rectifier cooling...

kasra · 19th December 2003, 09:40 PM

Im in middle of assembling a pair of Aleph 2:s.
On the cadpic you can see the arrangement of the inside.
Im planning to use 2 IXYS rectifierbridges per monoblock.
IXYS VBE 55-06NO7 68A

My question is how much cooling would the 2 bridges need?
The shelf over the transformer is 5x155x170mm aluminium, but id guess its not sufficent?
Perhaps on the underside of the big shelf in the middle? (the one going throu the whole enclosure, 5x300x170mm)

If also thats not efficent id guess the only remaining option is to put each one on the each of the main heatsinks.

Anyone who have any ideas?

/kasra

PRR · 19th December 2003, 11:12 PM

> how much cooling would the 2 bridges need?

Figure 7 Amps per channel, and assume 2 volts drop per bridge (it will be less, but this is conservative). 14 Watts. Then double that, because cap-input power supply RMS current is much higher than DC current. 30 Watts.

They seem to say you can let it run 125-150°C hot. Personally I prefer more like 50°C rise above ambient, especially when ambient may be hot as in a Class A amplifier.

30W/50°C= 0.6°C per watt, a very big heatsink. However only 1/10th the size of the main heatsinks in a dual 200W Class A amplifier.

You could probably just oversize the main sinks 10%, then stick the rectifier on the main sink. I can't recall ever seeing this done, so get a sink about 1/10th the size of the main sinks.

To be crude: 7x7-inch square of 1/8-inch aluminum will approach 0.6°C/Watt.

I suspect I have rounded-up a few too many times and you could get away with something much smaller. I bet the sink out of an old home hi-fi 30WPC stereo will do the job comfortably.

kasra · 20th December 2003, 12:08 PM

thanks for the reply.
The main heatsinks are already cut to size. Since i already expect them to get hot i rather nor put the bridges on them. Also i have some space issues inside the box.
I think i will try with the bridges attached to the 7"x12" 5mm thick shelves.

What is the main difference from running the bridges at 60deg vs 80-90deg?

/kasra

SkinnyBoy · 20th December 2003, 12:18 PM

hmm.. doesn't 30 watts of heat on a 0.6 degree celcius per watt heatsink equal:

30 X 0.6 = 18 degrees?

did I miss something?

edit.. your maths was around the wrong way, yeah? it should have been 50/30 = 1.67 dc/w heatsink...

grataku · 20th December 2003, 12:22 PM

kasra,
attaching the rectifier to the bottom plate of the box should be just fine.

kasra · 20th December 2003, 12:24 PM

grataku:

Which one are you refering too? The smaller one over the transformer(7"x7") or the one over it (7"x12")?
Because these are the options so far.

/kasra

peranders · 20th December 2003, 12:42 PM

Quote:

Originally posted by PRR
> how much cooling would the 2 bridges need?
To be crude: 7x7-inch square of 1/8-inch aluminum will approach 0.6°C/Watt.

0.6°C/Watt is a real big heatsink, hardly a flat aluminium plate

I must admit that it's hard to really estimate the power dissipation of a rectifier bridge when it's not resistiv load with no smoothing. Someone maybe have some handy thumb rule?

grataku · 20th December 2003, 01:02 PM

I was referring to the big plate, it will have to be able to radiate most of the heat outside the box. In my stereo AX I have a single PS with 2 rect mounted on the bottom plate. I experience no problem with excessive heating and I am running 10 amps.
If the case is a big hunk of Al and is thermally coupled, the heat generated in the bridge contributes to the overall heat generated by the case and everything gets equalized.

The only problem I can see would be with the electrolytics. They need NOT to be touching the hot case. I used a 1/4 in G10 plate for the base of the electrolytics.

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19th December 2003, 11:12 PM	#2
PRR diyAudio Member Join Date: Jun 2003 Location: USA	> how much cooling would the 2 bridges need? Figure 7 Amps per channel, and assume 2 volts drop per bridge (it will be less, but this is conservative). 14 Watts. Then double that, because cap-input power supply RMS current is much higher than DC current. 30 Watts. They seem to say you can let it run 125-150°C hot. Personally I prefer more like 50°C rise above ambient, especially when ambient may be hot as in a Class A amplifier. 30W/50°C= 0.6°C per watt, a very big heatsink. However only 1/10th the size of the main heatsinks in a dual 200W Class A amplifier. You could probably just oversize the main sinks 10%, then stick the rectifier on the main sink. I can't recall ever seeing this done, so get a sink about 1/10th the size of the main sinks. To be crude: 7x7-inch square of 1/8-inch aluminum will approach 0.6°C/Watt. I suspect I have rounded-up a few too many times and you could get away with something much smaller. I bet the sink out of an old home hi-fi 30WPC stereo will do the job comfortably.

20th December 2003, 12:08 PM	#3
kasra diyAudio Member Join Date: Nov 2001 Location: Sweden	thanks for the reply. The main heatsinks are already cut to size. Since i already expect them to get hot i rather nor put the bridges on them. Also i have some space issues inside the box. I think i will try with the bridges attached to the 7"x12" 5mm thick shelves. What is the main difference from running the bridges at 60deg vs 80-90deg? /kasra

20th December 2003, 12:18 PM	#4
SkinnyBoy Banned Join Date: Nov 2002 Location: Adelaide, Australia	hmm.. doesn't 30 watts of heat on a 0.6 degree celcius per watt heatsink equal: 30 X 0.6 = 18 degrees? did I miss something? edit.. your maths was around the wrong way, yeah? it should have been 50/30 = 1.67 dc/w heatsink...

20th December 2003, 12:22 PM	#5
grataku diyAudio Member Join Date: Dec 2000 Location: -	kasra, attaching the rectifier to the bottom plate of the box should be just fine.

20th December 2003, 12:24 PM	#6
kasra diyAudio Member Join Date: Nov 2001 Location: Sweden	grataku: Which one are you refering too? The smaller one over the transformer(7"x7") or the one over it (7"x12")? Because these are the options so far. /kasra

20th December 2003, 01:02 PM	#8
grataku diyAudio Member Join Date: Dec 2000 Location: -	I was referring to the big plate, it will have to be able to radiate most of the heat outside the box. In my stereo AX I have a single PS with 2 rect mounted on the bottom plate. I experience no problem with excessive heating and I am running 10 amps. If the case is a big hunk of Al and is thermally coupled, the heat generated in the bridge contributes to the overall heat generated by the case and everything gets equalized. The only problem I can see would be with the electrolytics. They need NOT to be touching the hot case. I used a 1/4 in G10 plate for the base of the electrolytics.