[R-390] OT: Pass Transistor Question

[2002tii]

Ian wrote:

>Has anyone had experience using power mosfets as pass elements? From 
>what I've read, equalizing resistors are not needed, but I wonder, 
>being REALLY CONSERVATIVE, if a small resistor, say .1 ohm, in each 
>source lead might be a good idea. I have a plan germinating to re-do 
>a big old GRC PSU putting out 28V at 50 A as a variable voltage PSU 
>using a multiturn pot, a three-terminal Voltage regulator and mANY 
>parallelled power fets as an improvement on an SCR pass element.

Lateral power MOSFETs (generally Hitachi devices, for example 
2SJ134/135 and 2SK49/50), as used in a number of '80s and '90s audio 
amplifiers, are self-equalizing and share current well.  They are 
typically operated in parallel with no emitter resistors.  I have 
never seen any reason to use resistors, and have never used them in 
my lateral MOSFET designs.  Vertical power MOSFETs (such as the IRF 
devices), which were designed for switching applications, are not so 
good about sharing current when operated in the non-saturated region 
(as you would be doing in a linear power supply).  (Lateral and 
vertical refer to the way current travels in the dies.)

Using many MOSFETs in parallel will result in substantial gate 
capacitance, and may be a real pain to drive without 
instability.  This can be dealt with, but it adds complexity and it 
is not clear what you get in return.  Much as I like power MOSFETs, 
and I have designed quite a few products and projects using them, my 
first instinct would be to use bipolar devices for your project.

Best regards,

Don