RT8487

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is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS8487-00

March

2015

10

Thus, a 0.1

F / 500V film capacitor can be chosen in

this case.

Inductor Selection

For high power factor application, the RT8487 operates

the converter in BCM (Boundary-Condition Mode). The

inductance range is defined by peak current of inductor、

maximum and minimum value of switching on time and

off time, for ensuring the inductor operates in BCM. The

peak current of inductor is showed as below :

 

PEAK

PEAK

2Pin

I

=

V

F a

OUT

PEAK

V

where a =

V

and

 





4

3

2

F a

-0.411a +0.296a -0.312a +0.638a-0.0000846,

a|0~0.7



The inductance range is showed as below :





PEAK

OUT

ON

OUT OFF

PEAK

PEAK

V

V

T

VT

L =

=

II



Where 0.5

The frequency at the top of the sine wave can be

calculated :

SW

ON

OFF

DELAY

1

f

=

T

+ T

+ T

AND pin , see Turn on delay time)

Turn On Delay Time

After the inductor current has reached zero, a

resonance will occur between the inductor and the

MOSFET drain-source capacitance.

In order to minimize the MOSFET switching losses,

RT8487 provides the flexibility to adjust the delay time

of next switch-on cycle in order to switch-on at the

maximum point of the resonance, which corresponds to

the minimum drain-source voltage value.

The delay time from zero current point to the maximum

of the switch resonance which can be calculated from :

resonance

SW

T

=

L1 C





mostly determined by the MOSFET drain-source

capacitance.

point to next MOSFET switch-on cycle can be adjusted

by the resistor value R3B connected between AND pin

and IC GND

R3B resister value in k

.

Forward Diode Selection

When the power switch turns off, the path for the

current is through the diode connected between the

switch output and ground. This forward biased diode

must have minimum voltage drop and recovery time.

The reverse voltage rating of the diode should be

greater than the maximum input voltage and the

current rating should be greater than the maximum

load current.

The peak voltage stress of diode is :









D

AC(MAX)

V

1.2

2

V

= 1.2

2

264 = 448V











will reach 448V.

MOSFET Selection

The peak current through this MOSFET will be over the

maximum output current. This component current

rating

should be greater than 1.2 times the maximum load

current and the reverse voltage rating of the MOSFET

should be greater than 1.2 times the maximum input

voltage, assuming a ±20% output current ripple.

The peak voltage rating of the MOSFET is :









Q1

AC(MAX)

V

= 1.2

2

V

= 1.2

2

264 = 448V









The current rating of MOSFET is determined by the