TM 11-6625-2958-14&P
ing current to the summing point, ensuring that the triac will fire at low output “voltages.
4-24 The summation of the input signals results in the generation of a voltage waveform at TP80 similar to that shown in waveform (A) of Figure 4-4.When the linear ramp portion of the waveform reaches a certain negative threshold voltage, diodes CR74 and CR75 become forward biased. The negative voltage is then coupled to the base of transistor Q72. Transistors Q72 and Q73 form a squaring circuit resembling a Schmitt trigger con- figuration. Q72 is conducting prior to firing time due to the positive bias connected to its base through R84, Transistor Q73 is cut off at this time because its base is driven negative by the collector of Q72.
4-25 When the negative threshold voltage is reached, transistor Q72 is turned off and Q73 is turned on. The conduction of Q73 allows capacitor C71 to discharge rapidly through pulse transformer T70 resulting in the generation of a firing pulse across the secondary winding of T70. As shown in
Figure 4-4.Preregulator Control Circuit Waveforms
waveform (C) of Figure 4-4,the firing pulse is quite narrow because Q73 saturates rapidly, causing the magnetic field surrounding T70 to collapse. Diode CR76 damps out positive overshoot.
4-26 Reset of the control circuit occurs once every
8.33milliseconds when the rectified ac voltage at the junction of CR77, CR78, and CR79 (TP82) in- creases to a level at which diode CR78 becomes forward biased. Summing capacitor C70 is then al- lowed to discharge through CR78. Diodes CR74 and CR75 become reverse biased at reset and transistor Q72 reverts to its “on” state. Consequently, Q73 is turned off and capacitor C71 charges up through R79 at a comparatively slow rate until the collector voltage of Q73 reaches approximately +11 volts. The above action causes the small positive spike that appears across the windings of pulse transform- er at T70 at reset time.
4-27SERIES REGULATOR AND DRIVER
4-28 The series regulator consists of transistors A4Q103 through A4Q108 connected in parallel. The transistors serve as the series or “pass” element which provides precise and rapid control of the out- put. Resistors A4R150 through A4R155 allow high output currents to be equally shared by the series regulator transistors. The conduction of the series transistors is controlled by signals obtained from driver A4Q102, which is connected in a Darlington configuration with the parallel-connected series regulator transistors. Thermal switch A4TS101 opens if the heat sink assembly temperature exceeds approximately 230°F, thus turning off the series regulator transistors. This feature protects critical components of the supply from excessive temperatures which could occur if cooling fan A4B1 failed. Diode CR50 provides a discharge path for the output capacitors when the supply is rapidly down- programmed; R57 limits the discharge current flowing through the diode and through error amplifier A4Q101. Diode A4CR105, connected across the regulator circuit, protects the series elements from reverse voltages that could develop across them during parallel operation if one supply is turned on before the other.
4-29SHORT CIRCUIT PROTECTION
4-30 This circuit acts to initially protect the series regulator against a simultaneous full-voltage, full- current conditions such as might occur if the output were shorted when the controls were set to deliver a high output voltage and current. Under this con- dition, Q20 goes into heavy conduction due to the increased voltage across the series regulator, putting R26 in parallel with the current controls and thus limiting the current to less than 10% of the supply’s rating. Within 10 milliseconds after the short circuit is imposed, the preregulator shuts off.
