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Old 10-22-2019, 03:14 PM   #1
Per Hansson
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Default Capacitive dropper supplies not lasting

I've noticed a trend of capacitive dropper supplies not lasting.
This supply is super simple: you use a capacitor in series with the load to limit the current and voltage the device can consume.
I have fixed several such power supplies now where the only problem is that the safety rated X2 capacitor used for the implementation has dropped in capacitance.
This of course starves the circuit of power:
In many implementations there is a relay being driven, and thus when the load of the relays coil is added the device simply turns off or rather resets.

I have a question regarding the safety of actually using a X2 rated capacitor to begin with in such an implementation.
X2 caps may fail shorted, that would mean the little MCU or whatever the dropper is powering sees full line voltage, 230VAC, not cool!
The obvious solution is to use Y2 capacitors instead, but of course they are more expensive and not available in as large capacitance values generally.

Maybe that problem is academic at best, I don't suppose using inrush limiting with an NTC would help either.
If they can even be found in so small values, since these devices are generally never switched off.
But then any type of non-polarized capacitor could be used, if we anyway do not care about the safety of the failure mode...

So the problem is the capacitor itself, the "self healing" properties they have mostly just means when the layers get damaged they automatically burn off an area in a safe way:
A better name for them would be capacitors with a controlled failure mode.
I have even seen some datasheets now specifically mention to not use certain X2 capacitors for capacitive dropper implementations.

EPCOS / TDK has a new model that is branded as ok for this use: B3293 (305VAC) and also an older model called B3292*H/J (305VAC)
They also have models B3265* MKP and B3267*L MKP but they are without safety approvals.
Vishay has models F1772 and F339X1

Do you know of any others? I got tired of looking myself...
Or maybe some other solution to make the capacitors last longer?
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Last edited by Per Hansson; 10-22-2019 at 03:16 PM..
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Old 10-22-2019, 04:13 PM   #2
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Default Re: Capacitive dropper supplies not lasting

Vishay calls it (capacitive dropper) "series impedance applications" and attributes the stress due to overvoltage spikes, 2.5kV for Cat. II. Safety is not so much an issue because a circuit has a fuse/fusible resistor present.
App note AC Film Capacitors in Connection with the Mains

X-capacitors have a series-plate construction. Example F1772-2 series is "extended" metalized film with internal series design: https://www.vishay.com/docs/26033/ge...al Information

Ripple current is extremely high which is why I think the caps age fast. There is no datasheet spec for this. The aluminium deposition is only 0.02-0.1 m thick and I imagine it cracks, giving low capacitance.
I get long life if I have a series resistor say 100R with the cap, it lowers ripple current (and lowers cap transient voltage according to Vishay) but this resistor sees very high peak power pulses, and tends to burn up before a fuse blows.

Some film cap failures were corrosion and the end-connections going bad: https://www.semlab.com/services/ecfa...ilmcapacitors/
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Old 10-23-2019, 05:16 AM   #3
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Default Re: Capacitive dropper supplies not lasting

i have seen film caps intended for solar psu's that have very high current handling,
but the whole point of a capacitive dropper is to make it dirt cheap.
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Old 10-23-2019, 05:31 AM   #4
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Default Re: Capacitive dropper supplies not lasting

... or even cheaper than dirt!
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Old 10-23-2019, 08:06 AM   #5
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Default Re: Capacitive dropper supplies not lasting

redwire: The two devices I have on my bench are both made by the Norwegian company ELKO.
They are thermostats for electric floor heating systems.
The older one of the two has a thermal fuse for protection, quite nice solution, and a 470Ω 3W resistor in series with the X2 capacitor.
The newer model has a digital display, but the dropper circuit is very similar.
They removed the fuse, and lowered the resistor to 56Ω
There is a bi-directional TVS diode on the primary side, this will conduct if the X2 capacitor fails.
That means the 3W resistor would be dissipating close to 1kW I guess it will not have much choice but become a fusible resistor

stj & CapLeaker: Exactly: they are too cheap to go for a transformer solution, so instead this.
But of course the proper Y2 capacitor is too expensive.
And the proper X2 capacitor that I and redwire linked above are also too expensive.
Basically a "proper" X2 cap for this job usually internally is an Y2 capacitor...
And the problem for me is to find a "better" X2 capacitor that can still fit inside the case, it seems almost like mission impossible...
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Old 10-23-2019, 08:44 AM   #6
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Default Re: Capacitive dropper supplies not lasting

I'd just say screw the X caps, just either use Y caps if you have money or use regular high voltage film caps... but that fuse or fuseable resistor is critical.
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Old 10-23-2019, 11:36 AM   #7
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Default Re: Capacitive dropper supplies not lasting

What's the original cap rating, as in X1 or X2 and VAC? over 2.2uF? Size?
X-caps have thicker dielectric for higher withstand voltage on mains transients. So they are naturally huge. If there is a MOV upstream to lower the spikes- then you can use a smaller ordinary film capacitor. Perhaps this technique was used by ELKO? The TVS I did not quite understand and wonder if has gone open.
The (fusible) resistor limits current the cap and zener will see. 1uF is 120mApk at 240VAC.

Overvoltage aside, I still suspect something going on with the metallization causing the low value with age.
Recently I saw chinese shielded cable and the aluminum flaked off the mylar shield, so it could be a quality problem.

EPCOS B3293* "heavy duty" X2 with no mention what they are better spec'd for.
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Old 10-23-2019, 11:49 AM   #8
Per Hansson
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Default Re: Capacitive dropper supplies not lasting

There is a picture of it in this thread I created.
I've had failures on other units too though, I also have another thermostat with a different X2 cap but same failure mode.

Bigclive mentions it in this video too, seems to be pretty common.
https://www.youtube.com/watch?v=qGc9-ToEiIQ
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Old 10-25-2019, 10:21 AM   #9
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Default Re: Capacitive dropper supplies not lasting

Response from Vishay Film Capacitors Division:
"It is because of Corona effect.
Corona effect is electrical discharge (partial breakdown) which results from ionization of air on the surface or between the capacitor plates in AC voltage applications or in rapid changing DC voltages (pulses). However, film capacitor has the feature called self-healing. After one discharge, one self-healing. As a result, loss increase and capacitance decrease.

When there is too many corona effect accumulated, the capacitor will be failed.

Therefore, if we make internal 'series construction', we can make the voltage between capacitor plates half. It will be much lower than corona start voltage, in order to avoid corona effect.

F1772 is internal series construction.

At last, I suggest you also read our attached application note.

Please note two tables in it.

We recommend that for continuous across line application and in series with the mains application, internal series construction capacitor should be used."
--------------------------------------------------------------------------

*Notice the X-cap's standard has a +/-10% over 1,000hours - much shorter than continuous use and I would say X-caps are also degrading/aging the same way but it's not noticed.
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File Type: png image003.png (75.7 KB, 5 views)
File Type: png image004.png (96.2 KB, 6 views)
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Last edited by redwire; 10-25-2019 at 10:26 AM..
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Old 10-25-2019, 01:54 PM   #10
Per Hansson
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Default Re: Capacitive dropper supplies not lasting

Yes, this is the document I based my initial post on.
Sorry I forgot to attach it then.
I think I agree with budm from my other thread:
Quote:
Originally Posted by budm View Post
Seems to me that it will lose it capacitance even if you use it as X cap.
I think this is 100% true, nobody really cared for the longevity in X caps before they started being used in critical areas for device function...
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Old 10-27-2019, 03:37 AM   #11
Per Hansson
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Default Re: Capacitive dropper supplies not lasting

Quote:
Originally Posted by Per Hansson View Post
EPCOS / TDK has a new model that is branded as ok for this use: B3293 (305VAC) and also an older model called B3292*H/J (305VAC)
They also have models B3265* MKP and B3267*L MKP but they are without safety approvals.
Vishay has models F1772 and F339X1

Do you know of any others? I got tired of looking myself...
Or maybe some other solution to make the capacitors last longer?
I found two really good capacitor series now:
Kemet F862 is a special series, directly quoting from the datasheet:
"Typical applications include connection in series with the mains, capacitive power supplies and energy meters, with special emphasis in automotive applications for severe ambient conditions."

Kemet F862 is physically larger than the old Kemet R46 series, therefore they have later introduced the F863 which has the same physical dimensions as R46 and cheaper then F862.
The capacitance change at a specific relative humidity test is 50% worse for this series, however if space is important in an existing application it is an obvious choice.

This is not communicated very well in the datasheets (you can't admit that your old series was crap) but here are two sources:
https://www.digikey.com/en/product-h...ion-capacitors
https://www.codico.com/en/News/long-...from-kemet.htm

Quote:
Codico news article linked above:
Long-term stable X2 film capacitors from KEMET Newly introduced by KEMET: The F863 Series!
Rapid loss of capacitance at X2-certified film capacitors especially occurs under the usage in harsh environmental conditions. This leads to an early failure of power supplies as well as a loss of reliable EMI-suppression.

Two X2 film capacitors series
In the range of long-term stable X2 film capacitors KEMET provides two series, F862 and F863. F862 has been tested 85C/85% r.h./240Vac for 1,000 hours and available on the marked for some time already. Recently KEMET has introduced a new series F863, which is a cheaper and partly smaller alternative to F862. So this special treated X2-capacitor has the same dimensions like the standard-X2 of R46-series. F863 has been tested 85C/85% r.h./240Vac for 500 hours.

Last edited by Per Hansson; 11-01-2019 at 11:42 AM.. Reason: Fixed mistake pointed out by mockingbird
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Old 10-28-2019, 10:33 PM   #12
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Default Re: Capacitive dropper supplies not lasting

Very informative and constructive post Per! Just to make a small correction the cap you reference is Kemet, not EPCOS.
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