Datasheet

SLUU087B
8
10-Watt Flyback Converter Using the UCC3809
GAP +
m
O
m
R
N
P
2
A
E
10
*2
L
P
where µ
o
is the permeability of free space equal to 4π×10
–7
H/m and µ
R
is the relative permeability of free space,
which is air, equal to 1. A
E
is the effective core area measured in cm
2
.
8 Soft Start
Soft start is used to reduce transformer and output capacitor stress and to reduce the surge on the input circuits
when the converter action starts. The considerable capacitance on the output lines should be charged slowly
so as not to reflect an excessive transient. Also, a wide initial pulse could result in saturation of the core, and
voltage overshoot on the output results if the inductor current is allowed to rise to a high value during start-up.
To program this rise time, referred to as t
SS
, a 10-nF capacitor is chosen so that the 9.1-µA of maximum
internal-charging current from the SS pin of the UCC3809 delays complete start-up for approximately 2 ms (800
cycles). When the voltage on this capacitor charges from 1 to 2 volts, the duty cycle is gradually increased from
narrow pulses to normal operating conditions. If pulled below 0.5 V the output driver is inhibited and the IC
reference is pulled low. The controller goes into its low start-up current of less than 100 µA. A simple NPN
transistor and two resistors allow the user to externally control this feature by application of a 5-V signal to J5
as shown in Figure 1.
9 Self-Biasing Winding
At first power up, the UCC3809 has a very low start-up current of approximately 100-µA. This current can be
supplied by a start-up resistor from the converter input line. But constant IC operating current, as high as 1.25
mA at 400 kHz at 25_C, no load, in addition to the average gate-drive current from this source would be
impractical for efficiency reasons. Therefore, a bias winding is added to the Flyback inductor. This winding is
essentially another secondary output but its return is common to the primary side. This winding must maintain
voltage above the undervoltage lockout (UVLO) threshold of the UCC3809 while supplying the IC with its
operating current as well as the average gate-drive current, as mentioned above. The turns ratio for the bias
winding is determined using the same procedure as the main secondary, explained previously, except that V
OUT
is replaced with VDD. A small leading edge blanking RC circuit is placed at the output of the transformer to filter
out the voltage spike due to leakage inductance of the transformer. A 100-V Schottky is used because the
device’s working peak reverse voltage rating must be able to handle the reflected primary voltage added to the
reflected main output voltage along with the VDD voltage that it is supplying. The bias winding capacitor must
be able to maintain the IC input voltage above the UVLO threshold, V
UVLO
, while supplying both the operating
current for the UCC3809, I
VDD
, and the average gate-drive current, I
GATE(avg)
. A minimum value for C
BOOT
can
be calculated as:
C
BOOT
+
ǒ
I
GATE(avg)
) I
VDD
Ǔ
t
SS
ǒ
VDD * V
UVLO
Ǔ
10 Selection of the Switching Element
The MOSFET switch is selected to meet the drain to source voltage stress resulting from the maximum input
voltage, V
IN(max)
, the reflected secondary voltages, equal to the output voltage, V
OUT
, plus the output diode
forward voltage drop, V
F
, and the voltage spike due to the leakage inductance, assumed to be 30% of the input
voltage:
V
DS(stress)
+
ƪ
ǒ
V
IN(max)
Ǔ
) N
ǒ
V
OUT
) V
F
Ǔ
ƫ
) 0.3 V
IN(max)