I have an isolated switching power supply which takes in 30V and outputs 55V 1.5A. It's a power source for a PoE+ powered device. When the device which is supposed to be powered is not connected (via RJ45), the only load present on the 55V rail is an LED which draws 6mA and the output voltage goes up to somewhere between 56V and 57V. When the powered device is connected (it draws ~400mA) I have a nice and stable 55V output.

Now, the transformer requires a minimum load of 20mA. So the question is, how can I add a temporary load on the output that would automatically disconnect when I connect my powered device? As mentioned above there is only a 6mA load, so I need to add a temporary 14mA load.

As the voltage increase when there is no load, I was thinking about adding a zener with a breakdown voltage of 55V in series with an LED and a resistor. So if the voltage was greater than 55V the zener would start conducting and I could adjust the resistor value accordingly to draw at least 14mA. However I couldn't find a zener with a Zv of 55V.

Any other ideas? I have limited space on the pcb so the component count/footprint is important.

Thanks in advance.

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    \$\begingroup\$ Do you mind having an extra constant 20 mA draw? If not, just add a resistor. \$\endgroup\$ – awjlogan Mar 23 '18 at 10:34
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    \$\begingroup\$ I wouldn't like to waste those 14mA, that's why I need it to be temporary. \$\endgroup\$ – A Silva Mar 23 '18 at 10:42
  • \$\begingroup\$ @A Silva - ok! Suggest you add that to your question, but Andy's answer will get you started :) \$\endgroup\$ – awjlogan Mar 23 '18 at 10:45
  • \$\begingroup\$ Is the actual goal to assure that the output is not above 55V, even with no external device connected? If so, there are potentially other ways of doing that instead of loading the supply so that it is regulating to 55V. \$\endgroup\$ – crj11 Mar 23 '18 at 13:13
  • \$\begingroup\$ @crj11 No, it's ok if the output is above 55V. The actual goal is to draw at least 20mA with no external device connected. Then disconnect this temporary load when the external device is connected. \$\endgroup\$ – A Silva Mar 23 '18 at 13:40

You could start by monitoring the load current like this: -

enter image description here

The output voltage (1 volt per amp of load current) could be used to detect that less than 20 mA is flowing into the load and turn on a transistor that shunts to ground whatever current is needed to make 20 mA flow through the 0.1 ohm shunt resistor.

If the load takes more than 20 mA then the transistor turns off.

  • \$\begingroup\$ Thank you Andy. As I have limited space on my pcb, I'm looking for a solution with the minimum amount of components as possible. I can simulate LT6015 with LTSpice and it might work but it would be very tricky adding the circuit to the board. \$\endgroup\$ – A Silva Mar 23 '18 at 11:40
  • \$\begingroup\$ @ASilva I hear what you say but consider this - you need a circuit that will take 20 mA from 55 volts for potentially long periods of time. The power associated with 55 volts and 20 mA is 1.1 watts and, if you have a very small space, whatever you build will rapidly overheat and destroy itself. How will you overcome this obstacle? \$\endgroup\$ – Andy aka Mar 23 '18 at 12:15
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    \$\begingroup\$ @ASilva Two solutions offered, both rejected after additional design criteria revealed. What I'm seeing is you're looking for a BandAid to fix the real problem: You have the wrong power supply for your PoE. Replace it with one that meets your criteria, which only you know. \$\endgroup\$ – AlmostDone Mar 23 '18 at 13:11
  • \$\begingroup\$ @AlmostDone What's with the hostility? If you read my post you can clearly see that the load has to be temporary and not permanent so the first comment by awjlogan was never a solution. Secondly, I did not reject the second solution - I said that it might work but it would be tricky due to the limited space I have (again, clearly stated on my question). And finally, I do not have the wrong power supply - If you read my post it says PoE+, not PoE. For IEEE Type 2 (PoE+) the min voltage is 51V and the maximum 57V. \$\endgroup\$ – A Silva Mar 23 '18 at 13:33
  • \$\begingroup\$ @ASilva maybe you should provide a link to the power supply data sheet? \$\endgroup\$ – Andy aka Mar 23 '18 at 13:42

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