The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Jack Smith
Subject: Re: low loss transformer availability?
X-Newsreader: Forte Agent 1.92/32.572
Date: Sat, 04 Jan 2003 20:50:53 GMT
NNTP-Posting-Date: Sat, 04 Jan 2003 15:50:53 EST
Organization: Cox Communications
On Sat, 04 Jan 2003 14:30:08 -0500, "Smile, it can only get WORSE"
>Good day all,
>I need help with a transformer design, or perhaps just a nudge in the
>My transformer needs to be small (physically), and have extremely low
>standby losses. It will operate around 3 mhz in a linear AC power
>supply (not switching mode). It's output capability will vary between
>1 microamp and 150 microamps and it only needs to supply 50 milliwatts
>of power. It should be capable of a 10 to one primary/secondary
>voltage ratio (12 volts in and 120 volts out). Since the output
>current is so small, a lot of secondary wire resistance is acceptable.
>I'm told that toroids are ok, but they still have a lot of stray
>capacitive coupling between the windings.
>I need to make sure the input current is low when the load current is
>minimal.... hence the interest in ultralow standby current (no
>secondary load current should mean no (minimum) input current to the
>I was wondering about alternative types of transformer
>technology...such as planar. Or, perhaps a ferrite core could be
>machined to allow power transfer into the core while creating
>isolation between the windings?
>I began my research with a signal generator and by plotting the input
>current versus the output current with varying loads on the output. As
>I got down to loads in the mid/high megohm range, I discovered that he
>input was still drawing significant input current, which IS NOT what I
>need. Perhaps I need a torroid based transformer with high input
>impedance so that it doesn't draw much current without a secondary
>load...but, I could not achieve this on the bench test setup.
>Any ideas? Am I missing something?
It seems to me that you are going to have difficulty here, regardless
of the transformer technology.
You want to deliver 120 volts at 3 MHz (I assume you meant Megahertz
when you used mHz instead of MHz) into a load that draws between 1 uA
and 150 uA.
This implies that the resistive load ranges between 800 Kohm (150 uA)
and 120 Mohm (1 uA), assuming your voltage and current specs are RMS.
Stray capacitance is going to give you fits here. Consider that 1 pF
at 3 MHz has an Xc = 53 KOhm.
With some care and a bit of luck, you might get your transformer
output capacitance, and stray wiring capacitance down to 1 pf or so.
So that's going to shunt your high resistance load with 53 KOhm of
If the frequency is fixed at 3 MHz, it may be possible to tune out the
strays with parallel inductance operate a resonant transformer.
There's a reason that RF work tends to be done into low impedances.
>PS: Please don't send me any switching power supply design messages.
>My input is AC, my output has to be AC and I can't use switching mode
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup