Re: Common collector load driver

On May 5, 2:24 pm, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Mon, 5 May 2008 09:57:01 -0700 (PDT), rich <rsoennich...@xxxxxxxxx>
wrote:



Hello all,
I need to to drive a 300mA resistive load with an variable voltage
of 0-30V. Right now I have a NPN transistor in the common collector
configuration.

The collector is tied to the supply voltage which can be 10-30V. The
base is driven from 0V up to the supply voltage.

This works just fine up to a supply voltage of about 20V. I can vary
the base voltage from zero to that and get a nice output without too
much heat.
Above this however I get way to much heat, even at small loads.

The transistor is in a sot-223 package with a pcb copper pour area as
a heatsink.

What is a better way to construct a voltage follower that will
dissipate less heat? Perhaps switching to a p-channel mosfet?

Thanks

Rich

The power will always be Iout * (Input_voltage - Output_voltage) for
a linear regulator. That's the same whether you use a MOSFET, a BJT or
a rheostat operated by a servo.

If I take the worst-case interpretation of your requirements (that it
might be required to supply 300mA into a 0-ohm load with a 30V supply)
then you need to dissipate 9W, which will require a substantial
heatsink.

You might consider using a switching regulator, but I can'tt think of
a simple single chip solution off the top of my head. Check out Linear
Technology's line, there may be something you can use if cost isn't a
factor.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@xxxxxxxxxxxx Info for manufacturers:http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Rich,

You can use a positive 28VDC reg chip such as a LM350TG 3 A,
Adjustable Output 1.2 to 33 V, Voltage Regulator TO-220 case. Instead
of connecting the gnd lead to gnd you would connect it to your
controlling voltage. This will vary the output as you want. The
bottom line is with a 300 ma load at 30 volts you will be generating 9
watts, thats the way it is. The output will be what ever you set it
for even if the load may vary.

Good luck,

Dennis
.

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