AP3965/66/71
Document number: DS36531 Rev. 3 - 2
7 of 13
www.diodes.com
November 2013
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AP3965/66/71
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Diodes Incorporated
Operation Description (cont.)
Constant Voltage Operation
The AP3965/66/71 captures the auxiliary winding feedback voltage at FB pin and operates in constant-voltage (CV) mode to regulate the output
voltage. Assuming the secondary winding is master, the auxiliary winding is slave during the D1 on-time. The auxiliary voltage is given by:
()
d
S
AUX
AUX
VVo
N
N
V +⋅=
…………(5)
Where V
d
is the diode forward drop voltage, N
AUX
is the turns of auxiliary winding, and N
S
is the turns of secondary winding.
Figure 3. Auxiliary Voltage Waveform
The output voltage is different from the secondary voltage in a diode forward drop voltage V
d
which depends on the current. If the secondary
voltage is always detected at a constant secondary current, the difference between the output voltage and the secondary voltage will be a fixed V
d
.
The voltage detection point is portion of Tons after D1 is turned on. The CV loop control function of AP3965/66/71 then generates a D1 off-time to
regulate the output voltage.
Constant Current Operation
The AP3965/66/71 is designed to work in constant current (CC) mode. Figure 4 shows the secondary current waveforms.
Figure 4. Secondary Current Waveform
In CC operation, the CC loop control function of AP3965/66/71 will keep a fixed proportion between D1 on-time Tons and D1 off-time Toffs by
discharging or charging the built-in capacitance connected. This fixed proportion is
2
4
Toffs
Tons
=
………… (6)
The relation between the output constant-current and secondary peak current Ipks is given by:
ToffsTons
Tons
Ipks
2
1
I
OUT
+
⋅⋅=
…………(7)
At the instant of D1 turn-on, the primary current transfers to the secondary at an amplitude of:
Ipk
N
N
Ipks
S
P
⋅=
…………(8)
