Hi all, I have an amplifier which has 6 Mosfets 3 BUZ900 3 BUZ905. The amplifier can do 450 watts rms at 4 ohm. I'm running it at 8ohm. The data sheet says its Mosfets have a maximum junction operating temperature of 150 deg C. I've had to remove its heat sink to fit it in a new case and I was hoping the case would compensate. Well unfortunately it still gets too warm for my liking, so I want to add a fan and use a thermostat to turn off the fan when the amp is cool or idling. What temperature would I use?
Thanks
Mike
Thanks
Mike
This question cannot be answered without more info. You will need to do a thermal resistance calculation for the answer. These are very simple resistor networks representing thermal performance. Your MOSFETs have a thermal resistance of 0.5K/W. I presume they have individual insulators which may add 0.2-0.4K/W. Let's say 0.9K/W in total. A class AB amp delivering 200W may dissipate 200W as well (need to know the PSU voltage), so the six devices will each be dissipating 33W or thereabouts. That will rise 30 degrees above the heatsink temperature. The heatsink or case needs to provide a thermal resistance that will keep the junction temperature to 150C, but that would be rather high with a case temperature of about 150-17 degrees or 133C, which is too hot to touch. If the case is able to be touched then a 50C limit is needed. Now you need to know your room temperature. Say 25C. so your case/heatsink/fan arrangement needs to dissipate 25 degrees C for 200W which is not trivial.
You can measure the thermal resistance of your case by driving the amplifier to a known power level (typically resistive load), measuring the input power, hence dissipated power, and measuring the temperature of the case. You then need to know the thermal resistance of the case to fan (moving air) which may be obtained from data in fan manufacturers' tables, or heatsink manufacturers' data, and determine what airflow you need.
It is not difficult, but does need thinking about, and 200W is not a trivial amount of heat to get rid of. YOu my need to repeat the measurement with a chosen fan to calibrate it too.
So I'd say to answer your question 50 degrees for an external case, higher if you can insulated this from being touched, but I am not sure you will be able to cool it sufficiently.
You can measure the thermal resistance of your case by driving the amplifier to a known power level (typically resistive load), measuring the input power, hence dissipated power, and measuring the temperature of the case. You then need to know the thermal resistance of the case to fan (moving air) which may be obtained from data in fan manufacturers' tables, or heatsink manufacturers' data, and determine what airflow you need.
It is not difficult, but does need thinking about, and 200W is not a trivial amount of heat to get rid of. YOu my need to repeat the measurement with a chosen fan to calibrate it too.
So I'd say to answer your question 50 degrees for an external case, higher if you can insulated this from being touched, but I am not sure you will be able to cool it sufficiently.
> maximum junction operating temperature of 150 deg C.
That may be OK steady-state. Note that this is NOT the case temperature!
In class B audio, an old rule of thumb is to limit *rise* to 50 deg C. The thump-thump nature of speech/music means the temperature is constantly changing. The materials are expanding and contracting constantly. This tends to crack the seals.
While seals have improved since I was younger, I note that most reliable amps still don't go over 50 deg C rise in hard speech/music work.
Assuming 25C room, 50c rise is 75 deg C (167 deg F). This is somewhat too hot to touch, and will skin-burn you if you hold on for many seconds.
The original heatsink was probably well-designed and it may be unwise to shorten/trim an amp's sinks. (No, "putting the heat into the case" never works as well as hoped.)
A 65 deg C (150 deg F) thermostat should take the edge off. However 200W is more than most PCs, so it may be several "PC Fans".
That may be OK steady-state. Note that this is NOT the case temperature!
In class B audio, an old rule of thumb is to limit *rise* to 50 deg C. The thump-thump nature of speech/music means the temperature is constantly changing. The materials are expanding and contracting constantly. This tends to crack the seals.
While seals have improved since I was younger, I note that most reliable amps still don't go over 50 deg C rise in hard speech/music work.
Assuming 25C room, 50c rise is 75 deg C (167 deg F). This is somewhat too hot to touch, and will skin-burn you if you hold on for many seconds.
The original heatsink was probably well-designed and it may be unwise to shorten/trim an amp's sinks. (No, "putting the heat into the case" never works as well as hoped.)
A 65 deg C (150 deg F) thermostat should take the edge off. However 200W is more than most PCs, so it may be several "PC Fans".
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