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AC
line
User supplied
Isolation
Transformer
V-
GND
+52V to +127V
+
+
LOAD
H Dy & EHT
VIDEO
142
SECONDARIES
136
166
137
V
REF.
Error Amp.
Comp.
OSC.
ENABLE
DRIVER
115
C5184
(-200V)
FET
V-
FLYBACK
DIODE
GND
Res.
+
V+
SIMPLIFIED POWER SUPPLY CIRCUIT, FUNCTION, DESCRIPTION.
The switching regulator includes the
power FET 136 which passes current from
V- to GND through the inductor
166 .
During the time the FET is on, the current
in the inductor is increasing and the
inductor is storing energy.
As can be seen from the waveforms, the
largest number of changes occur when the FET is
turned off. Also, the FET drain voltage switches
fast due to the high inductor current. To
minimize video interference from the power
supply, the power supply is synchronized to the
horizontal oscillator such that horizontal
blanking is coincident with the FET turn off time.
FET Gate Drive
FET Drain Voltage
Inductor Current
The first FET pulse is a full on pulse
which causes current to flow in the inductor.
After the FET is turned off the current in
the inductor drops much more slowly than
normal since the inductor is discharging
into a much lower than normal voltage. If
the FET were turned on for full power in the
88
the waveforms are:
FET drain voltage
Flyback pulse
Current in diode
Voltage across
137
Current in inductor Current supplying GND
Current from V-
Current added to the +127V line
142
When the FET is turned off, the stored
energy in the inductor continues supplying
current to GND. But in this case, the
current path is from V+ to GND, instead of
V-to GND. During this part of the cycle,
the current in the inductor is decreasing.
The C5184
115 is the series regulator
IC. All of the control circuits that are
built into this IC work together to produce
one output signal, which is the FET drive
signal. This signal can take on many
shapes depending on the load conditions of
the power supply. The waveforms for
normal operation are shown above.
Under normal conditions, the current will decrease to zero and the voltage will ring.
For the shorted +127V to GND condition, which also occur right on power up,
next cycle with current still flowing in the
flyback diode, a current spike of 6A would
occur, which is a power spike of 2,000W.
The reason for this is that the diode stores
charge when current flows which turns into
reverse current for a short time when the
voltage is reversed across the diode.
136
166
115