Theory of Operation
1720/1721
4–19
DIAGRAM 7 LOW VOLTAGE POWER SUPPLY
+5V
Line
rectifier
and filter
Pulse width
modulator
Kick
starter
Voltage
snubber
Base drive
and
switching
transistr
Current
limit
Opto
isolator
Error
Supply voltage
Isolated secondary
AC Line
Isolated secondary
The Low Voltage Power Supply converts the mains line voltage (90–250 VAC)
to supply the power requirements of the instrument. The voltages supplied by
the Low Voltage Power Supply are +40 V, "15 V, and +5 V.
The Low Voltage Power Supply is called a Flyback Switcher. When switcher
mosfet Q9 is turned on, its drain voltage drops to approximately 0 V. The
current through the 350 mH primary winding of T3 begins ramping up. The
voltages present at all secondaries is such that the rectifier diodes are reverse
biased. Energy is being stored in the magnetic field of T3. When Q9 turns off,
the drain voltage “flies back” in a positive direction. Current now flows in all of
the secondary windings and supplies power.
The input line voltage is filtered by the rear-panel connector to reduce the
electrical noise conducted into or out of the instrument. R89 limits the initial
charging current through the rectifier diodes and C54. CR21, CR22, CR23, and
CR24 form a bridge rectifier. C54 filters the 110 to 350 VDC rectifier output.
L4 filters the switching noise produced by the switcher. R102 reduces the
circulating current in the parallel circuit consisting of L4 and C44. DS4, R93,
and R94 form a line voltage indicator. R91 and R92 charge C42. C42 provides
power to U5 until the primary housekeeping winding provides power through
CR17.
Line Rectifier and
Filter
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