Datasheet

P
COND
= R
DS(on)
× K × I
2
RMS
Soft Start
C
SS
+
10 mA t
DELAY
7.5 V
Multiplier
I
MOUT
+ I
IAC
ǒ
V
VAOUT
* 1
Ǔ
K V
VFF
2
UCC2817-EP
UCC2818-EP
www.ti.com ........................................................................................................................................................................................... SLUS716 – DECEMBER 2008
Where:
Q
GATE
= Total gate charge
V
GATE
= Gate drive voltage
f
S
= Clock frequency
C
OSS
= Drain source capacitance of the MOSFET
I
L
= Peak inductor current
t
ON
and t
OFF
= Switching times (estimated using device parameters R
GATE
, Q
GD
and V
TH
)
V
OFF
= Voltage across the switch during the off time (in this case V
OFF
= V
OUT
)
Conduction loss is calculated as the product of the R
DS(on)
of the switch (at the worst-case junction temperature)
and the square of RMS current:
Where:
K = temperature factor found in the manufacturer's R
DS(on)
vs junction temperature curves
Calculating these losses and plotting against frequency gives a curve that enables the designer to determine
which manufacturer's device has the best performance at the desired switching frequency, or which switching
frequency has the least total loss for a particular power switch. For this design example, an IRFP450 HEXFET™
from International Rectifier was chosen because of its low R
DS(on)
and its V
DSS
rating. The IRFP450 R
DS(on)
of 0.4
Ω and the maximum V
DSS
of 500 V made it an ideal choice. A review of this procedure can be found in the
Unitrode™ Power-Supply Design Seminar SEM1200, Topic 6, Design Review: 140 W ( Multiple Output High
Density DC/DC Converter).
The soft-start circuitry is used to prevent overshoot of the output voltage during start up. This is accomplished by
slowly bringing up the voltage amplifier output (V
VAOUT
), which allows for the PWM duty cycle to slowly increase.
Use the following equation to select a capacitor for the soft-start pin.
In this example, t
DELAY
= 7.5 ms, which yields a C
SS
of 10 nF.
In an open-loop test circuit, shorting the soft-start pin to ground does not ensure 0% duty cycle. This is due to the
current amplifiers input offset voltage, which could force the current amplifier output high or low depending on the
polarity of the offset voltage. However, in the typical application, there is sufficient amount of inrush and bias
current to overcome the current amplifier offset voltage.
The output of the multiplier of the UCC2817 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 VAOUT, the voltage amplifier error signal, 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 as:
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