AP39811

Document number: DS39992 Rev. 5 - 2

10 of 18

www.diodes.com

June 2019

© Diodes Incorporated

AP39811

Operation Description (continued)

The energy stored in the magnetizing inductance LM each cycle is therefore:

2

2

1

PK

M

I

L

Eg







(3)

So the power transferring from the input to the output is given by:

SW

PK

M

f

I

L

P









2

2

1

(4)

Where, the fSW is the switching frequency. When the peak current IPK is constant, the output power would depend on the switching frequency fSW.

2.

A Special IV Curve for Shaver Charger

Figure 2. A Special IV Curve

Figure 2 is a typical IV curve special for shaver charger, comparing to a conventional charger. In addition to usual Constant Voltage (CV) part and

Constant Current (CC) part, shaver charger requires a unique Slanted Voltage (SV) part. To achieve this special IV curve, the AP39811 creatively

adopts a new control method, called Enhanced Cable Compensation method.

3.

Constant Voltage Control

The output voltage is proportional to the auxiliary winding voltage during tONS period indicated by Formula 5. This auxiliary winding voltage is

divided by resistors RFB1 and RFB2 before inputting to the FB pin. As Figure 3 illustrated, the AP39811 detects the FB voltage at the end of tSAMPLE

during tONS period, the detected voltage which reflects the output voltage is regulated to VFB of 2.4V with the help of the constant voltage control

block. To be compatible with different system designs and avoid the turn-off spike impact, the tSAMPLE is designed to be part of tONS, usually 55%

of tONS at light load and 60% of tONS at heavy load. For system design, adjusting the ratio of RFB1 and RFB2 can get the target output voltage value.





Vd

Vo

N

N

V

S

AUX

AUX







(5)

Figure 3. Auxiliary Voltage Waveform

0V

tONS

tSAMPLE

tSW

tOFF

Vo

Io

CV

SV

CC