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11/05

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Power Integrations

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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

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Figure 2. PoE Interface Circuit–Using a MOSFET Pass-Switch.

Figure 3. Detection Impedance V-I Curve.

Q51

IRF530N

(100 V / 0.11

Ω)

R51

24.9 k/1%

1/4 W

R52

51 k

1/8 W

R53

249 k

1/8 W

VR51

28 V

500 mW

VR52

15 V

200 mW

0 V to 57 V

DC-DC

RTN

0V

DC-DC

RTN

PI-3825-030304

+

Voltage (V)

0.80

0.60

23.75 k

Ω

0.20

0.40

0.00

0

2

4

6

8

10

12

Class 0

Detect Min

Detect Max

Detection

Voltage

Range

26.5 k

Ω

Figure 4. Classification Current (Class 0).

Figure 5. Pass-Switch Efficiency.

Voltage (V)

6

2

4

0

10

12

14

16

18

20

22

Class 0

Class 0 Max

Class 0 Min

Classification

Voltage Range

400

µA

@ 10 V

880

µA

@ 22 V

30

40

45

35

50

55

60

Input Voltage (V)

90

80

70

60

100

IRF530N

TIP29 Enhanced

the under-voltage threshold for the DPA-Switch is programed

bycomponentsR1,R21,R22,R23andQ20,settingtheturn-on

voltage to 42 VDC and turn-off voltage to 34 VDC.

Key Design Points

Bipolar Transistor Pass-Switch:

• Choose Q51 bipolar with sufficient current and voltage

capability and highest available DC-current gain.

• Choose R52 to give sufficient base-drive at turn-on, to

allow the DC-DC converter to start.

• Choose R54 (typically 10-20 Ω) to limit the capacitively

coupled current spike during switching.

MOSFET Pass-Switch:

• Choose R52 to limit power dissipation in VR51 and VR52.

• Choose R53 to ensure MOSFET turn-off below 28 VDC.

• Choose VR51 to block Q51 turn-on below <28 VDC.

• Higher values of R52, R53 and VR51 will limit dissipation.

• Choose VR52 to limit the maximum gate to source voltage

on Q51 (typically 15 VDC is a good choice).