Computing stuff tied to the physical world

A “beefy” power supply

In Hardware on Nov 14, 2011 at 00:01

In a comment on the daily weblog, Jörg pointed to a very interesting chip which can directly switch 220 V.

All the parts are available as through-hole, so I decided to give it a go:

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I used the LNK302, with a 2.00 kΩ / 2.32 kΩ 1% resistance divider to select the output voltage. At the left there’s a fusible 100 4700 Ω resistor, a diode, and a 3.3 µF (400V!) electrolytic cap for (high-voltage) DC input.

The circuit officially only works with input voltages above 70 V, but that’s a conservative spec. It actually works fine from my 30 VDC lab supply, which means I can safely poke around in it and see how it behaves.

Time to fire up the scope again. Here’s the output with a 1 mA load:

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Channel 1 (yellow) is the output, but AC coupled, i.e. just the fluctuations, while channel 2 (blue) is hooked up to the same pin but in DC-coupled mode.

As you can see, the output is roughly 3.8V with brief but fairly large spikes of almost 0.3V. Basically, the switching chip periodically connects the input voltage to the output (through an inductor, and charging a 100 µF cap).

The fun begins when you start loading the supply a bit. Here’s what it does at 10 mA:

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Similar spikes, at roughly 10 KHz (quite a bit of variation in timing). Now 25 mA:

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More of the same, the repetition rate doubles to around 20 KHz, and the voltage drops a bit. Let’s go for 50 mA:

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It’s getting a bit jittery now, doubling its frequency every once in a while. And here’s 75 mA:

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Nice and steady output, the ripple voltage is under 0.2V now. Still holding at 3.2V.

Can we pull more current out of this circuit? Not really, I’m afraid – see what happens at around 80 mA:

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Going full speed now at around 65 KHz, but there’s simply not enough energy: the output collapses to 1.32 V.

With roughly 70 mA @ 3.2 V, input power consumption is about 20 mA @ 30 V. This isn’t stellar (37% efficiency), but also not really indicative of what it will do at 220 V, since I’m running the chip way out of spec.

I’ll need to do some tests at the full 220 VAC to make sure this behavior is similar under real-world conditions, but from what I can tell, 50..65 mA is probably about the limit of what this circuit can supply at about 3.3V. Which would be plenty for a JeeNode in full transmit mode BTW, including some additional circuitry around it.

One problem is the fairly large ripple voltage. It would be better to dimension the circuit for a 5V output, or even 12V, and then add the usual linear regulator to get it down to 3.3V for the logic circuit. This could actually be quite practical in combination with a small 12V relay (which isn’t affected by such voltage fluctuations).

Note that a circuit like this – even if it were to supply only 5 mA – would be plenty to drive a JeeNode which sits mostly in low-power mode and only occasionally needs to activate its RFM12B wireless module.

So all in all: a very interesting (non-isolated) option!

Update – Also ok on 220 V: 65 mA @ 3.0 V (draws 1.25 W, i.e. 15 %). With 2 mA @ 3.7 V, power consumption is 0.40 W (vs. ≈ 8 mW delivered, i.e. 2 % efficiency). At 80 mA, the voltage drops to 2.5 V – above that it collapses.

  1. Wouldn’t 0.4mW vs 8mW be around 95% efficiency?

    • Careful with units – that’s 8 mW delivered with 0.4 W consumed, i.e. 8 vs 400 mW.

  2. JC, could there possibly be something wrong with your input power measurement? You might be measuring a lot of apparent power. Did you measure input current or input power? I would expect aroung >50% efficiency at 230V AC, this is a big difference.

    The 4k7 in the input could also be a bit big and, yes, the ripple is too high for using it as a direct controller supply (there should be at least more filtering or a linear regulator behind).

    • Could well be. It does sound odd – I’ll redo that in the coming days. I’ve got two different instruments to measure such low powers, but the one I used might well be too inaccurate for such high phase differences.

      The 4.7 kΩ is a bit high, I agree, but the 100 Ω I tried first immediately blew (well, that’s what I assume happened). Still, 1 mA should only be in the order of just 5 mW (and 100 mW during the start up peak).

  3. I find your readings weird, if I compare them to the specifications of the LNK302:

    EcoSmart®– Extremely Energy Efficient • Consumes typically only 50/80 mW in self-powered buck topology at 115/230 VAC input with no load (opto feedback) • Consumes typically only 7/12 mW in flyback topology with external bias at 115/230 VAC input with no load • Meets California Energy Commission (CEC), Energy Star, and EU requirements

    You’re measuring 0,4W in ‘idle and only 15% efficiency during load, where the spec claims 75%…

    Something doesn’t look right ;-)

    • Note that the spec says: 50/80 mW using opto feedback… which is not the case here. But yes, I suspect that my power measurement is off.

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