Power FETs with low Vgs?

[Fred Bartoli]

"Winfield Hill" <Winfield_member@newsguy.com> a écrit dans le message news:
c7g11t0228e@drn.newsguy.com...

Fred Bartoli wrote...

Small quizz :
I once had to build a +/- 20V@1A supply from a +15V rail with as
little components as possible and with as standard components as
possible (only buck switchers in stock).
How would you all do this ?

Generally to step up from say +15 to +20 we use boost switchers
to provide the required extra 5V from a flyback inductor. For
these switchers the voltage on the inductor is a 21V square wave,
so it's possible to add a capacitor and two diodes to obtain -20V.
Or one may use a second winding on the inductor, saving a diode
and one cap. Doing this job with a buck switcher (if that's what
you meant to say) would imply using a transformer rather than an
inductor anyway.

Yep, with a buck switcher (LM1374), but no transformer. I had the idea when
looking at the end of the datasheet... http://www.linear.com/pdf/1374fbs.pdf


Thanks,
Fred.

 

[Fred Bartoli]

"Ken Smith" <kensmith@violet.rahul.net> a écrit dans le message news:
c7fh4f$th7$1@blue.rahul.net...

In article <409a58fe$0$17900$626a14ce@news.free.fr>,
Fred Bartoli
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[....]
Small quizz :
I once had to build a +/- 20V@1A supply from a +15V rail with as little
components as possible and with as standard components as possible (only
buck switchers in stock).
How would you all do this ?

There are some makers that make inductors with 2 windings. They intend
you to put them in series or parallel to get different inductances but you
don't have to. You can use them for the two outputs.

Yep, that's part of the answer.

I hope your bucker chips are the ones that allow the switch voltage to
swing below ground or are rated at more than 35V.

Unfortunately no (LT1374).

If the switch voltage on the bucker can't go below ground, use the -V as
the ground of the switcher chip.

The simple (+) to (-) converter has 20V + a diode on the winding when it
is delivering power to the load side. The other winding can be delivering
the +20V

Is that what you did?

I can't figure how you'd wire that with the switcher GND shifted to -V : the
primary inductor would see +30V/-5V across its terminals. Plus your -20V
output depends directly on the -V supply, which in that case wasn't very
stable.

BTW I didn't use the -V rail.

Thanks,
Fred.

 

[Ken Smith]

In article <409bff64$0$18320$626a14ce@news.free.fr>,
Fred Bartoli <fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:

"Ken Smith" <kensmith@violet.rahul.net> a écrit dans le message news:
c7fh4f$th7$1@blue.rahul.net...
In article <409a58fe$0$17900$626a14ce@news.free.fr>,
Fred Bartoli
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[....]
I once had to build a +/- 20V@1A supply from a +15V rail with as little

The input is +15V and GND
[... not understood ...]

!----------! D1
---------! Switcher !-------------!<----------- -V output
! chip ! ! !
!----------! ) V
! ) --- D2
! ) !
-V GND GND


As far as the switcher chip is concerned, -V is the ground of the system.
D1 is the main recitifier. D2 is only needed for starting up.

The feedback pin of the switcher chip is run from a voltage divider across
either the -V to ground or the -V to the +V output I will discribe below.
If just -V and GND is used the -V is tightly regulated.


The +V output:
When the inductor is delivering energy to the -V output, the voltage
across it is 20V plus a diode drop. If a tightly coupled winding, such as
a Pico miniature power inductor would have, is used to make the +V the
resulting +V will be well if not tightly regulated. A bleeder load would
have to be included in both the +V and -V sides to ensure that neither
ever goes unloaded. At very light loads the regulation of the +V gets
bad.



--
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kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <c7g11t0228e@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
[....]

Generally to step up from say +15 to +20 we use boost switchers
to provide the required extra 5V from a flyback inductor. For
these switchers the voltage on the inductor is a 21V square wave,
so it's possible to add a capacitor and two diodes to obtain -20V.
Or one may use a second winding on the inductor, saving a diode
and one cap. Doing this job with a buck switcher (if that's what
you meant to say) would imply using a transformer rather than an
inductor anyway.

Don't rule out two inductors. A bucker chip can be used to make a Cuk
converter with a minus input voltage and a positive output. You can make
5V more than the 20V that way too.





--
--
kensmith@rahul.net forging knowledge

 

[Fred Bartoli]

"Ken Smith" <kensmith@violet.rahul.net> a écrit dans le message news:
c7o6m3$e85$1@blue.rahul.net...

In article <409bff64$0$18320$626a14ce@news.free.fr>,
Fred Bartoli
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:

"Ken Smith" <kensmith@violet.rahul.net> a écrit dans le message news:
c7fh4f$th7$1@blue.rahul.net...
In article <409a58fe$0$17900$626a14ce@news.free.fr>,
Fred Bartoli
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[....]
I once had to build a +/- 20V@1A supply from a +15V rail with as
little

The input is +15V and GND
[... not understood ...]

!----------! D1
---------! Switcher !-------------!<----------- -V output
! chip ! ! !
!----------! ) V
! ) --- D2
! ) !
-V GND GND

Ahmm, OK -V is the output voltage. Now I look like stupid

As far as the switcher chip is concerned, -V is the ground of the system.
D1 is the main recitifier. D2 is only needed for starting up.

The feedback pin of the switcher chip is run from a voltage divider across
either the -V to ground or the -V to the +V output I will discribe below.
If just -V and GND is used the -V is tightly regulated.


The +V output:
When the inductor is delivering energy to the -V output, the voltage
across it is 20V plus a diode drop. If a tightly coupled winding, such as
a Pico miniature power inductor would have, is used to make the +V the
resulting +V will be well if not tightly regulated. A bleeder load would
have to be included in both the +V and -V sides to ensure that neither
ever goes unloaded. At very light loads the regulation of the +V gets
bad.

That was almost what was done, except that :
- the buck was cascoded with a small PMOS to allow under GND voltage as
LT1374 input is limted to 25V max,
- a cap was added across the 2 supplies inputs to increase coupling and
cross regulation,
- regulation was done on the positive output side.

+15V
| .---------------------------------------.
| | |
.------------. |
|Vin BOOST | |
| OUT|----+^+---+-----+-------|<--+----|--->-20V
| | ||| | o| | |
| | === | C| --- |
| FB | | | C| L1 --- |
'------------' | | C| | |
| | .-. | | GND |
| | | | | GND |
GND | | | --- | L1,L2
coupled, 1:1 ratio
| '-' --- |
| | | |
| GND | |
| | o___ |
| +-----------UUU---+----+--->+20V
| | L2 | |
| - --- .-.
| ^ --- | |
| | | | |
| GND GND '-'
| | ___
'--------------------------------------+--|___|---.
|
GND

created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de


Thanks,
Fred.

 

[Ken Smith]

In article <40a02426$0$12740$636a15ce@news.free.fr>,
Fred Bartoli <fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[... I wrote ...]

!----------! D1
---------! Switcher !-------------!<----------- -V output
! chip ! ! !
!----------! ) V
! ) --- D2
! ) !
-V GND GND



[...]
That was almost what was done, except that :
- the buck was cascoded with a small PMOS to allow under GND voltage as
LT1374 input is limted to 25V max,
So you didn't have a LT1076 to work with. Too bad it would have saved a

part.

- a cap was added across the 2 supplies inputs to increase coupling and
cross regulation,

If you mean the one I marked C1, don't tell Mr. S. Cuk you did it. What
you have here is a coupled inductor Cuk converter. Back in the 1980's
IIRC he got a patent on this tyoe of converter. His patent was more about
getting the ripple current down to zero than just the topology.

- regulation was done on the positive output side.

+15V
| .---------------------------------------.
| | |
.------------. |
|Vin BOOST | |
| OUT|----+^+---+-----+-------|<--+----|--->-20V
| | ||| | o| | |
| | === | C| --- |
| FB | | | C| L1 --- |
'------------' | | C| | |
| | .-. | | GND |
| | | | | GND |
GND | | | --- | L1,L2
| '-' --- C1 |
| | | |
| GND | |
| | o___ |
| +-----------UUU---+----+--->+20V
| | L2 | |
| - --- .-.
| ^ --- | |
| | | | |
| GND GND '-'
| | ___
'--------------------------------------+--|___|---.
|
GND

--
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kensmith@rahul.net forging knowledge

 

[Fred Bartoli]

"Ken Smith" <kensmith@violet.rahul.net> a écrit dans le message news:
c7pbut$qm7$1@blue.rahul.net...

In article <40a02426$0$12740$636a15ce@news.free.fr>,
Fred Bartoli
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[... I wrote ...]
!----------! D1
---------! Switcher !-------------!<----------- -V output
! chip ! ! !
!----------! ) V
! ) --- D2
! ) !
-V GND GND



[...]
That was almost what was done, except that :
- the buck was cascoded with a small PMOS to allow under GND voltage as
LT1374 input is limted to 25V max,
So you didn't have a LT1076 to work with. Too bad it would have saved a
part.

- a cap was added across the 2 supplies inputs to increase coupling and
cross regulation,

If you mean the one I marked C1, don't tell Mr. S. Cuk you did it. What
you have here is a coupled inductor Cuk converter. Back in the 1980's
IIRC he got a patent on this tyoe of converter. His patent was more about
getting the ripple current down to zero than just the topology.


- regulation was done on the positive output side.

+15V
| .---------------------------------------.
| | |
.------------. |
|Vin BOOST | |
| OUT|----+^+---+-----+-------|<--+----|--->-20V
| | ||| | o| | |
| | === | C| --- |
| FB | | | C| L1 --- |
'------------' | | C| | |
| | .-. | | GND |
| | | | | GND |
GND | | | --- | L1,L2
| '-' --- C1 |
| | | |
| GND | |
| | o___ |
| +-----------UUU---+----+--->+20V
| | L2 | |
| - --- .-.
| ^ --- | |
| | | | |
| GND GND '-'
| | ___
'--------------------------------------+--|___|---.
|
GND

I don't think so.
While being quite close, the Cuk converter has the switch and L1 inductor
position swapped to allow for zero current ripple at both input and output.
Here, input ripple is the one of a flyback.

Thanks,
Fred.

 

[Ken Smith]

In article <40a08986$0$31942$626a14ce@news.free.fr>,
Fred Bartoli <fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[.. is it a Cuk or isn't it ..[

I don't think so.
While being quite close, the Cuk converter has the switch and L1 inductor
position swapped to allow for zero current ripple at both input and output.
Here, input ripple is the one of a flyback.

The classic Cuk converter has a plus input and minus output. If you take
your design and change all of the (+) to (-) etc, the only remaining
difference is where the output filter connects. I think your design
really is a Cuk. I wouldn't worry about it because I'm sure that part of
the patent isn't the important bit and could be killed on grounds of prior
art. Cuk's real improvement was actually seeing that zero ripple can be
done.

--
--
kensmith@rahul.net forging knowledge

 

[Terry Given]

"Ken Smith" <kensmith@violet.rahul.net> wrote in message
news:c7riip$tna$1@blue.rahul.net...

In article <40a08986$0$31942$626a14ce@news.free.fr>,
Fred Bartoli
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[.. is it a Cuk or isn't it ..[
I don't think so.
While being quite close, the Cuk converter has the switch and L1 inductor
position swapped to allow for zero current ripple at both input and
output.
Here, input ripple is the one of a flyback.

The classic Cuk converter has a plus input and minus output. If you take
your design and change all of the (+) to (-) etc, the only remaining
difference is where the output filter connects. I think your design
really is a Cuk. I wouldn't worry about it because I'm sure that part of
the patent isn't the important bit and could be killed on grounds of prior
art. Cuk's real improvement was actually seeing that zero ripple can be
done.
kensmith@rahul.net forging knowledge

I went to a seminar run by Rudy Severns in the early 90's, ostensibly about
magnetics. He ranted on at great length (bloody interesting stuff too) about
how patents are mostly meaningless, and fronted up with some prior art that
(seemingly) invalidates Cuk's patent - IIRC a 50kW "flying capacitor
converter" using Ignitrons, circa mid-1950.

I know a guy in NZ who patented using a resistor to sense current in an AC
drive, then got royalty payments (for many years) from a company that used
his technique....LOL. I have also seen a 1997 patent that claims, among
other things, the boost converter. Alas, prior art searches are inevitably
merely patent searches (and cost $$$ too!), a necessary but NOT sufficient
condition. Dwarfed, of course, by the USPTO's proclivity to patent any old
nonsense.....

I also went on one of Ed Blooms integrated magnetic seminars. Interesting
stuff, and 10 years later I am actually starting to design IM converters.

cheers
Terry