Datasheet

UCC28050, UCC28051
UCC38050, UCC38051
SLUS515F−SEPTEMBER 2002 − REVISED MARCH 2009
12
www.ti.com
APPLICATION INFORMATION
For low to medium power applications up to approximately 300 W, the CRM boost has an advantage in losses.
The filtering requirement is not severe and therefore is not a disadvantage. For medium to higher power
applications, where the input filter requirements dominate the size of the magnetics, the CCM boost is a better
choice due to lower peak currents (which reduces conduction losses) and lower ripple current (which reduces
filter requirements). The main tradeoff in using CRM boost is lower losses due to no reverse recovery in the
boost diode vs. higher ripple and peak currents.
Design Procedure
For a selected V
OUT
and minimum switching frequency, the following equations outline the design guidelines
for power stage component selection. Refer to the typical application diagram for reference designators.
Inductor Selection
In the transition mode control, the inductor value needs to be calculated to start the next switching cycle at zero
current. The time it takes to reach zero depends on line voltage and inductance and as shown in equation (1),
L determines the converter’s frequency range.
L +
ǒ
V
AC(min)
Ǔ
2
ǒ
V
OUT
* 2
Ǹ
V
AC(min)
Ǔ
2 F
s(min)
V
OUT
P
IN
where
D V
AC
= RMS line voltage
D V
AC(min)
= minimum AC line voltage
D P
IN
= maximum input power averaged over the ac line period
I
L(peak)
+ 2 2
Ǹ
P
IN
V
AC(min)
I
L(rms)
+
I
L(peak)
6
Ǹ
MOSFET Selection
The main switch selection is driven by the amount of power dissipation allowable. It is important to choose a
device that minimizes gate charge and capacitance and minimizes the sum of switching and conduction losses
at a given frequency.
I
Q(rms_crm)
+
1
6
*
ǒ
4 2
Ǹ
Ǔ
ǒ
V
AC(min)
9p V
OUT
Ǔ
Ǹ
IL
PEAK(crm)
V
Q(max)
+ V
OUT
(1)
(2)
(3)
(4)
(5)