Patent GB-1084565-A (GB)
Title: Transistor switching circuit
IPC: H02M3/156
Filed: 1965-03-23
Abstract: 1,084,565. Switching circuits; oscillators. GENERAL ELECTRIC CO. March 23, 1965 [April 6, 1964], No. 12278/65. Heading H3T. [Also in Division H2] An oscillator or switching circuit comprises a saturable transformer with one winding connected between base and emitter of a transistor across which is also a tunnel diode connected in like polarity to the base-emitter junction. In Fig. 4, consider the transistor 11 to be cut off and the transformer core 16 to be in negative saturation. Resistors 24, 25 will apply a small bias to the transistor and to tunnel diode 33 so that a small current will flow through each. A small collector current will flow through winding 15 and bring the core out of saturation, inducing a voltage across winding 18 positive at the dot end. This voltage increases regeneratively and eventually switches the tunnel diode to its high-voltage region and turns on the transistor fully to supply current to the load 17. When the core reaches positive saturation the voltage across winding 18 falls to zero and the current through resistors 53, 54 applies a bias to the transistor to turn it off. Diode 41 prevents this bias from being shorted by the tunnel diode. The collapse of current through winding 15 unsaturates the core and induces a voltage in winding 18 that is positive at the no-dot end, thus reverting the tunnel diode to its lowvoltage state. Diode 55 (which may be connected as shown dotted) protects the emitterbase junction from this reverse voltage. Diode 44 prevents impedance mismatch between the tunnel diode and the transistor. A voltage applied to the base of transistor 57 will control the point at which the core reaches positive saturation and hence the conduction period of transistor 11. Similarly a voltage applied to the base of transistor 59 will turn off transistor 11 instantly in case of possible damage. Further control windings can be added if desired. In a modification (Fig. 8, not shown) the regenerative back-coupling to the transformer is omitted and switching is effected by a square-wave signal continuously applied to a winding on the transformer. A further control winding switched by transistor 57 is also provided. In Fig. 9 this principle is applied to the control of a bridge inverter supplying either A.C. or D.C. to a load. Fig. 9 shows one half of the bridge, the other half being identical and the load being connected between the arrows on the two halves (Fig. 10, not shown). The saturable transformer is split into two, 16 and 16<SP>1</SP>, operating on alternate half-cycles of the square-wave signal applied to windings 15, 15<SP>1</SP>. The same signal is rectified at 64a and used as reverse bias for ensuring turn off of the transistor 1 1a. As the value of the voltage on the base of transistor 57 increases, the conducting period of transistor 11a falls from 360 degrees to zero. The rate of switching is twice the frequency of the square-wave signal. As in Fig. 4, multiple control inputs can be provided. The switching currents flow through pulse transformers 71, 72 which apply gating signals to SCR's 73, 74 connected to a similar tunnel diode and power transistor circuit 33b and 11b. When each SCR is turned on, a pulse of current is supplied from the same square-wave source 62 to the transistor 11b, which is provided with a reverse-bias circuit 64b identical to that of transistor 11a. The transistors 11a and 11b are arranged to conduct alternately, and the similar circuit forming the other half of the bridge is switched in synchronism so that current flows to the load through two power transistors in series (Fig. 10, not shown).