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3DQDVRQLF
Chapter
4
2.3. Operation
When the internal FET transistor of IC4801 conducts
the current flows via the primary winding P2 / P1 of
T4801 and IC4801 pin 3 (Drain) and pin 2 (source)
causing a voltage drop across R4805, R4807 to
develop, the voltage as this point being monitored by
the CROW BAR configured zener diode D4805, used
in case of an over voltage condition.
This voltage drop across R4805, R4807 is then fed
back to pin 1 of IC4801 via a noise filter made up of
R4808, C4807.
This feedback voltage at pin 1 (approx. 2.5V) is then
fed to an internal comparator that is used to detect
when the voltage at pin 1 exceeds the internally
generated (0.73V) reference signal. When this
condition is detected the internal FET transistor is
switched OFF.
At the same time once IC4801 begins to operate Vcc
pin 4 is supplied from the drive winding V1/V2 as
mentioned previously.
This voltage developed in the drive winding is also fed
via an RC Network consisting of D4809, R4810,
C4806 and D4807 (located between the drive winding
of T801 and pin 1 of IC4801) which is used to delay the
switch ON of the internal FET transistor, allowing zero
current switching. This reduces switching losses that
occur as a result of operating with high switching
frequencies.
When the internal FET transistor of IC4801 is
switched OFF, as described earlier, the current flow
via the primary winding P1, P2 stops. This results in
the collapse of the magnetic field of T4801 and the
energy stored in the primary winding is transferred to
the secondary windings.
During this period the voltage at pin 1 of IC4801
begins to fall at a rate determined by C4806.
When the internal comparator of IC4801 detects that
the voltage at pin 1 is below the internally generated
(0.73V) reference signal the FET transistor is once
again switched ON and the cycle is repeated.
2.4. Regulation
The power supply ON time is controlled by controlling
the feedback supply to pin 1 of IC4801. This is
achieved by the use of the photocoupler D4811 whose
conduction is varied in response to the load which is
monitored by IC4855.
To monitor the load, the regulator (R) terminal of
IC4855 is connected to the 3.3V supply via the voltage
divider resistors R4858 / R4859.
If any variations are detected by IC4855 via the R
terminal, the cathode (K) terminal will increase the
flow of current via the diode junction of the
photo-coupler D4811 and the cathode (K) and anode
(A) junctions of IC4855.
This in turn controls the flow of current via the collector
/ emitter junction of the photo-coupler D4811 and pin 1
of IC4801, controlling the power supply ON time.
The figure below shows how the ON time changes
against the current fed via D805 to pin 1 of IC4801.
0.900
0.800
0.700
0.600
Ã
v
ƒ
0.500
Ã
6
€
0.400
Ã
i
s
0.300
D
0.200
0.100
0
0
10
If either the A.C. mains input voltage to the SMPS gets
higher, or the load current on the secondary gets
smaller, the K and A junction of IC4855 sinks more
current causing the opto-islator to conduct more.
This causes the current flowing via the photocoupler
D4811 to pin 1 of IC4801 to increase, resulting in the
ON time of the internal FET transistor of IC4801 to
become shorter. This in turn causes the secondary
3.3V level to return to its nominal value.
52
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