AP3306

Document number: DS42803 Rev. 6 - 2

8 of 13

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May 2022

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AP3306

Operation Description (continued)

Frequency Modulation Strategy

The AP3306 operates with valley switching, ZVS mode, green mode, and burst mode to achieve high-efficiency performance.

In general, the AP3306 power system operates with first

“valley status” under low-line and full-load conditions, where the maximum primary peak

current and transformer flux density can occur. The power system designer is thus required to choose the 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 the AP3306 increases accordingly. In order to avoid

performance degradation at very high switching frequency operation, there is a fixed 125kHz maximum frequency limitation in the AP3306. Since

a too high switching frequency will lead to worsening performance, the AP3306 has a built-in reference in the FB pin voltage to adjust

“peak status”

for green-mode operation, as shown in Figure 5. When the FB pin voltage decreases to a modulating reference, the first

“valley status” is forced to

shift to another available

“valley status”.

The AP3306 has a minimum switching frequency limit of 22kHz to avoid audible noise issues. When the switching frequency decreases to 22kHz

with output load decreasing, the switching frequency will remain at 22kHz. When the FB pin voltage is lower than VBURST, the power system enters

burst mode to reduce power dissipation under very light load conditions.

Figure 5. Frequency Curve

HV Start-Up Circuit for AP3306 Only

A built-in HV start-up circuit in the AP3306 can help to simplify the power system design for ultra-low standby application. For AP3306, there are

two HV start-up charging currents: the ICHARGE-L when VCC is lower than 6V; and the ICHARGE-H when the VCC voltage rises above 6V, which can

prevent the IC from overheating when VCC short-to-GND fault occurs. The HV start-up circuit will stop working with no additional power dissipation

when the VCC voltage reaches the VST, at which point the AP3306 will begin working and will supply energy to VCC from auxiliary winding.

However, the charging process described above is only for the normal system startup condition. Once some system faults occur and the

protection process triggers, the AP3306 will shut down and VCC voltage will begin to decrease. The HV start-up circuit starts working again when

the VCC voltage decreases below VCC-UVLO, and charges the VCC capacitor with the current of ICHARGE-FAULT. This special design can hugely

reduce the input power dissipation when system faults occur, especially for output short conditions. The HV start-up circuit-working processes are

illustrated in Figure 6.