DI-70
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.
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).
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.
6
Class 0
• Choose R54 (typically 10-20 Ω) to limit the capacitively
coupled current spike during switching.
Class 0 Max
4
Classification
Voltage Range
VIN
DC-DC
0 V to 57 V
+
VR51
28 V
500 mW
R52
51 k
1/8 W
R51
24.9 k/1%
1/4 W
2
880 µA
@ 22 V
400 µA
@ 10 V
R53
249 k
1/8 W
VR52
15 V
200 mW
Q51
IRF530N
Class 0 Min
RTN
0V
DC-DC
RTN
(100 V / 0.11 Ω)
0
20
10
12
14
16
18
22
PI-3825-030304
Voltage (V)
Figure 2. PoE Interface Circuit–Using a MOSFET Pass-Switch.
Figure 4. Classification Current (Class 0).
0.80
Class 0
Detect Min
Detect Max
0.60
100
90
23.75 kΩ
Detection
0.40
Voltage
Range
80
IRF530N
TIP29 Enhanced
0.20
26.5 kΩ
70
60
0.00
10
0
2
4
6
8
12
30
35
40
Input Voltage (V)
Figure 5. Pass-Switch Efficiency.
45
50
55
60
Voltage (V)
Figure 3. Detection Impedance V-I Curve.
For the latest updates, visit www.powerint.com
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, Clampless, E-Shield, Filterfuse,
PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies.
©Copyright 2005, Power Integrations, Inc.
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11/05
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