transformerless power supplies - green?

[utimgr]

In the near future, it would appear the sale of the traditional wal-warts will be a thing of the past in California, and probably followed on by other states.

So, do new devices that are to be sold in California that might use a simple transformer type supply embedded, subject to the same restrictions? Just curious....

New design looking for a very compact non-isolated AC-DC supply. The circuit will be attached directly the AC line, and isolated by the plastic case, with all the standard NRTL safety considerations of course. But thats another issue.

The circuit is going to only need maybe 150mA total current (if that) at 5V, so I was looking at using a capacative type supply that uses just two diodes, two caps and a power resistor. Anyone built one of these for a product...what kind of issues did you run into if so?

Or if there is a better suggestion, open to that as well.

 

[itsmoked]

There is no way they will be dropped. There may be some changes but they aren't going anywhere.

Check

http://www.maxim-ic.com

they had some non-isolated regulators. Not sure they will go to 150mA but take a look.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

A capacitive dropper is not a bad plan. Make sure the cap is specifically rated for Class X service, which is basically mains duty where failure does not directly lead to risk to life. Class Y is the type to use where failure would have more serious consequences.

You must ensure that the circuit always draws power from the supply. A zener shunt regulator is the easiest method. You could also use a bridge rectifer - smaller smoothing cap, and the smallest bridges don't take up much more space than a pair of 1N4001's. Costwise there is probably little in it.

The circuits are not good for keeping mains-borne noise out. The series capacitor is effectively a short at HF. If your circuit is noise-sensitive you might be better off with a purpose-made switchmode module from one of the numerous vendors, rather than fighting a losing battle trying to keep mains-borne noise at bay. The cheap & cheerful solution might end up more expensive if it has to be heavily filtered.

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I don't suffer from insanity. I enjoy it...

 

[utimgr]

Scotty...I agree, sometimes the quick approach turns out to be more of a headache. The only problem I've seen uin using a switchmode type device is what appears to be semi-custom transformers, at least in the app notes. Are you familiar with any that have designs using off the shelf transformers?

 

[ScottyUK]

Not really - most designs use one of the standard ferrite cores with a custom winding. You are talking serious money to produce a custom core. If you use a standard core and bobbin the quantities don't have to be enormous before it is commercially viable. There are a lot of small winding companies who would love to have the business. It's a long time since I did this sort of work, but I doubt things have changed much.

What about something like

http://www.onsemi.com/pub/Collateral/NCP1200-D.PDF

According to EDN the magnetics are available from a couple of suppliers:

http://www.edn.com/articles/pdfs/EDN/20010118/02di11.pdf

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I don't suffer from insanity. I enjoy it...

 

[utimgr]

Perfect. And coilcraft is one of the best companies to work with, samples well and my local rep is great.

Thanks very much!!

 

[ScottyUK]

It's not easy being a genius . Hope it works out.

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I don't suffer from insanity. I enjoy it...

 

[itsmoked]

You realize the that the certifications on these can be expensive to acquire (both in money and time)! You might want to check out something like this too:

http://datasheet.astrodyne.com/PWB.pdf

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

Back to your irst idea,

"I was looking at using a capacative type supply that uses just two diodes, two caps and a power resistor."

These supplies, as said before, need something to limit the voltage and that will consume power. Unless you switch the AX nains and have a load that is always the same current, you will consume more power overall than a wall wart. This defeats the intent of the law. Without isolation, the entire powered device will have to be built to code.

X2 capacitors are the simple way to go. You can achieve the same results by placing two to three lower priced capacitors in series. This is internally what X2 capacitors are. A capacitors AC rating is not proportional to the DC rating. Under AC fields tiny air gaps in the insulation wrap cause corona and "light up" eventually destroying the capacitor. I've used garden variety capacitors in this way for 480V UL line operated equipment.

 

[benta]

I've built capacitor-drop power supplies for smaller loads and it's a good solution, but there's a couple of important things you need to look out for.

Basic circuit is an X2 capacitor feeding a bridge rectifier (4 x 1N4007 or a ready-made bridge). On the DC side of the bridge a smoothing capacitor in parallel with an appropriate Zener, the smoothed DC being fed to an LDO (low dropout voltage regulator) of the desired voltage.

1: remember to place a high-voltage capable resistor across the X2 capacitor, value 200...500 kohms. This is to bleed off charge when you unplug the supply, so you don't get a shock from the capacitor when touching the plug pins. Vishay VR37 types are a good choice.

2: remember to place a current limiting resistor in series with the X2 cap. This is to limit the surge when plugging the power supply in, as you otherwise would probably blow the Zener or the smoothing cap. Select a resistor with an appropriate Ws capability, I've used Vitrohm BW234 and BW235 types (wirewound). Value 50...200 ohms depending on how much current you need.

That's it. The main input current of this type of supply is reactive (leading), so it shouldn't give a lot of problems with power loss. It might even improve the situation in an environment where most loads are lagging.

Benta.

 

[Madcow]

Will the leakage current be a problem?

 

[benta]

"Will the leakage current be a problem?"

Question not understood. What leakage current?

Benta.

 

[Comcokid]

Utimgr - you could also check the components made by Power Integrations. These are ICs designed for constructing low wattage off-line power supplies. Yes - this does bring up the agency approval issues, but maybe the company can then direct you to a suitable product made by one of their customers.

 

[nbucska]

Don't forget a FUSE !!!! Any capacitor can be shorted
by a static/lightning etc.

If you connect cap's in series, it is not a bad idea to
add a R parallel with each to control/balance the leakage
currents.


Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[VEBill]

Sony external power supplies tend to be more advanced (lighter, more powerful) than old-fashioned wall-warts. Maybe take one apart and see what's inside...

 

[utimgr]

Just back to this....I actually am going with the ON Semi circuit, pretty straight forward, and the inductor is on the way (as are the ON parts). The problem with Power Integrations is the custom coils for it. Otherwise, nice product line.

Agency approval will be required anyway, for the entire design so as long as I do the layout properly, shouldnt be a big issue. I've been thru NRTL before. Client pays for it as well.

 

[itsmoked]

Thanks utimgr for the feedback.

May your design go well.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Comcokid]

Actually, the "custom transformer" issue may not be as bad as it seems. A quick search found "Power Integrations" at coilcraft, zettlermagnetics, and other transformer web sites. Some of these magnetics companies have part numbers for the most popular switcher reference designs available.

 

[electricuwe]

I don't think that you will achieve a compact design with a capacitor-drop circuit for that current.

But there are some alternaives:

Most of the integrated switching power supplies circcuits (from PowerIntegrations, ST (Viper) or Infineon (Coolset)) can also be confiured to work just with an inductor if no isolation is required. Check for ANs on that.

There are alos some circuits on the market using a different approach. See:

http://www.supertex.com/feature_sr03x.html

I have not used the Supertex circuits, but I have designed cicuits using the same approach with discrete parts long time ago and it worked well.