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marekm
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RB5009 power supply efficiency at 48V vs 24V

Sun Mar 12, 2023 6:21 am

RB5009 powered from CRS328-24P, the CRS auto-detects PoE voltage as high (52V) but can be forced low, the RB5009 works at both voltages. Power reading at the higher voltage is about 1W higher (7.5W vs 6.5W when mostly idle, and only the SFP+ port linked with a DAC). Is this just some CRS328-24P PoE-out power measurement inaccuracy, or is really the RB5009 power supply less efficient at high input voltage? It is usually the opposite (higher loss at lower input voltage), but it really depends on DC/DC converter design details (conduction vs switching losses).

BTW, RB5009 PoE-in it is also not isolated (negative ground), so not fully 802.3af/at compliant contrary to what the specification says, and not safe to use with -48V DC telecom power.
 
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mkx
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Re: RB5009 power supply efficiency at 48V vs 24V

Sun Mar 12, 2023 4:36 pm

Efficiency of typical step-down converters is worse with higher difference between input and output voltage (there are numerous articles about it on internet). So I believe that numbers you're seeing are at least indicative if not scientifically accurate.

Where higher voltages shine is power loss on long supply lines with non-negligible resistance (e.g. PoE) where lower power loss due to lower current over (relatively( high-resistance cables more than compensates the higher losses in DC-DC down coversion.

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