Multi-Cell Switch-Mode Power Amplifier with Closed-Loop Hybrid Output Voltage Filter

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1 PIM Erope 05, 9 May 05, remberg, Germany Mlti-ell Switch-Mode Power Amplifier with losed-oop Hybrid Otpt Voltage Filter H.. Votzi, H. Ertl niversity of echnology Vienna, Institte of Energy Systems and Electrical Drives, Power Electronics Section Gsshasstrasse 7-9, A-040 Wien, Vienna, Astria helmt.votzi@twien.ac.at, johann.ertl@twien.ac.at Abstract In this paper a promising novel otpt voltage filtering concept for mlti-cell switch-mode power amplifiers is analyzed. he proposed hybrid filtering concept is based on a passive filter enhanced by a linear-mode correction stage of small rated power. sing this principle, also "sb-harmonic" (i.e. low-freqency) switching noise distortions (typically appearing in real interleaved mlti-cell switching amplifiers) can be effectively sppressed which is not possible sing conventional filters. ombining the new hybrid filtering approach with a switch-mode power amplifier formed by an arrangement of low-voltage MOSFE switching cells finally reslts in a power sorce of high otpt voltage precision at high efficiency rates.. Introdction Mlti-ell Switch-Mode Power Amplifier Basics o demonstrate the advantage of the proposed hybrid filtering concept, the basic characteristics of mlti-cell switch-mode amplifiers and their limitations are described first. Sch power amplifiers based on cascaded H-bridge switching cells increasingly have been proposed and reported in the last decade (e.g., [-5]). he advantages of this topology (Fig.a) are that (i) the individal switching cells can be eqipped with highly efficient low-voltage semicondctors and (ii) the otpt ripple crrent sbstantially is redced de to interleaved PWM (Fig.c). As depicted in Fig.a the total otpt voltage = Σ ab,i of all cells shows only voltage steps of amplitde / at a period of / (=/f S, f S MOSFE switching freqency). All low-freqency harmonics f S (-)f S which appear at the cell otpts mtally compensate regarding the total otpt voltage. he system therefore acts as a switch-mode amplifier with an "effective" switching freqency of f S. In this case the maximm ripple PWM control nit switching cells i = Z i = s ab ab load Δ I 5A/div 00s/div = 4 (c) Fig. : Switch-mode power amplifier consisting of a series arrangement of fll-bridge switching cells ; advantageosly (in addition to the interleaved PWM mode of the individal cells) also the control signals s a, s b of each cell are interleaved; with this, the otpt voltage ab of each cell already shows 3-level characteristic); : eqivalent circit symbol; (c): demonstration of otpt crrent ripple redction according to. 57 VDE VEAG GMBH Berlin Offenbach

2 PIM Erope 05, 9 May 05, remberg, Germany 0 / Û I + I I.0 Δ I( m) ΔI max = tri, = =4 m s a, s b, ab, m Δ I max, Δ I = max Δ I max = 8 f S / ab, ab, 3 Δ max, π f0 = 3 f 3 S 0 / m / Δ I ab, i i ab, 4 i Fig. : Generation of the interleaved cell otpt voltages ab,i by PWM sing /-time-shifted trianglar-shape carrier signals (freqency f S = / ) and total otpt voltage = Σ ab,i of all switching cells (effective plse freqency = f S ) ; redction of the crrent ripple ΔI (in dependency on the modlation depth m) for increasing nmber of switching cells. Fig. 3: Measrements of a real interleaved mlti-cell system (4 cells) and D otpt voltage with slightly different cell voltages. h: Otpt voltage 0V/div; h: ipple of the indctor crrent 00mA/div; time: 5μs/div. he indctor ripple crrent shows the typical low freqency noise. Parameters: f Scell = 00kHz, = 0μH, = 400nF, = 9.4Ω. crrent amplitde ΔI max, for increasing nmber of cells redces according to (Fig.b, Fig.c), the maximm voltage ripple Δ max, at the filter capacitor even according to 3! he described sbstantial ripple redction of the mlti-cell principle relies on the smmation of partial voltages which are perfectly interleaved and of eqal amplitde. his, however, is not ideally tre for practical systems. De to slightly different D link cell voltages i /, gating time delays/errors and de to different semicondctor on-state voltage drops the described mtal harmonic cancellation in real systems will not be perfect reslting in residal noise components of basic switching freqency f S. nfortnately, it in general is not possible to reject this low-freqency switching noise by a passive otpt filter becase the noise appears within the pass-band of the filter. his effect (which can sally be observed as a ripple "envelope" of period /f S in the systems otpt signal measrement, cf. Fig.3) shall be discssed briefly on basis of a dimensioning example: As indicated in Fig.4a for an idealized 4-cell topology rnning at f S,cell = 00kHz per cell (interleaved PWM assmed), all low freqency harmonics will cancel ot and the first residal harmonics are groped arond 4 f S,cell = 800kHz. hese harmonics are properly sppressed by the -filter (Fig.4b, with filter ct-off freqency 300kHz). he ideal cancellation, however, is only valid if the cells are spplied with D spply voltages of perfectly eqal amplitdes. If these voltages do not show eqal 58 VDE VEAG GMBH Berlin Offenbach

3 PIM Erope 05, 9 May 05, remberg, Germany 000 SW 000 O filter passband region 000 [V] [V] [V] filter response O noise de to non-eqal D cell spply voltages [khz] [khz] [khz] (c) Fig. 4: Otpt voltage spectrm of an idealized mlti-cell switch-mode amplifier before and after passive filtering (4 interleaved switching cells, switching freqency f Scel l = 00kHz per cell). De to interleaved PWM the low-freqency harmonics p to 4 f Scell will cancel ot. If, however, the cells show non-eqal spply voltages (here it is assmed that one cell is operated at 0% and a second cell at 90% of the rated D voltage) lowfreqency harmonics appear (c) which cannot be rejected by the passive filter becase they are located within the pass-band of the filter. vales the cancellation is imperfect and noticeable residal harmonics arond 00kHz arise. hese "sbharmonic" noise components in principle can be not sppressed by the -filter, becase they show a freqency within the filter s pass-band region (Fig.4c)!. Mlti-ell Switch-Mode Power Amplifier with Hybrid (Active+Passive) Filter In principle passive filters are not able to solve the described sbharmonic noise problem becase they separate wanted to nwanted signal components exclsively on the basis of different freqency regions. o circmvent this limitation, a filtering concept has been developed (based on [6-8]) which rejects high-freqency switching noise by components, whereas noise in the pass-band is redced by sing a correction voltage stage inserted in series to the filter capacitor as depicted in Fig.5. he correction voltage is generated by a linear power amplifier. De to the fact that this amplifier has low otpt crrent (only the ripple of the switch-mode stage) as well as low otpt voltage (i.e., can be operated at a low-voltage spply) the active correction stage has small rated power (only a few % of the total amplifier power, hence the efficiency of the total system is only slightly lowered). A very essential featre of the presented concept frther is, that the correction voltage is generated by a kind of operational amplifier whose feedback loop incldes the capacitor of the passive filter, reslting in a closed-loop hybrid filter (Fig.6). onseqently, the otpt voltage of the total amplifier is defined directly by the linear stage whereas the switch-mode system covers the main part of the load crrent, the ripple of the switching stages otpt crrent i SW is "absorbed" by the linear amplifier. he filter capacitor takes over the "fndamental" component of. In Fig.7 the control of the system is drafted as control-oriented diagram. his concept gides the otpt voltage of the switch-mode part (mlti-cell stage) sch that the linear amplifier s otpt signal is minimized. For =, IP i load Fig. 5: Basic strctre of the proposed power amplifier with hybrid filtering consisting of a mlti-cell switching stage ( ), a passive filter and a voltage correction stage to sppress sbharmonic noise. optimizing the system response a damping path (capacitor crrent feedback) as well as an otpt crrent feed-forward (s) is implemented. As demonstrated by Fig.8, the idealized total amplifier shows excellent otpt voltage, in especial no sbharmonic noise (as known for standard mlti-cell, Fig.4c) occrs. Additionally it has also to be mentioned, that the system shows very good (i.e. low) otpt impedance characteristic, becase (on contrary to standard switch-mode amplifiers) the system s otpt impedance is not given by the -filter bt defined "actively" by the 59 VDE VEAG GMBH Berlin Offenbach

4 PIM Erope 05, 9 May 05, remberg, Germany k PWM k PWM + ' ' 0 i Z OAD linear stage + ' ' i Z OAD i SW O Fig. 6: Basic concept of the proposed active/passive filtering; implementation scheme sing a mlti-cell switch-mode power stage and an operational amplifier with sbseqent emitter-follower for realizing the linear stage. s m SW i SW i Fig. 7: Proposed scheme for controlling the switching stage otpt voltage in order to minimize the linear amplifier s otpt signal. linear stage. his may be a very important fact, especially if the amplifier is sed as a testing voltage sorce (e.g., for EM compliance tests) where it is reqired, that a non-sinsoidal load crrent may not lead to recognizable testing voltage harmonics. 3. Design onsiderations As illstrated in Fig.5, the essential part of the system contains of a passive filter and a voltage correction stage. For providing demonstration examples, in this chapter sitable dimensioning gidelines for the proposed active hybrid filter soltion are given. At first, however, as initial point a dimensioning example for a filter of standard switch-mode mlti-cell system shall be smmarized briefly. Assming a 4-cell system (=4) with D link voltages of / = 5V, a maximm otpt voltage of ±00V can be achieved. he cells now are eqipped with low-voltage MOSFEs ( DS,on only a few mω) operating at a switching freqency of f S = 00kHz), hence an effective switching freqency of f S,eff = 800kHz reslts for interleaved PWM. he ct-off freqency of the filter now is fixed to a vale being typical for switch-mode systems of f 0 f S,eff /0 = 80kHz. Assming a maximm load crrent of 0A for maximm otpt voltage of 00V yields a load resistance of = 00V/0A = 0Ω. For achieving a proper damping behavior, the characteristic impedance Z 0 = / of the filter is also fixed to Z 0 0Ω. he vales f 0 and Z 0 finally define =0μH and P =0.μF (index P denoting the passive filter). Based on this dimensioning and sing the eqations given in Fig., maximm ripple vales of I max,4 = 0.3A and max,4 = 0.3V appear. If the system actally is operated at maximm otpt voltage ±00V at 80kHz, however an additional peak reactive crrent of ±00V π 80kHz 0.μF = ±0A will flow throgh P. If sch a passive filter amplifier system now wold be extended to the proposed hybrid concept, the correction voltage sorce has to generate a rather low voltage level bt is loaded by considerably high crrents (0.3V/0A). Sch voltage/crrent levels do not fit very well to today s available power operational amplifiers (OP), especially also regarding otpt impedance considerations, which will be discssed sbseqently. he otpt capacitor of the passive filter therefore is 530 VDE VEAG GMBH Berlin Offenbach

5 PIM Erope 05, 9 May 05, remberg, Germany 000 [V] 00 0 no otpt voltage noise also in case of non-eqal D cell spply voltages! f = 0 khz 00V/div.5μs/div [khz] f = 0 khz 00V/div.5μs/div (c) f = 0 khz 00V/div.5μs/div Fig. 8: Simlation reslts of the proposed hybrid-filtering amplifier concept: Even in case of non-eqal cell spply voltages the otpt voltage does not show harmonic noise (a, b); the amplifier also shows good transient response if the d/dt is limited sch that an override of the linear amplifier is avoided (c, d). (d) operational amplifier model s I Z OP v 0 s f ω = f Fig. 9: a) Eqivalent circit of the voltage correction stage considering freqency response of OP gain (approximated by /s) and otpt resistance ; b) OP gain of a real operational amplifier. redced from P = 0.μF to = 0nF for the hybrid system reslting in more convenient OP otpt voltage/crrent level of 3V/A. onsidering adeqate voltage crrent margins reqired for covering transients, OPs with ±5V spply, which can provide otpt voltages of typically ±0V and are available also for the MHz-region, seem to be optimal. Besides the otpt voltage and crrent level a frther important parameter of the OP is its otpt impedance and the freqency dependency of otpt impedance and gain. In order to achieve a low residal otpt noise, the inner impedance of the OP-path (inclding capacitor X =/(ω)) shold be as low as possible. herefore, the OPs shold be very low and its loop-gain high to lower the inflence of X. For analyzing the relationships a simple OP eqivalent circit (Fig.9a) consisting of an integrator element /(s) representing the amplifier s transit freqency f =/(π) and an ohmic otpt resistance. With this, the systems closed-loop otpt impedance calclates to Z = (+s) / (s) s / (+s), which represents a series arrangement of a PI-characteristic originating from and (Fig.0a, ble crve, f =/(π)) and a high-pass behavior de to f (Fig.0a, red crve). Z finally shows lead-lag behavior (Fig.0a, green crve) which yields Z / for low freqencies ( f < f ), whereas at high freqencies ( f > f ) is defined by the otpt resistance of the OP Z. 53 VDE VEAG GMBH Berlin Offenbach

6 PIM Erope 05, 9 May 05, remberg, Germany Z + s f = 80MHz s Z + s s f = 8MHz a ) 0.μF 0.Ω ω 8MHz s P 80MHz ω s + s 0nF Ω s ω b) ω s P s + s s ω Z + s s f = 6. 7MHz + sk sk (s) G PI ω s + s ω k ω / k ω c) s P s d) ( + sk )( + s / k) G ( s) Fig. 0: Bode diagram of the otpt impedance of the proposed system for a) f =80MHz; b) f =8MHz and c) f =6.7MHz; d) voltage transfer fnction a mlti cell switched mode power amplifier system sing a passive filter and a standard PI-type otpt voltage controller. Assmption of the following system parameters f = 80MHz (OP transit freqency), = Ω (OP otpt resistance) and =0nF (filter capacitor) reslts in: =ns, =0ns, f = 8MHz, / = ns/0nf = 0.Ω. As indicated by the shaded triangle of Fig.0a hence the noise sppression in the freqency region <8MHz (i.e. the region where the dominant switching freqency harmonics of the mlti-cell switch-mode system appear) will be significantly higher for the proposed system in comparison to a simple passive filter P. his especially is tre regarding sbharmonics. Fig.0 however also demonstrates that high-bandwidth power OPs are essential for the implementation of the proposed concept (see redction of shaded improvement region for f = 6.7MHz or 8MHz., Figs.0bc). Specific focs therefore has to be set on the OP selection (f ) and on the design of the crrent bffer stage (). 4. omparison to onventional Filtering In mlti-cell switch-mode power amplifiers (Fig.a) de to D link voltage imbalances, switching time and interleaving errors, sbharmonics at f S or even f S may appear, despite the fact that the dominant switching noise is groped arond f S. he most beneficial fact of the proposed system is the ability to redce/sppress low freqency switching distortions of the system s otpt signal which cannot be achieved sing standard passive -filters. (emark: A sppression of sch distortions based on passive filters wold only be possible if the filter s ct-off freqency is redced correspondingly, which, however will lower the bandwidth of the amplifier.) egarding the dimensioning example sed in the prior chapter this yields that the passive -filter with = 0μH, P = 0.μF shows a ct-off freqency of f 0 = 80kHz f S,eff /0. he MOSFE switching freqency however is 00kHz and sch sbharmonics may appear in the otpt voltage of the mlti-cell chain in case of non-symmetries of the gate drivers and 00kHz sbharmonics occr in case on imbalanced cell D link voltages. Both components are not sfficiently rejected by the passive filter. For demonstrating the advantages of the proposed hybrid filtering for mlti-cell amplifiers now a comparison of residal otpt voltage ripple shall be given (Fig.). 53 VDE VEAG GMBH Berlin Offenbach

7 PIM Erope 05, 9 May 05, remberg, Germany a) m P io P b) OP model m A FB i io -filter P -filter P c) hybrid-filter t d) hybrid-filter t 0.V/div 4μs/div 0.V/div 4μs/div balanced cell D-link voltages (4x5V) imbalanced cell D-link voltages (4x5V ±0%) Fig. : omparison of a standard (open-loop) 800kHz mlti-cell amplifier with passive otpt filter, parameters: P = 0.μF, = 0μH to a mlti-cell amplifier sing the proposed hybrid filtering, parameters: = 0nF, = 0μH, FB = 0Ω, OP-model = Ω, f = 3MHz, v 0 = 0 4. (c) and (d) give the otpt voltage ripple for a D reference signal (constant dty cycle). In case of balanced D link voltages (4x5V) the otpt does not show any sbharmonic noise (c); nevertheless, the hybrid filter system shows an otpt voltage of lower ripple than the passive filter P ; at imbalanced D cell spply voltages (=0%, =00%, 3=00%, 4=90%) the advantages of the hybrid system appear prononced (d), becase the passive filter for this operation conditions show an excessive 00kHz sbharmonic component. onsidering the worst case an operation point prodcing maximm otpt ripple is chosen. According to Fig.b for a 4-cell system this is valid at a modlation index of m = (i.e. O = 87.5V) for 4x5V D link voltage). As given in Figs.b,c the passive filter shows an otpt voltage ripple of 0.3V for balanced D cell voltages which rises p to V for cell voltages of 0% imbalance. he proposed hybrid filter soltion is characterized by sbstantially lower ripple rates ( 50mV/80mV), especially for the imbalanced cell voltage case. he passive filter case analyzed before represents the otpt filter on an open-loop switch-mode amplifier. It might be arged that the appearing sbharmonics cold be alternatively redced by enhancing sch a system (mlti-cell amplifier with standard -filter) by an otpt voltage control loop which eliminates distrbances and therefore also the appearing sbharmonic components. Sch a control can be implemented e.g. by a simple PI-type controller as shown in Fig. and described in []. De to the fact that the -filter in the idealized case itself does not show any intrinsic damping, an active damping path (capacitor crrent feedback) is added which shifts the bondary-stable transfer fnction G (s) of the filter to a well-damped dynamic behavior G (s) by pole splitting (typical splitting factor k 5). ow the PI-controller is dimensioned sch, that it s zero cancels the slower of the two poles of G (s) i.e., G PI (s) = (+k)/(k). Analyzing the according Bode-plot (Fig.0d) it cold be realized that the open-loop characteristic G PI (s) G (s) of the system does not provide high loop-gain rates in the freqency region where the discssed mli-cell sbharmonics arise. onseqently, for a mlti-cell amplifier with passive filter the sbharmonics indeed wold be lowered by closed loop control, bt only very ineffectively in comparison to the proposed hybrid filter scheme. 533 VDE VEAG GMBH Berlin Offenbach

8 PIM Erope 05, 9 May 05, remberg, Germany I PI m FB Fig. : ontrol scheme of a non-hybrid passive filter system of a mlti cell switch mode power amplifier eqipped with closed loop otpt voltage control (nable to effectively sppress sbharmonics in the region of the transistor basic switching freqency f S ). i G ( s) = + s P G ( s) = ( + sk)( + s / k) PI-type controller: + sk G PI ( s) = sk 5. Smmary onclsions In general, mlti-cell topologies of series-connected switching cells increasingly are getting importance for many power electronic systems de to their basic advantages that low-voltage semicondctors can be sed and that significant ripple redction can be achieved by interleaved PWM. In practical systems however nfortnately also sbharmonic ripple components appear (i.e. harmonics of the transistor basic switching freqency f S ) de to non-symmetries of the cells (e.g., imbalanced D link voltages, switching time delays etc.) which cannot be sppressed effectively by the filter becase they appear within the filters passband region. his is also tre for closed-loop otpt voltage control. For solving this problem a hybrid filtering scheme is proposed based on the principle that the otpt voltage of the switch-mode amplifier actally is provided by a linear-mode power amplifier. he former filter now is sed as a kind of copling network to combine the mlti-cell switching stage and the linear stage in a way that the linear mode is brdened only by the small ripple crrent of the switching stage. Frthermore, the linear amplifier has to provide only small otpt voltage levels (typ. 5 0V), hence its dimensioning power is only a few % of the systems otpt power. It has to be mentioned, however, that the linear stage has to have a high bandwidth and has to be designed very careflly. 6. eferences [] H. Ertl, J.W. Kolar, F. Zach: "Analysis of a Mltilevel Mlti-ell Switch-Mode Power Amplifier Employing the "Flying-Battery" oncept", IEEE ransactions on Indstrial Electronics, Vol. 49, o. 4, (00), pp , DOI: 0.09/IE [] G. Gong, H. Ertl, J.W. Kolar: "A Mlti-ell ascaded Power Amplifier", st IEEE Applied Power Electronics onference (APE), Dalles, SA, March 9-3, 006, pp , DOI: 0.09/APE [3] Ying Zhang, QingDe Meng, Yaoha i, ing ai: "A High-Efficiency ascaded Mltilevel lass-d Amplifier with FPGA based on sliding mode control", 3rd Int. Symposim on Systems and ontrol in Aeronatics and Astronatics (ISSAA), Harbin, hina, Jne 8-0, 00, pp.86-90, DOI: 0.09/ ISSAA [4] H.. Votzi, H. Ertl: "High-Efficiency Battery Storage nit for enewable Energy D Micro-Grids", Int. onf. on Power onversion (PIM), ürnberg, Germany, May 7-9, 0, pp [5] H. Ertl, J.W. Kolar, F. Zach: "A ovel Mlti-ell D-A onverter for Applications in enewable Energy Systems", IEEE ransactions on Indstrial Electronics, Vol. 49, o. 5, (00), pp , DOI: 0.09/ IE [6] H. Ertl, F.. Zach, J. W. Kolar: "Dimensioning and ontrol of a Switch-Mode Power Amplifier Employing a apacitive opled inear-mode ipple Sppression Stage", Int. onf. on Power onversion (PIM), ürnberg, Germany, Jne 7-9, 005, pp [7] G. B. Yndt: "Series- or Parallel-onnected omposite Amplifiers", IEEE ransactions on Power Electronics, Vol., o., pp , 986, DOI: 0.09/PE [8] J. Pforr: "Switch-Mode rrent Amplifier with High Otpt rrent Qality Employing an Active Otpt Filter", Int. onf. on Power onversion (PIM), ürnberg, Germany, Jne 4, 999, pp VDE VEAG GMBH Berlin Offenbach

A Novel Concept for Mains Voltage Proportional Input Current Shaping of a VIENNA Rectifier Eliminating Controller Multipliers

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