Datasheet

UCC28050, UCC28051
UCC38050, UCC38051
SLUS515F−SEPTEMBER 2002 − REVISED MARCH 2009
13
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APPLICATION INFORMATION
Diode Selection
The effects of the reverse recovery current in the diode can be eliminated with relatively little negative impact
to the system. The diode selection is based on reverse voltage, forward current, and switching speed.
I
D(avg)
+ I
OUT(avg)
I
D(rms)
+ I
L(peak)
2
Ǹ
V
AC
p V
OUT
Ǹ
V
D(peak)
+ V
OUT
Capacitor Selection
The hold-up time is the main requirement in determining the output capacitance. ESR and the maximum RMS
ripple current rating may also be important especially at higher power levels.
C
OUT(min)
+
ǒ
2 P
OUT
t
HOLDUP
Ǔ
ǒ
ǒ
V
OUT
Ǔ
2
*
ǒ
V
OUT(min)
Ǔ
2
Ǔ
where:
D V
OUT(min)
= minimum regulator input voltage for operation
I
C(rms)
+
ǒ
I
L(peak)
Ǔ
2
2
Ǹ
V
AC(max)
p V
OUT
*
ǒ
P
OUT
V
OUT
Ǔ
2
)
(
ac rms load currents
)
2
Ǹ
Multiplier Set-Up
Select R
AC1
and R
AC2
so that their ratio uses the full dynamic range of the multiplier input at the peak line voltage
yet, their values are small enough to make the effects of the multiplier bias current negligible. In order to use
the maximum range of the multiplier, select the divider ratio so that V
MULTIN
evaluated at the peak of the
maximum ac line voltage is the maximum of the minimum dynamic input range of MULTIN, which is 2.5 V.
Choose R
AC1
so that it has at least 100-μA at the peak of the minimum ac operating line voltage.
R
AC1
R
AC2
+
ǒ
2
Ǹ
2.5
V
AC(max)
Ǔ
* 1
In extreme cases, switching transients can contaminate the MULTIN signal and it can be beneficial to add
capacitor C
AC1
. Select the value of C
AC1
so that the corner frequency of the resulting filter is greater than the
lowest switching frequency. Keep in mind that the low corner frequency of this filter may compromise the overall
power factor.
(6)
(7)
(8)
(9)
(10)
(11)