© 2007 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN6300 • Rev. 1.0.1

10

Operation Description

The FAN6300 of PWM controller integrates designs to

enhance the performance of flyback converters. An

internal valley voltage detector ensures power system

operates at Quasi-Resonant (QR) operation in a wide

range of line voltage. The following descriptions highlight

some of the features of the FAN6300 series.

Startup Current

For startup, the HV pin is connected to the line input or

bulk capacitor through an external diode and resistor,

Typical startup current drawn from pin HV is 1.2mA and

it charges the hold-up capacitor through the diode and

until the auxiliary winding of the main transformer

provides the operating current.

Valley Detection

The DET pin is connected to an auxiliary winding of the

transformer via resistors of the divider to generate a

valley signal once the secondary-side switching current

discharges to zero. It detects the valley voltage of the

switching waveform to achieve the valley voltage

switching. This ensures QR operation, minimizes

switching losses, and reduces EMI. Figure 17 shows

150k

Ω to 220kΩ to achieve valley voltage switching.

voltage is clamped to 0.3V.

Figure 17. Valley Detect Section

retriggering within 8µs after the gate signal going-low

system frequency being too high. Figure 18 shows a

typical drain voltage waveform with first valley switching.

Figure 18. First Valley Switching

Green-mode Operation

The proprietary green-mode function provides off-time

modulation to linearly decrease the switching frequency

the voltage feedback loop, is taken as the reference. In

finishes. Therefore, the valley detect circuit is activated

switching frequency and provides extended valley

voltage switching. However, in very light load condition,

initiates a new cycle start after a 9μs delay. Figure 20

and Figure 21 show the two different conditions.

Figure 20. QR Operation in Extended Valley Voltage

Detection Mode

Failure to Detect Valley Voltage (with 9µs Delay)