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Old 22nd June 2006, 07:56 PM   #1
bramht is offline bramht  Belgium
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Default heatsink replacement

hello,

I'm doing some mods to some bkelectronics amplifiermodules

one of which is to replace the ugly heatsinks with a nicer one of conrad heatsinks. and change from active to passive cooling

I have the mf450 modules: 450rms @ 4 ohm (300@8 ohm)
it has 6 * SML BUZ900 A001 6BY
the heatsink is in total 2000cm^2

because it is actively cooled I'm multiplying it by 3 to 6000cm^2 to calculate a passive replacement

using the 50/sqrt(cm^2) formula this results in 0.65 C/W,

so i have to find a heatsink of 0.65 C/W or better?

I'm planning on using the MF35-100 which has 0.28C/W (80C rise)

So my question is: Am I getting away with this?

resources: esp and harry's homebrew pages
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Old 22nd June 2006, 07:57 PM   #2
bramht is offline bramht  Belgium
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right hand view
there's supposed to be a fan in front of the heatsink
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Old 23rd June 2006, 08:47 AM   #3
djk is offline djk
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A fan usually gives 5:1 leverage on heatsink area.

Don't test at full power too long, 0.28C/W may be OK for music and home use. At full power the amplifier efficiency will be around 50% with an unregulated supply, so you will have at least 225W of heat to dump.
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Old 23rd June 2006, 10:09 AM   #4
bramht is offline bramht  Belgium
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with active <=> passief => 1:5 then it would be 0.5C/W

well it's never going to be full power, it's for stereo music and HT in the living room

i tested like it is now, without fan on movie viewing sound level (which is prety high) with bass-heavy music for 2-3hours, and the heatsink measured 38C, however thats was on a test speaker which is 91db and the eventual speakers (adire kit2641) is only 89db, so eventually it will work harder resulting in higher temps. But still i was supprised that it didn't get hot.

I finaly received answer of the constructor to the question of how much the heatsink rating should be passively: 0.15C/W is their answer but that is on full power for an extended period of time (it's often used as PA-amp)

At first this amp will be used for mains, but eventually it will probally be powering subs,

if don't see a problem with mf35-100 for mains, but maybe for subs just as a safer margin going to mf35-151.5 (0.21C/W) would be the smarter thing to do?

I must say that i was planning for the latter, but my test got me thinking that i could go lower in heigth as it would be more appealing (WAF) and get away with it
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Old 23rd June 2006, 06:19 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
that BK sink looks pretty cheap and very ineffective.

A ClassAB amp can usually approach 70% efficiency when at full power. At this peak power the heat dissipated by the sink is the other 30%. So the peak dissipation is about 30/70 * 450W =193Wpk
However some amps may only achieve about 60% efficiency giving a dissipation of 300Wpk.
Most designers would choose a sink that can dissipate about 60% of the maximum output power of the amp equating to 270W.

So there you have a dissipation range of 193Wpk to 300Wpk.
It also happens that the efficiency drops when delivering less than full power. At less than full power, proportionally more power goes into the sink compared to that sent to the load. However the peak dissipation is close to the maximum dissipation when driving at about 60% to 70% of maximum output voltage. The maximum dissipation is slightly worse than the peak dissipation. So we now need to dissipate about the same watts or slightly more (+10%) when the output is about half maximum.

Now some lateral thinking. What duty do you want? How often will it run at full power? How often will it run at half power? How long will the sinks take to heat up due to a prolonged transient?
Do you only play in a domestic environment? Will you want the amp to survive a party? will it ever be loaned out on disco duty?

You will gather from that array of questions that these all affect the decision on sizing your sink.

For domestic duty you can get away with a sink that dissipates about (20% of maximum + the quiescent dissipation)=about 100W to 150W depending on Iq. This relies on cool sinks most of the time and the thermal inertia of the massive sink material to absorb the short period transients. Then the music level returns to lower SPL and the sinks cool again awaiting the next big moment.

As you move up the scale to hard driven home use, through party to disco, you will need to move towards that 300W figure and ensure that the output stage keeps the transistor case temperatures at or below the designers maximum Tc for the SOAR his devices can survive long term.

Sorry, no quick and ready reckoner answers here.

Djk, your maths lets you down.

For a 450 watt amplifier running at 50% efficiency, when delivering maximum power then the input power will be double the output power ie 450/900=0.5 Subtract output power from input power and the remainder is the dissipated power, which in this case is 450W. Put another way, Pd=50/50*450=450W and this is a peak value when maximum power is just at the point of clipping.
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Old 23rd June 2006, 06:30 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
just seen your last post.

Assuming 250W dissipation and 0.15C/W the sink temp rise is about 38Cdegrees. Assuming an ambient of 25degC the sink temperature will be about 63degC and this will result in a transistor case temperature of about 73degC. This is getting pretty hot and with 200degC devices results in a derating of 0.73

All the currents extracted from the SOAR for values of Vce must be reduced by this multiplication factor.

If the devices are only 150degC then the derating factor is 0.62 This is much worse.

Reliability costs money. If you want continuous party levels when distortion due to clipping tends to pass unnoticed and the amp is forced to run at power levels from 25% to 100% of maximum with out a rest then go for the big sinks (0.15C/W).
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Old 24th June 2006, 08:41 AM   #7
bramht is offline bramht  Belgium
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it will never be on continious party levels, it's only for home stereo and HT

I doubt it will ever run on half power for a long period.
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Old 24th June 2006, 09:22 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
for domestic use you tend to listen to clean output. When doing this you are actually adjusting the system gain to avoid clipping.
This can allow loud music (and effects) that do not shout at you and yet only when someone tries to speak to you that you realise the music was turned up loud.

If your listening is based on a similar senario then it is likely that you will only exceed the 25% power rating of the amp for less than 10% of the time.

If you turn up the volume until the clean sounds start changing to aggressive, shouty and tiring then you have a much higher risk of running at more than 25% of max power and for very significantly more than 10% of the time.

It is during the other 90% than everything gets a chance to cool down again.

Domestic is an easy thermal loading.

Sinks designed for disco use to cover all (extreme) listening levels will feel cold in a normal domestic environment.

You can, therefore make a choice to adopt more acceptable sized sinks and delete the noisy and failure prone fan.
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Old 24th June 2006, 09:56 AM   #9
bramht is offline bramht  Belgium
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i'll go with my first idea and that the mf35-151.5 with 0.21C/W

I'll make up with the WAF some other time



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