Datasheet

SLUU087B
11
10-Watt Flyback Converter Using the UCC3809
14 Output and Input Capacitors
Output capacitors are selected based upon their capacitance value, equivalent series resistance (ESR),
equivalent series inductance (ESL), and capacitor ripple current rating. The capacitance value controls the peak
to peak output ripple voltage at the switching frequency. Assuming a linear decay of the capacitor voltage during
the off time, during which the capacitor must supply the load current, the minimum value of the output capacitor
may be calculated as follows:
C
OUT
+
ǒ
T * t
ON(max)
Ǔ
I
OUT
V
RIPPLE
where V
RIPPLE
is the acceptable peak-to-peak output-voltage ripple. Unfortunately there are practical limitations
to how low a single stage output filter can reduce the ripple voltage and sometimes an extra LC filter stage is
necessary. This second-stage filter would also reduce the output high-frequency noise. Parasitic resistance and
inductance in the output capacitors tends to make the ripple voltage much greater than expected based upon
the above equation. Using capacitors with the lowest possible ESR and ESL helps reduce the high-frequency
ripple. The rms ripple current that the output capacitors experience is not the same as the secondary-side rms
output current; it is the ac portion of it. The secondary-side rms current is in the shape of a clipped sawtooth,
or trapezoid, where as the output capacitor’s current waveform is in the shape of right triangle. Therefore, the
typical capacitor ripple current rating the output capacitors must meet is equal to:
I
RMS(cout)
+ I
PEAK(sec)
ǒ
t
RESET
T
Ǔ
ȧ
ȧ
ȧ
ȧ
ȱ
Ȳ
4 * 3
ǒ
t
RESET
T
Ǔ
12
ȧ
ȧ
ȧ
ȧ
ȳ
ȴ
Ǹ
where I
PEAK(sec)
is the peak-secondary current, and t
RESET
is equal to the off time of the switch. The same
selection criteria is used for the input capacitor, keeping in mind these capacitors must also be rated to handle
the maximum-input voltage.
15 Loop Compensation
The UCC3809 is a primary-side controller for use in isolated converters; therefore it does not contain an internal
error amplifier. The TLV431, a low-voltage adjustable-precision shunt regulator, is used on the secondary side
for the feedback control loop. This regulator is ideal for low voltage supplies because the output voltage can be
set to any value between its reference of 1.24 V and 6 V while operating from a lower voltage than the standard
TL431.
The output voltage is resistively divided and compared to the TLV431’s 1.24V reference voltage. When the
resistively divided-converter output voltage rises above this threshold, the TLV431 drives the optocoupler diode
on which, in turn, drives the FB pin on the UCC3809 to its 1-V threshold, turning off the output driver. The reverse
happens when the resistively divided-output voltage falls below the 1.24-V TLV431 reference voltage.
The feedback loop needs to be closed around the error amplifier by adding a compensation network. The
network components are selected so as to give the converter good dynamic response, acceptable line and load
regulation, and stability resulting from optimum closed-loop bandwidth. Before calculating the error-amplifier
compensation, the control to output gain, or transfer characteristic, along with the power-stage poles and zeros
must be determined. This is easily done using Lloyd Dixon’s Closing the Feedback Loop available in the
Unitrode Power Supply Design Seminar SEM–700. The output capacitor, because of its parasitic ESR,
contributes a zero in the frequency response at:
F
ESR(zero)
+
1
2 p ESR C
OUT