Re: Active bridge rectifier using cmos hex inverters, was Using zener diode to protect test circuit?

Jim Thompson wrote...
>
>> Winfield Hill wrote:
>>> Fred Bloggs wrote...
>>>> Winfield Hill wrote:
>>>>> Andrew wrote...
>>>>>
>>>>>> I am designing a test instrument that will be powered from the circuit
>>>>>> under test. Consequently there is a high probability of the incorrect
>>>>>> voltage or polarity being supplied to my circuit. I cannot protect
>>>>>> against reverse polarity by using a series diode as this will drop the
>>>>>> supply voltage too much.
>>>>>
>>>>> You didn't say how high the voltage is... If it's above 4V, you
>>>>> can use a bridge rectifier to present the correct polarity to your
>>>>> circuit. Not content to see a 1.2V drop in the bridge? Use four
>>>>> MOSFETs instead, two each p- and n-channel. I think this works.
>>>>> Tie the drains of both types to both inputs, with each FET's gate
>>>>> to the opposite input. Tie the p-channel sources together, this is
>>>>> the + output. The n-channel sources are the - output. Like this:
>>>>>.
>>>>>. IN --+------- D S ----+---- + out
>>>>>. | p G |
>>>>>. | _______| |
>>>>>. | | |
>>>>>. | +---- D S ----' low-voltage active bridge
>>>>>. | | p G Vin >> Vgs(th), and < 20V
>>>>>. |__| ______|
>>>>>. | |
>>>>>. +--| --- D S ----, uses two each p-channel and
>>>>>. | | n G | n-channel MOSFETs, as shown
>>>>>. | |_______| |
>>>>>. | | |
>>>>>. IN --| -+---- D S ----+---- - out
>>>>>. | n G
>>>>>. |__________|
>>>>>
>>>>> When power is first applied the FET's substrate diodes conduct
>>>>> to present the proper voltage polarity to the output. Two of
>>>>> the FET's also immediately turn on, shorting out their diodes
>>>>> for a low voltage drop. The opposite-polarity FETs remain off.
>>>>
>>>> Now I like that damned thing- and you can get the same performance
>>>> in a smaller package using some analog switches- I posted on this
>>>> a while back.
>>>
>>> That's a great idea, I'm sorry I missed your post. Unless I'm
>>> mistaken, all that's needed to implement the circuit above are
>>> two CMOS inverters...
>>>
>>>. _Vdd --+---- + out
>>>. / | |
>>>. IN ----O< |-----'
>>>. \_| Active bridge rectifier
>>>. unknown using cmos hex inverters
>>>. polarity _
>>>. / |
>>>. IN ----O< |-----,
>>>. \_| |
>>>. Vss --+---- - out
>>>
>>> Amazing. Looks positively spooky! A hex inverter can have its
>>> two three-section sets tied in parallel for better conductance.
>>>
>>> I'll bet much has been written about this circuit idea someplace.
>
> It works if you...
> Tie input of upper inverter to lower "IN"
> Tie input of lower inverter to upper "IN"
> Cute! I'll use it ;-)

That's right, thanks! I messed-up when translating my first
drawing to the second one. Here's the corrected schematic.

.. _Vdd ----- + out
.. / |
.. IN ---+--O< |--,
.. | \_| | Active bridge rectifier
.. unknown \________|_ using cmos hex inverters
.. polarity ________| |
.. / _ |
.. | / | |
.. IN ---+--O< |----'
.. \_|
.. Vss ----- - out

Again, someone must have invented this long ago. Sometime shortly
after the first cmos hex inverters came out in the mid 60s, or at
least soon after cmos became available in low-cost plastic packages
about 35 years ago... But now we can make it with a micro-miniature
ON Semi NL27WZ04 MiniGate? in a SC70-6 package, a 2.1 x 2.1mm dust
flake. See http://www.onsemi.com/site/content/0,,1241,00.html and
http://www.onsemi.com/pub/Collateral/NL27WZ04-D.PDF

Using the FETs in a low-voltage cmos process allows us to use this
technique down to 1.5V or even less, although of course the FET's
on resistance goes up at low voltages. Has anyone seen an Ron vs
Vdd plot for these low-voltage logic families? ON Semi's NL17SV04
is claimed to work well at 0.9V supply, and deliver 50MHz at 1.2V.

BTW, although the fabulous MiniGate logic sample kit is no longer
available free from ON Semi, it's offered by DigiKey for only $39.
Search on MINIGATEA-KIT. Break out those microscopes!


--
Thanks,
- Win
.

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