Re: 1000V input, 5V output, 5W, SMPS reference design needed

From: Fred Bartoli (fred._canxxxel_this_bartoli_at_RemoveThatAlso_free.fr_AndThisToo)
Date: 07/29/04 

Date: Thu, 29 Jul 2004 11:34:07 +0200

"Alexander Odyniec" <odyniec@uci.edu> a écrit dans le message de
news:ce9r6m$rik$1@news.service.uci.edu...
> My needs and concerns are similar to those described by Doug McNutt 7
years
> ago (below) but less stringent: I don't need a wide input range; just the
> fixed 480 V rms; non-isolated OK. However, I would prefer a solution that
> does not generate alot of EMI.
> Can anyone recommend a good design, please?
>
> Alex Odyniec
>
> From: dmcnutt@macnauchtan.com (Douglas P. McNutt)
> Subject: micropower 1000V MOSFET power
> Date: 1997/12/01
> Message-ID: <dmcnutt.1231365044A@news-2.sni.net>#1/1
> Organization: The MacNauchtan Lab
> Newsgroups: sci.electronics.design
>
>
> Mr. Hill's 1 kV design is of great interest to me because I have
encountered
> the same design in the real (commercial) world and solved it in much the
> same way. I do have some remaining problems though which I hereby put up
for
> comments.
>
> My goal is a power converter to run about 2 watts of microprocessor and
> analog preprocessing from AC line voltages from 48 to 480 volts rms in a
> single instrument. When the allowable fluctuations in the 480 volt line
are
> included 1 kV peak is just about right.
>
> I use the Motorola MOSFET in a D2Pak which is acceptable to UL at 1 kV
only
> if the entire module is potted. Since long life is important, aluminum
> electrolytics are out of the question and that gives me size and cost
> difficulties. After a bridge rectifier I need a filter capacitor. Said
> capacitor is discharged between charging pulses at constant power (That's
an
> interesting refresher course in separable differential equations.) An
0.47uF
> plastic film capacitor needs a minimum of 90 volts across it to keep the
> pulse width modulator (PWM) happy at 120 Hz. Even so it's the biggest
thing
> in the box and the cost is frightening. I can't reach my 48 volt goal.
>
> Startup of the PWM is a pain. Supertex has some nice solutions which
involve
> a depletion mode MOSFET with an internal reference and comparator. They
only
> go to 600 volts and I have burned out all my samples at less than that.
OK,
> I probably abused them, but that happens in the real world of 480 volt
power
> too.
>
> So I use another 1 kV MOSFET and a string of 1 Meg surface mount resistors
> feeding the gate via a zener and a transistor which are only used for 10
mS
> each time power is turned on. Again I object to the board space required.
> Running power is taken from a winding on the flyback transformer-choke.
>
> There is ripple on my input capacitor - 120 or 100 Hz - which I need to
get
> rid of. The LT1241 PWM can do a lot of it but I still find I need a linear
> regulator to make a clean 5 volts for downstream electronics. Mr. Hill's
75%
> efficiency is unreachable after that.
>
> I need a lot of isolation in the transformer for reasons of personal
safety.
> 4500 volts rms is the design goal. That makes it hard to get good coupling
> and makes for spikes on the drain. I find I'm dissipating a quarter watt
in
> the snubber. The idea of allowing the MOSFET to avalanche (Is that a
verb?)
> is intriguing. Has anyone actually done it? I'm afraid.
>
> How can I provide for serious voltage excursions at the input? I would
> really like to survive a lightning-induced spike of several kV on the
power
> line. Teccor and Raychem have nice products for shunting and automatic
> resetting but they don't address the 1 kV realm. Spark gaps have wide
> variability. We have some specially made aluminum wire in a silicone
> insulation typically used for heating elements. The aluminum will melt
> before the silicone burns. Installation on a 480 volt circuit requires no
> fuses which would otherwise be bigger than the product. But service is
> required after an event.
>
> I'm currently looking at a preregulator to take advantage of the AC
> character of the power I start with and switch it on only when the
> instantaneous voltage is in range. I have done that before and you might
be
> interested in:
>
> ftp://ftp.sni.net/macnauchtan/LivingPower.pdf
>
> where I did it for direct unisolated conversion from 120 V rms to 5 VDC
> using an SCR. I think ATT microelectronics and possibly Unitrode have done
> similar things with GTO devices.
>
> I do it because I enjoy it. I really do. Don't I?
>
> Doug McNutt
> Colorado Springs
>
>
>

What about using a transformer ? At 5W power, it will be small enough.
In case you don't want a customed transformer, you can do it with 2 240V
ones : primaries in serie, secondaries serial or parallel. 

-- 
Thanks,
Fred.

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