Panasonic EURO 4 Chassis Technical Manual page 129

3DQDVRQLF
Chapter 4
1.4. Operation
When the internal MOS-FET transistor of IC801
conducts the current flows via the primary winding P1 /
P2 of T801 and IC801 pin 3 (Drain) and pin 2 (source)
causing a voltage drop across R810, R811 to develop.
This voltage drop across R810, R811 is then fed back
to pin 1 of IC801 via a noise filter made up of R809,
C815.
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 MOS-FET transistor
is switched OFF.
At the same time once IC801 begins to operate Vcc
pin 4 is supplied from the drive winding B2/B1 as
mentioned previously.
This voltage developed in the drive winding is also fed
via an RC Network consisting of D804, R815, C819
and D806 (located between the drive winding of T801
and pin 1 of IC801) which is used to delay the switch
ON of the internal MOS-FET transistor, allowing zero
current switching. This reduces switching losses that
occur as a result of operating with high switching
frequencies.
When the internal MOS-FET transistor of IC801 is
switched OFF, as described earlier, the current flow
via the primary winding P2, P1 stops. This results in
the collapse of the magnetic field of T801 and the
energy stored in the primary winding is transferred to
the secondary windings.
During this period the voltage at pin 1 of IC801 begins
to fall at a rate determined by C819.
When the internal comparator of IC801 detects that
the voltage at pin 1 is below the internally generated
0.73V reference signal the MOS-FET is switched ON
and the cycle is repeated.
1.5. Regulation
The power supply ON time is controlled by controlling
the feedback supply to pin 1 of IC801. This is achieved
by the use of the photocoupler D805.
The photocoupler current varies in response to the
level at pin 2 of the comparator IC IC850.
IC850 is used to monitor the B+ supply by comparing
the B+ secondary voltage with an internally
established reference voltage within IC850.
The figure below shows how the ON time changes
against the current fed via D805 to pin 1 of IC801.
0.900
0.800
0.700
0.600
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0.500
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0.300
D
0.200
0.100
0
0 10
If either the A.C. mains input voltage to SMPS gets
higher, or the load current on the secondary gets
smaller, pin 2 of IC850 sinks more current causing the
opto-islator to conduct more.
This causes the current flowing via the photocoupler
D805 to pin 1 of IC801 to increase, resulting in the ON
time of the internal MOS-FET transistor of IC801 to
become shorter. This in turn causes the secondary B+
level to return to its nominal value.
46
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