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Old 17th April 2015, 12:11 PM   #1
chongwl is offline chongwl  United States
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Default SMPS overload protection specs

Hi, I'm trying to understand overload protection specs on switching power supplies.

I often see "105% - 150% rated power. Automatic recovery."
  1. Does that mean a 36V, 4.17A, 150W PSU can deliver 150*150% = 225W peak power?
  2. How does this overload relate to over-voltage protection? Does it deliver 225W through higher current (36V, 6.25A) or higher voltage?
  3. Is there a standard time for how long it can deliver peak power before going into protected mode?
  4. What does the lower number (105%) mean?

Thank you!
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Old Yesterday, 09:59 PM   #2
infinia is offline infinia  United States
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Location: Californication
it's simply a tolerance of the current limit, usually sensed on the primary.

practically speaking the target trigger point being set @ ~ 128 % rated power. wider range is for manufacturing tolerances and reference temperature drift combined.
1) peak power within the range given, once CL is triggered it's gone into "protect mode".
2) it doesn't! it regulates voltage until CL is triggered.
3) its a set point, u can only count on 5% 'bonus' power. practically it'll be closer to 128% or 5.3A peak. time isn't spec'd but it's very small like beyond the SMPS loop bandwidth, faster over current peaks supported are related to output capacitance and any local decoupling placed near your dynamic loads. note> Often you cant simply add more C to the outputs to provide higher current as it reduces the loop bandwidth and may cause unstable regulation.
4) lower spec limit on CL set point.
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust

Last edited by infinia; Yesterday at 10:24 PM.
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Old Today, 08:59 AM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
This is one area where I have tried to understand what an SMPS is capable of doing, as it relates to the very varied current demands of a Power Amplifier.
The vast majority of Power Amplifiers draw a variable current from the supply.
If the amplifier were a 150W into 4ohms type fed from this 150W supply, then one would expect on normal music/audio signals to draw between 0amps and 8.66Apk when feeding a resistive test load.
Will an SMPS be able to deliver that 8.66Apk to the 4r0 dummy load?
Can one design the SMPS to meet that transient demand and still not overheat when the average currents are well within the 4.17A specification?

Note that in the above that two 150W SMPS are needed to supply the 150W amplifier. The average current draw from each SMPS is ~3.06Arms
regards Andrew T.

Last edited by AndrewT; Today at 09:03 AM.
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