A SOFT COMMUTATION CONSTANT HIGH FREQUENCY LINK DC/AC CONVERTER OPERATING WITH SINUSOIDAL OUTPUT VOLTAGE Dalton L. R. Vidor, Arnaldo J. Perin Federal University of Santa Catarina Dept. of Electrical Engineering Power Electronics Laboratory P.O.Box 5 119 - 88040-970 - Florianopolis - SC - Brazil Phone: 55-482-3 1.9204 - Fax: 55-482-3 1.9770 - E. Mail:
[email protected] Abstract - The purpose of this paper is to introduce and analyze a high frequency link DC-AC converter for small power UPS applications, with the following properties: operation at constant frequency, soft commutation, isolation by a high frequency transformer, bi-directional power flow capability, absorption of the parasitic elements in the commutation process, simple control circuitry, no need to detect semiconductor conduction state, and low harmonic distortion at the output voltage.
These properties are achieved with a system composed by a high frequency ZVS inverter and a high frequency ZCS cvcloconverter interconnected each other by a high frequency tiansformer. The output voltage is regulated b; phase-shik between the two converters.
This converter is controlled in a very simple way by the phase difference between the two pairs of switches. One difficulty found in the mentioned circuit is the commutation of the output converter, when it takes place in a conventional way, causes switching stress upon the power semiconductors. This paper presents a new phase-controlled high frequency link DC/AC converter with no switching losses neither in the input converter nor in the output cycloconverter. Furthermore, a control strategy is described that allows a voltage with low harmonic distortion. Such concepts are presented and discussed in the subsequent sections of the paper.
In addition to the analysis results, experimental results taken from a laboratory prototype rated at SOOW are also presented in this paper.
I. INTRODUCTION Many applications, such as micro-computers, office, and home electronics equipment, require. small size and inexpensive UPS’S. Conceiving and designing such systems to meet size and cost specifications represent a challenge that most engineers face in the present days.
A simple technique suitable for such applications has been presented in reference [l], which power circuit is shown in Fig. 1.
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The power stage diagram of the proposed converter, which is a generalization of a D O C converter introduced in [2], is shown in Fig. 2, where L, represents the transformer magnetizing inductance and L incorporates the transformer leakage inductances. The circuit is composed by two stages. The first one is a half-bridge inverter, switching at constant frequency with duty-cycle equal to 0.5. This inverter is formed by E, Q1, Q2, D1, D2, C1, Cz, and L,. It operates at zero voltage switching thanks to the presence of Lm, C1, and C2.
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Fig. 2 - Basic circuit of the DC/AC converter with high frequency link.
Fig. 1 - Phase-controlled DC/AC converter introduced in reference [l]. 0-7803-1859-5/94/$4.00
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+ The second stage is configured as a cycloconverter, composed by four bi-directional switches, an input inductor L and an output capacitive filter. As it is described hereafter, this stage operates in a discontinuous current mode which implies that the switches commutate at zero current. The bi-directional switches are represented by thyristors because they are the power semiconductors that naturally perform commutation at zero-current. The load, not represented in Fig. 2 to simplify the operation description, is connected across the filter capacitor C. The operation of the circuit is described as follows, with the aid of the theoretical waveforms shown in Fig. 3. Let us assume that a positive current is flowing through the output capacitor. In this case the positive group of switches of the cycloconverter is properly activated, which is constituted by T 1 's and T2k. Transistors Q1 and Q2 are gated complementary with duty-cycle equal to 0.5 so that the voltage across a and b is rectangular as shown in Fig. 3.g. The current that flows through Lm, also shown in Fig. 3.g, has a triangular shape and ensures the ZVS of the inverter. First stage (tl, T/2), Fig. 4.a: at the instant tl, the semiconductors TI'S are gated on. The current iL start increasing linearly through E, QI, TI'S, L, and C and reaches its maximum value when t = T12. It is very important lo notice that TI'S are closed with no commutation losses, at zero current. Second stage (tz), Fig. 4.b: at the instant t2. Q1 is turned off. The currents iLm plus iL are deviated softly
from Q1 to D2, after charging C1 and discharging C2. This commutation takes place in a time interval much lower than the switching period. Therefore, it is assumed that it does not affect the power transfer to the load. Besides, during this short interval iL and I L are ~ ~ assumed constant. Third stage (t2, t3) Fig. 4.c: during this time interval Vab is negative, as D2 assumes the current i ~ Therefore . the current iL start decreasing linearly and reaches zero when t = t3. During this time interval Q2 is gated on at both zero voltage and zero current. Fourth stage (t3, t4) Fig. 4.d: during this stage there is no current through the switches of the cycloconverter. The power load is supplied by the output capacitor. Fifth stage (t4, T): at the instant t = t4, the switches T2's are gated on. Again, due to the presence of L, the current increases linearly from zero and the switches commutate with practically no losses. In the subsequent stages, the converter operates cyclically in a similar way. Therefore, they will not be described here. The corresponding topological states are shown in Fig. 4. What determines which group of switches of the cycloconverter is activated is the direction of the current ic through the capacitor. If ic is to be positive, the positive group (TI and T2) is activated, whereas as the negative one (T3 and T4) is gated to get negative current. The output voltage is regulated by the phase difference between the pulses of the bridges control, defined by DT in Fig. 3 .
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Fig. 4 - Topological states for a switching period.
3. THEORETICAL ANALYSIS (4)
In order to simplify the mathematical analysis of the converter, the following assumptions are made:
In the time interval A t2 = t3 - t2,the current IL is given
The transformer magnetizing inductance is very large,
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by:
The commutation is instantaneous,
1
All the semiconductors are ideal,
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The transformer tuns ratio is equal to the unit,
1 The switching frequency is very high in comparison with the output AC frequency, so that the output voltage can be assumed constant during a switching period.
The time interval A t2 = t3 - t2 is given by:
To analyze the steady state circuit behavior the following variables are defined:
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A t, = t2 - t,
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The output average current is represented by expression (7). (2)
At, I , =I&).(-+-) T (3)
Att,
(7)
T
Therefore with expressions (l), ( 3 ) and (4), we obtain:
Where D is defined as the duty-cycle. During the time interval A tl = tz - t,, the current IL is given by: Thus,
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(9)
Where:
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Therefore the inductance value must be:
LI The switching frequency is represented by fs. Expression (9) represents the DC conversion ratio of the converter in steady state, over a switching period, which is shown in Fig. 5 .
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And the maximum output power is obtained when
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Fig. 5 Output characteristic for the positive group.
The maximum duty-cycle takes place when:
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1
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T + At2 = -
Fig. 6 The maximum average instantaneous power transferred to the load.
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Using expressions (l), (3), (4), (5) and (1l), the maximum dutycycle is obtained and given by expression (12).
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The maximum voltage across the switches of the primary side converter is equal to E, whereas the switches of the cycloconverter are subjected by a maximum theoretical voltage given by expression (17).
The output instantaneous average power in the steady state operation is given by equation (13) and represented in Fig. 6.
Where n is the transformer turns ratio. Y
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4. CONTROL STRATEGY The proposed converter must be able to supply power to a load with low output voltage harmonic distortion. This requires an appropriate control strategy. In this paper the control of the instantaneous average output voltage has been adopted, which scheme is shown in Fig. 7. Vo*is a generated sinusoidal reference voltage. The amplitude of voltagc Vc at the output of the PI controller determines the phase delay of the cycloconverter gate signals by means of DT,while the polarity of Vc determines which semiconductors group of the cycloconverter is gated. In other words, DT determines the value of the cycloconverter output current, whereas Vc determines whether th~scurrent is positive to supply charge to the output capacitor, or negative to extract electrical charge from it.
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Fig. 8 a) main waveforms of the implemented control and b) associated phasor diagram, for operation at no load.
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