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Theory of Operation
Power Supply Overview
4–12
the VITS 200 tries to lock to program input or EXTernal 1 input. This line
switches the input to the genlock A/D (A1U43, schematic 5), the back porch
signal to the A/D, and the comp sync to the genlock with A1U70 (schematic 3).
A3U61 pin 6 drives the remote indicator output. A3U61 pin 5 goes to A1U14
(schematic 1) and allows the controller to inhibit insertion of test signals while
still passing program signal.
The power supply is located on the Inserter board and is found on schematics
<8> and <9>. This type of power supply is called a current-mode-controlled,
discontinuous, flyback, switching power supply. The current output is distrib-
uted between the four supplies as follows:
+12V
0.5 Amps max
+5V
4 Amps max
5V
1.5 Amps max
12V
0.2 Amps max
The power inductor, T1 (schematic 8), is driven by switching the voltage to its
primary winding on and off at a rate of approximately 45kHz. T1 is not used as
a transformer, but as an energy storage device; the energy is stored in the primary
during the first half of the switching cycle, while voltage is being applied. On
the second half of the switching cycle, voltage to the primary is switched off and
the energy stored in T1 is transferred to the secondaries.
Regulation is accomplished through feedback from the +5V supply to the Pulse
Width Modulator, U78 (schematic 8), controlling voltage to the primary. This
varies the length of time that voltage is applied to the primary, causing it to store
either more or less energy.
There is also circuitry to provide for operation from both 110 and 220 VAC
supplies, overvoltage protection (crowbar) on the +5V supply, and shutdown
circuitry which forces a restart of the supply if it remains in current limit for
more than a short period of time (<1 sec).
WARNING. All primary voltages are referenced to a floating ground, not chassis
ground. An isolation transformer or a differential amplifier is therefore required
for troubleshooting the circuitry in the primary and the Pulse Width Modulator,
and in their supporting circuits.
VITS 200
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