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!

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.

Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.

You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.

Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.

--
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!

Dave Platt wrote:
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.


Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.


You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.


Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.


I built a cheap ps using a bunch of 2n3055's as pass transistors. (think
I had 5 or 6
of them in parallel). Each had a 0.1 ohm resistor in the emitter lead
to balance
out the current (or one might try to hog it all if the transistors
aren't well
matched). They well darlington connected to another 2n3055 which acted
as a driver (and helps increase the gain at high collector current).
Yet another
transistor was used to drive the driver (TO-5 si, probably a 2n3053 or
equal)
from the regulator IC. A large heat sink was used and I mounted surplus
computer power supply fans on it to add cooling.

I used a heavy transformer and the input voltage to the pass regulator was
about 15-16 volts for 13.6 out. Key here is that with a large
transformer and
enough filter C the voltage won't droop on high current (guess that means
it's well regulated without the regulator!) , and we don't ask the
transistors to disapate too much power.

Course' such a supply is HEAVY, a switcher is much lighter, but a good
switcher with low noise is hard to come by! (Not to mention problems
of rf feed back!).

Dave Platt wrote:
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.


Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.


You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.


Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.


I built a cheap ps using a bunch of 2n3055's as pass transistors. (think
I had 5 or 6
of them in parallel). Each had a 0.1 ohm resistor in the emitter lead
to balance
out the current (or one might try to hog it all if the transistors
aren't well
matched). They well darlington connected to another 2n3055 which acted
as a driver (and helps increase the gain at high collector current).
Yet another
transistor was used to drive the driver (TO-5 si, probably a 2n3053 or
equal)
from the regulator IC. A large heat sink was used and I mounted surplus
computer power supply fans on it to add cooling.

I used a heavy transformer and the input voltage to the pass regulator was
about 15-16 volts for 13.6 out. Key here is that with a large
transformer and
enough filter C the voltage won't droop on high current (guess that means
it's well regulated without the regulator!) , and we don't ask the
transistors to disapate too much power.

Course' such a supply is HEAVY, a switcher is much lighter, but a good
switcher with low noise is hard to come by! (Not to mention problems
of rf feed back!).

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.

Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.

You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.

Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.

--
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!

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