Panasonic GP 31, Technical Manual

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signal. When this condition is detected, the internal MOSFET transistor is switched
OFF.
4. When the internal MOSFET transistor of IC801 is switched OFF, the current flow
via the primary winding P1, P2 stops. Energy stored in the primary winding is
transferred to the secondary windings.
5. When the internal comparator of IC801 detects that the voltage at pin 7 is below the
internally generated 0.94V reference signal, the internal MOSFET transistor of
IC801 is switched ON and the cycle repeated.
6. The voltage developed in the driving winding V1/V2 is also fed via an RC Network
consisting of D824, R824, C830 and D823 which is used to delay the switch ON of
the internal MOSFET transistor, allowing zero current switching. This reduces
switching losses that occur as a result of operating with high switching frequencies.
7. C825, R830 construct the filter circuits that prevent malfunctions caused by the surge
current generated during the switch ON of the internal MOSFET transistor.
8.3 Regulation
1. The power supply ON time is controlled by controlling the feedback supply to pin 6
of IC801. This is achieved by the use of the photocoupler IC860.
2. The photocoupler current varies in response to the current drawn via pin 2 of IC802.
3. IC802 is used to monitor the +B supply by comparing the +B secondary voltage with
an internally established reference voltage within IC802, which in turn controls the
current drawn via pin 2.
4. If either the AC mains input voltage gets higher or the load current on the secondary
gets smaller, pin 2 of IC802 sinks more current causing the photocoupler to conduct
more.
5. This causes the current flowing via the photocoupler IC860 to pin 6 of IC801 to
increase, resulting in the ON time of the internal MOSFET transistor of IC801
becoming shorter. This in turn causes the secB level to return to its nominal
value.
8.4 Over Voltage Protection Circuit
1. When the voltage imposed between Vin and Ground terminals exceeds 27.7V, this
circuit starts its operation and triggers the latch circuit, and the control IC stops its
oscillation.
2. Although this circuit basically functions as protection of the Vin terminal, it also
prevents overvoltage at the secondary output (ie open circuited or by some other
event), since the Vin terminal voltage is supplied from the drive winding of the
transformer, whose voltage is proportional to the output voltage from the secondary
windings.