FAN4810 Datasheet, PDF (10/14 Pages) Fairchild Semiconductor – Power Factor Correction Controller

FAN4810
PRODUCT SPECIFICATION
Oscillator (RAMP 1)
The oscillator frequency is determined by the values of RT
and C_T, which determine the ramp and off-time of the
oscillator output clock:
f_OSC
=
-------------------------1---------------------------
t_RAMP + t_DEADTIME
(2)
The dead time of the oscillator is derived from the following
equation:
t_RAMP
=
C_T
×
R_T
×
I
n
-V----^R----^E---^F----–-----1---.-2---5--
V_REF – 3.75
(3)
at V_REF = 7.5V:
t_RAMP = C_T × R_T × 0.51
The dead time of the oscillator may be determined using:
t_DEADTIME
=
---2---.--5---V-----
5.5mA
×
C_T
=
450 × C_T
(4)
The dead time is so small (t_RAMP >> t_DEADTIME) that the
operating frequency can typically be approximated by:
f_OSC
=
-------1---------
t_RAMP
(5)
EXAMPLE:
For the application circuit shown in the data sheet, with the
oscillator running at:
f_OSC
=
100kHz
=
-------1---------
t_RAMP
Solving for R_T x C_T yields 1.96 x 10^-4. Selecting standard
components values, C_T = 390pF, and R_T = 51.1kΩ.
Clock Out (Pin 11)
Clock output is a rail to rail CMOS driver. The PMOS can
pull up within 15 ohms of the rail and the NMOS can pull
down to within 7 ohms of ground.
The clock turns on when the CLKSD pin is greater than
1.25V and the PFC output voltage is at rated operation value.
The clock signal can be used to synchronize and provide on/
off control for downstream DC to DC PWM converters.
CLKSD (Pin 5)
A current source of 25µA supplies the charging current for a
capacitor connected to this pin. Start-up delay can be pro-
grammed by the following equation:
C_dly
=
t_DELAY
×
-2---5---^µ----A---
1.25V
(6)
where C_dly is the required soft start capacitance, and t_DELAY
is the desired start-up delay.
It is important that the start-up delay is long enough to allow
the PFC time to generate sufficient output power for the
PWM DC converter. The start-up delay should be at least
5ms.
Solving for the minimum value of C_dly:
C_dly
=
5ms × -2---5---^µ----A---
1.25V
=
100nF
(6a)
Generating V_CC
The FAN4810 is a voltage-fed part. It requires an external
15V, ±10% (or better) shunt voltage regulator, or some other
V_CC regulator, to regulate the voltage supplied to the part at
15V nominal. This allows low power dissipation while at the
same time delivering 13V nominal gate drive at the PFC
OUT output. If using a Zener diode for this function, it is
important to limit the current through the Zener to avoid
overheating or destroying it. This can be easily done with a
single resistor in series with the Vcc pin, returned to a bias
supply of typically 18V to 20V. The resistor’s value must be
chosen to meet the operating current requirement of the
FAN4810 itself (7mA, max.) plus the current required by the
gate driver output and zener diode.
EXAMPLE:
With a V_BIAS of 20V, a V_CC of 15V and the FAN4810
driving a total gate charge of 38nC at 100kHz (e.g., 1
IRF840 MOSFET ), the gate driver current required is:
I_GATEDRIVE = 100kHz × 38nC = 3.8mA
(7)
R_BIAS
=
-V----^B----^I--^A---^S-----–----V-----^C---^C--
I_CC + I_G + I_Z
(8)
R_BIAS
=
---------------2---0---V------–----1---5----V----------------
7mA + 3.8mA + 5mA
=
316Ω
Choose R_BIAS = 330Ω.
The FAN4810 should be locally bypassed with a 1.0µF
ceramic capacitor. In most applications, an electrolytic
capacitor of between 47µF and 220µF is also required across
the part, both for filtering and as part of the start-up bootstrap
circuitry.
Typical Applications
Figure 4 is the application circuit for a complete 125W
power factor corrected power supply, designed using the
methods and general topology detailed in Application Note
42046.
10
REV. 1.0.12 9/24/03