AP3983BCD datasheet(7/13 Pages) DIODES

7 / 13 page

AP3983B/C/D

Document number: DS36784 Rev. 4 - 2

7 of 13

www.diodes.com

July 2016

AP3983B/C/D

Operation Description (Cont.)





AUX







………(5)

Where VD is the diode forward drop voltage, NAUX is the turns of auxiliary winding, and NS is the turns of secondary winding.

See equation 5

VAUX

Portion of tONS

tONS

Figure 3. Auxiliary Voltage Waveform

The output voltage is different from the secondary voltage in a diode forward drop voltage VD 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 VD.

The voltage detection point is portion of tONS after D1 is turned on. The CV loop control function of AP3983B/C/D then generates a D1 off-time to

regulate the output voltage.

Constant Current Operation

The AP3983B/C/D is designed to work in constant current (CC) mode. Figure 4 shows the secondary current waveforms.

See equation 7

tONS

tOFFS

IOUT

Figure 4. Secondary Current Waveform

In CC operation, the CC loop control function of AP3983B/C/D 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



OFFS

ONS

……… (6)

The relation between the output constant-current and secondary peak current IPKS is given by:

OFFS

ONS

PKS

OUT







………(7)

At the instant of D1 turn-on, the primary current transfers to the secondary at an amplitude of:

PKS





………(8)

Thus the output constant current is given by:

OUT





………(9)

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