AP3128

Document number: DS43487 Rev. 3 - 2

7 of 12

www.diodes.com

September 2021

© Diodes Incorporated

AP3128

Operation Description

The AP3128

’s Multi-Mode Operation includes Burst mode, QR mode, and CCM mode (which are specifically designed for off-line AC-DC power

supplies used in LCD monitors), notebook adapters, and battery charger applications. At medium load, the IC will enter QR mode with frequency

foldback to improve system efficiency and EMI performance. It offers a cost-effective solution with versatile protection functions.

Figure 3

Multi-Mode Operation

The switching frequency curve in Figure 3 shows three operation modes.



Burst Mode

At no load and light load, the system will operate at burst mode. In burst mode, the switching frequency is fixed at about 22kHz to avoid

audible noise. When the FB voltage is lower than 0.6V, the controller will not provide driving signal and the FB voltage will rise above 0.8V,

then the driving signal will resume. By this control strategy, the system will eliminate a bunch of pulses and the power loss is reduced.



QR Mode

QR stands for Quasi-Resonant, which means that the power MOSFET is forced to turn on at valleys of VDS. With QR control, the switching

power loss will be reduced by lower voltage stress of MOSFET. The VDS valley is detected by the DEM pin through the voltage divider network

of RDEM and RSOVP. Once the divided voltage is less than 100mV during the internal turning off of the MOSFET, the counter in the AP3128

will count it as one valley. At light load and medium load, the system will enter QR mode. As the load changes, the trend of the switching

frequency in QR mode is modulated by the AP3128.



CCM Mode

With the load increases, if the switching frequency decreases below 80KHz in valley1 mode, CCM mode is implemented to achieve high

efficiency. Usually at low line voltage, the system will enter CCM mode at heavy load, while at high line voltage the system may still operate

at QR mode with the 1st or 2nd valley on.

105KHz

80KHz