Service manual

Explanation for circuit operation

Specifications

ITEM Terminal Symbol

MIN TYP MAX

Unit

Voltage via drain-source 1 - 2 VDSS 650 - - V

Drain leakage current 1 - 2IDSS - - 300

ON resistance 1 - 2 RDS(ON) - - 3.90 OHM

Heat resistance - ch-F - - 1.63 C/W

Switching time 1 - 2 Tf - - 250

SEC

4) Explanation for operation

When the VIN terminal (Pin No.4) voltage of STR-F6656, which is charged in C815 and C822

via starting resistance (R809+R817) and (R823+R824) at the early starting, reaches 10V, a

control circuit in SRT-F6656 starts.

Since the circuit current before 6656 operation is limited to 400 (MAX), a high starting

resistance can be used.

Because a sustaining current of latch circuit in 6656 is set to 500 (MAX), this starting

resistance needs to be set so that 500 or more current flows though AC input power is the

lowest.

Because the time until control circuit in 6656 starts is determined after AC input voltage is

input by (R809+R817), (R823+R824) and C815, C822 product, the power start gets slow if

C815 and C822 is set too high.

The voltage via a coil D (d1) of trans (T802. T803) is rectified and smoothed after operation

of 6656 and a stable voltage is supplied for Vin terminal. For coil D1, approx. 15.5V voltage is

rectified and smoothed via R813, R818, D810, D813, C815 and C822 and approx. 15V

voltage is supplied for Vin terminal of 6656.

Since VIN (OFF) voltage is set to 10V (TYP) in 6656, the terminal voltage supplied for Vin

terminal should not be less than 10V (15V at present). The voltage of coil d1 should be set

over operation stop voltage (VIN(OFF)=10V(MAX)) and under OVP operation voltage

(VOVP=16V(MIN)) at normal operating state.

5) Protection circuit

5-1) OCP circuit

This is an overcurrent protection circuit of pulse by pulse type that the peak of MOSFET

drain current is detected by 1 pulse and the output of oscillator is reversed.

To detect the MOSFET drain current, R810 and R820 are connected between source

terminal (Pin No.2) and GND terminal (Pin No.5) of MOSFET and the voltage drop is input to

OCP/FB terminal. The threshold voltage of OCP/FB is set to approx. 0.78V for GND.

5-2) Latch circuit

This circuit stops the operation of power circuit, keeping the oscillator output low when the

overvoltage protection circuit and overheat protection circuit are operated.

Because the circuit current gets less than 400uA when Vin terminal voltage is lower than

operation stop voltage (10V), it begins to increase. Because the circuit current increases

again when it reaches operation start voltage (16V), it decreases. Therefore, this circuit

prevents the Vin terminal voltage from rising, making it goes up and down between 10V and