Assuming that so far you have only considered bi-polar transistors perhaps
it would be beneficial to look into the application of Power MOSFETs. In the
'on' state they have a very low resistance ,hence at high currents they
dissipate less heat .

Consequently you would need smaller heat sinks.

dont have any idea how to work with mosfet

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.

It's true that modern power MOSFETs have a very low series resistance
when switched "fully on". This allows for low losses, and relatively
low heat dissipation, if they're used in an application where "fully
on, or fully off" is suitable. "Hard" power switching, or pulse-
width-modulated drive applications such as a switching regulator or a
DC motor-speed controller are good places for MOSFETs (or IGBTs).

However, a linear regulator isn't this sort of application. In these
regulators, you aren't driving the pass transistor "hard on" -
because, if you did, the regulator would simply pass the full input
voltage to its output and wouldn't regulate at all! Instead, you're
switching the transistor on only partway, so that it's passing only
the amount of current needed to "pull up" the outputs to the
regulation voltage.

The rest of the voltage (input minus output) is going to appear across
the pass transistor (and possibly one or two other components in
series with it, such as ballast resistors) and will be dissipated as
heat. This will be true regardless of whether the regulating element
is a MOSFET, bipolar transistor, or whatever. If you're sucking 30
amperes, and dropping 10 volts between input and output, your pass
elements will be dissipating 300 watts.

The only ways I know of to reduce the heat dissipation are to either
lower your input voltage (but not enough to cause the circuit to drop
out of regulation), or move to a lower-loss architecture such as a
buck-mode switching regulator. The latter can be quite efficient
(often 90% or better), but you have to work REALLY HARD to build one
which won't emit enough radiated and conducted RF switching noise to
totally yngvi up your HF reception.

I agree with the other posters, who suggest that it's probably
preferable to use a sufficient number of simple, easy-to-buy
pass transistors (2N3055 being the classic example). Use enough
so that you can use 'em well within conservative ratings - don't
go anywhere near the edge of the "safe operating area" - and
ballast them so that they share current properly.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

rob34 wrote:

Paul Burridge wrote:
On Mon, 20 Oct 2003 11:20:20 -0400, rob34 wrote:


If you want to keep it to a couple transistors they would be expensive.
You are better off using either more NTE-180 or more mj2955, but I would
cut the voltage down to around 18 volts instead of 24 volts as this
higher voltage leads to more heat for the transistors. Rob


How come no one's yet recommeded that great old PSU war-horse, the
2N3055? Six of them on a decent heat sink and you're sorted!

2N3055 are great, but 2N3771 are better! Rob

True, but you can find 2N3055, or house numbered versions for free in
a lot of old linear power supplies pulled from junked equipment.
--


Michael A. Terrell
Central Florida

rob34 wrote:

Paul Burridge wrote:
On Mon, 20 Oct 2003 11:20:20 -0400, rob34 wrote:


If you want to keep it to a couple transistors they would be expensive.
You are better off using either more NTE-180 or more mj2955, but I would
cut the voltage down to around 18 volts instead of 24 volts as this
higher voltage leads to more heat for the transistors. Rob


How come no one's yet recommeded that great old PSU war-horse, the
2N3055? Six of them on a decent heat sink and you're sorted!

2N3055 are great, but 2N3771 are better! Rob

True, but you can find 2N3055, or house numbered versions for free in
a lot of old linear power supplies pulled from junked equipment.
--


Michael A. Terrell
Central Florida

On Mon, 20 Oct 2003 19:33:02 +0100, Paul Burridge
wrote:

On Mon, 20 Oct 2003 11:20:20 -0400, rob34 wrote:

If you want to keep it to a couple transistors they would be expensive.
You are better off using either more NTE-180 or more mj2955, but I would
cut the voltage down to around 18 volts instead of 24 volts as this
higher voltage leads to more heat for the transistors. Rob

How come no one's yet recommeded that great old PSU war-horse, the
2N3055? Six of them on a decent heat sink and you're sorted!

The main reason I wouldn't use a 2N3055 in that application is the
gain falls of dramatically at high Ic. Design needs to accommodate
this in terms of both driver current capability and loop gain.

On Mon, 20 Oct 2003 19:33:02 +0100, Paul Burridge
wrote:

On Mon, 20 Oct 2003 11:20:20 -0400, rob34 wrote:

If you want to keep it to a couple transistors they would be expensive.
You are better off using either more NTE-180 or more mj2955, but I would
cut the voltage down to around 18 volts instead of 24 volts as this
higher voltage leads to more heat for the transistors. Rob

How come no one's yet recommeded that great old PSU war-horse, the
2N3055? Six of them on a decent heat sink and you're sorted!

The main reason I wouldn't use a 2N3055 in that application is the
gain falls of dramatically at high Ic. Design needs to accommodate
this in terms of both driver current capability and loop gain.

Dave Platt wrote:

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.
. . .

Someone mentioned the 2 to 4 amps of base current you'd have to supply
for bipolar pass transistors. You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.

Roy Lewallen, W7EL

Dave Platt wrote:

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.
. . .

Someone mentioned the 2 to 4 amps of base current you'd have to supply
for bipolar pass transistors. You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.

Roy Lewallen, W7EL

Hi, from Ken,
"rob34" wrote in message
...
Paul Burridge wrote:
On Mon, 20 Oct 2003 11:20:20 -0400, rob34 wrote:

2N3055 are great, but 2N3771 are better! Rob

2N3773 are better still ( check out current ).

If you want a power supply to deliver round about 40amps the the first thing
to remember is that you MUST have the correct amount of pass transistors to
handle the current.
If you use 2N3055's ( very good and cheap ) you will need at least 10 of
these, don't think for one minute becouse you see the current stated as
being 20amps that one transistor will be able to handle this amout of
current, it won't, it does not work like that.
The best you can hope for ( current wise ) from one 2N3055 is FIVE AMPS. So
for a forty amp supply you would need at least, eight and a couple for that
le-way we all try to allow.

One other thing you must have good heat sinks.

Regards the transformer, it should be able to handle the current and should
( ideally for a 13.8Volt supply ) be rated between 17.5 and 18.5 Volts of
load.
Once regulated this voltage will rise to about, 24 Volts + or - 1 or 2
Volts.

Regards regulation the most common way is to use the LM723 voltage regulator
chip complete with short circuit protection.

Smoothing caps should be another point of concern.
Try to find two large ripple handling units rated at about 40 Volts and
allow two thousand mfd
per one amp of current required.

One final point to remember when building your own power supply is to use
some form of ELECTRONIC OVER-VOLTAGE PROTECTION ( I.e., the MC3423 chip is
perfect ).
The number of cheap commercial power supplies on the market without
overvoltage protection fitted is a crime ( but they never tell you this in
the Ads..right..?. ).

Hope the above helps in some small way. Good luck, Ken, G4KIR.



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Hi, from Ken,
"rob34" wrote in message
...
Paul Burridge wrote:
On Mon, 20 Oct 2003 11:20:20 -0400, rob34 wrote:

2N3055 are great, but 2N3771 are better! Rob

2N3773 are better still ( check out current ).

If you want a power supply to deliver round about 40amps the the first thing
to remember is that you MUST have the correct amount of pass transistors to
handle the current.
If you use 2N3055's ( very good and cheap ) you will need at least 10 of
these, don't think for one minute becouse you see the current stated as
being 20amps that one transistor will be able to handle this amout of
current, it won't, it does not work like that.
The best you can hope for ( current wise ) from one 2N3055 is FIVE AMPS. So
for a forty amp supply you would need at least, eight and a couple for that
le-way we all try to allow.

One other thing you must have good heat sinks.

Regards the transformer, it should be able to handle the current and should
( ideally for a 13.8Volt supply ) be rated between 17.5 and 18.5 Volts of
load.
Once regulated this voltage will rise to about, 24 Volts + or - 1 or 2
Volts.

Regards regulation the most common way is to use the LM723 voltage regulator
chip complete with short circuit protection.

Smoothing caps should be another point of concern.
Try to find two large ripple handling units rated at about 40 Volts and
allow two thousand mfd
per one amp of current required.

One final point to remember when building your own power supply is to use
some form of ELECTRONIC OVER-VOLTAGE PROTECTION ( I.e., the MC3423 chip is
perfect ).
The number of cheap commercial power supplies on the market without
overvoltage protection fitted is a crime ( but they never tell you this in
the Ads..right..?. ).

Hope the above helps in some small way. Good luck, Ken, G4KIR.



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.525 / Virus Database: 322 - Release Date: 09/10/2003

Thanks Jim for posting the flaws in that design !

Here is a better schematic http://www.wa8dbw.ifip.com/rs-70a.gif

for an Astron 70a linear ps.

clarke


Jim wrote:

On Mon, 20 Oct 2003 13:57:46 GMT, in
, clarke wrote:

Hi, check this link and see what you need to do,
one or two pass trans. will not do the job for you,
easier to use 6-10 pass trans to share the load.

http://www.mitedu.freeserve.co.uk/Ci...er/1230psu.htm

Is your transformer able to supply necessary power to this supply ?

I'm not sure anyone has ever built the PSU shown at the above
URL, and if so, I'd guess they haven't run it at the full 30-amp
output for more than a few seconds.

The basic design demonstrates a generally acceptable approach to
designing a power suppy, but some of the components chosen just
don't seem up to meeting the design criteria of 24 volts in, 12
volts out at 30 amps.

The ballast resistors are rated at 1 watt, but will be expected
to dissipate roughly 2.5 watts at the full 30-amp output.

The text analyzing the regulator performance has me baffled. It
correctly describes a current gain of about 35 for the pass
transistors, which at a glance suggests the regulator has to
handle a bit under 1 amp. The web page author says 871 ma is the
current into the regulator input and 866 ma is the current out,
for a net current of 5 ma and a cool regulator. Looks to me like
that poor thing is going to be asked to dissipate about 10 watts
and will run about 110 degrees C even if mounted on an *infinite*
heat sink. I'd redesign to include a transistor driver.

I'd say the designer of this supply never ran it at full output
for more than a few seconds.

--
Jim Higgins, KB3PU

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