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DI-102

A

8/05

Power Integrations

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Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does

not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND

SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A

PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and

circuits external to the products) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to

Power Integrations. A complete list of Power Integrations' patents may be found at

www.powerint.com. Power Integrations grants its customers a license

under certain patent rights as set forth at http://www.powerint.com/ip.htm.

The PI logo,

TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, EcoSmart, PI Expert and PI FACTS are trademarks of

Power Integrations, Inc. Other trademarks are property of their respective companies.

Copyright 2005, Power Integrations, Inc.

For the latest updates, visit

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Design Formulae

Component values are calculated as follows:

IEEE 802.3af overvoltage requiements are:

V

Input voltage turn-off

V

IN(th)

= 60 VDC

OVSD becomes active

From the DPA-Switch data sheet we have the following:

I

This is the L-pin current at which

the device turns off

V

L-pin voltage at I

V

Control-pin voltage

Assumptions:

V

Transistor base-emitter voltage

β = 100

Transistor minimum current gain

K = 10

This is the ratio of transistor bias

versus collector current (larger K

gives stronger bias)

Resistor values R4, R5 and R6 are calculated as:

Key Design Points

• 1 % accuracy resistors should be used to maintain the

highest accuracy for the OV on and off thresholds.

• To avoid interfering with the under-voltage thresholds, the

voltage V

levels.

• The D

increasing input voltage (increasing L-pin current), when

using only resistor R

However when the overvoltage threshold is modified with

additional circuitry, this will effectively change the D

MAX

limitproportionallyatvoltagesabovethethresholdvoltage

(V

sure that the power supply can still deliver the required

power with the reduced maximum duty cycle at high line.

R

I

K

V

V

V

_

_

(

)

OV OFF

OV OFF

L

Q BE

4

1

$

$

$

-

-

b

^

h

R

V

V

V

V

V

R

_

(

)

(

)

OV OFF

L

Q

L

Q

BE

BE

5

1

1

4

$

$

-

-

+

^

h

V

V

R

R

I

R

V

V

_

_

LS

L

LS

OV OFF

C

OV OFF

L

6

$

$

=

-

-

-

^

h

V

R

V

V

R

R

( )

(

)

IN th

L

Q BE

5

1

4

5

$

=

+

+

^

^

h

h

I

R

V

V

R

V

V

_

_

OV OFF

OV OFF

C

L

LS

L

6

$

=

-

-

+

b

l