A NEW BIDIRECTIONAL ZVS SWITCH FOR DIRECT AC/AC CONVERTER APPLICATIONS Marco V. M. Villaqa* and Arnaldo J. Perin
Federal University of Santa Catarina - Dept. of Electrical Engineering Power Electronics Laboratory - P. 0. Box 5 119 88.040-970 - Florianopolis - SC - Brazil Tel.: (55) 482-319204 - Fax: (55) 482-319770 - E.Mail:
[email protected] Abstract - This paper introduces a new bidirectional soft commutation switching cell aimed to use in direct AClAC converters. The proposed technique features no dead time between commutations of different switches and over-current protection thanks to the use of the principles of the dual thyristors. Because all the switches are driven at the same time, this cell allows to design a simple gate signal drive circuitry. As a consequence, all types of AC modulations can be used and the bidirectional switches can operate at high frequency of commutation. Another advantage of this technique is that only two power switches are conducting at the same time with no increasing of the conduction losses. A prototype of an AC chopper operating at 20Khz using IGBT was implemented in laboratory. Analysis and design are also presented in the paper.
only two power switches are conducting the main current at the same time. This means that the implementation of the ZVS commutation will not give more conduction losses than the traditional hard commutation switch. 2. THE PROPOSED CIRCUIT AND PRINCIPLE OF OPERATION
Consider the ac chopper shown in Fig. 1.a where a high frequency output filter and a series LR load are also included and the switches SI and Sz are bidirectional ones. r
(a>
1. INTRODUCTION
The main problem to use the A U A C direct converters are the commutation between the switches. Normally it is necessary to use a dead-time between the gate signals to switch off one switch and to switch on another one. In order to protect the components of the converter from voltage spikes, during the switching off operation of an inductive circuit, it is common to use a clamping circuit. As a result, a big portion of the source energy is deviated from the load and usually is dissipated, reducing the efficiency of the converter. The ZVS bidirectional switch proposed in this paper is naturally protected against over current and does not need dead-time interval during commutations thanks to the use of the Dual Thyristors technique [l]. Furthermore, the use of soft commutation ZVS principles [2] allow to increase the maximum switching frequency of this type of converters without increase currentholtage stresses on switches. The proposed ZVS bidirectioIial switch is implemented using hvo bidirectional active sw tches, one auxiliaq active switch as well some passive elements. The advantage of this structure, in comparison with other proposition [J] is that
Fig. 1. - a) Chopper A C and b) the bidirectional switch with the commutation cell
The bidirectional switch u.ith the commutation cell is shown in Fig. 1.b. The circuit can be divided in tlvo sections. The first one is the main bidirectional switch composed by QpI, QF2, D1 and D2. The second section is a commutation cell composed by the auxiliary switch QA, the resonant inductor L,, the resonant capacitor C,, the auxiliary diodes D3 and D4and the auxiliary voltage source E.
* hiarco V. hi. VillaFa is a Lecturer at Federal Technical School of S. Catann? He is currently working on his Ph.D. thesis at Federal University of S. Catarina.
In order to simplifi the description of the circuit
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0-7803-2482-Xl95 $4.00 0 1995 IEEE
operation, all components are assumed ideal, the circuit is in the steady state operation and during the commutations there are no variations either in the input voltage nor in the output current. All the active switches are turned on and off simultaneously.
0
0
There exist four operation modes determined by combining the polarity of the resonant capacitor and the output current (Fig. 2). Mode 1 : IVCRlI ( E , "CRl negative a d I positive lvc~ll( E , VCRl positive and I negative
Tlurd Stage (t2,t3) : Linear Stage - LR starts to discharge linearly while D1conduct a current equal to ILR - I. At the instant t3, ILR= I and the switch Q p l start to conduct (Fig 3.c). Fourth Stage (t&) : The transistor Qpl conducts a current equal to I - ILR and LR is still linearly discharging. At instant b the resonant inductor current is equal to zero and I,, = I, finishing the commutation process (Fig 3.d).
Obsewing the state-plane shown in Fig. 4, the capacitor CR and the main switch @I and Q p 2 are submitted to a voltage stress proportional to the output current. This problem can be solved using a coupled inductance as shown in Fig. 5 .
Or
. IVcRll ( E , VCR~ positive and I positive or
Mode 2 .
IVCRll ( E , VCRl negative and I negative L'CR 1 +I
Mode 3 : IVCRll ) E , V C R ~ positive and I positive or IVCRII ) E , VCRl negative and I negative
cR1'
1 -
cm
Fig. 2 - Equivalent circuit of the Chopper AC with the Bidirectional ZC5 switch.
The operation of Mode 2 is described as follows. The correspondmg topological states are shown in Fig. 3 and the theoretical idealized relevant waveforms in Fig. 4. 0
0
First Stage (t,,,tl) : Linear Stage - S: is opened and SI is closed simultaneously. CR, charges linearly with U2 until Vml = E, at the instant tl (Fig 3.a). Second Stage(tl,t2) : Resonant Stage - At the instant tl a resonant stage comprising LR1CR1and CR2begins. At the instant tZ, Val = 0 and D1starts to conduct (Fig 3.b).
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Fig. 3 - Stages of operation for operation .\lode 2 of the Bidirectional ZI 5 switch.
To guarantee the ZVS operation the converter must be designed respecting the equation (1) in the range of the load variation.
J ( E - V C R ) ~ +( I . Z o ) 2
\
J
t
2
E
This restriction can be eliminated using the variation of the ZVS bidlrectional switch shown in Fig. 6. In this case, the voltage E is not present in the resonant stage operation and the voltage stress now is still present and can be reduced combining the proposition of Fig. 6 with the solution presented in Fig. 5 .
I LZ I
vCRlmaX
CRI
Fig. 4 - Theoretical idealized relevant waveforms of Operation Mode 2 and Bidirectional ZVS switch state plane.
I
Q ~2
I
1
I E
Vc+ E
Fig. 5 - Bidirectional Z VS switch with a voltage clamping and the corresponding state-plane.
'CRl
Fig. 6 - A variation of the bidirectional ZVS switch and the corresponding state plane.
The three previously proposed cells hold as inconvenience the presence of one auxiliary voltage supply that absorbs the energy deriving from commutation of the main switches. The auxiliary supply E can be suppressed by adding in parallel with auxiliary switch a small capacitor (snubber capacitor). In t h s new cell (Fig. 7) the auxiliary switch is commutated off after the voltage of the resonant capacitor drops to zero. The energy accumulated in the inductor is transferred to the snubber capacitor. The inconvenient of t h s cell is the loss in the auxiliary switch because there is no ZCS commutation any longer. In the commutation cell presented in Fig. 7 exists only two
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Assuming that all the components of the circuit are ideal, the equations describing each operation stage of the cell are given as follows.
operation modes:
Mode
'
VCRl negative and I positive or V C R ~ positive and I negative
--
"CR 1
VCRl positive and I negative or Mode : VCRl negative and I positive
+I I -
I IC,
I
Fig. 7 - A simplified commutation cell
The operation of mode 1 is described as follows: e
a First Stage (t&): Resonant Stage - At instant resonant stage comprising LR, CR1and CR2begins. At instant tl, Vml = 0 and D1 starts to conduct. The capacitor Cs2 does not participate in this stage because it is charged with a voltage higher than the maximum voltage attained for C u . (Fig. 8.a) Second Stage (tl,tz): Freewheeling Stage - Diode D1 conducts a current equal to iLR4and the current of the inductor is kept constant. At instant t2, the auxiliary switch is commutated off. (Fig. 8.b) Third Stage (t2,t3): Resonant Stage - The inductor L R begins to discharge in a resonant way. This stage closes at t = t3 when iLR= I and the transistor Qpl starts to conduct. (Fig 8.c)
Fig. 8 - Stages of operation f o r operation Mode 1 of the Bidirectional ZVS Simplified Cell.
The equations that describe the voltage of C R ~and the current of LR, during the first stage are:
Fourth Stage (t3,f4): Resonant Stage - The transistor Qpl conducts a current equal to I - iLRand the inductor current drops to zero at the instant t+ when the transistor Qpl assumes all the load current, finishing the commutation process. (Fig 8.d) 3. MATHEMATICAL ANALYSIS
Were:
In this section a mathematical analysis of the operation mode 1 of the cell presented in Fig. 7 is realized.
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So the maximum values of the voltage of the resonant capacitor and the current of the inductor are given by: VCRI,,,, = J(I'zO)2+[VCR1(0)I2
(6)
4. DESIGN EXAMPLE
We shall work out a numerical example to calculate the circuits parameters of an ac chopper with the following specifications:
(7)
The maximum duration of the first stage is given by equation (8). At1 = (tl -to) =-
vi = 220
- Input frequency:
fi=60Hz
- Output voltage:
vo= 1 1 o v
-Outputcurrent:
Io = 13.6 A
(8)
Z, = 38.2 R
For the third and fourth operation stages the snubber capacitor voltage and the inductor current are gwen by:
2. By setting Z, = 38.2 R in (7), one obtains the maximum inductor current: IL-
(9)
iLR (t) = ILR,,
.sino n*t
v
1. Assuming a maximum voltage equal to 800 V across the main switches, from (6) one obtains:
n
00
- Input voltage:
= 40.2
A
3. To come together (8) and (15) and determining the maximum duration of the resonant stages, the resonant frequency can be determined:
(10)
oo= on= 4 . 7 ~ 1 0 rads ~
1
5. EXPERIMENTAL RESULTS
The proposed operation sequence is valid if: ILR,,
* z n2 VCRl(0)
In order to vellfy in laboratory the operation of the bidxectional ZVS switch, a prototype of a chopper ac converter has been designed and assembled with the following specifications:
(13)
Resulting in:
- Vi = 220 sen (2nfi t), fi = 60 Hz
(input voltage source);
- & = 0 + 2 0 0 and L = 10,4 mH Adopting C S = 2*cR,the third and fourth operation stages present a duration described in equation (15).
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- f, = 20-
(switching frequency)
(output load);
L
Fig. 9 - Power stage diagram of the implemented bidirectional ZvSswitch.
The corresponding power stage diagram is shown in Fig.
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9.
400- .
The components used to build the prototype are as follows:
- QAlrQA2:ID2260a2 (IGBT Module); - CR1, cm = 3.3 nF; C S l , cs*= l0nF
- 16QI0.0
10.0
5.0 Time ( ps)
5601 480-
$
400-
I
320~
1x20
I
,
OJ
