FAN6300DY Fairchild Semiconductor, FAN6300DY Datasheet - Page 10

• Manufacturer Part Number: FAN6300DY
• Description: Fan6300 - Highly Integrated Quasi-resonant Current Mode Pwm Controller
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN6300 • Rev. 1.0.3
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,
R
Typical startup current drawn from pin HV is 1.2mA and
it charges the hold-up capacitor through the diode and
resistor. When the V
startup current switches off. At this moment, the V
capacitor only supplies the FAN6300 to maintain V
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
divider resistors R
150kΩ to 220kΩ to achieve valley voltage switching.
When V
voltage is clamped to 0.3V.
The internal timer (minimum t
retriggering within 8µs after the gate signal going-low
transition. The minimum t
system frequency being too high. Figure 18 shows a
typical drain voltage waveform with first valley switching.
HV
, which are recommended as 1N4007 and 100kΩ.
AUX
Figure 18. First Valley Switching
Figure 17. Valley Detect Section
(in Figure 17) is negative, the DET pin
DET
DD
and R
voltage level reaches V
OFF
A
. R
OFF
time limit prevents the
DET
time) prevents gate
is recommended as
DD-ON
, the
DD
DD
Green-Mode Operation
The proprietary green-mode function provides off-time
modulation to linearly decrease the switching frequency
under light-load conditions. V
the voltage feedback loop, is taken as the reference. In
Figure 19, once V
increases linearly with lower V
detection signal does not start until the t
finishes. Therefore, the valley detect circuit is activated
until the t
switching frequency and provides extended valley
voltage switching. However, in very light load condition,
it might fail to detect the valley voltage after the t
expires. Under this condition, an internal t
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
Figure 21. Internal t
10
Failure to Detect Valley Voltage (with 9µs Delay)
OFF-MIN
Figure 19. V
time finishes, which decreases the
FB
TIME-OUT
is lower than V
Detection Mode
FB
vs. t
Initiates New Cycle After
FB
OFF-MIN
, which is derived from
FB
. The valley voltage
N
Curve
, the t
TIME-OUT
OFF-MIN
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OFF-MIN
OFF-MIN
signal
time
time