Hi,
I was thinking earlier, and somewhere I read that the devices should go near the geometrical center of the sink. As I thought about this, I realized that if you put the devices near the top, under natural convection it would be a sort of counterflow heat exchanger, with the bottom being the coolest and the top being the hottest. If the devices were mounted near the center, then the air flowing past the top would not take more heat from the sink because it has already passed the hottest center section. Is this not correct? Thanks for the help.
I was thinking earlier, and somewhere I read that the devices should go near the geometrical center of the sink. As I thought about this, I realized that if you put the devices near the top, under natural convection it would be a sort of counterflow heat exchanger, with the bottom being the coolest and the top being the hottest. If the devices were mounted near the center, then the air flowing past the top would not take more heat from the sink because it has already passed the hottest center section. Is this not correct? Thanks for the help.
Heat transfer is much greater between the device and the heatsink and the heatsink and the air, so devices should be mounted centrally.
The difference in air temperature between the top and bottom of the sink will be a few degrees at most, wheras the temperature between the heatsink and the air will be in the tens of degrees. The laws of thermodynamics show that conduction is relative to difference in temperature, so you want to get as much heat into the sink as possible, and to do that, you mount the devices in the middle. The effect of convection is purely to keep the air moving over the heatsink, the few degree temperature difference between air top and bottom will not noticably affect the amount of heat that the air takes from the sink.
The difference in air temperature between the top and bottom of the sink will be a few degrees at most, wheras the temperature between the heatsink and the air will be in the tens of degrees. The laws of thermodynamics show that conduction is relative to difference in temperature, so you want to get as much heat into the sink as possible, and to do that, you mount the devices in the middle. The effect of convection is purely to keep the air moving over the heatsink, the few degree temperature difference between air top and bottom will not noticably affect the amount of heat that the air takes from the sink.
Hi,
if you have a single device put it in the middle across the width and about 40% to 45% up from the bottom.
For two or three devices form a row across the width spacing them out evenly with the edge gap about 40% to 60% of the centre to centre gaps.
For four devices the options increase. Two columns of two need each about 25% in from each side. The upper row should be about 60% to 70% up from the bottom and the lower row should be about 25% to 30% up from the bottom.
The reason for the slight downwards location is to make use of the cold air coming in from the bottom having most effect on the hottest part of the sink. The upper row have more sink area dedicated to them due to having pre warmed air coming from the bottom passing over their part of the sink.
A lot more data is available in a paper by Wakefield on their site.
if you have a single device put it in the middle across the width and about 40% to 45% up from the bottom.
For two or three devices form a row across the width spacing them out evenly with the edge gap about 40% to 60% of the centre to centre gaps.
For four devices the options increase. Two columns of two need each about 25% in from each side. The upper row should be about 60% to 70% up from the bottom and the lower row should be about 25% to 30% up from the bottom.
The reason for the slight downwards location is to make use of the cold air coming in from the bottom having most effect on the hottest part of the sink. The upper row have more sink area dedicated to them due to having pre warmed air coming from the bottom passing over their part of the sink.
A lot more data is available in a paper by Wakefield on their site.
You can model the effect of different placings for the transistor on the heatsink at this site:
http://www.frigprim.com/online/natconv_heatsink.html
Input the dimensions of your heatsink and the expected transistor dissipation then move the transistor around and see what happens to the temperature gradients within the heatsink. Multiple heat sources can also be modelled.
http://www.frigprim.com/online/natconv_heatsink.html
Input the dimensions of your heatsink and the expected transistor dissipation then move the transistor around and see what happens to the temperature gradients within the heatsink. Multiple heat sources can also be modelled.
Hi Darkfenriz,
something like this - http://members.dodo.com.au/~gregball/guru_004.htm
The little ones regulate input stage current sources to track the Vgs of the MOSFETs. I would mount a compact module such as this (4" x 2") slightly below centre on a large heatsink and ventilated below on the inside of the chassis.
Cheers,
greg
something like this - http://members.dodo.com.au/~gregball/guru_004.htm
The little ones regulate input stage current sources to track the Vgs of the MOSFETs. I would mount a compact module such as this (4" x 2") slightly below centre on a large heatsink and ventilated below on the inside of the chassis.
Cheers,
greg
Hi Darkfenriz,
Thanks. I'm REALLY pleased with how it's turned out.
The TO92's are actually diode connected BC546B/556B and they are simply bent forward to spring load their flat face against the heatsink when the MOSFETs are mounted. These 'diode' sensors are connected in the emitter sense leg on the 2BJT CCs on the input stages such that they have a comparable tracking rate for the MOSFET bias.
It works a treat.
Cheerrs,
Greg
Thanks. I'm REALLY pleased with how it's turned out.
The TO92's are actually diode connected BC546B/556B and they are simply bent forward to spring load their flat face against the heatsink when the MOSFETs are mounted. These 'diode' sensors are connected in the emitter sense leg on the 2BJT CCs on the input stages such that they have a comparable tracking rate for the MOSFET bias.
It works a treat.
Cheerrs,
Greg
Hi Geoff
thank You for this useful link!!
Heinz
http://www.frigprim.com/online/natconv_heatsink.html
thank You for this useful link!!
Heinz
http://www.frigprim.com/online/natconv_heatsink.html
mastertech said:
You absolutely need air flow on heatsinks.
A very simple indicator of this is to compare the
thermal rating of a given heatsink that has a
comparison of convection versus fan forced.
Even the slowest fan forced air flow will increase the
thermal capacity 3 x or better.
So impeding the little airflow convection offers
will derate the heatsinks a lot. Get your heatsinks
outboard unless there is very little thermal load on them.
T
Hi Mastertech,
the internal location will increase all these temps Ts, Tc, Tj. This will reduce the life of the output stage. Starting from Tj=100degC I saw a figure that life doubles for each reduction of 10Cdeg. So Tj=70degC gives eight times the life even though the hot one is designed to not exceed the manufacturer's ratings.
A further downside is that all other internal components will run hotter due to both convection and radiation. Capacitors and semis will suffer.
the internal location will increase all these temps Ts, Tc, Tj. This will reduce the life of the output stage. Starting from Tj=100degC I saw a figure that life doubles for each reduction of 10Cdeg. So Tj=70degC gives eight times the life even though the hot one is designed to not exceed the manufacturer's ratings.
A further downside is that all other internal components will run hotter due to both convection and radiation. Capacitors and semis will suffer.
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