AP8012 Ver la hoja de datos (PDF) - Unspecified

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AP8012

OFF LINE SMPS PRIMARY SWITCHER GREEN POWER

Unspecified 

AiT Semiconductor Inc.

www.ait‐ic.com

AP8012

OFF LINE SMPS PRIMARY SWITCHER

GREEN POWER

The Power MOSFET delivers a sense current which is proportional to the main current. R2 receives this

current and the current coming from the COMP pin. The voltage across R2 VR2 is then compared to a fixed

reference voltage. The MOSFET is switched off when VR2 equals the reference voltage.

Leading Edge Blanking (LEB)

At the instant the internal Sense FET is turned on, there usually exists a high current spike through the Sense

FET, caused by the primary side capacitance and secondary side rectifier diode reverse recovery.

Excessive voltage across the sense resistor would lead to false feedback operation in the current mode PWM

control. To counter this effect, the device employs a leading edge blanking (LEB) circuit. This circuit inhibits

the PWM comparator for a short time (typically 500ns) after the Sense FET is turned on.

Under Voltage Lock Out

Once fault condition occurs, switching is terminated and the Sense FET remains off. This causes VCC to fall.

When VCC reaches the UVLO stop voltage, 8V, the protection is reset and the internal high voltage current

source charges the VCC capacitor. When VCC reaches the UVLO start voltage, 14.5V, the device resumes

its normal operation. In this manner, the auto-restart can alternately enable and disable the switching of the

power Sense FET until the fault condition is eliminated.

Thermal Shutdown (TSD)

The Sense FET and the control IC are integrated in the same chip, making it easier for the control IC to detect

the temperature of the Sense FET. When the temperature exceeds approximately 170°C, thermal shutdown

is activated, the device turn off the Sense FET and the high voltage current source to charge VCC. The device

will go back to work when the lower threshold temperature about 140°C is reached.

Over Voltage Protection (OVP)

In case of malfunction in the secondary side feedback circuit, or feedback loop open caused by a defect of

solder, the current through the opto-coupler transistor becomes almost zero. Because excess energy is

provided to the output, the output voltage may exceed the rated voltage, resulting in the breakdown of the

devices in the secondary side. In order to prevent this situation, an over voltage protection (OVP) circuit is

employed. If VCC exceeds 42V, OVP circuit is activated resulting in termination of the switching operation. In

order to avoid undesired activation of OVP during normal operation, VCC should be properly designed to be

below 42V.

REV1.0 ‐NOV2008 Released ‐

‐7‐

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