TM 11-6625-2958-14&P
and optional resistor R 117. The other input of the differential amplifier (pin 4) is connected to a summing point (terminal A6) at the junction of the programming resistors and the current pullout resistors R30 and R31. Changes in the output current due to load changes or changes in the voltage at the summing point due to manipulation of the CURRENT controls produce a difference voltage between the two inputs of the differential amplifier. This difference voltage is amplified and appears at the output
of the differential amplifier (pin | 6) as an error volt- |
age which ultimately varies the | conduction of the |
s cries regulator. | |
4-42 Resistor R30 serves as a trimming adjustment for the programming current flowing through A5R123 and A5R124. If the supply is equipped with Option 021, resistor R115 and potentiometer R116 provide a means of adjusting the programming current. As in the constant voltage comparator circuit, a variable input bias (from resistor R118 and potentiometer R119) is provided to allow the output current to be adjusted to exactly zero when the supply is programmed for zero output. Diode CR21 limits excessive voltage excursions at the summing-point input to the differential amplifier.
4-43VOLTAGE CLAMP CIRCUIT
4-44 The voltage clamp circuit keeps the constant voltage programming current relatively constant when the power supply is operating in the constant current mode. This is accomplished by clamping terminal A2, the voltage summing point, to a fixed bias voltage. During constant current operation the constant voltage programming resistors are a shunt load acress the out put terminals of the power sup- ply. When the output voltage changes, the current through these resistors also tends to change. Since this programming current flows through the current sampling resistor, it is erroneously interpreted as a load change by the current comparator circuit. The clamp circuit eliminates this undesirable effect by maintaining this programming current at a constant level.
4-45 The voltage divider, Z2A, Z2B, and VR1, back biases CR2 and Q1 during constant voltage operat ion. When the power supply goes into constant current operation, CR2 becomes forward biased by the voltage at pin 12 of Z 1. This results in conduction of Q1 and the clamping of the summing point at a potential only slightly more negative than the normal constant voltage potential. Clamping this voltage at approximately the same potential that exists in constant voltage operation results in a constant voltage acress, and consequently a constant current through, the current pullout resistors R3, R4, and R5.
4-46MIXER AND ERROR AMPLIFIERS
4-47 The mixer and error amplifiers amplify the error signal from the constant voltage or constant current input circuit to a level sufficient to drive the series regulating transistors. Mixer amplifier Q41 receives the error voltage input from either the constant voltage or constant current comparator via the OR-gate diode (CR1 or CR20) that is conducting at the time. Diode CR1 is forward biased and CR20 reverse biased during constant voltage operation. The reverse is true during constant current opera- tion.
4-48 Transistor Q40 provides a constant current to the collector of Q41 and also generates a negative going turn-off signal for the series regulator when the unit is first turned off. Feedback network C41, R47, and R53 shapes the high frequency rolloff in the loop gain response in order to stabilize the series regulator feedback loop.
4-49 Error amplifiers Q42 and A4Q101 serve as the predriver elements for the series regulator. In addi- tion, transistor A4Q101 allows faster down-programming by providing a discharge path for output capacitors A3C3 and C19, and by supplying a bleed current for the series regulator (thus keeping it in its linear, active region) when the supply is set for zero output current. Diode CR44, in the base circuit of transistor A4Q101, prevents the base from going more negative than -3 volts. This action limits the current through R57 to a relatively low level, thus protecting A4Q101 from damage in the event a voltage higher than the programmed output voltage is placed across the output terminals (such as might occur in Auto-Parallel or battery charging applications).
4-50OVERVOLTAGE PROTECTION CROWBAR
4-51 The overvoltage protection circuit protects delicate loads from high voltage conditions such as might result from the failure of the series regulator transistor. It accomplishes this by shorting the output of the supply. Under normal operation (no overvoltage), Q92 is conducting since CR91 is reverse biased and Q91 is off. Thus no trigger signal is received by SCR A4CR110 and it acts as an open circuit, having no effect on normal output voltage.
4-52 A5R125 (OVERVOLTAGE ADJUST) adjusts the bias of Q92 with relation to -S. It establishes the point at which CR91 becomes forward biased and Q92 is turned off. Zener diode VR90 provides a stable reference voltage with which the -S potential is compared; R95 sets the upper crowbar trip limit. When Q92 turns off, Q91 begins to conduct, sending a positive going trigger pulse to A4CR110, causing it to create a near short circuit across the