TM 11-6625-2965-14&P

series

regulator so

that

the output current is limited

to the

proper

value.

 

 

4-6 The reference circuit provides stable refer-

ence

voltages

used

in

the constant voltage compar-

ator

and current limit

circuits. The bias circuit

provides the less critical bias voltages used in the supply.

4-7 The meter circuit provides a continuous indication of output voltage or current in both ranges.

4-8 DETAILED CIRCUIT ANALYSIS

4-9 FEEDBACK LOOP

4-10 The feedback loop functions continuously to keep the output voltage constant during normal operation of the supply. For purposes of this discus- sion, assume that the output voltage instantaneously rises (goes positive) due to a variation in the external load circuit. Note that the change may be in the form of a slow rise in the output voltage or a positive going ac signal. An ac signal is coupled to summing point A6 through capacitor Cl and a dc voltage is coupled to A6 through R 10.

4-11 The rise in output voltage causes the voltage at A6 and thus the base of Q1A to decrease (go neg- ative). Q1A now decreases its conduction and its collector voltage rises. The positive going error voltage is amplified and inverted by Q3 and fed to the base of the series transistor(s) via emitter follower Q4. The negative going input causes the series transistor(s) to decrease its conduction so that it drops more of the line voltage, reducing the

output

voltage to its

original

level.

 

4-12

If

the external

load

resistance

decreases to

a certain

crossover point,

the

supply

will operate

in the current limiting mode. In the current limit mode, Q1O conducts sending a negative going, turn-down signal to the series regulator via driver Q4 .

4-13 SERIES REGULATOR

4-14 The series regulator consists of transistor stage Q7 (and Q6 on Model 6206 B). The regulator serves as a series control element by altering its conduction so that the output voltage is kept constant and the current limit is never exceeded, The conduction of the transistor(s) is controlled by the feedback voltage obtained from driver Q4. Diode CR11, connected across the regulator circuit, protects the series transistor(s) against reverse voltages that could develop across it during parallel or auto-parallel operation if one supply is turned on before the other.

4-15 CONSTANT VOLTAGE COMPARATOR

4-16 The circuit consists of the coarse and fine programming resistors (Rl0A and R 10 B), and a differential amplifier stage (Ql and associated com-

ponents). Transistor Q1 consists of two transistors housed in a single package. The transistors have matched characteristics minimizing differential voltages due to mismatched stages. Moreover, drift due to thermal differentials is minimized, since both transistors operate at essentially the same temperature.

4-17 The constant voltage comparator continuously compares a fixed reference voltage with a portion of the output voltage and, if a difference ex- ists, produces an error voltage whose amplitude and phase is proportional to the difference. The error output is fed back to the series regulator, through the (mixer) error and driver amplifiers. The error voltage changes the conduction of the series regulator which, in turn, alters the output voltage so that the difference between the two input voltages applied to the differential amplifier is reduced to zero. The above action maintains the output voltage constant.

4-18 Stage Q1B of the differential amplifier is connected to a common (+S) potential through impedance equalizing resistor R5. Resistors R6 and R8 are used to zero bias the input stage, offsetting minor base-to-emitter voltage differences in Q1. The base of Q1A is connected to a summing point at the junction of the programming resistors and the current pullout resistors, R12 and R13. Instantaneous changes in output voltage result in an increase or decrease in the summing point po- tential. Q1A is made to conduct more or less, in accordance with summing point voltage change.

The resultant output

error voltage is fed back

to

the

series regulator

via

the remaining

components

of

the feedback

loop.

Resistor

Rl, in

series

with

the

base Q1A,

limits the current

through the

pro-

gramming resistors during rapid voltage turn-down. Diodes CR1 and CR2 form a limiting network which prevent excessive voltage excursions from over driving stage Q1A. Capacitor Cl, shunting the programming resistors, increases the high frequency gain of the input amplifier. Resistor R1 3, shunting pullout resistor R12, is factory selected so that all of the + 6.2 Volt reference is dropped across R12 and R13. Linear constant voltage programming is assured with a constant current flowing through R1O. C20 stabilizes the feedback loop and may be removed to avoid current surges and increase the programming speed.

4-19 ERROR AMPLIFIER AND DRIVER

4-20 The error and driver amplifiers amplify the error signal from the constant voltage comparator circuit to a leve1 sufficient to drive the series regulator transistor(s). Driver Q4 also receives a current limiting input if Q10, the current limiting transistor, conducts.

4-2