>
> Robert Scott wrote:
> > There is no magic way to make a MOSFET into a low-drop diode. The
> > bridge rectifiers made out of MOSFETs use additional active circuitry
> > to drive the gates appropriately. It is not as simple as just
> > shorting two pins of the MOSFET.
>
> Hmmm, makes me go and look it up in my PSU book...
>
> "Power Supplies, Switching Regulators, Inverters, and Converters"
>
> I was wrong, you don't just short two pins - you sort of cross them over
> to the opposite side, but the "MOSFET synchronous rectifying bridge"
> depicted on page 422-423 doesn't require additional active circuitry.
>
> Um.. difficult to draw this circuit in ASCII. I'll try.
>
> O-------+-----------------------+------------+
> \ | |
> \ +----------------N/ \P-------------+
> \/ / \ | |
> AC IN /\ -ve O---+ +---O +ve | |
> / \ \ / | |
> / +---------------N\ /P----------+ |
> / | |
> O------+------------------------+---------------+
>
> Got that? Where I've put an N or a P, that means there's an N-channel or
> P-channel power MOSFET there, with the source connected closest to the
> DC outputs. The inputs that cross over and go to the other side are of
> course the gates, leaving the drains as the outputs in the diagonal
> lines opposite the sources, obviously.
This may look good on paper, but if you are worried about large
transients, then the gate is likely to not survive. I guess you can
find MOSFETs with large gate to drain breakdown voltages, but the ones I
have seen are not very high. For example, I don't think you could use
this on a 120 VAC circuit, as I don't know of any parts that will take
that high a gate voltage. Am I wrong?
However, if you use a zener diode and resistor at the gates, you can
limit the gate voltage to something that will fully turn on the MOSFET
and not blow it out. Or if you are just looking for reverse polarity
protection, then you only need one P-channel FET connected like the one
on the upper right (and of course the zener to protect the gate).
Reply by Alan Holt●January 23, 20042004-01-23
Look at the LM2940. It was designed for vehicular use; it has short-circuit
and reverse voltage protection.
Reply by Alaric B Snell●January 23, 20042004-01-23
Robert Scott wrote:
> There is no magic way to make a MOSFET into a low-drop diode. The
> bridge rectifiers made out of MOSFETs use additional active circuitry
> to drive the gates appropriately. It is not as simple as just
> shorting two pins of the MOSFET.
Hmmm, makes me go and look it up in my PSU book...
"Power Supplies, Switching Regulators, Inverters, and Converters"
I was wrong, you don't just short two pins - you sort of cross them over
to the opposite side, but the "MOSFET synchronous rectifying bridge"
depicted on page 422-423 doesn't require additional active circuitry.
Um.. difficult to draw this circuit in ASCII. I'll try.
O------+-----------------------+------------+
\ +--------------- | |
\ / N/ \P-------------+
\/ / \ | |
AC IN /\ -ve O---+ +---O +ve | |
/ \ \ / | |
/ \ N\ /P----------+ |
/ +-------------- | |
O-----+------------------------+---------------+
Got that? Where I've put an N or a P, that means there's an N-channel or
P-channel power MOSFET there, with the source connected closest to the
DC outputs. The inputs that cross over and go to the other side are of
course the gates, leaving the drains as the outputs in the diagonal
lines opposite the sources, obviously.
ABS
Reply by Michael Anton●January 23, 20042004-01-23
"Jay" <eatthisandide@127.0.0.1> wrote in message
news:_ZUPb.15984$xu6.9047@fe02.usenetserver.com...
> In article <100tgb5p7aom585@corp.supernews.com>,
> manton@nocompusmart.abspam.ca says...
>
> I am actually facing the design of a power supply for automotive use as
> well.
>
> I came up with the following and would appreciate comments...
>
> Take 12V DC into a bridge rectifier (~ terminals). Output of rectifier
> "+" through poly fuse to power supply circuit. Place a TVS (such as the
> above MR2535L) across the "+" & "-" terminals of the rectifier.
>
As someone else also stated, do not use a bridge, just a single diode
in the positive supply line. You should have your circuit referenced
to the vehicle ground.
> My thinking is if there is an overvoltage the MR2535 alone will eat the
> spikes. However, if for some reason there are negative spikes on the
> 12V, the TVS alone won't do anything (and other solutions use a second
> TVS to shunt negative spikes into the positive terminal).
There is no issue with the negative spikes, as the MR2535L is not
bidirectional.
It looks like a standard zener diode, so since its forward voltage is around
a volt, that is what it will clamp your negative spikes to.
>
> With a rectifier all spikes are positive going & combined with the TVS,
> they should be clamped...right?
Sure, but only if your rectifier bridge is fast enough, and rated for the
high
instantaneous currents involved. But the method above is better.
Mike Anton
Reply by Robert Scott●January 22, 20042004-01-22
On Thu, 22 Jan 2004 19:52:14 +0000, Alaric B Snell
<alaric@alaric-snell.com> wrote:
>Jim Stewart wrote:
>
>> Personally, I'd prefer a single series diode for
>> reverse polarity protection. I'd not want the
>> ground of my device to be a diode drop above
>> battery ground. Of course, if you do it my way
>> you have to deal with those negative spikes.
>
>You can make a low-drop bridge recitifier out of MOSFETs... use a MOSFET
>with gate and drain (or source? I can't remember) wired together as a
>diode. Um, I'd need to look in my transistor book to remember which way
>round it would then work, but I'm sure a *real* electronics engineer
>will just know :-)
There is no magic way to make a MOSFET into a low-drop diode. The
bridge rectifiers made out of MOSFETs use additional active circuitry
to drive the gates appropriately. It is not as simple as just
shorting two pins of the MOSFET.
-Robert Scott
Ypsilanti, Michigan
(Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
Reply by Alaric B Snell●January 22, 20042004-01-22
Jim Stewart wrote:
> Personally, I'd prefer a single series diode for
> reverse polarity protection. I'd not want the
> ground of my device to be a diode drop above
> battery ground. Of course, if you do it my way
> you have to deal with those negative spikes.
You can make a low-drop bridge recitifier out of MOSFETs... use a MOSFET
with gate and drain (or source? I can't remember) wired together as a
diode. Um, I'd need to look in my transistor book to remember which way
round it would then work, but I'm sure a *real* electronics engineer
will just know :-)
ABS
Reply by Jim Stewart●January 22, 20042004-01-22
Jay wrote:
> In article <100tgb5p7aom585@corp.supernews.com>,
> manton@nocompusmart.abspam.ca says...
>
>
>>On a 12V system, usually you have to be able to handle at least 24V
>>on the input side. Put in a reverse polarity protection diode, and a
>>fuse (or polyswitch), then follow that with a 24V power zener to
>>ground to limit incoming voltage spikes. We used to use a MR2535L
>>part from Motorola which was meant for this specific purpose.
>>A datasheet can be found here:
>>http://www.onsemi.com/pub/Collateral/MR2535L-D.PDF
>
>
> I am actually facing the design of a power supply for automotive use as
> well.
>
> I came up with the following and would appreciate comments...
>
> Take 12V DC into a bridge rectifier (~ terminals). Output of rectifier
> "+" through poly fuse to power supply circuit. Place a TVS (such as the
> above MR2535L) across the "+" & "-" terminals of the rectifier.
>
> My thinking is if there is an overvoltage the MR2535 alone will eat the
> spikes. However, if for some reason there are negative spikes on the
> 12V, the TVS alone won't do anything (and other solutions use a second
> TVS to shunt negative spikes into the positive terminal).
>
> With a rectifier all spikes are positive going & combined with the TVS,
> they should be clamped...right?
Personally, I'd prefer a single series diode for
reverse polarity protection. I'd not want the
ground of my device to be a diode drop above
battery ground. Of course, if you do it my way
you have to deal with those negative spikes.
Reply by Jay●January 22, 20042004-01-22
In article <100tgb5p7aom585@corp.supernews.com>,
manton@nocompusmart.abspam.ca says...
> On a 12V system, usually you have to be able to handle at least 24V
> on the input side. Put in a reverse polarity protection diode, and a
> fuse (or polyswitch), then follow that with a 24V power zener to
> ground to limit incoming voltage spikes. We used to use a MR2535L
> part from Motorola which was meant for this specific purpose.
> A datasheet can be found here:
> http://www.onsemi.com/pub/Collateral/MR2535L-D.PDF
I am actually facing the design of a power supply for automotive use as
well.
I came up with the following and would appreciate comments...
Take 12V DC into a bridge rectifier (~ terminals). Output of rectifier
"+" through poly fuse to power supply circuit. Place a TVS (such as the
above MR2535L) across the "+" & "-" terminals of the rectifier.
My thinking is if there is an overvoltage the MR2535 alone will eat the
spikes. However, if for some reason there are negative spikes on the
12V, the TVS alone won't do anything (and other solutions use a second
TVS to shunt negative spikes into the positive terminal).
With a rectifier all spikes are positive going & combined with the TVS,
they should be clamped...right?
Thanks,
Jay.
Reply by Michael Anton●January 21, 20042004-01-21
"Jake McGuire" <jamcguir@yahoo.com> wrote in message
news:dbb82eb7.0401201254.2ffe6b9a@posting.google.com...
> Sadly, I'm running up against the limits of my meager mixed-signal
> design knowledge. I'm trying to put together an embedded device
> intended for use in an automotive (well, motorcycle) environment, and
> am trying to figure out how to power the damn thing.
>
> A few of the components I need to use (video overlay chip, MEMS
> accelerometers, op-amps) require +5V. And since I'm going to to be
> doing analog sampling and then A/D conversion, I suspect the power has
> to be fairly clean. None of my signals are going to be above 100Hz,
> so I might even be wrong on that point.
>
> Anyway, as near as I can tell, my options include powering from the
> vehicle +12V system via a linear regulator, using a lithium-ion
> battery along with one of the nifty USB-powered charger chips and a
> switching regulator to step up to +5V, or just whipping up a 6-cell
> NiMH or NiCad pack and regulating it down to +5V, then using an
> external charger to recharge the thing.
>
> The switching option seems like the best combination of operational
> and design simplicity, but I'm worried about the effect of the 50mV
> P-P 1MHz power supply ripple on my already poorly-understood analog
> circuitry. It seems like I should be able to LC notch filter that
> out, but is this going to mess up the feedback on the power supply
> chip?
You can put an LC notch filter or just a lowpass filter after the regulator,
but AFTER the feedback. Do not include the filter inside the feedback
loop, or it will likely oscillate. You could also put a linear regulator
after
the switcher, and if you use a low dropout part, the losses aren't very
high. Just remember to set the output voltage of the switcher high
enough to give you the headroom required for the linear reg.
>
> The vehicular +12V supply seems the next simplest (operationally), but
> I've heard all sorts of nasty things about the quality of that power,
> and how it's full of noise and spikes, occasionally reverses polarity,
> etc etc. Is handling this just a matter of putting in appropriately
> rated caps and a reverse-polarity protection diode, or is there more
> to it?
On a 12V system, usually you have to be able to handle at least 24V
on the input side. Put in a reverse polarity protection diode, and a
fuse (or polyswitch), then follow that with a 24V power zener to
ground to limit incoming voltage spikes. We used to use a MR2535L
part from Motorola which was meant for this specific purpose.
A datasheet can be found here:
http://www.onsemi.com/pub/Collateral/MR2535L-D.PDF
>
> Is implementing this going to be so much of a pain in the ass that I
> should just solder together a bunch of AA NiMH cells and accept the
> need to swap out battery packs every now and again?
>
> (note: I'm not planning on actually selling this to strangers, so I'm
> not concerned about UL testing or functioning 100% of the time, just
> "mostly working".)
>
> -jake
I would think that you should be able to get standard regulators to
do what you want, without having to resort to a battery supply.
Mike Anton
Reply by Gary Pace●January 21, 20042004-01-21
It depends on your power requirements.
If it's a few hundred mA or less, I'd use a series diode, a reservoir
capacitor and a linear regulator (78M05 etc). You could include a series L
to reduce "noise".
The limits of this circuit will likely be dissipation and cooling of the
regulator ((12v - 0.7v -5v)* Current)
For more than this, use a monolithic switcher (Linear Tech make some very
nice ones and their website has afree simulator for designing with them).
You can get wound components for these from many suppliers (e.g. Coilcraft)
"Jake McGuire" <jamcguir@yahoo.com> wrote in message
news:dbb82eb7.0401201254.2ffe6b9a@posting.google.com...
> Sadly, I'm running up against the limits of my meager mixed-signal
> design knowledge. I'm trying to put together an embedded device
> intended for use in an automotive (well, motorcycle) environment, and
> am trying to figure out how to power the damn thing.
>
> A few of the components I need to use (video overlay chip, MEMS
> accelerometers, op-amps) require +5V. And since I'm going to to be
> doing analog sampling and then A/D conversion, I suspect the power has
> to be fairly clean. None of my signals are going to be above 100Hz,
> so I might even be wrong on that point.
>
> Anyway, as near as I can tell, my options include powering from the
> vehicle +12V system via a linear regulator, using a lithium-ion
> battery along with one of the nifty USB-powered charger chips and a
> switching regulator to step up to +5V, or just whipping up a 6-cell
> NiMH or NiCad pack and regulating it down to +5V, then using an
> external charger to recharge the thing.
>
> The switching option seems like the best combination of operational
> and design simplicity, but I'm worried about the effect of the 50mV
> P-P 1MHz power supply ripple on my already poorly-understood analog
> circuitry. It seems like I should be able to LC notch filter that
> out, but is this going to mess up the feedback on the power supply
> chip?
>
> The vehicular +12V supply seems the next simplest (operationally), but
> I've heard all sorts of nasty things about the quality of that power,
> and how it's full of noise and spikes, occasionally reverses polarity,
> etc etc. Is handling this just a matter of putting in appropriately
> rated caps and a reverse-polarity protection diode, or is there more
> to it?
>
> Is implementing this going to be so much of a pain in the ass that I
> should just solder together a bunch of AA NiMH cells and accept the
> need to swap out battery packs every now and again?
>
> (note: I'm not planning on actually selling this to strangers, so I'm
> not concerned about UL testing or functioning 100% of the time, just
> "mostly working".)
>
> -jake