Patent GB-1325031-A (GB)
Title: Converter circuit
IPC: H02M7/53862
Filed: 1970-07-09
Abstract: 1325031 Transistor inverter and monostable circuits TEKTRONIX Inc 9 July 1970 [24 July 1969] 33448/70 Heading H3T [Also in Divisions G3 and H2] A D.C. to D.C. converter comprises a resonant circuit 20, 22 connected to a D.C. supply 38, 40 through transistors 70, 72 and the primary winding 42 of a transformer 44 having secondary windings 46, 48, 50 whose outputs are rectified, these outputs being maintained at a desired voltage by comparing a signal represenative of the voltage across winding 42 with a reference signal to obtain a control signal which determines the instant of turn-on of transistor 70 or 72 relative to the zero crossing point of the waveform of the current through members 20, 22, 42. The transistors 70, 72 are alternately switched on and off at corresponding zero crossing points by the windings 78, 80 of a transformer 66, whose primary winding 64 is in series with members 20, 22. This arrangement, in the absence of the control system to be described, switches transistor 70 off and transistor 72 on at one zero crossing point and vice versa at the next point, Fig. 2 (not shown). The control system includes a winding 90 on transformer 66 which prevents the turning-on of the OFF transistor for a period of time following the zero crossing point determined by the unstable period of a monostable multivibrator comprising transistors 174, 176. This period is initiated by each zero crossing of the current in a transformer 62 whose secondary winding 100 momentarily turns-on transistor 162 or 164 to turn off transistor 176 and turn on transistor 174. This period is terminated by turn-on of transistor 176 when a capacitor 180 has charged to a particular voltage in a time dependent on a transistor 182, whose base is responsive to the voltage across winding 42 (via the sensing circuit 102, 104, 106, 194, 196, 190) and which is connected to a reference circuit 198, 200, 202 of which the Zener diode 198 determines the voltage about which regulation occurs. Variation of the voltage across winding 42 above or below the reference value thus causes the clamping period of the winding 90 to be respectively lengthened or shortened to delay or advance the turn-on of the OFF transistor. This has the effect of increasing or decreasing the frequency of the current in members 20, 22, 42, thus decreasing or increasing the voltage across winding 42 to maintain it at the desired value, Figs. 5 and 6 (not shown). If the current reaches an excessive value, the output winding 100 of transformer 62 turns on a transistor 152 through diode 154 to clamp the winding 90 to earth and so limit the current. Members 156, 158, 160 provide transient voltage protection. A starting circuit 108 makes transistor 72 conductive through a charging capacitor 118 and three layer diode 124, subsequent operation of the transistor discharging capacitor 118 every half cycle through a diode 126. By-pass capacitors 112, 114 decouple switching transients from the D.C. source 38, 40, further protection being provided by a choke 116 having coupled coils oppositely wound to provide flux cancellation. Chokes 128, 130 minimize turn-on losses in the transistors 70, 72 and have energy-absorbing circuits 132, 134 and 136, 138. Diodes 140, 142 assist in turning off the transistors.