Patent GB-1261838-A (GB)
Title: Power converter circuits having a high frequency link
Inventor: MCMURRAY WILLIAM
IPC: H02M5/16
Filed: 1969-04-09
Abstract: 1,261,838. Static converters. GENERAL ELECTRIC CO. 9 April, 1969 [16 April, 1968], No. 18197/69. Addition to 1,261,392. Heading H2F. [Also in Division G3] In a power converter as claimed in parent Specification having a control circuit for selectively rendering conductive the solid state switching means in primary and secondary current paths at a switching rate frequency which is high compared with the supply frequency, there is further provided a phase shift circuit coupled to the control circuit to render conductive the primary and secondary current paths at a selected phase angle one with respect to the other, whereby to control the output voltage. The switches may be transistors as shown, Fig. 3, or Gate-turn-off thyristors (Fig. 10, not shown). Converter circuit, Fig. 3.-As in the parent Specification the load voltage e 2  is at the same low. frequency as the input side voltage e 1 , the input current being chopped by transistor switches and applied in opposite directions alternately through a ferrite core transformer 14 at a high frequency switching rate, e.g. 5-10 kHz. The h.f. current at the secondary side is reformed into a low frequency waveform by the secondary side transistor switches also operating at the h.f. rate. Reversibility of power flow through the converter is possible. Each switch means is bidirectional and as shown, Fig. 3, comprises an anti-parallel pair of' transistors Q1-Q2, Q3-Q4, Q5-Q6, Q7-Q8 respectively. Both transistors in a pair preferably receive base current from a control circuit 52 simultaneously and the circuit conditions determine which conducts or alternatively the base signals may be applied via logic circuits. Thus during the positive half cycles of low frequency input, Q2 conducts via diode 29 and in negative half cycles thereof, Q1 conducts via diode 28 and the transformer primary. In normal operation (i.e. 0 degree phase shift between input and output side switches) Q1, Q2 receive base signals at the same time as Q5, Q6 the transistors Q3, Q4, Q7 and Q8 being OFF. During the next h.f. half-cycles Q3, Q4, Q7, Q8 receive base signals and the others are OFF, and so on. D.C. supply.-The connection of the converter to a D.C. source is envisaged wherein the phase shift control is modulated to obtain a desired output waveform to a load. To obtain a sine wave the phase shift is changed between 0 and 180 degrees and then between 180 and 0 degrees according to a sine wave function. Alternatively the output may be D.C. Control circuit, Fig. 9.-An oscillator 58 supplies clock pulses at twice the h.f. switching rate to drive a flip-flop 59 connected to an amplifier 61 which provides base signals for the input side transistors Q1 to Q4. The oscillator pulses are also fed to a phase shifter 62 which actuates a second flip-flop 63 connected to a base signal drive amplifier 64 for the output side transistors. The output drive 63 is thus synchronized with the input drive 59 but is appropriately phase delayed by the shifter 62. Voltage regulation.-The output voltage is sensed by a transformer 48, is rectified and compared in a differential amplifier 68 with a pre-set level VR eg  and fed to the phase shifter 62 which causes the output side transistor to change conduction states at a different time from the input side transistors, in either leading or lagging sense. Fig. 4 (not shown) illustrates the output voltage and transformer voltage waveforms for different degrees of phase difference between input and output transistor switches. For 90 degrees phase difference for instance the output voltage is zero. Current regulation.-Load current is sensed by a C.T. 49 and compared with a desired value IR eg  in a circuit 71 and a difference value output operates the phase shifter 62 to adjust the output voltage and hence load current. Alternatively the load current signal is compared with a desired current limit value <SP>I</SP>Lim in circuit 71, the phase shifter operating to reduce the output voltage and current, overriding the voltage regulator. Overcurrent protection.-If overcurrent sensed persists for a time determined by a timer 73 (e.g. 20 low frequency input cycles) a base signal modifying circuit 74 sends a signal to drive amplifiers 61, 64 to cut off base signals to all the transistor switches, or to the input side transistors only, preferably at load current zero to minimize reactive current. Alternatively, circuit. 71 may operate the phase shifter 62 if excess current persists for a pre-set time, to give a full phase shift of 90 degrees between input and output side switching causing zero output voltage. After a time delay to allow reactive current to die all the transistors are turned OFF.