AP3108LS

Document number: DS42105 Rev. 3 - 2

9 of 16

www.diodes.com

February 2021

© Diodes Incorporated

AP3108LS

Operation Description (continued)

Switching Frequency Control Strategy

The AP3108LS works in fixed frequency (65kHz) under heavy load, and decreases the switching frequency to improve the efficiency at light load

and middle load through green mode. In green mode, the switching frequency is a function of VFB and the relationship is shown as Figure 2. If the

VFB is lower than VTH-GREEN-0.2V, the switching frequency is fixed at about 22kHz to avoid audible noise.

Burst mode is a traditional method used to reduce the standby power at no load and extremely light load. In burst mode, the controller will stop

outputting switching pulses. As shown in Figure 3, when VFB drops below VBURST because of the light load, the system will enter burst mode and

there is no more power transferred to the output, causing output voltage to decrease and VFB to recover to VBURST +110mV. Then, the system will

recover and begin outputting switching pulses again, causing the output voltage to rise and VFB to drop again, which will start a new cycle.

fs

22KHz

65kHz

VTH- GREEN+0.2V

VTH- GREEN-0.2V

VFB

Load

FB

Heavy

Load

Light Load

VBURST

VBURST +110mV

fs = 22kHz

GATE

Figure 2

Figure 3

Maximum Output Current Limit (Primary Constant OCP Control)

The traditional primary cycle-by-cycle peak current limit method works well for overload protection situation, but the output short current (peak

value) is still too high, which will result in a higher safety risk. In order to reduce output short current (peak value) and keep normal startup

performance, the AP3108LS creates a new primary current control method to get a constant output current limit, the method is known as primary

constant OCP control. The output current both for CCM and DCM can be both described as:

Io =

VCSM

RCS

∗

NP

NS

∗

tONS

t

Where

RCS is the primary current sense resistor, VCSM is the middle voltage of the current sense voltage across RCS, NP is the primary winding turns,

NS is the secondary winding turns, tONS is the conduction time of secondary rectifier, t is the switching period of the system. In primary constant

OCP control mode, to get a constant output current, the product of

VCSM and

tONS

t

is kept as a constant value equaling to VREF, so the output current

equation can be rearranged as:

Io =

VREF

RCS

∗

NP

NS

∗ KCC

Where KCC is 1/8, an inner parameter used to balance the relationship between the current sense voltage and the primary constant OCP control

signal. For a specific power design, output current can be set by adjusting the value of

RCS. The primary constant OCP control module only

monitors the product of

VCSM and

tONS

t

. A peak current limitation of primary side is also set by VCS-MAX to avoid transformer saturation under some

transient conditions.

The AP3108LS samples the middle current of the primary side to calculate the output current. The detecting time takes the GATE signal as the

reference shown as Figure 4, at the half-on time of the GATE, the AP3108LS will record the VCS value as

VCSM. In the actual system, the primary

current will be greatly impacted by the turn-off delay time which mainly contains MOSFET charging time, resulting in an error between the

detected

VCSM and the actual VCSM. The error varies depending on line voltage, generally increasing with the line voltage. To get a precise VCSM and

keep the output current constant when primary constant OCP module active, the AP3108LS adopts a line compensation technology and the

control block is illustrated in Figure 5. The current flowing through R1 when the primary MOSFET is on reflects the line voltage. Scale down the

current and multiply it with RC and RF, then a compensation signal is formed. The external resistor R1 can be used to adjust the compensation

according to different delay time. The calculating formula is:

R1 =

2 ∗ LP

tD

∗

NA

NP

∗

(RC + RF)

RCS

∗

1

m

Where LP is the inductance of the transformer, tD is the turn-off delay time, NA is the auxiliary winding turns, NP is the primary winding turns, RC is

the inner compensation resistor which is 2.1k

Ω, RF is the filter resistor of SENSE pin, RCS is the primary-current sense resistor, m is the inner

proportional parameter which is 21.