An idea that disposes with the need for matching mosfets

LBHajdu

Member
2002-02-18 7:53 pm
I noticed that from evolving from the Aleph 5 to the Aleph 60 the number of mosfets went up, and the size of the heat sinks when down. This is in my belief because the intense heat (100c or so) in the die of the mosfet is distributed among more mosfets. This means the heat sinks can be smaller end run hotter. On the other hand it also means that more matched sets of mosfets are needed. This is because the “sense” is only connected at the first mosfet and all the others must follow closely.

My idea is, for the diy hobbyist is; why not use only one mosfet per rail. In the case of the Aleph 5 each mosfet would draw 2 amps and give off 75w of heat. The speck sheet for the IRFP244 says this can be done. The heat sink must be bigger and therefore more costly. I an not an thermal engineer so I don’t even know is such a passive heat sink made out of aluminum exists. I also think the power resistor may have to be changed. I don’t know if this idea is practical or not, however I would like to here other peoples ideas about this. I hope I got this concept right in order to avoid making a fool of myself.


Another thing that puzzles me is that an Intel® Pentium® 4 processor dissipates about 45 watts of heat and an AMD processor gives off about 70 watts. So how can they cool them with such small heat sinks?


Levente Hajdu


Oh I almost forgot, while browsing the Internet for heat sinks I found this picture, anyone see what I see?

[IMGDEAD]http://www.geocities.com/wpowersw/heatsink[/IMGDEAD]

If you can't see the image just click here : http://www.geocities.com/wpowersw/
 
Seems like a whole lot of trouble and expense to avoid the relatively simple task of matching some mosfets. It would be cheaper (and more reliable) to take the extra money you would save by getting smaller heatsinks by purchasing more output devices so that you ensure better matching.

Processor heatsinks have fans. Forced air convection is going to be alot more effective at moving heat than passive heatsink-only cooling. Fans aren't all that common in hifi amps, as the cost of noise (both acoustical and electrical) are general considered unacceptable. They are a necessity in high-power, high-reliablity PA gear.

A properly isolated fan probably wouldn't be a big deal, and could do wonders in cooling those class-A amps. As could water cooling.

-Won
 
also, you will find that given the thermal resistances, power deratings with die temp etc, it is not possible to run the typical fets such as 240/244's @ that amount of dissipation. You could of course use active cooling ie. a peltier cooler but that costs quite a bit and has it's own requirements. In a nutshell, your far better off just matching the right number of devices and be done with it.
 

UrSv

Member
2002-01-31 5:48 pm
Sweden
I am no expert on the Aleph 5 or the Aleph 60 but as far as I can tell the numbers are pretty much the same. The output devices in the Aleph 5 (at "slightly greater than" 2 A bias) dissipates a little more than 34*2*2 W which is 136 W and the Aleph 60 (at "sligtly less than" 2.5 A bias) dissipates 35*2.5*2 W which is 175 W. The Aleph 5 is a stero amp in a single chassis as far as I know and the Aleph 60 is a mono amp in a single chassis which means that the need for less heatsinking is right. As for the number of output devices I could imagine that it is a matter of reliability (as individual dissipation and heat goes down in the devices) as well as performance considering that distortion goes down by adding devices and damping goes up.

As for your suggestion it pretty much what the original Zen and Return of Zen is all about.

/UrSv