FAN6300A Fairchild Semiconductor, FAN6300A Datasheet - Page 11

• Manufacturer Part Number: FAN6300A
• Description: The highly integrated FAN6300A/H of PWM controller provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6300A / FAN6300H • Rev. 1.0.1
Operation Description
The FAN6300A/H 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 across a
wide range of line voltage. The following descriptions
highlight some of the features of the FAN6300A/H.
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 the HV pin is 1.2mA
and it charges the hold-up capacitor through the diode
and resistor. When the V
the startup current switches off. At this moment, the V
capacitor only supplies the FAN6300A/H 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 (3µs for H version) after the gate
signal going-low transition. The minimum t
prevents 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
and R
DD
voltage level reaches V
A
. R
OFF
DET
time) prevents gate
is recommended as
OFF
DD-ON
limit
DD
DD
,
11
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
linearly with lower V
signal does not start until t
valley detect circuit is activated until t
which decreases the 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
t
delay (with 5µs delay for H version). Figure 20 and
Figure 21 show the two different conditions.
TIME-OUT
Figure 20. QR Operation in Extended Valley Voltage
Figure 21. Internal t
t
O F F -M I N
38/13 μ s
2 .1 m s
8 /3μ s
signal initiates a new cycle start after a 9μs
OFF-MIN
(with 5µs Delay for FAN6300H version)
Figure 19. V
Failure to Detect Valley Voltage
1 .2 V
expires. Under this condition, an internal
FB
is lower than V
TIME-OUT
Detection Mode
FB
. The valley voltage detection
FB
OFF-MIN
vs. t
Initiates New Cycle After
FB
OFF-MIN
, which is derived from
finishes. Therefore, the
2 .1 V
N
, t
Curve
OFF-MIN
OFF-MIN
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increases
finishes,
V
F B