AP3720 데이터 시트보기 (PDF) - BCD Semiconductor

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AP3720

CURRENT MODE PWM CONTROLLER FOR BJT

BCD Semiconductor 

Preliminary Datasheet

CURRENT MODE PWM CONTROLLER FOR BJT

AP3720

Operation Description (Continued)

Protection Circuit

The AP3720 has several self-protective functions

such as over load protection (OLP), open loop

protection, over voltage protection for VCC, thermal

shutdown (TSD). Because these protection circuits

are fully integrated into the IC without external

components, the reliability can be improved without

increasing cost. Once the fault condition occurs,

switching is terminated and the power transistor

remains off. This causes VCC to fall. When VCC

reaches the UVLO stop voltage of 8V, the current

consumed by the AP3720 decreases to the start-up

current (typically 2.5µA) and the current supplied

from the DC link charges the VCC capacitor. When

VCC reaches the start voltage of 20V, the AP3720

resumes its normal operation. In this manner, the

auto-restart can alternately enable and disable the

switching of the power transistor until the fault

condition is eliminated.

Over Load Protection: In this situation, the

protection circuit should be activated to protect the

SMPS. However, even when the SMPS is in the

normal operation, the over load protection circuit can

be activated during the load transition. To avoid this

undesired operation, the over load protection circuit

is designed to be activated after a specified time

(typically 64ms) to determine whether it is a transient

situation or an overload situation. Because of the

cycle-by-cycle current limit, the maximum peak

current through the BJT is limited, and therefore the

maximum input power is restricted. If the output

consumes exceeds this maximum power, the voltage

decreases below the rated voltage. This reduces the

current through the opto-coupler LED, which also

reduces the opto-coupler transistor current, thus

increasing VFB. If VFB exceeds 4.85V and holds the

situation beyond 64ms, the over load protection

circuit is activated.

Open Loop Protection: In this abnormal situation,

OLP or OVP will be activated.

Over Voltage Protection: If the control loop is open,

the current through the opto-coupler transistor

becomes almost zero. Then VFB climbs up to the over

load situation and the over load protection is

activated. But maybe the output voltage exceed the

rated voltage before the over load protection is

activated, resulting in the breakdown of the devices

in the secondary side and the primary side. To

prevent this situation, an over voltage protection

circuit is employed. In general, VCC is proportional to

the output voltage. If VCC exceeds 26.5V, an OVP

circuit is activated resulting in the termination of the

switching operation. To avoid undesired activation of

OVP during normal operation, VCC should be

designed to be below 20V.

Thermal Shutdown

Thermal shutdown is provided by a precision analog

circuit that turns the power transistor off when the

junction temperature exceeds the thermal shutdown

temperature (typically 160ºC). When the junction

temperature drops about the hysteretic temperature,

normal operation resumes.

Jul. 2009 Rev. 1. 0

BCD Semiconductor Manufacturing Limited

10

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