Cold Temperature Start Up of Low Cost Power Supplies with Inrush Thermistors

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I usually get requested the query: "Relating to the TDK-Lambda low value power provide that's rated from -25°C to 70°C, will it begin up at -40°C?" I often reply, "It relies upon."

Most low value, low wattage power provides keep away from giant surges of present being drawn when the AC enter is first utilized by utilizing a thermistor in collection with the AC line (see determine 1). This device is a sort of resistor that when chilly has a a lot increased resistance than when heat.

Determine 1: Thermistor

When the AC is first utilized, the thermistor limits the quantity of inrush present that fees the majority storage capacitor. As soon as the power provide starts-up and delivers power, the thermistor self heats and reduces in resistance to enhance the power provide effectivity and operation.

At ambient temperatures beneath freezing, these thermistors have very high resistances, and if the provision is “rated” to start-up at a chilly temperature, it ought to have been examined through the design stage to make sure right start-up at full load and at minimal AC enter. If the power provide will not be particularly rated for a chilly temperature start-up, there's a chance that the power provide will flip on, attempt to ship power, however the thermistor is not going to have self-heated because of the very chilly ambient and therefore can have a big voltage drop throughout it, inflicting the power provide to change off once more. The power provide will attempt to restart once more, inflicting “blips” on the output. (Fig. 2, prime hint). In some circumstances, the power provide may not begin up in any respect. These makes an attempt to restart could cause system issues.

Determine 2: Cold Temperature Start Up

If the output load is gentle, nevertheless, the power provide might be able to begin up accurately.

So again to my reply to the preliminary query! "It relies upon on what the loading might be at -40°C. If the application has say 20% loading, then often the reply is sure." There are different points with chilly temperature operation, however I shall cowl that sooner or later.