[R-390] Re: R-390 Voltage Regulator

[Bob Camp]

Hi

If you are going to try the solid state R390 not an A regulator 
conversion here's a starting point on the pass device.

For a whopping $5.67 you can get a IRFPE30 from International 
Rectifier. Data sheet is at 
http://www.irf.com/product-info/datasheets/data/irfpe30.pdf.

Into an infinite heat sink the part is rated at 125 watts at room 
temperature. At 70C the part will handle 75 watts. Since we are running 
20 to 30 watts that seems about right. The heat sink we attach it to 
probably will need to be a bit better than a radio bulkhead, but still 
smaller than the entire rest of the radio.

With an 800 volt rating out to 2 amps overload and over voltage should 
not be a major issue. It also has a built in reverse protection diode 
so that's one less part you have to wire in.

Looks like the only major issue is the power dissipation.

Without a lot of work 30 watts seems like a reasonable starting point. 
The MOSFET has a lot better saturation characteristics than the tube. 
That means that we don't need quite as much headroom on the regulator. 
A simple trick is to put a resistor in series with the transistor to 
soak up part of the power. If half of the power is in the resistor that 
cuts our heat sink down to the 15 watt region. With a fan on it that 
can be a fairly small  chunk of metal. With no fan you will need a 
fairly large chunk of extrusion. Without going to far on heat sink 
design the part should be about 6 X 6 X 3 if it's a normal multi fin 
chunk of aluminum extrusion. That will keep the temperature down to the 
point that you won't cause burns if you bump into it. If you don't use 
the resistor trick then the heat sink goes to about 10X4X3. That's a 
pretty big heat sink ...

Bottom line is that the tubes run quite a bit hotter than the 
semiconductors. That makes them easier in the heat removal department. 
For the same margin in the design the volume of the semiconductor plus 
heat sink will be at least as large or larger than the tubes it 
replaces.

Still hanging on to my box of 6082's ....

	Take Care!

		Bob Camp
		KB8TQ