Tektronix 1705A Low VoltagePower Supply Diagram 7

Theory of Operation
4-14 1705A Spectrum Monitor
internal pulsewidth modulator. Q588 is the currentsense, sampling current
across R690 to provide a voltage level to the sensing input of U583. Ifthe level
on the over currentsense input (Sen) gets too high, U583 shuts down to prevent
damage.
L695 and C697 are theoutput filter providing a dc output that can be switched
onto the rear-panelL--Band connectorto power a Block Down Converter.
DS698 is an LED indicator light whichis lighted when the +18 V supply is
operating into anormal or no-load condition. If the +18 V output is shorted,the
light will extinguish.
Low VoltagePower Supply
Diagram 7
The Low VoltagePower Supply converts the mains line voltage (90 --250 Vac)
to supply the power requirementsof the instrument. The voltages suppliedby
the Low VoltagePower Supply are +40 V, 15V,and+5V.
The Low VoltagePower Supply is called a Flyback Switcher. Whenswitcher
mosfet Q9 is turnedon, itsdrain voltage drops to approximately 0 V. The
currentthrough the 350 H primary winding of T3 begins ramping up. The
voltages presentat all secondaries is such that the rectifier diodes are reverse
biased. Energyis b eing stored in the magnetic field of T3. When Q9 turns off,
the drain voltage “fliesback”in a positive direction. Current now flows in all of
the secondarywindings and supplies power.