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
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