UCC3817D UNITRODE, UCC3817D Datasheet - Page 8

• Manufacturer Part Number: UCC3817D
• Description: POWER FACTOR CNTRLR, 3817, SOIC16
• Manufacturer: UNITRODE

Specifications of UCC3817D

• Primary Input Voltage: 15V
• No. Of Outputs: 1
• Output Voltage: 5.5V
• Voltage Regulator Case Style: SOIC
• No. Of Pins: 16
• Operating Temperature Range: 0°C To +70°C
• Operating Temperature Max: 70°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

8
SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009
power stage
L
where D is the duty cycle, ∆I is the inductor ripple current and f
circuit a switching frequency of 100 kHz, a ripple current of 875 mA, a maximum duty cycle of 0.688 and a
minimum input voltage of 85 V
equation are at the peak of low line, where the inductor current and its ripple are at a maximum.
C
The value of capacitance is determined by the holdup time required for supporting the load after input ac voltage
is removed. Holdup is the amount of time that the output stays in regulation after the input has been removed.
For this circuit, the desired holdup time is approximately 16 ms. Expressing the capacitor value in terms of output
power, output voltage, and holdup time gives the equation:
In practice, the calculated minimum capacitor value may be inadequate because output ripple voltage
specifications limit the amount of allowable output capacitor ESR. Attaining a sufficiently low value of ESR often
necessitates the use of a much larger capacitor value than calculated. The amount of output capacitor ESR
allowed can be determined by dividing the maximum specified output ripple voltage by the inductor ripple
current. In this design holdup time was the dominant determining factor and a 220-µF, 450-V capacitor was
chosen for the output voltage level of 385 VDC at 250 W.
Power switch selection:
to be made. When selecting a power switch, it can be useful to calculate the total power dissipation in the switch
for several different devices at the switching frequencies being considered for the converter. Total power
dissipation in the switch is the sum of switching loss and conduction loss. Switching losses are the combination
of the gate charge loss, C
where Q
source capacitance of the MOSFET, I
(estimated using device parameters R
off time, in this case V
BOOST
OUT
L
C
P
P
P
BOOST
OUT
GATE
COSS
ON
: Two main criteria, the capacitance and the voltage rating, dictate the selection of the output capacitor.
GATE
) P
: The boost inductor value is determined by:
+
+ Q
+ 1
+
OFF
V
is the total gate charge, V
2
OUT
GATE
V
2
+ 1
IN(min)
C
( DI
2
OSS
2
OFF
P
* V
OUT
V
fs )
OSS
V
OUT(min)
= V
GATE
OFF
As in any power supply design, tradeoffs between performance, cost and size have
V 2
D
OUT
loss and turnon and turnoff losses:
RMS
OFF
Dt
APPLICATION INFORMATION
.
I
L
gives us a boost inductor value of about 1 mH. The values used in this
2
fs
GATE
GATE
L
fs
is the peak inductor current, t
t
ON
, Q
is the gate drive voltage, f
GD
) t
www.ti.com
and V
OFF
TH
) and V
fs
S
OFF
is the switching frequency. For the example
S
is the voltage across the switch during the
is the clock frequency, C
ON
and t
OFF
are the switching times
OSS
is the drain