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Old 18th January 2007, 02:49 PM   #1
zilog is offline zilog  Sweden
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Default over-temperature protection for audio smps

In my design of an SMPS for audio use, I dont use any temp-sensing of my main transformer. The SMPS is designed to handle 30-40% of full audio power contonuously, and to handle 100% peaks for short periods time. Transistors, diodes etc have cooling enough to withstand 100% abuse, but the thing that limits continuous power to 30% is the area of the transformer windings.

How do professional PA-amps implement this? I know that this will never pose any problem for me as a DIY:er as the amp will be used for audio signals ony, but it would be nice to know if the professionals also take this scenario into consideration.

One thing I have thought of is the possibility of placing an NTC resistor between windings, but I dont see how I would be able to measure its temperature without interrupting the switching for a short period of time due to magnetic interference. Another possibility would be to use a microprocessor to keep track of used input current all the time, and from this value estimate core and winding temperatures.

If I were to design smps:es and sell them for use with audio amplifiers, what kind of testing would they be forced to undergo? It feels silly to stress such an smps to 100% output power for temperature rise tests as I have read about other commercial smps:es being tested with.

/Daniel
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Old 18th January 2007, 07:36 PM   #2
Nordic is offline Nordic  South Africa
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Maybe use a thermal switch?

Click the image to open in full size.

Thermal sensors encapsulated in TO18 type packages with electrically isolated mounting tabs
Resistance changes from 100k to 100R at centre temperature for a 10C change (approx) (with small changes outside this transition range)
Maximum working temperature 120C
Length 25.4mm, Width 6mm, Body Height 4.2mm
Hole diameter 3.5mm, Lead length 12.7, Lead spacing 2.54mm

The one pictured is a 75C switch...
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Old 18th January 2007, 07:54 PM   #3
zilog is offline zilog  Sweden
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But where to put it? keep in mind that it will be placed inside the high flux density of a 55-80 kHz switching smps transformer core where one turn of winding will give 3-5 volts.
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Old 18th January 2007, 08:13 PM   #4
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I've seen temperature sensors attached to the core of a switching transformer (I think it was an EE-55 core in a QSC amplifier power supply) for thermal protection, but never inside sandwiched with the windings.

Although they are not measuring the hot-spot of the transformer, it will give an average transformer temperture- enough to prevent a melt down!
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Old 19th January 2007, 01:23 AM   #5
Eva is offline Eva  Spain
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Stick a small NTC to the windings, it can be placed externally if you account for the fact that the inner windings will be hotter and you lower the temperature limit accordingly. Capacitive and inductive interference from transformer are not a problem at all since the temperature signal is almost DC and the interferences are high frequency AC, so a low pass filter is enough to tame them.
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Old 19th January 2007, 01:51 AM   #6
zilog is offline zilog  Sweden
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Quote:
Originally posted by Eva
Stick a small NTC to the windings, it can be placed externally if you account for the fact that the inner windings will be hotter and you lower the temperature limit accordingly. Capacitive and inductive interference from transformer are not a problem at all since the temperature signal is almost DC and the interferences are high frequency AC, so a low pass filter is enough to tame them.
Ok, will do so for an experiment. Will be interesting to see what inner winding temperatures that arise from using an ETD34 core for 10-16V to 2x45V conversion at different power levels.
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