Simplest Way to step down the 220AC with more than 10% efficiency

Question

Objective

I want to design a tiny single packaged mains-powered temperature sensor. Possibly as a multi-chip module (MCM) on one small PCB board.

Constraints

The temperature sensor I plan to incorporate:

1. Operates (Iq) between 1VDC, 10mA and 3VDC, 30mA
2. It is a 65nm Samgsung fabraction process. So it is very small.

Power source:

1. around 220V mains powered (single-phase)

2. Operation without a secondary source (batt, bulk cap, et. al.) desired to minimize size.

3. The efficiency demanding is just >10% and that is enough for me.

Ideas now

I was intended to design my power converter into a small chip, thanks alot for people's suggestions . I think about charge pump, and different IC modules, the RF powerharvesting, and the possibility for implementing such a device on nm Si process. Above all, microchips that can convert 220V ac to low dc is big-size for me, and also have lots big caps around module; charge pump from Olin's idea, I dont think I can design diode that can support high-voltage on chip, if not on chip, it will be big size(I think); for powerharvesting, I did some research before, and honestly it is good, but RF energy is not very solid, and there are some limitations about the distance. So I go back to the original ideas how to convert voltage. Hope your guys can check if they are right. I will use some regulating stuff(off chip) to regulate the voltage at last, so i just wanna check if the thoughts are right.

1> Linear with the resistors. Input 220Vac to the two resistors,one is 1Mohm, the other is around 10Kohm,figure below:

I know the efficiency is verylow, about 1%, so I drop it. But it is my earliest idea.

2> linear with the capacitor. Since the first way efficiency is so low, I wonder can I use two capacitors to replace the resistors, (figures below) one cap is 1pf, the other is 10fF (the exact values are not certain).

also I wanna design the big capacitor on my pcb board by myself like figure below,so I can mini-size. If the cap value is not big, I can design it on chip.

First I dont know if the fundamental of circuit (the first figure) will work or not, because I never see this before, but as I know, I hope it works. Assume it works, how to design it exactly ( I mean how to design the cap value properly and how to choose the switch frequency properly).

3> The conventional linear isolating converter:

My question is, in this conventional way, the output current in other designs usually needs big, so the transformer is big, but I just need it very small(1mA~30mA), so according to your view, how big it will be, I know little about transformer design. the voltage ratio is 45:1, and the current is 1:45, how many coils on both sides are good ? How big it will.

if the transformer above is not so big, Furthermore, my design method for the transformer is improved, and I wanna design transformer on PCB board by myself, like figures below:

in my first figure, I just use one stage transformer, if size is big to design 220V-5V converting, maybe I can design two or more stages to convert like 220V-48V-5V. I hope the transformer (each stage, hope one stage is enough) is less than 1cm×1cm. If that is possible, I WILL do it. And I think the efficiency is good, and also safe.

Above all, I want to improve my design in the No 2 and No 3 method, but I dont know if my thoughts are right or not. Ready to get judgement now.

Answer 1

What you are looking for is called an off-line power supply. A quick search reveals the Fairchild Semi FSAR001. Putting in 80 - 240 Vac yields 5Vdc at 35 mA max.

There are many more around.

• it is NON-isolated!!! meaning this is not a design to put into devices that people will handle - Full Stop.

let me repeat that, this is a lethal circuit, but perfectly reasonable to use under the right conditions.

Here is a snip from page 2 of the datasheet.

This answer does not address your question concerning a chip design. Which I can answer but I'm hoping that the real problem is solved with this lead and direction.

Answer 2

No getting around physics. You will need a "wall wart". Either you get a wall wart as a packaged product or you implement it yourself on your PCB.

Here's why your ask isn't feasible:

A wall-wart...

• performs rectification (converting bi-polar AC power to uni-polar AC power)
• followed by filtration (converting uni-polar AC to an approximation of DC)

Those are the basic steps to get from AC to DC. Any alternate approach will include those steps in some form. You can get much smaller (lower output power) units if that would suit your needs better. You can also get them as PCB modules (google for "open-frame" power supply) instead of packaged products.

For example, these.

Answer 3

The FSAR001 circuit suggested by rawbrawb is a clever one that will do what you want better than almost any other circuit IF designed well. But it can be very unreliable if badly designed. This is because transients or unintended signals can turn on the input-to-output switch when it should not be one. Mains is then connected to output directly. This is usually 'a bad idea' [tm].

The FSAR001 circuit efficiency will be high when considering efficiency as
(Vin x Iin) / (Vout x Iout) . BUT it's power factor is poor - which often does not matter. ie it draws all its power when the mains cycle is at low voltage and none at all when voltage is high so the waveform is VERY distorted compared to a sinusoid. Regulatory authorities are increasingly unhappy with such systems BUT when power levels are very low (as here) it may be considered acceptable.

It works by turning on a switch when Vin_mains is low and ~- Vout, and turning it off when Vin_mains >> Vout. If your IC process can withstand mains peak voltage you can build it into your IC. If not then you can add it using one high voltage switch (bipolar transistor etc) plus a low voltage IC. This is a very clever circuit and also easily unreliable if designed badly. It will do what you want if designed well.

A design which can be small and which is potentially safer is to rectify mains and then to use a very high frequency oscillator to transfer energy via a magnetic core. The higher the frequency the smaller the core. Some modern converters operate in the 1 MhZ - 10 Mhz range to get size low. Even higher frequencies are possible with due care.

A recent approach is to generate RF at extremely high frequencies - 1 GHz + in some cases, and to use very very small capacitors as energy coupling devices. This can result in extremely small systems but complexity is higher.

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Korea? A visit there would be interesting ... :-).

Answer 4

Now to answer the "chip design" side of the question. This answer cannot by necessity cover all the details. You'll have research individual areas your self, which I hope to provide leads to in the text.

The first step you'll need to do is to find a process that can handle the high voltages that are required.There are limits to the materials that are used here that scale with electric field strength. There are Si processes that span from 1000V down to 1 V so we'll assume you'll find a Si process (Bipolar, BiCMOS or CMOS) that can handle the voltage.

You seem fixated on 65 nm process technology. Doing a rough calculation and assuming 1V operation, to scale this design to handle 600 V you'd need a 39,000 nm process node = 39um. And that is to support the lateral e-field from Source to Drain. That in it self is a big hint that this process wouldn't be used. In fact higher voltage process nodes use slightly different devices, like DMOS. The offline controller chip is most likely made in a 1 or a 2 or a 3 um process, and may actually be SOI.

The highest voltage, smallest process node I am aware of is ~ 50V on 0.18u CMOS process, - automotive qualified. There could be others out there. Look around. Since you are in Korea look at Magnachip and Dongbu Hightech. as fabs.

Assuming you've now picked a process that can handle the voltage and the 65 nm process node is long gone from your thoughts. You are now a hero because the NRE for the process has gone from $1M (65 nm node) to maybe $60K (3u node).

So can we put inductors on chip? Absolutely. but they are HUGE, and very hard to make in a way that has good yield. RF guys use them for Tank circuits and filters. But the thing to remember is that the inductor sizes used in RF circuits are about 1/1,000,000 the inductance that you will need to do a good SMPS convertor. And NO you can't put a high permittvity material down to boost inductance, you are stuck with SiO2 and it's various variations. So POWER inductors are out of the equation now too.

Next, capacitors. Based upon a known process node - 180 nm, supports 1.8 Volts and has 8.8 fF per um^2 capacitance. Lets scale this to 600 V by increasing gate oxide thickness. => 60um thick gate oxide to prevent rupture. (E-field stays the same). Capacitance is 1/333 => 26.4 aF/um^2. For 10 uF you need 3.8e11 square um to get that capacitance. => 0.4 m^2 notice that is a die that is roughly 0.6 m X 0.6m on a side. I think that cost starts to become an issue then. That offchip capacitor now starts to look very reasonable.

Now all the design constraints are in place. Using an old, high voltage process node, with no access to on chip inductors or capacitors. But it's inexpensive! And you get proper analog transistors vs. the digital ones you'd get in the 65nm process.

The only solution I can think off, since you can't use any capacitors off chip, is to build a full wave rectifier and ONLY operate the circuit when the the input voltage is above the 3 V threshold of operation. Have the circuit shutdown during the AC waveform zero crossing. That way you don't need the "big" hold capacitors. Once the AC waveform is above the 3V range around zero crossing you'll have plenty of power. You could put much smaller filtering, charge holding capacitors on the bias circuits (that don't consume much power) to allow the operating point of the circuit to stay fixed during the varying power supply. And you can reduce power. You should be able to get a good bandgap circuit that operates on less than 1 uA so that means far smaller capacitors. But it means that you can only communicate during the AC voltage "high" phase.

Answer 5

Isolated power step-down and rectification from mains power line, as well as power, analog and digital blocks on a single die are possible, and are being done by at least a couple of manufacturers, Analog Devices being noteworthy in this regard.

Analog Devices isoPower iCoupler devices achieve 5 kV of isolation, with single-power-source operation, through their in-chip microtransformer technology. While their current isoPower portfolio does not apparently offer any microcontroller or temperature sensor devices, the technology concept proof should serve to point a chip designer in the right direction.

The paper referenced above provides detail on isolation geometries, gaps and materials parameters for their designs.

Some salient points from the paper:

• The microtransformers are built on a CMOS substrate, and a 20 µm thick polyimide layer in between the primary and the secondary provides HV isolation up to 5 kV.
• Rectification of the secondary is achieved through integrated Schottky diodes
• A linear regulator on the secondary regulates output power, thus allowing the devices to output regulated power to supply additional logic-level components.

In short, the isoPower line is almost an ideal match for the power aspect of the requirements stated in the question.

Once single-chip isolated regulated power is achieved, the temperature sensing and display functionality can be addressed as a more conventional chip design / MEMS problem.

Answer 6

Essentially - no. You will find a very few off-line power supplies on silicon - but they are formed on non-standard processes tuned specifically for high voltage transistors, and not suitable for microcontrollers or general analog circuits. As a chip designer you will not have access to these processes unless you talk to a specialist manufacturer - International Rectifier, Ixys etc.

If you can design your whole system - including sensor - so that it can be completely isolated from access by a consumer - "double-insulated" - then you can probably use a non-isolated off-line power supply like the Fairchild part mentioned above. Then you can devote perhaps a square inch of PCB space to the offline PSU - your sensor and its electronics might live on the same board.

But a temperature sensor, isolated from the environment as it must be for safety reasons, and physically close to a warm-running power supply, sounds fairly useless to me...

This is the reason for the persistent questions about exactly what your sensors are and we still don't have the information from you to answer your question properly.

Answer 7

You are simply not going to put a line-powered power supply on a chip. The voltages are too high to allow for a reasonable size, and you need other components that require enough energy storage to make them impossible.

I assume this can be a isolated supply since you are apparently trying to build a stand-alone unit that does not electrically connect to the outside world except to the power line. In that case, I still think a charge pump is your best option. Yes, it will be external to the chip, and compared to a chip it will be huge. That's the way it is.

Here is a basic charge pump:

When the top AC input goes negative with respect to the bottom, C1 is charged up to negative the peak line voltage thru D2. As the voltage swings back to positive, it is discharged thru D1 and charges up C3 somewhat. With no load, the DC output voltage is the peak line voltage, which is not what you want. However, the current is well limited, so the simplest thing would be to follow this with a shunt regulator. That will dip between peaks, so you either design the reset of the circuit to tolerate that, or you make the shunt regulator a little higher than what you want and follow that by a normal linear regulator.

One drawback to this approach is that the current you get is depressingly low for nice size capacitors at line frequency. You can make smaller capacitors allow more current, but you'd have to then rectify the AC line and chop it yourself with active circuitry.

There is no free lunch as you seem to be wishing. If what you are asking for was reasonbly possible, others would have done it long ago.