AP3303

Document number:DS41016 Rev. 5 - 2

9 of 16

www.diodes.com

May 2019

© Diodes Incorporated

AP3303

Operation Description

Quasi-Resonant (QR) Mode

Quasi-Resonant operation is regarded as a soft switching technology which always turns on the primary MOSFET at the valley status of Drain-to-

Source voltage (VDS). Compared to traditional hard switching, QR switching-on can reduce the switching power loss of MOSFET and achieve

good EMI behavior without any additional BOM cost.

Available “Valley status”

to turn on the MOSFET

Figure 1

Figure 1 shows the primary MOSFET VDS waveform. When the secondary-side current flows to zero, the primary inductance LM and the effective

MOSFET output capacitor COSS begin to resonant. The valley is detected by DEM pin through a pair of voltage divider. At primary MOSFET

turning off time, once the voltage on DEM pin is detected below 50

mV, one “valley status” is counted. To prevent the false trigger of the VDS ring

caused by leakage inductance, the valley detection function is blanked within the tSAMPLE (2ms, refer to Figure 6) when primary MOSFET turns off.

Each “valley status” of MOSFET VDS will be detected and counted by DEM pin. According to the frequency control strategy of AP3303, one proper

“valley status” will be selected to turn on the MOSFET.

Frequency Modulation Strategy

The AP3303 operates with QR mode, green mode and burst mode to achieve the high efficiency performance.

In general, the AP3303 power system operates with first

“valley status” under low line and full load condition, in which the maximum primary peak

current and transformer flux density occur. The power system designer is required to choose transformer size and switching frequency according

to this worst case condition.

With output load decreasing from full load in the first

“valley status”, the switching frequency of AP3303 increases correspondingly. In order to

avoid performance degrading at very high switching frequency operation, there is a fixed 120kHz maximum frequency limitation in AP3303. Since

too high switching frequency will lead to the worse performance, the 120kHz frequency limitation is not preferred to reach in system design.

Actually AP3303 has built-in reference in FB pin voltage to adjust

“valley status” for green mode operation, as shown in Figure 2. When FB pin

voltage decreases to a modulating reference, the first

“valley status” is forced to shift to other available “valley status”.

The AP3303 has the minimum switching frequency limit of 24kHz to avoid audible noise issue. When the switching frequency decrease to 24kHz

with output load decreasing, the switching frequency will keep at 24kHz. When FB pin voltage is lower than VBURST, the power system enters burst

mode to reduce the power dissipation under very light load condition.