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SG6848D

Low Cost Green-Mode PWM Controller for Flyback Converters

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Low Cost Green-Mode PWM Controller for Flyback Converters

Product Specification

SG6848

OPERATION DESCRIPTION

SG6848 devices integrate many useful designs into

one controller for low-power switch-mode power supplies.

The following descriptions highlight some of the features

of the SG6848 series.

Start-up Current

The start-up current is only 5uA. Low start-up

current allows a start-up resistor with a high resistance

and a low-wattage to supply the start-up power for the

controller. A 1.5 MΩ, 0.25W, start-up resistor and a

10uF/25V VDD hold-up capacitor would be sufficient for

an AC-to-DC power adapter with a wide input range

(100VAC to 240VAC).

Operating Current

The operating current has been reduced to 2mA. The

low operating current results in higher efficiency and

reduces the Vcc hold-up capacitance requirement.

Green-Mode Operation

The proprietary green-mode function provides

off-time modulation to linearly decrease the switching

frequency under light-load conditions. On-time is limited

to provide stronger protection against brownouts and

other abnormal conditions. The feedback current, which is

sampled from the voltage feedback loop, is taken as the

reference. Once the feedback current exceeds the

threshold current, the switching frequency starts to

decrease. This green-mode function dramatically reduces

power consumption under light-load and zero-load

conditions. Power supplies using the SG6848 can easily

meet even the strictest regulations regarding standby

power consumption.

Oscillator Operation

A resistor connected from the RI pin to ground will

generate a constant current source for the SG6848. This

current is used to charge an internal capacitor. The

charge-time determines the internal clock speed and the

switching frequency. Increasing the resistance will reduce

the amplitude of the input current and reduce the

switching frequency. A 95kΩ resistor Ri results in a 50uA

constant current Ii and a 70kHz switching frequency. The

relationship between Ri and the switching frequency is:

fPWM = 6650 (kHz )

RI (k Ω )

The range of the oscillation frequency is designed to

be within 50kHz ~ 100kHz.

Leading-Edge Blanking

Each time the power MOSFET is switched on, a

turn-on spike will inevitably occur at the sense-resistor.

To avoid premature termination of the switching pulse, a

250 nsec leading-edge blanking time is built in.

Conventional RC filtering can therefore be omitted.

During this blanking period, the current-limit comparator

is disabled and it cannot switch off the gate driver.

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 following a short

propagation delay tD. This propagation delay will

introduce an additional current proportional to tD*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. At high input line

voltages, the output power limit will be higher than at low

input line voltages.

To compensate for this output power limit variation

across a wide AC input range, the threshold voltage is

adjusted by adding a positive ramp. This ramp signal rises

from 0.85V to 1.0V, and then flattens out at 1.0V. A

smaller threshold voltage forces the output GATE drive to

terminate earlier. This reduces the total PWM turn-on

time and makes the output power equal to that of low line

input. This proprietary internal compensation ensures a

constant output power limit for a wide AC input voltage

range (90VAC to 264VAC).

© System General Corp.

-8-

Version 1.4(IRO33.0010.B2)

www.sg.com.tw

Apr.08, 2004

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