Please criticize my layout

[AudioAngel]

Hello!

I've layouted my first PCB for an Aleph-X (49W @ 8ohms). Now I want to hear your suggestions to make it better. Different from other designs it is singelsided (easier to make for hobbyist) and the mosfets are mounted on the heatsink through a cutout in the PCB. Why? Because I can mount the complete PCB on the heatsink while soldering the mosfets on the PCB, no cables are necessary. The demensions are: 70mm x 240mm, therefore choosing an enclosure is a littelbit easier for example 19"-enclosures.
I'm thinking about filling the space with copper and connect it with GND. Would this be good? What about a copper plane on the top layer by using a doublesided PCB?

Thanks for your help.

Greetings,

AudioAngel

Aleph-X-Layout

[Holger Barske]

Hmm... Let me see: If the board is about 70 mm wide, the cutout for the power transistors can't have more than 20 mm. Are you sure this is enogh for gettig the FETs mounted?
Second is: All your power transistors will be mounted side by side and very close; so there will be a lot of heat concentrated on a very small heatsink area. You might run into hot spot problems very easily.

[Bensen]

Hello AudioAngel,

There are also other method's for mounting FET's to a heatsink without using cables. I used a method where I placed a RVS bar between the PCB and the FET's. So, now I can solder the FET's to the PCB, and mount later the PCB with two bolts to the heatsink. The FET's are than between the heatsink and the RVS bar.
See how I done it:

I'm going crazy, please help, Pic's

Greetz

Ben

[AudioAngel]

Ok, thanks for your ideas. I'll chage may board. Is there any disadvantage in placing the mosfets with more space between them concerning the thermal coupling? I've seen this very close mounting of transistors in a power amp of Burmester, so I thought it would be a good idea.

AudioAngel

[kilowattski]

It is quite the opposite. Space them as far as possible. It helps spread the heat over more of the heat sink. I have seen some very poor commercial designs in my day. If something doesn't look right or smell right, chances are it isn't right, no matter who designed it.

[AudioAngel]

So, is it right to say spacing the mosfets as far as possible for a better cooling is more important than thermal coupling? I'm asking because Q4 and Q9 should be matched an placing them very close together will lead to avoiding different temperature of transisitors.
But I see I'll have to make a new PCB-design on weekend.

AudioAngel

[kilowattski]

If I am not mistaken, I think that Q5 and Q7 should be matched and thermally coupled, but not on the heat sink. Q4 and Q9 do not have to thermally track each other. Some suggest simply bolting Q5 and Q7 together with a mica insulator in between, using a nylon screw. Just to make sure we are talking about the same transistors I have enclosed the schematic I referenced.

[AudioAngel]

Yes, we are talking about the same schematic. I've read on the Aleph-X-diyAudio-Wiki-Page that besides Q5 and Q7 also Q2 and Q11 should be matched. So I made the mistake to think Q2 and Q11 should also be thermal coupled. In my previous statement I mixed up Q4 and Q9. Sorry.
If Q2 and Q11 don't have to thermally track each other, it should be possible to mount each on a seperate heatsink to get a better thermal resistance.

AudioAngel