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Service manual WP 895/895F, CP885/885F
When the MOSFET is ON, the STR-F6654 internal capacitor C1 is charged at the constant voltage 6.5V.
At the same time, the voltage at pin 1 (OCP / FB) increases with the same waveform as the MOSFET drain current.
When the pin 1 voltage reaches the threshold voltage V
TH1
= 0.73V, the STR-F6654 internal comparator 1 starts
operating. The STR-F6654 internal oscillator is inverted and the MOSFET turns OFF.
When the MOSFET turns OFF, charging of STR-F6654 internal capacitor C1 is released and C1 starts discharging
by the STR-F6654 internal resistance R1. So, C1 voltage starts falling in accordance with the gradient regulated by the
constant discharging time of C1 and R1. So, this means that the fixed time determined by C1 and R1 is the OFF-time
of the MOSFET.
When C1 voltage falls to around 3.7V, the STR-F6654 internal oscillator is reversed again and the MOSFET turns
ON. C1 is quickly charged to around 6.5V
The MOSFET continues to oscillate by repeating the above procedure.
5-9-2-3
STR-F6654 protection circuits
• overcurrent protection function (OCP)
Overcurrent protection is performed pulse by pulse detecting at STR-F6654 pin 1 (OCP) the peak of the MOSFET
drain current in every pulse.
• latch circuit
This circuit sustains an output low from the STR-F6654 internal oscillator and stops operation of the power supply
when overvoltage protection (OVP) and thermal shutdown (TSD) circuit are under operation
• thermal shutdown circuit (TSD)
This circuit triggers the latch circuit when the frame temperature of STR-F6654 IC exceeds 140
°
C
• overvoltage protection circuit (OVP)
This circuit triggers the latch circuit when the V
in
voltage exceeds 22V (typ.)