AP3306

Document number: DS42803 Rev. 6 - 2

10 of 13

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May 2022

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AP3306

Operation Description (continued)

Brown-In/Out Protection

To avoid potential high-current stress at low-line voltage, the AP3306 introduces a reliable brown-out protection. The AC line voltage is detected

through the HV pin. A pair of high-voltage diodes are connected to the AC line, which will rectify the AC input voltage to a double-frequency

positive voltage referring to GND. A 20k

Ω resistor is recommended to be added to improve surge immunity. When the voltage across the HV pin is

higher than VBR-IN for about 100µs once tBR-IN and VCC reaches VST, the GDL pin will output drive signals and the system will start to work. If the

HV pin voltage falls below VBR-OUT and lasts for 50ms of tBR-OUT, the GDL pin will turn off and the system will shut down until the line voltage rises

over its brown-in voltage again.

SOVP/SUVP Protection

The AP3306 provides output OVP and UVP protection functions. The auxiliary winding voltage during the secondary rectifier conducting period

reflects the output voltage. A voltage divide network is connected to the auxiliary winding and DEM pin. The DEM pin will detect the equivalent

output voltage with a delay of tSAMPLE from the falling edge of the GATE driver signal, as shown in Figure 8. The detected voltage will be compared

to the SOVP and SUVP threshold voltage VTH-SOVP and VTH-SUVP. If the SOVP threshold is reached continuously by seven switching cycles,

SOVP protection will trigger. If the SUVP threshold is reached continuously and the FB voltage is pulled high, SUVP protection will trigger. The

AP3306 will shut down after protection is triggered and the system will restart when the VCC voltage falls below the UVLO voltage.

To prevent a false-trigger of SUVP during the start-up process, a blank time of tBLANK-SUVP is set, during which the SUVP protection function is

ignored.

VAUX

VDEM

0V

Gate

tSAMPLE

System Protection

LOVP, FOCP, SSCP, VCC OVP, OTP

The AP3306 provides versatile protections to ensure the reliability of the power system. LOVP achieves line-voltage overvoltage protection. If the

detected AC line voltage is higher than VLOVP for seven switching cycles, the LOVP protection will trigger.

FOCP protection is an ultra-fast short-current protection, which is helpful to avoid catastrophic damage of the system when the secondary rectifier

is short. The primary peak current will be monitored by the CS pin through a primary sense resistor. Whenever the sampled voltage reaches the

threshold of VTH-FOCP for seven switching cycles continuously, FOCP protection will activate to shut down the switching pulse.

SSCP may trigger at ultra-low DC bus voltage conditions or other failure conditions that short the CS pin to ground. The SSCP module senses the

voltage across the primary sense resistor with a delay of 3µs after the rising edge of primary GATE signal. This sensed signal is compared with

VTH-SSCP. If it is lower than VTH-SSCP for seven switching cycles, the SSCP protection will trigger and the driving signal will disable.

All the protections described above will restart the system when the VCC voltage falls below UVLO.

Figure 8. Secondary-Side Voltage Sampling