shown in Fig. 9. It can be seen that the distortion is high and the THD is 20% which does not meet many international standards. The spectrum of the output voltage of the W I  using the proposed zero-voltage switching modulation method is shown in Fig 10. It can be seen that the harmonic distortion is very small. The THD is 9% which meets many international standards. The proposed modulation method using frequency modulation hence the switching frequencies are distributed between 12.5kHz and 40kHz. This components can be easily be filtered by LF and CF. 1076 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 15, 2009 at 00:00 from IEEE Xplore.  Restrictions apply.VII. CONCLUSIONS A novel variable frequency modulation scheme is proposed for the Resonant Pole Inverter. The proposed method enables that the inverter has zero-voltage switching for a wide load range. The harmonic distortion is low and the circuit is simple. The frequency and pulse width modulation method proposed is also very simple and easily to be implemented in an inverter. The reliability of the circuit is high because of the small component count. Theoretical, simulation and experiment has demonstrated the operation of the circuit and verified the feasibility of the circuit. It is therefore suitable for many industrial applications and even for aerospace actuation. ACKNOWLEDGEMENT The author gratefully acknowledges the financial support of The Hong Kong Polytechnic University. REFERENCES 1.  2. 3. 4. 5.  D.M.Divan., “The resonant dc-link converter - a new concept in static power conversion,”, IEEE Trans. Industry Applications, Vol. 25, NO. 2, pp. 317-325. . D.M.De Doncker. and J.P.Lyons, ‘The auxiliary resonant commutated pole converter”, IEEE-IAS Conf. Proceeding, 1990, pp. 1228-1235. D.M.Divan and GSkibinski, ‘Zero-switching-loss inverters for high- power applications”, IEEE Trans. Industry Applications, Vol. 25, No. 4, 1989, pp. 634-643. B.J.C. Filho,. T.A.Lipo, “Space-vector analysis and modulation issues of passively clamped quasi-resonant inverters”, IEEE Transactions on Industry Applications, Volume: 34 4 , July-Aug. 1998, Page@): 861 -869. B.M.Song S.R.Lee and J.S.Lai, “An improved three-phase auxiliary resonant snubber inverter for AC motor drive applications”, IEEE PESC, 1998, pp. 423-428. 1077 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 15, 2009 at 00:00 from IEEE Xplore.  Restrictions apply.

Frequency and pulse width modulation for zero-voltage-switching resonant pole inverter

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