CR6842 Просмотр технического описания (PDF) - Unspecified

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CR6842

Green-Power PWM Controller with Freq. Jittering

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CR6842

hysteresis window ensures that the hold-up capacitor can adequately supply VDD during start-up.

Gate Output / Soft Driving

The CR6842 output stage is a fast totem pole gate driver. Cross-conduction has been avoided to

minimize heat dissipation, increase efficiency, and enhance reliability. The output driver is clamped by an

internal 18V Zener diode in order to protect the power MOSFET transistors from any harmful over-voltage

gate signals. A soft driving waveform is implemented to minimize EMI.

Slope Compensation

The sensed voltage across the current sense resistor is used for peak-current-mode control and

cycle-by-cycle current limiting. The built-in slope compensation function improves power supply stability

and prevents peak-current-mode control from causing sub-harmonic oscillations. Within every switching

cycle, the CR6842 controller produces a positively sloped, synchronized ramp signal.

Constant Output Power Limit

When the SENSE voltage across the sense resistor RS reaches the threshold voltage, the output

GATE drive will be turned off following a small propagation delay TPD. This propagation delay will result in

an additional current proportional to TPD*VIN/LP. The propagation delay is nearly constant regardless of the

input line voltage VIN. Higher input line voltages will result in larger additional currents. Thus, under high

input-line voltages the output power limit will be higher than under low input-line voltages.

The output power limit variation can be significant over a wide range of AC input voltages. To

compensate for this, the threshold voltage is adjusted by the current IIN. Since the pin VIN is connected to

the rectified input line voltage through the start-up resistor, a higher line voltage will result in a higher

current IIN through the pin VIN.

The threshold voltage decreases if the current IIN increases. A small threshold voltage will force the

output GATE drive to terminate earlier, thus reducing total PWM turn-on time, and making the output power

equal to that of the low line input. This proprietary internal compensation feature ensures a constant output

power limit over a wide range of AC input voltages (90VAC to 264VAC).

VDD Over-voltage Protection

VDD over-voltage protection has been built in to prevent damage due to over voltage conditions.

When the voltage VDD exceeds the internal threshold due to abnormal conditions, PWM output will be

turned off. Over-voltage conditions are usually caused by open feedback loops.

Limited Power Control

The FB voltage will increase every time the output of the power supply is shorted or over-loaded. If the

FB voltage remains higher than a built-in threshold for longer than TLPS, PWM output will then be turned off.

As PWM output is turned off, the supply voltage VDD will also begin decreasing.

When VDD goes below the turn-off threshold (eg, 12.4V) the controller will be totally shut down. VDD

Dec.2006 V 1.3

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Chengdu Chip-Rail Tech.Co.,Ltd

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