the supply and, if it exceeds approximately 120% of maximum rated output, sends a turn-down signal to the preregulator control circuit. Hence, the output voltage of the supply is limited to a “safe” value despite any possible failure in the series regulator feedback loop.
4-14 The turn-on control circuit is a long time constant network which allows the supply to achieve a gradual turn-on characteristic. The slow turn-on feature protects the preregulator triac and the series regulator from damage which might occur when ac power is first applied to the unit. At turn- on, the control circuit sends inhibiting voltages to the preregulator control circuit and the s cries regulator (via the error and driver amplifiers). A short time after the unit is in operation, the inhibiting voltages are removed and the circuit no longer exercises any control over the operation of the supply.
4-15 The reference supply provides stable reference voltages used by the constant voltage and current comparators. Less critical operating voltages are obtained from the bias supply.
4-16DETAILED CIRCUIT ANALYSIS (See Figure 7-11)
4-17PREREGULATOR CONTROL CIRCUIT
4-18 The preregulator minimizes changes in the power dissipated by the series regulating transistors due to output voltage or. input line voltage var- iations. Preregulation is accomplished by means of a phase control circuit utilizing triac A2CR1 as the switching element.
4-19 In order to understand the operation of the preregulator, it is important to understand the operation of the triac. The triac is a hi-directional device, that is, it can conduct current in either direction. Hence, the device fires whenever it receives a gating pulse regardless of the polarity of the input a c that is applied to it. The triac is fired once during each half-cycle (8.3 3 millisec- onds) of the input ac (see Figure 4-3). Notice that when the triac is fired at an early point during the half-cycle, the ac level applied to the power transformer is relatively high. When the triac is fired later during the half-cycle, the ac level is relatively low.
4-20 Normally the ac input signal must be at a certain minimum potential before the triac will con- duct. However, A2R1 and A2C1 provide a holding current that allows the triac to conduct at any time during the ac input cycle. RFI choke A2L1A/A2L1B (in series with the triac) slows down the turn-on of the triac in order to minimize spikes at the output of the supply. Components A2CR1, A2R1, A2L1A/ A2L1B, and A2C1 are all mounted inside a shielded
T M 1 1 - 6 6 2 5 - 2 9 5 8 - 1 4 & P
Figure 4-3.Triac Phase Control Over
AC Input Amplitude
box (assembly A2) to minimize radiated and reflected RFI. Further RFI suppression is provided by bypass capacitors C110 and C111.
4-21 The preregulator control circuit samples the input line voltage, the output voltage, and the voltage across the series regulator transistors. It generates firing pulses, at the time required, to fire the triac. This action maintains the ac input voltage across the primary winding of T I at the desired level.
4-22 The inputs to the control circuit are algebraically summed across capacitor C70. All inputs contribute to the time required to charge C70. The input line voltage is rectified by CR81, CR82, CR83, and CR84, attenuated by voltage divider R86 and R83, and applied to the summing point at the collector of Q71 (TP81) via capacitor C70. Capacitor C73 is used for smoothing purposes.
4-23 Transistor Q71, connected in a common base configuration, provides a charging current for the summing capacitor varying in accordance with the input signals applied to its emitter. Resistor R78, connected between the negative output line and the emitter of Q71, furnishes a signal which is proportional to the output voltage. Resistors R75 and R76 sample the voltage across, and the current through, the series regulator. Capacitor C72 and resistor R82 stabilize the entire preregulator feedback loop. Resistors R70 and R80 are the source of a constant offset current which sustains a net negative charg-
