Citizen CLP-621, Technical Manual

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2-5. Power Supply
2-59
CLP-621 & CLP-631
The following circuit reduces the switching noise. So the output noise and the EMI
(electromagnetic interference) noise are suppressed, and the reversal voltage applied to Q1,
DS2 and DS3 is also reduced.
• C13 and R25 connected between drain and source of Q1 / D1, C6, R7 and R5 connected to
the primary winding N1 / C18, C21, R28, FB2 connected to DS3 / C29, C30, R26, FB1
connected to DS2
(2) Driver circuit
The switching FET (Q1) is driven by the control IC (IC1). When power is turned ON, power is
applied to pin 6 (VCC) of IC1 through R3 and R4. Then, the current starts to flow into the
primary winding N2 and voltage is produced there. When the voltage at pin 6 (VCC) reaches
approx. 16V DC, IC1 is activated and a pulse is output from pin 5 (OUT) of IC1 to drive Q1.
When Q1 starts switching, approx. 20-25V DC is supplied from N2 to pin 6 (VCC) of IC1 to
stably activate IC1. During stable oscillation, the switching pulse is continuously output from
pin 5 (OUT) of IC1 to Q1 through R17, R18 and D3. The oscillation frequency is determined by
R12 connected to pin 1 (RT), and the conduction-angle control is made by the control voltage
fed from the secondary circuit to pin 2 (FB) of IC1. The following timing chart shows the control
waveforms at various points.
0V
0V
0A
0A
0V
Vds of Q1
Pin 2 of IC1 (FB)
ID of Q1
Secondary current i
flowing into DS2/DS3
Pin 5 of IC1 (OUT)
Vds: Voltage between the drain and source of Q1
Id: Current flowing in the drain of Q1