KX-FT21RS
The following is an overview of how the power supply unit is controlled.
The control method of this power supply unit is pulse width modulation.
When Q1 is ON, the energy is charged in the transformer primary coil according to E1. When Q1 is OFF, the energy is output from the secondary transformer as follows.
L → D1 → Load → L
Then the power is supplied to the Load. When Q1 is ON, power is not output from the secondary side. The output voltage is fed back in the control IC according to the error amp rectifier. Then depending on how TON is controlled, stabilization occurs. Also, when the current load becomes too large, in order to decrease the voltage output, the increase in 
is controlled and the output voltage is stabilized.
Therefore, basically the timing: Ton/Toff of Q1 controls the output voltage.
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