Panasonic EURO 4 Chassis Technical Manual page 14

3DQDVRQLF
8.4. Operation
When the internal MOS-FET transistor of
conducts the current flows via the primary winding
P1 / P2 of T801 and IC801
(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
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
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
switched OFF as described earlier the flow via the
primary winding P2, P1 stops. This results in the
collapse of the magnetic field and the energy stored
in the primary winding is transferred to the
secondary windings.
During this period the voltage at pin 1 of
begins to fall at a rate determined by C819.
When the internal comparator of
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.
pin 3 (Drain) and pin 2
via a noise filter made up of
IC801 begins to operate Vcc
IC801
IC801 detects that
8.5. Regulation
IC801
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 150V (B+) supply by
comparing the 150V 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
Ã
v 
à ƒ
0.500

6
0.400
€
à 
 i
s
0.300
D
0.200
0.100
0
0
is
If either the A.C. mains input voltage to SMPS gets
higher, or the load current on the secondary gets
smaller, pin 2 of
IC801
This causes the current flowing via the photocoupler
D805 to pin 1 of
ON time of the internal MOS-FET transistor of
to become shorter. This in turn causes the
secondary B+ level to return to its nominal value.
14
10 20 30
Uv€rÃÈÆrpÃ
IC850 reduces.
IC801 to increase, resulting in the
40 50
IC801