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building a long-term capacitor PSU

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bernie79:
Hi,
I'm trying to build a capacitor power supply for some small Atmel/Pic applications.
For test proposes I came up with a LTspice schematic (see attachment. the .png is for readers, who dont want to check the .asc out)).
V1 is the typical 230VAC 50Hz mains we all know here in Europe.
C1,R1 and R2 is the core capacitive impedance, limited by the Zener D5.
D1-D4 builds a standard rectifier that is supported by C2 for smoothing the voltage out.
IC1, D4 and C3-C6 is the standard-circuit for the 7805.
R6 and R7 simulates the AVR and a AVR switched small relay.
L1 limits the burst from 630mA (without coil) to 310mA without starts a swinging.

In the diagram, the upper part with the legend is hidden, so:
Red = voltage at 7805 pin "in" (label "sec")
Green= voltage at 7805 pin "out"(label "out")
light blue= current out of 7805 "out"

This looks as good as I want this, exept the current burst on the start.
I peaks out at 310mA and thats well below the 1000mA, the D2Pack-7805 should able to deliver, but I'm not shure.

This little PSU will be the basis of some diffrent Atmels/PICs PCBs around in my house and should working at least 10 years without failure.
Yes I know there should be at least a fuse, a VDR and a TVS been added, but thats not for the Spice-simulation.

The switch-on-burst are my main concern. The whole board (including the PSU) will be switched on and off about two times a day so it will see about 7000 switches in its expected lifetime. I think this 310mA its not good in the long run, but how bad will it be? Has someone more experience?

P.S.
For all readers concerning electric hazard: I know that also the low voltage parts are on mains potential. thats okay because:
1. no outer connection besides connections that are considered as mains.
2. to reach the PCB you have to unplug all cabling from the PCB first. and its closed in an isolated wall box.
So that problem is fully covered.

jwet:
300 mA surge isn't a ton these days for a lot of AC powered devices.  It will stress the input cap mainly.  That should be an X/Y type film cap from a good supplier- Kemet is a safe bet.  In order to get your ten year life, you should over design/de rate-components especially HV caps.

The other thing to look at is a NTC type thermal surge device in place of or with R2.  Since the steady state current will be very low eventually, you could have a considerable delay- make sure that your uC has a reset circuit that will work if 5v comes up slowly- most modern PICS have a UVLO type detector on board.

As an FAE for Maxim, I saw a lot of this direct impedance type off line stuff in wall socket gadgets even in some fairly sophisticated size constrained gear.  Maxim had a part for a while called the MAX610 that had a full wave bridge and linear regulator integrated on board for these circuits.  All you had to do was provide a cap and a layout and you could make tiny AC-DC supplies for ~15 mA apps.  It was discontinued not because it was unsafe but customers wouldn't follow the apps guidelines and could make some really scary stuff.  You sound like you know what you're doing and know the pitfalls.

Google the MAX610 data sheet, it had a lot of good advice in it and might give you some ideas.

Good luck.

bernie79:
> That should be an X/Y type film cap from a good supplier- Kemet is a safe bet.  In order to get your ten year life, you should over design/de rate-components especially HV caps.

Yes, thats the first point on my list. Even checking whats happening when each of the caps are loosing their value over time (thats the initial reason for putting this into LTspice).

Replacing R2 with an NTC or a fuseable kind of resistor was also on my mind, and I havent got a decision to this question.
I got the Max610 in my hands years ago, but havent find a case where I should use this. There was always another solution.

Thats for your thoughts. Nice to see I'm on the right path.
Yet I'm still unsure about the surge. Maybe I should build a prototype and burn it with nasty overvoltages that are possible in the mains supply and see whats happening.

jwet:
Definitely prototype it.  You might also talk to a capacitor apps engineer about what you're doing.  The reliability will be dominated by that cap.

Jon_S:
At a previous place we made a high-volume product with a capacitive-dropper style power supply, it had a 10 year warranty.

Out of the returns with power-supply failure the overwhelming majority were failure (open-circuit) of the series resistor not the cap. From memory, we started off with a string of 2512 thin-film, switched to thick-film, and finally pulse-rated MELF.

The designs were independently tested to EN61000-4-5 and also abused as part of our in-house accelerated life testing on our own surge generator, but it appears that the mains in some areas is exceedingly lumpy indeed!

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