AP3302

Document number:DS38882 Rev. 4 - 2

9 of 17

www.diodes.com

March 2017

© Diodes Incorporated

AP3302

PIN3 Utilization for BNO/SOVP/SUVP/OCP COMP

DEM

VCC

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-

good EMI behavior without any additional BOM cost.

Available “Valley status”

to turn on the Mosfet

Figure 1

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

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

Frequency Modulation Strategy

The AP3302 operates with QR mode, green mode and burst mode to achieve the high efficiency performance.

In general, the AP3302 power system operates with first

“valley status” under low line & 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, the switching frequency of AP3302 increases correspondingly in first

“valley status” operation. In order

to avoid performance degrading at very high switching frequency operation, there is a fixed 120kHz maximum frequency limitation in AP3302.

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

Actually AP3302 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 the secondary “valley status”, and the switching frequency

decrease accordingly. When output load continues decreasing, the secondary

“valley status” will change to the third “valley status”, the fourth

“valley status until the fifteenth “valley status”. When the “valley status” number is higher than 15, the valley turn on function will be disabled since

the benefit of valley turn on is weak enough to ignore. AP3302 uses an advanced

“valley lock” technology to avoid system oscillation and audible

noise issue under the

“valley status” shift condition, in which there is loading value hysteresis when two “valley status” increasing and decreasing

occurs with loading changing.

The AP3302 has the minimum switching frequency limit of 22kHz to avoid audible noise issue. When the switching frequency decrease below

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