GB2196193A

GB2196193A - Series resonant inverter

Info

Publication number: GB2196193A
Application number: GB08718456A
Authority: GB
Inventors: Michael Peacock
Original assignee: Advance Power Supplies Ltd
Current assignee: APC DC Network Solutions UK Ltd
Priority date: 1986-08-05
Filing date: 1987-08-04
Publication date: 1988-04-20
Anticipated expiration: 2007-08-04
Also published as: GB2196193B; GB8619035D0; GB8718456D0

Abstract

A series resonant inverter comprises two inverter circuits (10, 11) connected in parallel. Both of the inverter circuits have the same resonant frequency and are arranged to operate at identical fixed repetition frequencies. Control means are provided which are operable to vary the phase difference between the inverter circuits to produce a required output current. <IMAGE>

Description

SPECIFICATION Series resonant inverter The present invention relates to series resonant inverter circuits.

Series-resonant inverter circuits are often used because of the inherent short-circuit protection that they provide, and also because of the lower RFI they exhibit when compared to non-resonant inverter circuits.

One disadvantage however is that since the high frequency current in the inverter flows for a time dependent only on the values of inductance and capacitance used to made the circuit resonant, we then have a "fixed-mark, variable space" system of energy transfer.

This in itself is not a problem to implement, but operation of one inverter can become audible at light loads. A high resonant frequency can help here, but at higher powers, where SCR's are needed, the speed of turn-off of the devices limits operation to frequencies below about 100 KHz. Since the mark/space ratio and hence the repetition frequency is dependent on output current, this means that for an inverter having a resonant frequency of 100 KHz then a minimum load of 20% of maximum is required to prevent operation becoming audible. On high power units, an internal "bleed" load of 20% can be a distinct embarrassment if not impossible to provide economically.

The system proposed here overcomes this difficulty by running the inverter at a fixed repetition frequency, by making use of the fact that the series-resonant inverter circuit is essentially a current source. The present invention provides a series resonant inverter comprising two series resonant inverter circuits connected in parallel, both of the inverter circuits having the same resonant frequency and being arranged to operate at identical fixed repetition frequencies; and control means operable to vary the phase difference between the inverter circuits to produce a required output current.

This system becomes especially useful if, as is often the case, inverters are being run in parallel in order to provide full output current, as no extra power components are then necessary.

Features and advantages of the present invention will become apparent from the following description of embodiments thereof given by way of example with reference to the accompanying drawings, in which: Figure 1 shows a block diagram useful for explaining the basis of the present invention; Figure 2 shows a circuit diagram of a halfbridge inverter according to an embodiment of the present invention; Figure 3 shows a circuit diagram of a full bridge inverter according to an embodiment of the present invention; Figure 4 shows a circuit diagram of a bidirectional switch arrangement for use in the circuits of Figs. 3 and 4; Figure 5 shows a waveform diagram for explaining the operation of the circuits of Figs. 3 and 4; and Figure 6 shows a further waveform diagram for explaining further operation of the circuits of Figs. 3 and 4.

Refering to Fig. 1, this shows a block diagram which is useful for explaining the basis of the present invention. Two identical seriesresonant inverter circuits 10 and 11 are connected in parallel across a load 12. Since both inverter circuits are current sources, they can be run in parallel which results in l,=i, +i2 If we require l,=O then i=-i2 (i.e. 1800 phase difference).

Fig. 2 shows a circuit diagram of how the circuit arrangement shown in Fig. 1 can be implemented and the same reference numerals are used for the same parts. It will be seen that each inverter is connected across a D.C.

supply and comprises a series circuit formed by a first switching arrangement A a first inductance LR, a second inductance LR and a further switching arrangement B. The junction between the two inductances LR is fed via a capacitor CR to one terminal of primary winding of an output transformer 14 the secondary of which is connected in a conventional manner to the load 12.

Although simple switches have been shown in Fig. 2, it will be understood that the switching arrangements A and B must be bidirectional and this is best achieved using semi-conductor switching arrangements. Normally these will be bi-polar transistors, SCR's, MOSFET's or GTO's with an anti-parallel diode as shown in Fig. 4.

The resonant frequency of each inverter is given by

The circuit operation is as follows. Both inverters 10, 11 run at exactly the same repetition frequency, and since both inverters have the same inductance and capacitance values they both have the same resonant frequency.

Both inverters have current waveforms which are identical in amplitude and shape as shown in Fig. 5, but the phase displacement (0ì between ii and i2 can be varied. With 0=0, I=i1+i2=2it=2i2 i.e. the waveforms add constructively.

With 0=180 , I=i1+i2=O i.e. the waveforms add destructively. For 00 < < 1800, I varies in a complex manner between these two extremes.

Fig. 6 shows a sawtooth waveform which can be used as the basis of the control of the switches in the inverters. By varying the control voltage between the upper and lower limits of the ramp of the sawtooth, the phase displacement between the inverter currents can be varied.

Fig. 3 shows a full wave inverter constructed from two half wave inverters each as shown in Fig. 2. In this case the inverter arrangement can be likened to two inverters in parallel across the primary winding of the transformer 14 with one half of each inverter on one side of the primary and the other half of each inverter on the other side of the primary. Switches A and D will be operated simultaneously and switches B and C will be operated simultaneously.

The full wave inverter of Fig. 3 will have the same repetition frequency as that of Fig. 2 and the same peak resonant current flows.

The switches A, B, C and D must be bidirectional as before.

Claims

1. A series resonant inverter comprising two series resonant inverter circuits connected in parallel, both of the inverter circuits having the same resonant frequency and being arranged to operate at identical fixed repetition frequencies; and control means operable to vary the phase difference between the inverter circuits to-produce a required output current.

2. A series resonant inverter as claimed in claim 1 in which each inverter circuit is connected across a d.c. power supply and comprises two inductors connected in series, the junction between the inductors being fed via a capacitor to one terminal of a primary winding of an output transformer.

3. A series resonant inverter as claimed in claim 2 in which the control means comprises switching means in each inverter circuit comprising a first switch arrangement connecting one inductor to one terminal of the power supply and a second switch arrangement connecting the other inductor to the other terminal of the power supply.

4. A series resonant inverter as claimed in claim 3 in which each switch arrangement comprises a controlled rectifier device with an antiparallel diode.

5. A series resonant inverter circuit substantially as hereinbefore described with reference to the accompanying drawings.