
An efficient power converter design reduces resting power consumption by 50% - werediver
http://news.mit.edu/2017/efficient-power-converter-internet-of-things-0217
======
Animats
This is progress, but in a small part of the power supply. It looks like they
have a charge-pump type power supply, and they turn off the charge-level
checking circuit except during polls. They also adjust the polling rate down
when not much is happening. The question is how fast they can respond to a
sudden demand for output power. Can a sudden load drain the capacitor before
the input side notices and pumps it up again?

Excessive standby power consumption is a big problem. There are lots of
devices, especially TVs, which consume far too much power when supposedly off.
(Of course, some of them are constantly listening to you, phoning home, and
possibly spying on you.)

One of the more important recent developments in switching power supplies is
the elimination of electrolytic capacitors. They're the biggest point of
failure. This is already happening in LED lightbulbs.[1][2] Electrolytic caps
inside LED lamp units are the primary cause of failure. They only last about
10,000 to 20,000 hours, while high-output LEDs are good for 40,000 hours. If a
power supply can be built with lower capacitance, ceramic capacitors can be
used. That makes it possible to get the lifetime of all the power components
above 100,000 hours, so the LEDs will burn out first.

[1]
[https://hub.hku.hk/bitstream/10722/164083/1/Content.pdf](https://hub.hku.hk/bitstream/10722/164083/1/Content.pdf)
[2] www.rle.mit.edu/per/wp-content/uploads/2014/10/Chen-Electrolytic-Free.pdf

~~~
semi-extrinsic
> Excessive standby power consumption is a big problem. There are lots of
> devices, especially TVs, which consume far too much power when supposedly
> off

Is this really a problem anymore? I have a five year old 50" plasma, and it
draws 0.12 W in standby. That's not "excessive", it's a rounding error. It's 1
kWh per year, or roughly 0.6 kg of CO2; the same as idling your car for an
extra 2 seconds every day.

~~~
philipkglass
The mean power consumption is significantly higher than the median because the
worst offenders are drawing tens of watts in standby mode. For example, this 4
year old Philips TV that is at 30 watts in standby:
[http://www.supportforum.philips.com/en/showthread.php?15754-...](http://www.supportforum.philips.com/en/showthread.php?15754-Philips-
violatig-EU-law-1275-2008-for-power-consumption-in-Standby-mode-\(30-Watts!\))

Cable/satellite boxes also frequently draw multiple watts in standby or "off".
If every bit of consumer electronics were as thrifty with standby as your TV,
it wouldn't be a problem anymore.

(I'll add: my iPod Touch can "wake up" instantly when I interact with it after
_days_ sitting unplugged, and that's drawing from a tiny battery. Even 120 mW
is far above the lower bound of what's necessary to build a complex device
with fast wakeup upon user interaction.)

(Second edit: the power converter that started this discussion is obviously
targeted at _much_ lower power applications than even an iPod, so it's not
really relevant to anything that can be plugged in at home.)

------
lutusp
Quote: “In the low-power regime, the way these power converters work, it’s not
based on a continuous flow of energy,” Paidimarri says. “It’s based on these
packets of energy. You have these switches, and an inductor, and a capacitor
in the power converter, and you basically turn on and off these switches.”

To me that's a failed attempt to create a layperson-accessible explanation. In
my NASA Space Shuttle work in decades past I also created efficient power
supplies and I used similar methods -- but I think I can explain it more
effectively.

The trick to making a modern power supply efficient is to control the phase
relationship between voltage and current. This is normally performed in
reactive elements like an inductor, a capacitor, or both.

Put another way, instead of changing a voltage level with a resistance (which
would waste power), you need only manipulate a reactive element so the phase
angle between voltage and current is something other than zero degrees. For
example, at a phase angle of 90 degrees, you can have substantial voltage as
well as current, but no power dissipation (except where it's needed).

That's the secret to power supply efficiency -- change the voltage without
using any power-dissipating elements.

~~~
dragontamer
But that's not the issue at 500pA to 1mA. Buck and Boost converters are
already over 90% accurate __easily __by modern technologies.

The issue at 500pA to 1mA is the quiescent current of the device. The
researchers here have basically created a very, very low quiescent current
device by turning off even more things than typical. Even the voltage-divider
which detects the presence of voltages has been turned off.

So I'd expect a bad transient response and maybe a brownout if the connected
devices suddenly requested a bunch of power.

------
mulmen
Would it be more efficient to have a secondary bus in the home that provides
lower voltage and current for IoT type devices? Something like the USB ports
built in to the power outlet but that actually has a single converter in the
home with more efficiency.

~~~
Obi_Juan_Kenobi
Generally this doesn't work too well because low-voltage distribution is very
inefficient. Even at very modest currents, you're going to need huge
conductors or else deal with lots of voltage drop.

Compare that to a typical switch-mode power supply, which is already very
efficient. The quiescent current of a modern, reasonably well designed power
supply is basically negligible. The problems of 'vampire draw' have basically
been solved for the actual power supply (devices that don't 'sleep' are still
an issue, but that's no fault of the supply).

Recently I modified some LED fairy lights with a LDR/transistor setup. The
quiescent draw during the day is about a tenth of a watt, vs. 6 watts under
load. Left plugged in for a year in 'off' mode, that would cost me less than
10 cents.

This 'variable clock' switch-mode supply is all about squeezing every bit of
battery life you can with a device. It's about making sensors that can be
deployed without a power source that last for years instead of months.

~~~
mulmen
This is exactly the type of response I was hoping for, thank you. Is this a
similar reason to why long distance power lines are of a much higher voltage?

~~~
euyyn
Correct. And why AC won over DC: It's much easier to step the voltage up for
power transmission.

~~~
zkms
> It's much easier to step the voltage up for power transmission.

It _used_ to be much easier to step the voltage up with AC, but now we have
advanced enough power electronics to make the inherent increased efficiency of
HVDC be actually worth it.

------
gens
Short story they measure the output voltage every once in a while, depending
on the load, instead of always measuring it.

> If no device is drawing current from the converter, or if the current is
> going only to a simple, local circuit, the controllers might release between
> 1 and a couple hundred packets per second.

> .. To accommodate that range of outputs, a typical converter — even a low-
> power one — will simply perform 1 million voltage measurements a second; on
> that basis, it will release anywhere from 1 to 1 million packets.

^ Normal switched-mode converter, though a million is not realistic and they
don't always work like that

Considering that today's computers power supplies work at 70kHz, a megahertz
is probably too much. (granted PSUs do PWM so i may be wrong, but it depends
on the size of the capacitor at the output; then again chemical capacitors are
bad for efficiency and block capacitors are huge compared to their capacity so
block capacitors would have to be filled more often meaning more pulses
meaning more losses on the FETs but then again transistors now have much lower
capacitance and resistance then ever before but .. ramble ramble ramble etc)

> Paidimarri and Chandrakasan’s converter thus features a variable clock,
> which can run the switch controllers at a wide range of rates.

Theirs just varies the rate of (in programmers terms) pool()ing. Like clocking
down a cpu when there is no load.

(note: "packets" means pulses, see switched-mode power supply on wikipedia)

~~~
marcosdumay
A bit longer story, they add a transistor before the voltage divider that is
doing that measurement, and keep the circuit open most of the time:

> so in the MIT researchers’ chip, the divider is surrounded by a block of
> additional circuit elements, which grant access to the divider only for the
> fraction of a second that a measurement requires

That's not something you'd do in any high power PSU. It is really an IoT
thing. It will decrease the full-on efficiency, while increasing the sleep
efficiency.

I also don't think they are using a 1 MHz PWM anywhere. That number probably
leaked into the journalist notes from some irrelevant detail.

------
dragontamer
Lets look at this practically:
[http://www.ti.com/product/tps62240/description](http://www.ti.com/product/tps62240/description)

This TPS62240 has an efficiency of 95% across any load above 1mA. Below 0.1mA,
its 15uA quiescent current kills its efficiency... but it still has over 70%
efficiency at 0.1mA. That is, the converter uses say... 0.13mA while
outputting only 0.1mA. (I mean, it really should be in terms of Watts. But
since P = VxI, the mA estimation is a good enough indicator).

A single NiMH AA battery has 2000mAh of energy storage capacity, Eneloops
actually are closer 2500mAh. [https://www.amazon.com/Panasonic-BK-3HCCA4BA-
Eneloop-Pre-Cha...](https://www.amazon.com/Panasonic-BK-3HCCA4BA-Eneloop-Pre-
Charged-Rechargeable/dp/B00JHKSL28)

So in effect, a SINGLE AA NiMH batteries can run at 0.13mA for around 2
__years __. In practice, the self-leakage of these batteries are the major
problem on those time periods.

At some point, reducing quiescent power consumption isn't a major problem
anymore. I think the ridiculously efficient 500pA draw is unlikely to be used
in any design where a AA battery is sufficient.

Smaller coin-batteries have issues ramping up to the ~50mA to transmit a radio
signal. So I'd bet that in practice, AA batteries (or larger 18650 cells) will
continue to be used... and therefore a hugely power-efficient design from
these researchers won't have a major competitive edge.

\-------

As an FYI: These efficient power converters enter "sleep" modes while the
voltage output is above 1% of the voltage. Once the voltage drops to the
nominal voltage, the TPS62240 "wakes up" and starts injecting charge into the
capacitor / inductors... before entering another sleep cycle.

Furthermore, at higher currents, I have my doubts that a charge-pump design
would be superior to the capacitor/inductor design of a typical buck (or
boost) converter. So sure... this charge-pump design might be more efficient
at 0.1mA, but I bet you that the capacitor/inductor is more efficient at 50mA.
(The TPS62240 hitting 95% efficiency in the >1mA range)

> new power converter that maintains its efficiency at currents ranging from
> 500 picoamps to 1 milliamp

Hmmm... so this design really is designed for below 1mA currents. I really
wonder what applications they are trying to use. Below 1mA, I say... just be
wasteful. If it takes two months for a AA battery to run out of charge... is
there really an issue?

~~~
londons_explore
There are uses for this, but not in the applications you're thinking of.

Think recovering energy from radio waves, solar powered sand-size devices,
etc.

------
electricant
Maybe this is relevant:

[https://youtu.be/1vYJq4GeXPM](https://youtu.be/1vYJq4GeXPM)

Dave has recently busted another device claiming 0 standby power consumption.

------
ars
Would it make sense to have two power supplies in the IoT device? One for the
nanoamp draw and a second one used only when the radio is on?

------
csense
What kind of stuff uses 0.7-0.9V for its power supply? Is that even enough to
switch a transistor?

------
throwawayish
I recently powered an old Acer laptop on that I didn't use for a while. It
informed me that "/dev/sda2 has gone 941 days without being checked", after it
finished that and booting fully up (a quick affair, even on that machine with
no SSD), the i3 status bar further informed me, that the battery still holds a
57 % charge :)

