Hello,
I am designing a data logger for 16 sensors, and I need to power sensors separately from isolated power supplies (each supply must be isolated from each other).
Now I need to design the sensors' supplies, and I need some help.
Requirements:
- my power source is a 24VDC
- the sensors have to be powered at 24V and the maximum load of each sensor is 100 mA (@24V)
- budget limitations: the total BOM cost for the single sensor supply should be under € 3 for 100 pcs (that means, for 16 sensors each board, the need to buy a total of 1600 identical circuits); this is not a strict limit, but I cannot go much higher than that
- the design should be as small as possible
- I need some kind of Enable pin (or something similar... but, well, I could use an input MOSFET to do that, if I find a suitable solution missing the EN pin)
- I need to measure the output current, but this has to be done on the input side; as I don't have strict requirements on the current accuracy, I can also estimate the circuit efficiency in order to have a better value...
My problem is I never designed an isolated power supply; I gave a look at many flyback switching regulators data sheets, as well as at other solutions...
I finally found LM5017, which seems to be a good solution. I also found PMP7315, which is a reference design apparently meeting all my requirements in terms of output specifications.
Now I have some questions about it:
- Can I use the UVLO input as an enable pin?
- How can I measure the output current? Looking at the PMP7315 schematics, I was thinking about putting a small precise resistor in series to the inductor in the non-isolated side, and using an opamp + MCU's ADC to read the current... but I don't know if that resistor can somewhat alter the circuit.
- How accurate is PMP7315 output? I mean, LM5017 is a buck converter, and the transformer is 1:1; that should mean I cannot get Vin = Vout... or am I missing something?
- Can I use LM5018 or LM5019 with the very same circuit to save money?
- And what about LM25017, LM25018 and LM25019?
Many thanks in advance for your help.
Stefano Infante