Operation description – Diodes AP3968/69/70 User Manual
Page 8
AP3968/69/70
Document number: DS36759 Rev. 2 - 2
8 of 15
December 2013
© Diodes Incorporated
AP3968/69/70
A Product Line of
Diodes Incorporated
Operation Description
(Cont.)
Where f
SW
is the switching frequency. When the peak current Ipk is constant, the output power depends on the switching frequency f
SW
.
Constant Voltage Operation
The AP3968/69/70 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
V
Vo
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.
See equation 5
V
AUX
0V
Portion of Tons
Tons
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 AP3968/69/70 then generates a D1 off-time to
regulate the output voltage.
Constant Current Operation
The AP3968/69/70 is designed to work in constant current (CC) mode. Figure 4 shows the secondary current waveforms.
Is
0A
See equation 8
Tons
Toffs
Iout
Figure 4. Secondary Current Waveform
In CC operation, the CC loop control function of AP3968/69/70 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
3
4
Toffs
Tons
………… (6)
The relation between the output constant-current and secondary peak current Ipks is given by:
Toffs
Tons
Tons
Ipks
2
1
Iout
…………(7)