Tektronix 1705A +18 VoltSupply

Theory of Operation
1705A Spectrum Monitor 4-13
The +18 V supply is of the Buck-Regulator type. It uses a Switched Mode
Power Supply Control Circuit, U583. Q783 is a bufferfor slow startup and duty
cycle limit. R681 and R680 arethe limit resistors. The voltage on C684 ramps
up to provide forslow startup when power is initially applied. The Internal
Zener reference (VZ) for U583 is approximately 8V and is present on pin 2.
R586 and C583 are the frequencydetermining components for the IC’s internal
sawtooth generator,which in this instance runs at approximately 50 kHz. R686
and R685 are thevoltage setting components on the feedback input; C683, C684,
and R682 form a frequencycompensation network to prevent the IC from
oscillating.
+40V
+40V
+40V
+18V
+18V
+18V
--0.7V
--0.7V
--0.7V
LOW LOAD
AVERAGE
LOAD
HIGHLOAD
Figure4 -2: Output duty cycle of the pulse width modulator used in the +18 V Power
Supply
Q481 and Q482 drive theswitching transistor. Q482 provides forward base
currentto the switching transistor, Q596. Q483 provides reverse base current to
effectivelyturn off Q596. Q596 operates with voltage levels between --0.7V and
+40 V. CR594 is the commutating diode that sets the voltage level when Q596
is off (--.07 V) and current is still flowing in L591. The input filter circuit,
composed of L591 andC 690, is effectivelydriven with a square wave; however
it is not a true square wave and is dependent on the loading of the +18 V supply.
See Figure 4-2. The output voltage is fed back to the FB input of U583, which
comparesit toan internal reference voltage to determine the on time for the
+18 VoltSupply