Lambda LP Series - Theory of Operation; General Circuit Description; Functional Description

THEORY OF OPERATION
GENERAL
The Lambda power supply consists of an AC input circuit and transformer; a bias
supply consisting of an auxiliary rectifier and filter, and preregulator; a main regulator
circuit consisting of the main rectifier and filter, a series regulator, emitter follower
driver(s), a current comparator, a voltage comparator, a voltage amplifier, current and
voltage sensing networks and a voltage reference circuit.
The circuit arrangement is shown in block diagram form, · Figure 10. The circuitry
is discussed with reference to the block diagram and the schematic diagram.
FUNCTIONAL DESCRIPTION
Single phase input power is applied to transformer Tl through the input circuit con-
taining a built-in thermostat and fuse Fl, which protect the supply against overheating and
internal faults.
The main rectifier, a full-wave rectifier, provides the power which is filtered by
capacitor C9 and then regulated via a series regulator and delivered to the output. Half-
wave auxiliary rectifier CRl provides voltage filtered by capacitor C2 for voltage amplifier-
Q4 and preregulator Ql. Preregulator Ql provides regulated power for zener diode CR3,
which is the reference element for the current sensing circuit, for current comparator Q2,
Q3,
voltage comparator Q5, Q6, and for Q7 and zener diode CR7, which together function
as the voltage reference for constant voltage operation. CR8 provides voltage compensation
which acts to reduce the effect of temperature changes on the emitter-base bias of Q7.
Constant voltage or constant current crossover circuit operation is determined by
changes in the load. A change in the output voltage is sensed by voltage sensing divider
R30A, B, which has a constant current of 5 milliamperes flowing in the divider element ,
determined by Q7 and the setting of calibration control R20. This output voltage variation
causes a change to the input of Q6 of the voltage comparator, which compares it with the
reference voltage determined by CR7, resulting in an error signal at the output of Q5. In
constant voltage operation, the voltage across R29 is less than the preset voltage across
R6A, B; Q2 and CR4 are at cutoff; Q3 is in saturation and CR6 is conducting. If the load
current decreases, causing a rise in output voltage, Q6 conducts less, Q5 is biased toward
cutoff and the current from R18 flows through OR gate diode CR6 instead of Q5. This action
causes amplifier Q4 to conduct more heavily, reducing drive current to the base of driver
Q8, causing the series regulator impedance to increase and the output voltage to decrease
accordingl y. · ·
Simultaneously, Q2 of the current comparator samples the load current through
current sensing resistor R29 and compares it with the reference voltage across current
sensing divider R4, R5 and R6A, B, determined by zener diode CR3. In constant current
crossover operation, the voltage across R29 is approximatel y equal to the preset voltage
across R6A, B, Q2 will conduct and bias Q3 toward cutoff, causing the current from RlO to flow
through OR gate diode CR4 instead of Q3. Similarly, this action will cause amplifier Q4 to
conduct more hea v ily, reducing drive current to the base of QS, causing the series regulator
impedance to increase and the output voltage to decrease accordingly, effectively maintaining
a constant current. This output voltage decrease, sensed by the voltage sensing network,
causes Q6 to be reverse biased, turning it off and causes turn-on of Q5 till saturation occurs
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