MT-7933 데이터 시트보기 (PDF) - Unspecified

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MT-7933

Single Stage, High PFC, AC-DC LED Driver

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Maximizing IC Performance

MT7933

Single Stage, High PFC, AC-DC LED Driver

Auxiliary Sensing

The turning-on of power MOSFET is controlled

by sensing DSEN’s waveform, which is sampled

from Auxiliary windings by the resistor-divider. As

DSEN’s waveform goes below 0V at OFF time,

which indicates the drain of power MOSFET

damps to its valley or nearby, MT7933 turns on

the power MOSFET. Besides, minimum OFF

time control provide noise immunity.

Furthermore, MT7933 features over-voltage

protection (OVP), short-circuit protection (SCP),

and over-current protection (OCP) functions.

Those protections are triggered by sensing the

auxiliary winding waveform information, as the

auxiliary winding voltage is proportional to the

output voltage (secondary winding voltage)

during the OFF time period. The auxiliary

winding voltage is sampled by DSEN pin, one

LEB (Leading Edge Blanking) time right after

DRV signal is turned off.

Auxiliary sensing function is shown in Fig.3.

DRV

DSEN

Sampling Point

LEB=2uS

(typ)

Fig.3 Auxiliary Signal Sensing

Over-voltage Protection

MT7933 is implemented with two over-voltage

protection schemes: (1) If DSEN pin’s voltage is

detected above 3.2V for three times, (refer to

Auxiliary Sensing section), MT7933 turns off

the PWM switching signal, and VDD voltage

gradually drops to UVLO threshold, and the

system will be re-started. The threshold voltage

of over-voltage protection VOUT_OV, can be easily

defined as (refer to the application circuit in page

1):

VOUT _OV

 3.2  (1 

R3)  Ns

R4 Na

 VD3

Where Ns is the secondary winding, Na is

auxiliary winding, VD3 is the forward bias of the

secondary side rectifier diode.

(2) If VDD pin’s voltage exceeds 24V three times,

MT7933 turns off the PWM switching signal, and

VDD gradually drops to UVLO threshold, and

then the system will be re-started. It is highly

recommended to set up the VDD voltage

between 12V and 22V by designed a proper Na

to Ns ratio of the transformer.

Short-circuit Protection

The short-circuit protection is triggered if the

DSEN pin voltage is detected below 400mV at

OFF period for a continuous time of 5 to 10ms.

The gate drive switching will be turned off, and a

restart process will be kicked off when the VDD

voltage drops below the UVLO threshold.

This re-start process will repeat if the short-circuit

condition continues to exist.

Over-current Protection

MT7933 immediately turns off the power

MOSFET once the voltage at CS pin exceeds

1.8V. This cycle by cycle current limitation

scheme prevents the relevant components, such

as power MOSFET, transformer, etc. from

damage.

MT7933 Rev. 1.20

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Copyright © 2012 Maxic Technology Corporation

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