DC/DC isolator with small minimum load requirement

[leonerd]

Can anyone suggest an isolated (>= 1kV) 5V-5V DC isolator, that can cope with almost no load, maybe down to the single-digits of mA?


I'm looking to make an RS-485 isolator. Part of that requires power supply isolation to the isolated-side MAX481 or similar chip.

While I'm normally a fan of things like the Murata NME or NKE series (e.g. NME0505SC http://uk.farnell.com/1021456), I'm aware these things have a minimum load requirement. E.g. the 1W converter outputting 5V wants to see a minimum load of 20mA. The logic isolator (ADuM1401) says it wants 1mA per channel, so 4mA, and the MAX481 wants basically nothing as compared that. So I'm still 16mA short of the minimum load. Without that minimum load, the power isolator has a tendency to output excessive voltage.

So my options here seem to be any of

• Burn the extra load off in a big resistor. Seems a bit wasteful
• Aim at a higher output voltage and pass that through a linear regulator - e.g. 9V then 7805
• Find a DC converter with a lower minimum load requirement

The final of these three options seems the most elegant to me, but I'm not sure what products would be available there.

Can anyone recommend anything?

[Rerouter]

hook up a power indicator led, and call it a day?

In reality your talking about burning 80 milli-watt, so not that wasteful unless your aiming for very low power, (0.016x5)

So lets say a power led at 5mA, and a 470 ohm resistor across the supply to soak up the rest,

[Fungus]

So my options here seem to be any of
Burn the extra load off in a big resistor. Seems a bit wasteful

Only a tiny bit.

(unless you're running off batteries)

Aim at a higher output voltage and pass that through a linear regulator - e.g. 9V then 7805

...which would be 50% inefficient overall, but that didn't seem wasteful? 

[leonerd]

I think I've found another solution. Rather than separate MAX481 / DC isolator / logic isolator, Maxim make an almost-allinone chip; the MAX14855.

This contains the logic isolator and RS-485 line driver/receiver on the isolated side, and driver/regulator to handle a small power transformer. All you do is add an isolator transformer and a couple of rectifier diodes, and there you have it.

The data sheet for this chip doesn't make mention of any sort of minimum load requirement - does that mean it should be fine whatever? It talks about the maximum load through the LDO regulator being 300mA, but there's no comment on minimum. I imagine since there is a regulator there, that should keep the output voltage stable at small loads; and likely the entire arrangement is designed for this sort of setup in the first place, being an all-in-one chip like this.

Does that seem preferable? It wins in price terms too

[Giaime]

We use ADM2587 a lot at work for this application, it's all in one. Check it, maybe that's all you need 

[leonerd]

We use ADM2587 a lot at work for this application, it's all in one. Check it, maybe that's all you need 

Yes; I'm aware of the all-in-one isolated logic+power RS-485 transceivers by AD; such as that one. They're insanely expensive though - the MAX chip + separate transformer comes in at around half that cost. It is an option, but I think the MAX + transformer is a better one.

[chris_leyson]

I've used the NME0505SC before but wasn't worried about minimum load requirements. They're just simple Royer converters with no secondary regulation. What you have to watch out for in this type of converter is Vin rise time. Make the rise time too long, 10s of milliseconds for example and it lets the magic smoke out because the bias network expects th Vin transient to start the oscillator.
I tip for young players, read the 3rd or 4th page of the data sheet because in this instance startup time does really matter.

[leonerd]

Actually, having now just measured the performance of my actual device, it seems with the current draw I get 5.3V out of the 5V output anyway, which seems good enough.

[nctnico]

Also the really cheap DC-DC converters are not short circuit proof. I'm phasing them out for that reason.

[David Hess]

Pulse transformers like from Coilcraft can be used as part of a simple inverter which will have no minimum output current requirement.  Ethernet isolation transformers can also be used for this at low power levels.

If you need a tightly regulated output, then that can be done as well with some added complexity.

[ebclr]

http://www.aliexpress.com/item/B0505S-2W-5V-to-5V-isolated-power-isolated-DC-DC-power-module/32621723618.html?spm=2114.01010208.3.100.UTZ0nT&ws_ab_test=searchweb201556_8,searchweb201602_4_10037_10017_507_10033_10032_401_10040,searchweb201603_3&btsid=470b0247-3747-47a8-89f0-48e20bdcb1c1

[Jeroen3]

Build one yourself and use a sot-23-5 LDO, like the mic5205.
http://www.ti.com/lit/ds/sllsea0g/sllsea0g.pdf

[ebclr]

liked this SN6501  appear to be a nice ic to drive VFD transformers, I will make a try