Datasheet

SLUU087B
7
10-Watt Flyback Converter Using the UCC3809
5 Determine the Maximum on Time (D
MAX
) for the Primary Switch
Maximum duty cycle occurs at minimum input voltage and maximum load. Programming D
MAX
clamps the
maximum on–time of the switch. This maximum on–time should incorporate enough of a margin so that control
is well maintained at minimum input voltage. The UCC3809 has a programmable maximum duty-cycle clamp
up to 70%. Core saturation is prevented and discontinuous mode is ensured by establishing a dead time, usually
approximately 20% of the switching period. For this converter it is assumed D
MAX
is 40% and the clamp is set
at 50% Setting a maximum duty cycle protects the magnetics from saturating during startup and brown outs.
Selecting RT1 equal to RT2 results in 50% duty cycle, the value of CT is selected to satisfy the following
equation:
1
f
SW
0.74
(
RT1 ) RT2
)
* 27 pF + 141.919 pF
where f
SW
is the desired switching frequency of the converter. A standard value of 150 pF results in a measured
switching frequency of 380 kHz.
6 Determine the Turns Ratio of the Flyback Inductor
By equating the volt–second on product, which is equal to the minimum input voltage, V
IN(min)
, minus the drain
to source forward voltage drop, V
DS
, multiplied by the maximum on–time, t
ON(max)
= D
MAX
T, to the reset
volt-second product, equal to the output voltage, V
OUT
, added to the forward voltage drop of the output diode,
V
F
, multiplied by the reset time, t
RESET
= (0.8–D
MAX
)T, the core is prevented from drifting up or down its
hysteresis loop. The turns ratio of the transformer can be calculated by using this steady state volt–second
approach:
n +
ǒ
V
IN(min)
* V
DS
Ǔ
D
MAX
T
ǒ
V
OUT
) V
F
Ǔ ǒ
0.8 * D
MAX
Ǔ
T
This application utilizes a turns ratio of 7, the primary consists of 14 turns while the secondary has 2 turns.
7 Primary Inductance
Magnetic design is a major part of any switch-mode power supply. The flyback transformer is actually a coupled
inductor, acting as an energy storage unit as well as performing the usual transformer functions. Crucial
considerations include primary inductance, working flux density swing, gap length, winding scheme and wire
diameter. The primary inductance, L
P
, for a discontinuous mode flyback converter can be calculated according
to the following relationship:
L
P
+
n
ƪ
ǒ
V
IN(min)
* V
DS
Ǔ
t
ON(max)
ƫ
2
2 T V
OUT
I
OUT
where n is the assumed efficiency of the converter and I
OUT
is the output current. This converter design requires
a primary inductance of approximately 15 µH, typical. With a required output power of 10W and assuming 70%
efficiency, the core should be sized to safely handle at least 14 W. The ferrite core should have high saturation,
low residual flux density, and low losses. An RM5 core of 3F3 material proved to be suitable for this application.
Hysteresis loss is minimized in this design by restricting the flux density to 800 gauss. Selecting a core material
with high permeability is not crucial because the energy stored in the flyback transformer is actually stored in
the air gap. Gapping the core also reduces the residual flux density. The size of the air gap is calculated by
applying the following equation: