Transformerless power supply.

[buzzp]

I have the need to convert 120VAC to 24VAC. A 1VA transformer is to big for the package. I need to design a supply to source about .75 VA. I really don't want to use a resistive supply as this generates alot of heat (5-7W) and is very inefficient. I am thinking of a capacitive circuit, cap in series with the AC to limit the current using the reactance of the cap. I believe they use these types of circuits in appliance control.
Below is one design link using half wave rectifier. This is microchips app note AN954 and is good for about
15mA.

http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

(forgot how to insert pics)

Below is yet another link using full wave rectifier from microchips document TB008.

http://www.e-sonic.com/whatsnew/Microchip/power/TB008.pdf

I have always known about a capacitive supply but never really dove into it much until now. I believe my requirements are feasible using such a design, just not sure if I am going to gain much board space using this method over a transformer depending on the requirements of the front end cap. I can come up with the capacitance, voltage, etc on my own just not sure if UL has any requirements for this cap to be class X capacitors. I always thought these were more for suppression but I thought of using them but the footprint is large compared to standard eletrolytics. I am thinking class X would be overkill, especially if I use transient protection such as an MOV and/or other suppression. Microchip does mention the second circuit requires R1 and R2 in order to achieve UL approval. R1 is for current limiting and R2 is for bleeding off the charge. I don’t believe the fuse is necessarily required as it only protects for the line and neutral from being miswired.
I will not be using either one of these circuits directly. However, I will likely be using full wave rectifier with the components on the line side to limit the traces with 120Vac on them (spacing). I need about 55mA at 12VDC but I am thinking I will change by biggest load (relay coil @12VDC is about 37mA) to operate on a different voltage (maybe as much as 115) to reduce the current requirements.
Anyway, I am just looking for some advise on the front end cap, as far as if I should use class X caps (I think not necessary) as well as any other advise from others who have used these types of circuits. Yes I kow the isolation is gone going this route without the use of a transformer or optics. Building a switching supply probably won't save me much space either and I would rather go the route of a resistive supply than building a switcher for this design. Thanks in advance.

 

[OperaHouse]

Interesting that they put the voltage dropping components in the neutral, you should reverse that. I have used this a lot in commercial products. Capacitors don't like line AC, using 2 caps in series will reduce corona problems.

 

[buzzp]

Yes, I mentioned I would not be reversing them as microchip has.
Did you use normal aluminum electros on the front end (with correct voltage rating of course)?

I am not sure I understand your comment about corona. The only issues I see are with line transients and I will keep those in check with other components. Is this what you were referring too, internal capacitor corona?
Thanks

 

[itsmoked]

I was going to suggest the MAX610/611/612 parts but it seems they have abandoned them. ????!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Renovator1]

buzzp - You can use regular electrolytics for the smoothing caps BUT you must keep in mind that the voltage presented to them will be inversely proportional to the load current. Thus, a shunt regulator, such as a zener diode, is pretty much a necessity here. Now, what construction you choose for the series capacitor depends a lot on whether or not this widget you are making needs to meet safety agency approval (i.e., UL, VDE, etc.). If it does, then practically speaking the series capacitor must be an X type (there are some exceptions, but you'll want to consult your legal dept./a lawyer if you plan on invoking any of them).

itsmoked - this is precisely why I am very loathe to use MAXIM products in any of my designs - they seem to be the worst by far at churning out new products then killing them off a year or so later.

 

[itsmoked]

Renovator1; I actually haven't noticed them killing off any parts before but they over package each part into a dozen packages which is the equivalent of directly increasing their parts offerings. Then with their rented fabs - not ever having stock on anything you want when it comes to production.

But we digress.

Buzzp; I have seen products that tried to not use X types in that regard. They were explosive and generated some lawsuits if I recall correctly.

The company's lame argument was that the caps were rated at 1kV and it was only a 240V system. They were using, like, SM1410 caps. They were potted and the whole products smoked while buried in several, 24 million dollar machines, in clean room fabs. C.Gavazzi junk. Self CE'd no less.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

X series caps are actually two caps in series internally. Even 1000V DC caps suffer from corona when used on AC. Corona can occur in tiny air spaces in the insulating layer at voltages over about 80V. Over time this corona can destroy the insulating layer. So, reliability can be increased if two ot more capacitors are used in series. I've taken dozens of products through UL with non X caps. They only care if it will catch fire.

 

[itsmoked]

Nice to know Opera. I'll keep that in mind.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[benta]

Don't forget a resistor in parallel to the series capacitor to discharge it when disconnecting the mains.

Benta.

 

[MacGyverS2000]

Maxim parts are a running joke in the halls here... one engineer took a real shine to anything Maxim, designing in components that weren't in stock. "But it does what we need!" Since it's next to impossible to get most Maxim parts in quantity, we had a lot of last-minute redesigns ;-)

Since he disappeared, we only use Maxim parts when we can't find anything else (and it still has to be in stock immediately). If we do, extras are always ordered from the beginning.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[Renovator1]

OperaHouse - would you be so kind to share some specific part numbers of those non-X capacitors? That might come in handy now and then.

 

[buzzp]

I think using non X caps is fairly standard in appliance control(in the US and Canada), at least according to what I have been reading. The X caps seem to have a higher voltage rating compared to standard electrolytics and seem to be geared more for CE marking or for marks using IEC standards. I see no reason not to use standard electrolytics, two in series as suggested along with appropriate transient protection to prevent the cap voltage from being exceeded (current limiting and cap bleeder Rs too, as already mentioned).
As far as some one getting sued over the design with 1000V caps AND putting it in potting is plane stupid. In fact, anyone who puts a front end cap, MOV or other such device in potting is asking for trouble, if its hooked to the mains.
Thanks for the input.

 

[OperaHouse]

The Microchip example gave a value of 47 ohms to the series resistor. A 220, 470 ohm or even higher would be preferred and better protect against line transients. Remember that resistors have voltage limits too as to flash over. Use at least 1/2 watt or higher. AC line can see spikes of 6KV. If most capacitors weren't self healing, we would all be in trouble.

 

[Madcow]

buzzp,
I don't understand why potting front end caps. is bad.
I've used canned emi filters with out problems. The x and
y caps are potted in them. I'm looking at the same circuit
only at 10 mA. The customer wants to pot it for enviromental
concerns. Is there something I should watch out for?
Its an agency aproved rtv potting compound.

Thx

 

[buzzp]

I dont have experience with canned EMI filters to answer your question explicitly. My point was to protect the components for the nasty high transients that can be on the line. If it gets to high, caps, MOV's etc can actually explode. If the component is covered in potting, the energy has to go some where and it will blow apart the potting in some cases, causing potential for more damage than letting it hang in free air. Now I understand sometimes potting is the only option depending on environment. I don't know your conditions but I would always look at other options first such as conformal coating (spray or dipping), etc.

I posted another link to a cap type power supply from ST. It is somewhat different than the one posted earlier. It looks strange but I think they are just using current flow from negative to positive in their diagram. Otherwise, it is the same as microchips (AN954).

http://www.st.com/stonline/books/pdf/docs/8313.pdf

 

[benta]

Buzzp, the ST circuit is quite elegant. However, compared to full-wave rectification after the series cap you only get half the current, or you need a capacitor double the size to get the same current.
In my opinion, the expensive part in such circuits is the series cap, so saving a few pcents or a couple of diodes makes little sense.

Benta.

 

[Skogsgurra]

A word of caution.

Trafoless supplies are OK. I used to love them for small things like speed controlled bath-room fans and the like (X-cap, PTC, diode, E-cap, zener, PIC+triac). They are out there in zillions.

But, I have recently been engaged in a case where the the PTC goes high-Z and the thing stops working. The reason is that there is so much HF noise from VFDs on the mains voltage so that the PTC overloads (the PTC is required to prevent fire if the X-cap fails). Obviously, the impedance of the X-cap is very low at the ca 10 kHz PWM residues compared to the 50 or 60 Hz it is supposed to see. Instead of being a 10 - 20 kohm series impedance it is a 50 - 100 ohms series impedance. You do not need much HF amplitude to have an overload situation there.

I can imagine what a plain 10^2-ohmish 1/2W series resistor (for transient protection)would do in such a situation. Make sure it doesnt produce flames that can start a fire when it gets too hot!



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[benta]

Good one Skogs, if I knew how I'd give you a star for that one.

Benta.

 

[OperaHouse]

All those new CE drives were supposed to solve that problem. I used to sell RC arc suppressors for motors and about 10 years back got a report of a 3 phase network heating up from a drive on another motor. A 10V 10KHZ noise signal shouldn't bother a several hundred ohm resistor in a low current application. That much noise is unusual and cause many other problems. This is a constant design problem where you can't control what is out there. I had a client that used 50 photo eyes on a machine. These had a phase triggered power supply. Fine if you use just one or two. 50 triggering all at once drove the supply line crazy.

 

[buzzp]

I don't anticpate such an environment. In any case, I am not going the route of a capacitor coupled supply. I don't see getting the currents I need without a large footprint cap (especially if use X class cap, which might be required at the voltage and capacitance rating I calculated). Since I am stuck with through hole parts, initially, I am going to have to use a resistive, zener type supply. I hate too but size might dictate it. I wanted to avoid using a switching supply but I may have too.
Has anyone used the small switchers from ROHM? Here is a link to an isolated unit

http://www.rohm.com/products/databook/pm/pdf/bp5710-1.pdf

They also have non-isolated

http://www.rohm.com/products/databook/pm/pdf/bp5039a.pdf

I was hoping to avoid using a switching supply but these little critters might be a good option, (cost very comparable to a transformer in quantity).