IEEE POWER ELECTRONICS LETTERS, VOL. 3, NO. 3, SEPTEMBER 2005 101
Commutation Technique for High-Frequency LinkCycloconverter Based on State-Machine Control
Robert S. Balog, Member, IEEE, and Philip T. Krein, Fellow, IEEE
Abstract—A technique for commutating the load current of ahigh-frequency link, pulse-width modulated cycloconverter is pre-sented. As the load current makes a sign transition, switch op-eration becomes critical to avoid commutation failure. Previoustechniques required either dead time or the use of large induc-tors to limit shoot-through current, thereby compromising per-formance and efficiency. The technique presented here identifiesthe switching sequence that is necessary to allow load current tocommutate naturally without distortion. The switching sequenceis managed with a state machine.
Index Terms—AC–AC power conversion, cycloconverter, cyclo-converter control, high-frequency link converters, state machineswitch control.
I. INTRODUCTION
THE high-frequency link, pulse-width modulated (HFPWM) cycloconverter has been shown to be a reduced-
cost inverter solution that produces familiar PWM waveforms[1]. A multisignal modulation method [2] alleviates the his-torically difficult control of a cycloconverter and generatesoutput PWM waveforms directly from a square-wave input[1]. However, this technique is still plagued by a difficult issuewith cycloconverters: current commutation. As the load currentchanges sign, switch operation becomes critical. Improperswitching can cause commutation failure where either thesource is short-circuited or the load current is interrupted.Commutation failure often produces high voltage that candamage the equipment.
Methods reported in the literature to prevent commutationfailure in cycloconverters require either the insertion of deadtime or the use of large inductors to limit shoot-through current,compromising converter performance or efficiency [3]. Thispaper contributes a commutation technique suitable for theHF PWM cycloconverters. The cycloconverter is controlled,as determined by the characteristics of the load, by propersequencing of the output switches. Commutation failure is pre-vented and zero-cross distortion is mitigated without additionalcomponents. The technique facilitates implementation of aHF PWM cycloconverter as an inverter suitable for alternativeenergy systems or uninterruptible power supplies (UPS).
Manuscript received May 2, 2005; revised August 1, 2005. This paper wasrecommended by Associate Editor S. K. Mazumder.
The authors are with the Grainger Center for Electric Machinery and Electro-mechanics, Department of Electrical and Computer Engineering, University ofIllinois at Urbana-Champaign, Urbana, IL 61801 USA (e-mail: [email protected];[email protected]).
Digital Object Identifier 10.1109/LPEL.2005.858422
Fig. 1. Cycloconverter output bridge.
A. Background
In an ac-to-ac converter, bilateral switches are required toconduct ac current and block ac voltage [4]. The high-frequencylink cycloconverter shown in Fig. 1 uses anti-parallel thyristors.The switches, always gated in pairs to maintain a continuouscurrent path, are separated into two groups based on the polarityof the link voltage: the positive converter (Q1, Q2) and (Q5, Q6)and the negative converter (Q3, Q4) and (Q7, Q8) [3]. There arefour possible switch configurations derived from combinationsof link voltage and load current. When the load current is pos-itive (flowing in the direction indicated in the figure), switches(Q1, Q2) conduct positive HF link current and switches (Q3,Q4) conduct negative HF link current. Similarly, when the loadcurrent is negative, switches (Q5, Q6) conduct positive HF linkcurrent and switches (Q7, Q8) conduct negative HF link current.Conducting switch pairs that will soon be turned off are calledthe outgoing converter and the next switches to be turned on arecalled the incoming converter [3].
During commutation, the incoming converter must be chosento support the link voltage and prevent commutation failure.When the load current is positive, commutation from the out-going converter to the incoming converter involves turning onswitches (Q3, Q4) then turning off switches (Q1, Q2). A con-tinuous current path is maintained and the opposite polarities ofthe switching devices prevent HF link shorting. A similar pat-tern is followed for negative load current.
Commutation of the load current across a current zero is morecomplicated. For a positive HF link voltage, switches (Q1, Q2)conduct positive load current. As the current smoothly crossesthrough zero, switches (Q5, Q6) are required to turn on to con-duct negative current. However, if (Q5, Q6) turn on while (Q1,Q2) are still on, the HF link is shorted. Further, time must beallowed for reverse recovery to avoid a commutation failure. Ifdead time is added to avoid the problems, the load current be-comes discontinuous, as shown in Fig. 2, and zero-cross distor-tion appears.
102 IEEE POWER ELECTRONICS LETTERS, VOL. 3, NO. 3, SEPTEMBER 2005
Fig. 2. SCR output bridge current zero-cross with dead-time, HF Link voltage,and converter gating signals. (Color version available online at http://ieeexplore.ieee.org.)
B. Literature Review
Load current commutation from the positive converter to thenegative converter has always been a challenge in cyclocon-verters. Well-known techniques include an intergroup reactor(circulating current mode) or dead time (noncirculating currentmode) to mitigate shoot-through current [3]. Newer techniquesuse sophisticated control circuits to indirectly detect the polarityof the load current and set the converter switching action [5]–[7].One method [8] uses a controllable resonant link as an interme-diate stage so the cycloconverter input voltage can be adjustedto zero during load current commutation. Another technique [9]improves this by using the output bridge to maintain current flowwhile the link is zero volts during current commutation. Thesenewer techniques function as dead-time controls, since the loadcurrent decays to zero prior to activation of the incoming con-verter. An ac–ac converter, similar to the HF PWM cyclocon-verter, is used as an audio amplifier in [10]. The commutationmethod there pertains to the HF link, not the output bridge.
II. THEORY OF OPERATION
The new contribution in this paper is the identification ofthe incoming converter configuration such that no commutationfailure occurs. The next switch pair to be turned on is foundby considering the polarity of the HF link and properties of theload current. Switching can then occur such that no considera-tion for reverse recovery is required. Thus the dead time is elim-inated and smooth commutation of the load current through zerois produced, as shown in Fig. 3. The essence of the technique isto choose the switch pair, based on a threshold current, that isrequired to turn on the instant the load current crosses zero andthat will support the polarity of the HF link voltage during theswitching cycle.
The technique is based on five assumptions about the opera-tion of the thyristor-based HF PWM cycloconverter in Fig. 1.
1) For every HF link commutation, the load current min-imum always occurs during the freewheeling mode.
2) There exists a unique switch pair that is gated for everytransition in HF link polarity.
3) Switching occurs only once per HF link commutation.4) There exists a threshold current level such that no HF link
commutation occurs prior to the load current zero.
Fig. 3. SCR output bridge current zero-cross without dead-time, HF Linkvoltage, and converter gating signals. (Color version available online at http://ieeexplore.ieee.org.)
5) There exists a threshold current less than the ripple com-ponent of the load current but greater than the thyristorholding current.
Assumption 1 is verified by inspection of Fig. 2 where the mag-nitude of the load current is always decreasing during a free-wheeling interval (when the voltage imposed on the load is neg-ative) and increasing during a gated interval (when the voltageimposed on the load is positive). Assumption 2 is satisfied bythe requirements of providing a continuous current path andavoiding shorting the HF link. Assumption 3 prevents commu-tation failure from reverse-recovery problems. Assumption 4,shown in Fig. 3, is fulfilled by choosing the threshold limits ar-bitrarily close to zero. Assumption 5 implies that the thresholdlimits should be set near the holding current. The polarity ofthe load current is required but can be obtained by a variety ofknown techniques. Current polarity signals are defined such that
and .There are four possible sequences for commutating the load
current through a zero. The two for positive-to-negative com-mutation are shown in Fig. 4; the other two are mirror images.The details corresponding to Fig. 4(a) are shown in Fig. 3 andcan be compared directly with the previous dead-time techniquein Fig. 2. In both figures, prior to the HF link changed po-larity and current is decreasing in magnitude, freewheeling viathe outgoing converter (Q3, Q4). At time the incoming con-verter SCR pair (Q1, Q2) is turned on according to the PWMprocess in [2]. The HF link voltage across the load is in the pos-itive direction and results in an increase in the magnitude of theload current. At time , the HF link voltage becomes negativeand gate pulses to the SCR pair are halted. The load current con-tinues to flow, freewheeling through the previously gated SCRs.When dead-time control is used, as is Fig. 2, the load currentfreewheels until it decays to zero at time . The load currentremains zero until time when the next SCRs are gated on bythe PWM process. This dead time is required to allow reverse re-covery when Q5–Q8 are gated on as one group. In the new tech-nique, the load current freewheels after time until it crossesthe positive current threshold at time . At this point theSCRs of the incoming converter (Q7, Q8) are gated on to allowa conduction path for current reversal. However, the load cur-rent continues to flow through the freewheeling SCR pair (Q1,
BALOG AND KREIN: COMMUTATION TECHNIQUE FOR HIGH-FREQUENCY LINK CYCLOCONVERTER 103
Fig. 4. Load current zero-cross commutation sequence. (a) Positive to negativeload current commutation for negative HF link voltage. (b) Positive to negativeload current commutation for positive HF link voltage.
Fig. 5. Cycloconverter state machine.
Q2) until when the current commutates to the incoming con-verter (Q7, Q8). Thus occurs naturally as determined by theload characteristics, not by imposed switching. The incomingconverter of both techniques carries increasing current prior to
when the HF link changes polarity, the current freewheels(decreases in magnitude), and the load current zero-cross com-mutation is complete.
Managing the HF link and current zero-cross commutationswitch sequences is easily accomplished with a state-machine,shown in Fig. 5. Each of the four states is the currently con-ducting switch pair (the outgoing converter). The input signalsdetermine the next state (the incoming converter switch pair)
Fig. 6. Cycloconverter bridge waveforms and logic signals. (Color versionavailable online at http://ieeexplore.ieee.org.)
and are comprised of the HF polarity (defined to be “1” for pos-itive voltage and “0” for negative voltage) and and (previ-ously defined). The transition from Q1Q2 to Q7Q8 correspondsto a change in the HF link polarity from positive (“P”) to nega-tive (“N”) and a load-current commutation from positive (“P”)to negative (“N”), as in Fig. 4(a). State transitions are clocked bythe HF link switching to enforce assumption 3. During commu-tation of the HF link, only the link polarity changes. It is shownas state transitions between vertical states where the delay PWMsignal is used for positive load current and the advance signalfor negative current [1]. Load-current zero-cross commutationis a transition between horizontal states. To prevent commuta-tion failure, only the state-transitions shown are allowed.
III. EXPERIMENTAL RESULTS
The state-machine from Section II was implemented in aXilinx CPLD with additional states added to accommodateerror-correction and startup/shutdown operation. The HFPWM cycloconverter output-bridge waveforms for a R-L load(25 mH, 2.5 ) are shown in Fig. 6 along with the logic signalsfrom the state machine. In practice, an output filter would beused to smooth the ripple. Signals AD and DE are derived froma 2 kHz ramp, as in Fig. 13 in [1]. The state machine steers theAD and DE pulses to create output signals Q1Q2S, Q3Q4S,Q5Q6S, and Q7Q8S which trigger multivibrators to providethe four gate-drive pulse trains.
The load-current zero-cross and the HF link voltage areshown in detail in Fig. 7. The load current smoothly commu-tates from the positive converter to the negative converter, as inFig. 4(a), subject to the holding current of the SCR (shown as asmall discontinuity at the zero-cross.) What appears to be dis-continuous current during the positive HF voltage immediatelyfollowing the zero-cross is due to the dead time introducedby the voltage-sourced push-pull converter generating the HFlink. Thus the proper sequencing of the SCR switches preventscommutation failure in a noncirculating current mode cyclo-converter without requiring dead-time.
104 IEEE POWER ELECTRONICS LETTERS, VOL. 3, NO. 3, SEPTEMBER 2005
Fig. 7. Load current zero-cross in the cycloconverter bridge. (Color versionavailable online at http://ieeexplore.ieee.org.)
IV. CONCLUSION
A technique for controlling an HF link cycloconverter waspresented. It eliminates commutation failure without using ei-ther additional components that lower the energy conversion ef-ficiency or switching dead time that introduces zero-cross dis-tortion. By identifying the proper sequence of switching in theoutput bridge, load current is allowed to change polarity (zerocross) naturally as determined by the load characteristics. Al-though the technique is presented within the context of the HFLink PWM Cycloconverter with SCRs as the switching devices,
it can be applied to any HF link cycloconverter using unidirec-tional switch elements.
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