UCC2817A, UCC2818A

UCC3817A, UCC3818A

SLUS577C − SEPTEMBER, 2003 − REVISED MARCH 2009

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APPLICATION INFORMATION

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

P

V

GATE

f

S

P

1

2

C

OSS

OFF

f

S

P

1

2

V

OFF

I

L

t

f

S

and the square of RMS current:

P

K

RMS

Calculating these losses and plotting against frequency gives a curve that enables the designer to determine

either 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

can be found in the Unitrode Power Supply Design Seminar SEM1200, Topic 6, Design Review: 140 W, [Multiple

Output High Density DC/DC Converter].

Softstart

The softstart circuitry is used to prevent overshoot of the output voltage during start up. This is accomplished

slowly. Please use the following equation to select a capacitor for the softstart pin.

C

10

mA

t

DELAY

7.5 V

In an open-loop test circuit, shorting the softstart 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’s offset voltage.

(4)

(5)

(6)

(7)

(8)