SG6841D Ver la hoja de datos (PDF) - Unspecified

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SG6841D

High-integrated Green-mode PWM Controller

Unspecified 

High-integrated Green-mode PWM Controller

Product Specification

SG6841

Built-in Slope Compensation

The sensed voltage across the current sense resistor

is used for current mode control and pulse-by-pulse

current limiting. Built-in slope compensation will

improve stability or prevent sub-harmonic oscillation of

peak current mode control. SG6841 inserts a

synchronized positive-going ramp at every switching

cycle. Vs-comp = 0.33V.

Constant Output Power Limit

When the SENSE voltage, across the sense resistor

Rs, reaches the threshold voltage, around 1.0V, the output

GATE drive will be turned off after a small propagation

delay tD. This propagation delay will introduce an

additional current proportional to tD*Vin/Lp. Since the

propagation delay is nearly constant regardless of the

input line voltage VIN. Higher input line voltage will result

in a larger additional current and hence the output power

limit is also higher than that under low input line voltage.

To compensate this variation for wide AC input range, the

threshold voltage is adjusted by the VIN current. Since

VIN pin is connected to the rectified input line voltage

through the start-up resistor, a higher line voltage will

generate higher VIN current into the VIN pin. The

threshold voltage is decreased if the VIN current is

increased. Smaller threshold voltage, forces the output

GATE drive to terminate earlier, thus reduce the total

PWM turn-on time and make the output power equal to

that of low line input. This proprietary internal

compensation ensures a constant output power limit for

wide AC input voltage from 90VAC to 264VAC.

„ Thermal Protection

An NTC thermistor Rntc in series with a resistor Ra

can be connected from pin RT to ground. A constant

current IRT is output from pin RT. The voltage on RT pin

can be expressed as VRT = IRT × (Rntc + Ra) in which IRT =

2 x (1.3V / Ri).

At high ambient temperature, Rntc will be smaller

such that VRT will decrease. When VRT is less than 0.65V

(VOTP,START), the PWM duty cycle will be decreased to

lower the internal temperature of power supply. If the

over heating situation still exists such that VRT decreases

further to 0.62V (VOTP,STOP), the PWM will be completely

turned off.

Limited Power Control

Every time when the output of power supply is

shorted or over loaded, the FB voltage will increase. If the

FB voltage is higher than a designed threshold, 4.2V, for

longer than 31msec, the PWM output will then be turned

off eternally. VDD, the supply voltage for SG6841, will

decrease due to the supply current for SG6841. When VDD

is lower than the turn-off threshold such as 10V, SG6841

will be totally shut down. Due to the start up resistor, VDD

will be charged up to the turn-on threshold voltage 16V

until SG6841 is enabled again. If the over loading

condition still exists, above protection will take place

repeatedly. This will prevent the power supply from being

overheated under over loading condition.

Noise immunity

Noise on the current sense or control signal may

cause significant pulse width jitter, particularly in the

continuous conduction mode. Slope compensation helps

alleviate this problem. Good placement and layout

practices should be followed. Avoiding long PCB traces

and component leads, locating compensation and filter

components near to the SG6841, and increasing the power

MOS gate resistance will always help.

©System General Corp.

Version 2.1 (IRO33.0001.B5)

- 10 -

www.sg.com.tw

Jun.15,2006

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