Datasheet

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SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
16
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TYPICAL APPLICATION
peak current limit
Resistor divider R14 and R29 along with current sense resistor R5, devise the peak-limit comparator of the
UCC38500 and are used to protect the boost switch Q3 from excessive currents. Proper preparation of this
comparator requires that it not interfere with the boost converter’s power limit or the forward converter’s
pulse-by-pulse current limiting. For this design example the forward converter is selected to go into
pulse-by-pulse current limiting at approximately 130% of maximum output power. The power limit of the boost
converter is set at 140% of the maximum output power. The peak current limit for the boost stage was selected
to engage at 150% of the maximum output power to ensure circuit stability.
The following equation is used to select the current-sense resistor R5, where the current-sense resistor is
selected to operate over a 1-V dynamic range (V
DYNAMIC
). The current-sense resistor required for the design
needed to be approximately 0.43 Ω.
R5 + R
SENSE
+
V
DYNAMIC
I
PK
)
(
0.5
)
DI
^ 0.43 W
The following equation is used to size resistor R14 properly by first selecting R29 to be a standard resistance
value. For this design resistor R29 was selected to be 10 kΩ. With a typical reference voltage (V
REF
) of 7.5 V
gives a calculated value of approximately 1.91 kΩ for resistor R14.
R14 +
ǒ
P
OUT
1.5 2
Ǹ
V
IN (min)
h1 h2
) DIǓ R5 R29
V
REF
multiplier
The output of the multiplier of the UCC38500 is a signal representing the desired input line current. It is an input
to the current amplifier, which programs the current loop to control the input current to give high power-factor
operation. As such, the proper functioning of the multiplier is key to the success of the design. The inputs to the
multiplier are V
VAOUT
, the voltage amplifier output, I
IAC
, a representation of the input rectified ac line voltage,
and an input voltage feed forward signal, V
VFF
. The output of the multiplier, I
MOUT
, can be expressed:
I
MOUT
+
I
IAC
ǒ
V
VAOUT
* 1
Ǔ
K
ǒ
V
VFF
Ǔ
2
Where K is a constant typically equal to 1 / V.
The I
IAC
signal is obtained through a high-value resistor connected between the rectified ac line and the IAC
pin of the UCC3850X. This resistor (R
IAC
) is sized to provide the maximum I
IAC
current at high line. For the
UCC3850X the maximum I
IAC
current is about 500 µA, and a higher current can drive the multiplier out of its
linear range. A smaller current level is functional, but noise can become an issue, especially at low input line.
Assuming a universal line operation of 85 V
RMS
to 265 V
RMS
gives a R
IAC
value of 750 kΩ. Because of voltage
rating constraints of the standard 1/4-W resistor, this application requires a combination of lower value resistors
connected in series to give the required resistance and distribute the high voltage amongst the resistors. For
this design example two 383 kΩ resistors are used in series.
(9)
(10)
(11)