Datasheet

SLUU087B
9
10-Watt Flyback Converter Using the UCC3809
The switch must be able to conduct the repetitive peak-primary current as determined by:
I
PEAK(primary)
+
ǒ
V
IN(min)
* V
DS
Ǔ
t
ON(max)
L
P
The primary-current waveform of a discontinuous mode flyback converter is triangular in shape, therefore, its
root mean square (rms) current is calculated by:
I
PRMS
+
I
PEAK(primary)
3
Ǹ
t
ON(max)
T
Ǹ
The chosen device should also have a low R
DS(on)
value because the conduction losses of the device are
proportional to the square of the primary rms current through the device. Selection of a device that has a peak
current rating of at least three times the peak-primary current usually ensures acceptably low conduction losses.
P
CONDUCTION
+ I
PRMS
2
R
DS(on)
Switching losses are the result of overlapping drain current and source voltage at turn off. The drain voltage
begins to rise only after the Miller capacitor of the device begins to discharge. This discharging time is a function
of the external gate resistance, R
GATE
, and the gate to drain Miller charge, Q
GD
, as shown in the following
equation:
t
MILLER
+
Q
GD
R
GATE
VDD * V
TH
where V
TH
is the turn on threshold voltage of the gate.
The power loss due to the external capacitance of the MOSFET also contributes to the total switching losses,
which can be calculated as shown:
P
SWITCHING
+ f
SW
ȧ
ȡ
Ȣ
C
OSS
V
DS(stress)
2
2
) V
DS(stress)
I
PEAK(primary)
t
MILLER
ȧ
ȣ
Ȥ
During turn on there is no overlap of drain voltage and current because there is no current in a discontinuous
current-mode converter at turn on. Minimal losses also occur during the off-time of the FET due to the leakage
current:
P
OFF(time)
+
ǒ
1 * D
MAX
Ǔ
I
LEAK
V
DS(stress)
11 Current Sense
The ground referenced sense resistor is selected such that the maximum peak-primary current trips the 1-V FB
pin threshold when this current is 10% higher than its normal operating peak value at the minimum input voltage.
This limits the peak-primary current in the event of an output short circuit. This resistor must have a power rating
to meet the I
RMS
2
R requirement, where I
RMS
is the root mean square (rms) primary current. Because this
resistor defines the maximum peak-primary current, the input energy to the transformer is defined and equal
to (1/2)L
P
I
PEAK
2
. This defined energy in a fixed frequency discontinuous-mode flyback results in a fixed output
power. The advantage of current-mode control is that the output voltage is held constant despite changes in the
input voltage because the peak-primary current remains constant; the slope of this inductor current and its pulse
width are adjusted. Leading edge spikes or noise are caused by the reverse recovery of the rectifier, equivalent
capacitive loading on the secondary, and parasitic circuit inductances. A small low pass RC filter is added to
the current-sense signal to filter out these spikes so the comparator does not assume an overload condition is
present during switch turn on. To avoid excessive phase lag on the current-sense signal, the low pass filter
corner frequency is selected to be at least a decade above the switching frequency.