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AN6846

Applying SG6846 to Control a Flyback Power Supply with Surge Current Output

Fairchild Semiconductor 

AN-6846

APPLICATION NOTE

Figure 13.Universal Line Voltage Compensation for

Constant Output Power Limit

Two-level Over-Current Protection (OCP)

The cycle-by-cycle current limiting shuts down the PWM

immediately when the switching current is over the peak-

current threshold. In addition, an over-current protection

circuit is built-in. When the switching current is higher than

the OCP threshold, the internal counter starts counting up.

When the switching current is lower than the OCP threshold,

the internal counter counts down. When the total

accumulated counting time is more than OCP delay time

(SG6846A: 1600ms; SG6846C: 110ms), the controller is

latched off.

By adjusting the RS resistance to let the peak-current at

maximum load under OCP threshold and over this threshold

at peak load condition, the two-level OCP protection is

enabled. These functions are especially designed for an

SMPS with surge current output, such as for printers,

scanners, and motor drivers.

Thermal Protection

A constant current, IRT, is provided from the pin RT.

The resistor connected to pin RI determines its magnitude:

IRT = 1.8V / RI

(8)

For example, IRT = 70µA if RI = 26KΩ.

For over-temperature protection (OTP), an NTC thermistor

RT in series with a resistor Ra can be connected between the

RT pin and ground. When the voltage of the RT pin drops

below 1.065V, PWM output is latched off. A debounce time

around 100µs is added to prevent false triggering. After the

latch is reset and cleared (SG6846CX: VRT>1.165V;

SG6846LX: AC unplugged), PWM turns on again. If the RT

pin is not used, connect a 100kΩ resistor between the RT

pin and ground to disable this thermal protection function.

Beside NTC and Ra, a capacitor should be connected to RT

pin to eliminate switching noise. This capacitor’s value is

less than 1nF (shown as Figure 14).

Figure 15.T C1 (<1nF) Connect to RT Pin

Figure 14.Timing Chart for Two-level Over-Current

Protection (OCP)

VDD Over-Voltage Protection (OVP)

VDD over-voltage protection prevents the controller from

over-voltage destruction. The VDD voltage rises when an

open-loop failure occurs. Once the VDD voltage exceeds

23.6V for around 100µs, the power supply latches off.

Lab Note

Before rework or solder/desolder on the power supply,

discharge the primary capacitors by an external bleeding

resistor. Otherwise, the PWM IC may be destroyed by

external high voltage during solder/desolder.

This device is sensitive to ESD discharge. To improve

production yield, the production line should be ESD

protected according to ANSI ESD S1.1, ESD S1.4, ESD

S7.1, ESD STM 12.1, and EOS/ESD S6.1.

© 2008 Fairchild Semiconductor Corporation

Rev. 1.3.2 • 9/26/08

6

www.fairchildsemi.com

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