Patent GB-1291455-A (GB)
Title: A static dc converter
Inventor: LINNMAN SVEN NILS JOHANNES
IPC: H02M3/337
Filed: 1969-10-14
Abstract: 1291455 Static converters S N J LINNMAN and P E C UDDEN 14 Oct 1969 [16 Oct 1968 19 March 1969] 50425/69 Heading H2F] In a D.C./A.C./D.C. converter (e.g. for arc welding or battery charging) comprising a switching means (transistor) Q 1  (Fig. 1) which intermittently and at relatively high frequency connects the D.C. source U e  across a transformer T 1  primary and A.C. produced from its secondary S 2  is rectified and smoothed, the transformer core is pre-magnetized to saturation, in a direction opposite to that produced by current through the switching means Q 1  when conducting, to displace the B/H curve to a position, such as curves b, c or h (Fig. 8), whereat the permeability of the core is low when the magnetizing force is below a certain value and is high when this value is exceeded such that when the core operates in its unsaturated state the flux density increases considerably. Thereby current through the switching means is caused to exhibit a substantially rectangular waveform. The pre-magnetization may be produced by a permanent magnet in the core as Fig. 9 (not shown); or by an extra winding; or by an arrangement Fig. 12, (not shown), wherein a supplementary D.C. source (E 20 ) sends a magnetizing current I m  through the primary of T 1  when Q 1  is not conducting; or as in Fig. 13 (not shown), wherein a diode (D 20 ) is connected in series with the primary of T 1  and in a brief period when the transformer is self-oscillating with C 11  a voltage occurs across the diode which causes a magnetizing current I m  to flow. The pre-magnetization of the core causes the transistor Q 1  to operate with a collector current I c  just blow the maximum permissible value during the period energy is being transferred from the source to the core resulting in improved utilization of the transistor. In addition, the core may have an air-gap, Fig. 10 (not shown), giving a decreased rate of change of collector current I c . Current and voltage waveforms of a converter operating with a pre-magnetized core are illustrated in Fig. 7. Control of output.-The transistor Q 1  (Fig. 1) is controlled via a transformer T 2  from a triggering circuit (not illustrated) including an oscillator. The inverter output is controllable by varying the triggering frequency. Thus by increasing the frequency the ON time of Q 1  is reduced whereby the maximum current value reached by the collector current I c  is reduced and hence the max. and mean current through diode D 1  will also be less. Alternatively, the output may be controlled by varying the premagnetization or by employing a transformer having several secondaries with different transformation ratios, each secondary connected to C 2  or directly to the load via thyristors or electro-mechanical switches (not shown). The output may also be controlled by pulse width modulation i.e. by increasing or decreasing the self-oscillation period during which there is no output.  In Fig. 15 (not shown) two transistors are operated in anti-phase. The transformers T 1  in the converters need not be operated to saturation if an auxiliary choke is provided in parallel with the primary winding thereof and which becomes saturated before the transformer core.