Re: using mosfets as rectifiers?

From: Albert (pass_at_pass.com)
Date: 06/11/04 

Date: Thu, 10 Jun 2004 22:40:14 -0400

On Wed, 09 Jun 2004 20:30:53 -0500, Jim Adney <jadney@vwtype3.org>
wrote:

>On Tue, 08 Jun 2004 12:06:05 -0400 Albert <pass@pass.com> wrote:
>
>>I've already found (and ordered) a 2.2 volt turn on mosfet for my
>>switching transistor. This should give me the the head room I need to
>>drive the gate with a lower voltage.
>
>I don't follow this stuff much, so I didn't know that such things
>existed. I still have to ask: Is 2.2V where the turn-on starts, or
>where it is really fully on?
>

It's where it starts to turn on. However, even if the resistance was
only 100 ohms between the source and drain, it would probably be
enough to key the transmitter (keying line on a modern ham
transceiver). At best, I need to control a milliamp of output
current and worst case would be 50 ma of output current needed. So I
can use a partially conducting mosfet.

>Thinking about this, I gather that you intend to run these MOSFETs
>backwards, so that the "rectifier" is on when it is parallel to the
>body diode and off when in the usual forward direction. Does that
>work? I never considered the possibility....

No.....

I asked about mosfets as diodes. I was hoping I could bias the gate
high and the mosfet would only conduct with positive input
voltage...but it appears they are somewhat unidirectional. Since I
need to control the input to the gate when positive input voltage is
detected, I probably can't use this method as I don't have a steady
supply voltage as the unit is self powered (no external dc input
voltage).

I am however going to use a mosfet to switch my load, which is the
keying line on the radio transceiver.

So, mosfets as rectifiers is out. Mosfet as a switch to pull the
keying line low when I get enough audio voltage WILL be used.

>
>>My shottky diodes are already dropping just under 4 tenths of a volt,
>>so switching to a ger diode gets me very little and might not help at
>>all as some of them have very high leakage currents.
>
>In my limited experience, Schottkys have a lower forward drop than
>even Ge.
>

Even is the Ge drops a slightly lower voltage, the leakeage current is
much higher, which is one reason why we use silicon. There just isn't
anything to gain by switching to Ge.

>I don't know your application, but I wonder if you could transformer
>couple your signal and boost it a bit, just enough to give you the
>extra voltage you need to keep this job simple.
>

With a bipolar transistor acting as a switch, I am probably limited
severely by the transformer step up ratio. But, the mosfet needs very
little real power applied to the gate to make it switch, so looking
for a higher turns ratio audio type transformer might be the answer as
well.

I have to say that I've been reading all the responses and I've
learned alot. But, you (Jim) seem to have the clearest understanding
of my original question and my reason for seeking help here.

I wonder what the practical limits are for turns ratio for 8 ohm input
impedance? If I could get a 50 to 1 step up ratio, I would have no
problem generating enough voltage to turn my mosfet switch on! I don't
think transformers can be built with that much of a step up ratio (and
still be a relatively wide range audio frequency device).

I thank all who have commented.

Regards,

A

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