EPR-86 Datasheet, PDF (8/28 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP 86 - 6.6 W Multi-Class Powered Device (PD) for Power over Ethernet (PoE) Using DPA-Switch-(DPA423G)

EP-86 – 6.6 W, 3.3 V, 2 A PoE Powered Device
13-Apr-2006
Figure 5 – Table of PoE Classifications and Power Levels.
Figure 6 – Classification Current (Class 2: R34 = 69.8 Ω; Class 3: R34 = 45.3 Ω).
Zener diode VR5 conducts above 27 VDC, raising the gate voltage on the pass-switch
MOSFET (Q8), turning it on when the gate-threshold voltage is exceeded. Pull-down
resistor R25 limits the current through VR5 while pull-down resistor R24 keeps Q8 turned
off, unless it is being actively driven on. Zener diode VR4 limits the maximum gate-to-
source voltage on Q8 to 15 V. When VR5 conducts, it also turns on Q6 through R23.
Transistor Q6 pulls down on the base of Q7, which turns off the main classification
current source (although the bias current source of 350 µA will continue to conduct).
4.6 Wide Hysteresis Under-Voltage Lockout
If there were no other components connected to the L pin, then resistor R5 would set the
under-voltage turn-on threshold to approximately 35 VDC and the turn-off threshold to
approximately 33 VDC.
However, in the case of PoE, the turn-on voltage is much higher than the turn-off voltage.
This requires more under-voltage hysteresis. When the power supply is operating
normally, the bias voltage is approximately 14 VDC. Resistors R15 and R16 form a
voltage divider that turns off the base of Q2, once the DC-DC converter has begun
switching and the bias voltage is present. At start-up, when the bias voltage is absent,
Power Integrations
Tel: [phone] Fax: [phone]
www.powerint.com
Page 8 of 28