low voltage drop transistors for power supply

John Popelish · Apr 27, 2004

"Watson A.Name \"Watt Sun - the Dark Remover\"" wrote:

Has anyone tried to do this? Series aiding secondary to boost the ACV.
(view with courier font)

o-----+--+
120VAC | | +-------o
| )|| |
120VAC Pr'y | )||(
12VAC 3A Sec'y | )||(
| )||(
| )|| | 132VAC
| | | 3A max.
o--+--)---+ | to PS
| | |
| +--------+
|
+--------------------o

One time I used a similar setup to make a 220 VAC guitar amp run off 120VAC.

Nothing wrong with it at all.

--
John Popelish

budgie · Apr 27, 2004

On Mon, 26 Apr 2004 09:20:24 -0700, Tim Wescott <tim@wescottnospamdesign.com>
wrote:

What is it using for a regulator? You may want to consider boosting the
main supply by putting in a little 6.3-volt transformer + bridge to make
a base supply for your pass transistor. This will let you drive the
pass transistor into saturation, which will give you 1/2 to 1 volt more
headroom.

Using a PNP (or multiples) removes the need for this auxiliary high-side drive
arrangement. I have built 20A cts PSU's using an LM723 as the regulating device
and a pair of 2N4399's as the pass elements.

Tim Wescott · Apr 27, 2004

budgie wrote:

On Mon, 26 Apr 2004 09:20:24 -0700, Tim Wescott <tim@wescottnospamdesign.com
wrote:

What is it using for a regulator? You may want to consider boosting the
main supply by putting in a little 6.3-volt transformer + bridge to make
a base supply for your pass transistor. This will let you drive the
pass transistor into saturation, which will give you 1/2 to 1 volt more
headroom.

Using a PNP (or multiples) removes the need for this auxiliary high-side drive
arrangement. I have built 20A cts PSU's using an LM723 as the regulating device
and a pair of 2N4399's as the pass elements.

I have, too, and the high voltage gain of the PNP gave me fits trying to
stabilize the loop for an arbitrary load. I'd do it again in a minute,
but I wouldn't recommend it to someone who _I_ don't know has a pretty
solid grounding in closed-loop stability theory _and_ good circuits
knowledge (not to mention a supply of low-ESR electrolytics).

Actually I think the next supply like this that I build will have an
auxiliary supply up to 24 volts or so, and will drive the gate of a nice
low on-resistance FET. This should give you millivolts of dropout
instead of volts, and it'll fit in the power supply range of a '723.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tweetldee · Apr 27, 2004

"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com> wrote
in message news:c6k5o0$6rn19$1@hades.csu.net...

Tweetldee wrote:

"Albert" <pass@pass.com> wrote in message
news:8daq809a40983ebqc6abs9te4hja7vn2p6@4ax.com...

know what he was doing, and may have put some wrong parts in there.

I am sure someone with a low level of competancy repaired it the last
time it blew. Many of the soldered connections left a lot to be
desired although I did not find any connection that actually failed.

It says 25A on the front, but looking at the innards, it is clear that
it probably never produced 25A. I suggested to the owner that we
rebuild it as a 15 A constant duty with 20A intermittent output
because it probably can't produce 25A no matter what we do to it. So,
we are going to rebuild it from scratch.

My hope was to gain some extra head room with a quick and dirty pass
transistor and rectifier swap and build a simple regulator from the
ground up.

Should I be looking at a switching supply or try to squeek out a few
extra volts of headroom by maintaining the origianl design with some
improved parts?

Thanks for all comments.

Well, that only adds to the confusion!!! As I said in my earlier post,
the
MJ2955 is a PNP unit, whereas the 2N3055 is NPN. They can't operate as
series pass elements in parallel!!!!!!
Is the output of the supply regulated now? Is it holding the output
constant with changing load?
If the supply is operating (albeit limping) with only the MJ2955 in the
circuit, then I have to ask for a schematic, or at least a sketch of the
circuit. I'm curious as to how this supply is built. What's the
make/model
of this supply?
If it's simple enough to sketch, that may allow us to offer more
intelligent
answers to your dilemma.

Cheers!!!!!

Maybe they're a Sziklai pair?

http://www.ampslab.com/c200cfp.htm

Nope... If you will remember, the OP said that the MJ2955 and the 2N3055
are in parallel.... the configuration as posted on the ampslab page have two
NPN units in parallel.. not a PNP in parallel with an NPN.

--
Dave M

Good judgement comes from experience, and a lot of that comes from bad
judgement.

Watson A.Name \"Watt Sun · Apr 27, 2004

John Popelish wrote:

Albert wrote:

I was just at the Fairchild semi site looking for big diodes. The only
ones they have are in TO-220 packages and although they have 30 and
40A ratings, the power dissipation is only 3 watts. How can the thing
drop .7 volts at 40A and only dissipate 3 watts of heat?? Now I'm
confused:>:

For instance:

http://www.fairchildsemi.com/ds/MB/MBR3050PT.pdf

That rating applies if you just have the diode sticking up into air,
with no heat sink. The thermal resistance spec applies when you mount
it in a heat sink and need to figure out how hot the junction gets
compared to the heat sink. You need heat sink date to figure out how
hot the heat sink gets.

Who else makes high current schottky diodes??

International rectifier makes lots of big ones. So does Diodes
Incorporated. Digikey sells both these brands.

(snip)

Another use for high current diodes is for dual battery charging systems
in RVs and such. They can handle a lotta current.

One thing I haven't seen mentioned is the zero voltage drop diode, which
is actually a power FET operating like a diode.

Watson A.Name \"Watt Sun · Apr 27, 2004

John Popelish wrote:

Albert wrote:

I was just at the Fairchild semi site looking for big diodes. The only
ones they have are in TO-220 packages and although they have 30 and
40A ratings, the power dissipation is only 3 watts. How can the thing
drop .7 volts at 40A and only dissipate 3 watts of heat?? Now I'm
confused:>:

For instance:

http://www.fairchildsemi.com/ds/MB/MBR3050PT.pdf

That rating applies if you just have the diode sticking up into air,
with no heat sink. The thermal resistance spec applies when you mount
it in a heat sink and need to figure out how hot the junction gets
compared to the heat sink. You need heat sink date to figure out how
hot the heat sink gets.

Who else makes high current schottky diodes??

International rectifier makes lots of big ones. So does Diodes
Incorporated. Digikey sells both these brands.

(snip)

Those dirty SOBS! They've included John Popelish's post in their web
page!

I do a google search for "zero voltage drop diode" and up pops this URL:
http://www.cyber-spy.com/electronics-design/electro-03420-29917.html

But I looked thru the web page and found nothing about it. When I
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<title>Re: Lowest voltage drop diode</title>
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<center><pre>
Message-ID: <3D91094E.C564E59C@rica.net>
From: John Popelish <jpopelish@rica.net>
Organization: This space not available for advertising.
X-Mailer: Mozilla 4.7 [en] (Win98; U)
X-Accept-Language: en
Newsgroups: sci.electronics.design
Subject: Re: Lowest voltage drop diode
References: <b73441b7.0209241627.9a47314@posting.google.com>
Date: Wed, 25 Sep 2002 00:54:52 GMT
NNTP-Posting-Host: 24.51.150.249
NNTP-Posting-Date: Tue, 24 Sep 2002 20:54:52 EDT

oguz wrote:

I am looking for a very low voltage drop diode ( or equivalent circuit
) that has a forward voltage drop of 0.2 Volts or less. Maximum
current to be conducted will be less than 5mA.

Currently, I am using a BAT85 Schottky diode from Philips. This has a
forward voltage drop of 0.3-0.4 Volts maximum. I have looked at some
Germanium diodes that have a drop of 0.3 Volts.

Is there a way to do this with a MOSFET or Transistor ?

Is a one-way conducting device of 0.2 Volt drop or less achievable by
any means ?

Oguz Murtezaoglu
BiPOM Electronics
oguz@bipom.com
www.bipom.com

This one has a .2 volt drop at 100 ma:
http://www.irf.com/product-info/datasheets/data/95sq015.pdf

In some cases, you can use a mosfet as an active rectifier by turning
the thing on when it has reverse voltage applied to the drain. At low
currents, the drop can be very close to zero. Even a tiny BZ170 would
only drop about 25 millivolts with its 5 ohms channel resistance.

--
John Popelish
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R.Legg · Apr 27, 2004

~3V.

C*V=I*t; solve for C.

V = 3V (ripple drop)
I= 20A
t=8.33ms (60Hz line, full wave rectifier)

C= (20 * 0.0083)/3 = 0.0055 FARAD!

You better have a 60,000uF 25V capacitor in there, do you?

..0055F = 5,500uF

RL

Frank Bemelman · Apr 27, 2004

"R.Legg" <legg@magma.ca> schreef in bericht
news:e715b5cc.0404271126.1537fc6e@posting.google.com...

~3V.

C*V=I*t; solve for C.

V = 3V (ripple drop)
I= 20A
t=8.33ms (60Hz line, full wave rectifier)

C= (20 * 0.0083)/3 = 0.0055 FARAD!

You better have a 60,000uF 25V capacitor in there, do you?

.0055F = 5,500uF

But C= (20 * 0.0083)/3 = 0.055 FARAD

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)

John Popelish · Apr 27, 2004

"Watson A.Name \"Watt Sun - the Dark Remover\"" wrote:

Those dirty SOBS! They've included John Popelish's post in their web
page!

I do a google search for "zero voltage drop diode" and up pops this URL:
http://www.cyber-spy.com/electronics-design/electro-03420-29917.html

But I looked thru the web page and found nothing about it. When I
search for any of those words inside the web page, I get not found. So
I viewed the page source and I get: (scroll down about halfway to find
the usenet post)
(snip)
Message-ID: <3D91094E.C564E59C@rica.net
From: John Popelish <jpopelish@rica.net
Organization: This space not available for advertising.
X-Mailer: Mozilla 4.7 [en] (Win98; U)
X-Accept-Language: en
Newsgroups: sci.electronics.design
Subject: Re: Lowest voltage drop diode
References: <b73441b7.0209241627.9a47314@posting.google.com
Date: Wed, 25 Sep 2002 00:54:52 GMT
NNTP-Posting-Host: 24.51.150.249
NNTP-Posting-Date: Tue, 24 Sep 2002 20:54:52 EDT

oguz wrote:

I am looking for a very low voltage drop diode ( or equivalent circuit
) that has a forward voltage drop of 0.2 Volts or less. Maximum
current to be conducted will be less than 5mA.

Currently, I am using a BAT85 Schottky diode from Philips. This has a
forward voltage drop of 0.3-0.4 Volts maximum. I have looked at some
Germanium diodes that have a drop of 0.3 Volts.

Is there a way to do this with a MOSFET or Transistor ?

Is a one-way conducting device of 0.2 Volt drop or less achievable by
any means ?

Oguz Murtezaoglu
BiPOM Electronics
oguz@bipom.com
www.bipom.com

This one has a .2 volt drop at 100 ma:
http://www.irf.com/product-info/datasheets/data/95sq015.pdf

In some cases, you can use a mosfet as an active rectifier by turning
the thing on when it has reverse voltage applied to the drain. At low
currents, the drop can be very close to zero. Even a tiny BZ170 would
only drop about 25 millivolts with its 5 ohms channel resistance.

--
John Popelish
(snip)

That is bizarre.
--
John Popelish

Franc Zabkar · Apr 28, 2004

On Mon, 26 Apr 2004 10:14:18 -0400, Albert <pass@pass.com> put finger
to keyboard and composed:

My neighbor brought his 25A (12 volts) power supply over for repair
last evening. I found the bridge rectifier shorted and one of the 2
pass transistors shorted. The 20A fuse in series with the output was
not blown, the 4A line fuse was blown.

I patched it enough to test it, putting in a 10A bridge rectifier and
using only one of the pass transistors (they were in parallel).

I found the transformer which looked hugh, was only putting out 17
volts no load (dc, measured at the bridge output) and 15.5 at 10A
load. While the transformer probably has the current capacity, it
probably loads down under full load (at 20A). I couldn't test at 20A,
but he confirmed that the ham radio transmitter had some quirky
behavior, probably due to the voltage drop off at the higher load
currents.

It looks like someone (previous owner) disconnected the metering in
order to get around the voltage drop associated with the meter.

I'd like to put the thing back together for him, and have it operate
properly.

It had a 2N3055 and an MJE2955 pass transistors-not sure why they were
different types, they were in parallel.

I need suggestions for low collector to emitter voltage drop pass
transistors that will handle 15A each. I also wonder if a bridge
rectifier made from schottky diodes. Suggestions for the schottky
diodes??

C 0--+---\ /---/\/\/---+--0 E
| _\_/_ R |
| | |
+---\ | /---/\/\/---+
| _\|/_ R |
| | |
+---\ | /---/\/\/---+
_\|/_ R
|
|
0
B

I would share the load between 4 transistors of the same type, and
preferably from the same batch, with additional load equalising
resistors in the emitter circuits (NPN transistors shown). For a 5A
load (per transistor) I would use 0.033 or 0.047 ohm 2W resistors. I
would also ensure that all transistors were subject to the same
thermal conditions.

- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

Apr 29, 2004

Albert wrote:

My neighbor brought his 25A (12 volts) power supply over for repair
last evening. I found the bridge rectifier shorted and one of the 2
pass transistors shorted. The 20A fuse in series with the output was
not blown, the 4A line fuse was blown.

I patched it enough to test it, putting in a 10A bridge rectifier and
using only one of the pass transistors (they were in parallel).

I found the transformer which looked hugh, was only putting out 17
volts no load (dc, measured at the bridge output) and 15.5 at 10A
load. While the transformer probably has the current capacity, it
probably loads down under full load (at 20A). I couldn't test at 20A,
but he confirmed that the ham radio transmitter had some quirky
behavior, probably due to the voltage drop off at the higher load
currents.

It looks like someone (previous owner) disconnected the metering in
order to get around the voltage drop associated with the meter.

I'd like to put the thing back together for him, and have it operate
properly.

It had a 2N3055 and an MJE2955 pass transistors-not sure why they were
different types, they were in parallel.

I need suggestions for low collector to emitter voltage drop pass
transistors that will handle 15A each. I also wonder if a bridge
rectifier made from schottky diodes. Suggestions for the schottky
diodes??

Thanks,

A

Operate properly as defined by whom?
The supply was marginal, at best, when new.
A 25 amp supply with a 20 amp secondary fuse
is a clue, as is the voltage sag, and the
fact that there are only 2 output transistors.
Others have mentioned that the 2N3055 is a
bit wimpy for this. Two transistors in
parallel is wrong - don't know if that was the
original design, but the two you mentioned
can't be paralleled - and in any event, each
should have its own emitter resistor.

Get the original schematic and restore it to that,
using beefier transistors than the schematic
specifies. That's likely the best you can do,
without bearing "speculative" costs. By that I
mean - there is no point in building for 25 amps
if the power transformer can't provide it.

low voltage drop transistors for power supply

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