Efficiency in Power Delivery Analysis
Explore the impact of efficiency on power delivery in systems like PD power, Vpse, Rcable, Cable current, Max Current, and Max PD power. Understand the importance of considerations between Rcable and VPSE in optimizing power delivery efficiency. Dive into detailed equations, solutions, and graphical representations to enhance your understanding.
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Presentation Transcript
PD power, Vpse and Rcable Chad Jones Cisco Systems
Cable current The equation for Icable is a quadratic: R * I2 VPSE* I + PPD= 0 => I = ((VPSE SQRT(VPSE2 4 * R * PPD)) / 2 * R) The plus solution is an unstable mode, use on the minus solution PPSE= VPSE* I PPSE= VPSE( (VPSE SQRT(VPSE2 4 * R * PPD)) / 2 * R ) 2
Max Current, Max PD power The max of that quadratic can be found by taking the derivate wrt I, solving for I. But I don t really care about IMAX as much as I want to know PPD_MAX PPD_MAX occurs when PPD = PCABLE (PPD) VPSE * I R * I2 = R * I2 => VPSE * I = 2 * I2 * R => I = VPSE / 2 * R And PPD = VPD* I = (VPSE / 2)* VPSE / 2 * R = VPSE2 / 4 * R PPD_MAX = VPSE2 / 4 * R 3
Which is more important, RCABLEor VPSE PD power delivered 120.00 50V 40V 30V 100.00 24V 12V 80.00 Ppd 60.00 40.00 20.00 0.00 6.25 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Rcable 4
Efficiency The problem with the previous analysis is it ignores efficiency It s not ignored, it s just 50%. STANDARD/TYPE VPSE RCABLE PPSE PPD EFFICIENCY AF 44V 20 Ohms 15.4W 13W 84% AT 50V 12.5 Ohms 30W 25.5W 85% BT Type 3 50V 6.25 Ohms 60W 51W 85% BT Type 4 52V 6.25 Ohms 91W 71.3W 78% BT Type 4* 52V 6.25 Ohms 100W 75W 75% *802.3bt does not have a 100W PPSE mode, this is the theoretical max for demonstration purposes 5
How does efficiency affect power delivered? 60% Efficiency 65% Efficiency 120.00 120.00 12V 12V 100.00 100.00 24V 24V Delivered PD Power Delivered PD Power 30V 30V 80.00 80.00 40V 40V 60.00 60.00 50V 50V 40.00 40.00 52V 52V 20.00 20.00 0.00 0.00 6.25 12.5 20 40 6.25 12.5 20 40 Rcable Rcable 70% Efficiency 75% Efficiency 100.00 90.00 12V 12V 90.00 80.00 24V 24V 80.00 70.00 Delivered PD Power Delivered PD Power 70.00 30V 30V 60.00 60.00 50.00 40V 40V 50.00 40.00 50V 50V 40.00 30.00 52V 52V 30.00 20.00 20.00 10.00 10.00 0.00 0.00 6.25 12.5 20 40 6.25 12.5 20 40 6 Rcable Rcable
How does efficiency affect power delivered? 80% Efficiency 85% Efficiency 80.00 60.00 12V 12V 70.00 50.00 24V 24V Delivered PD Power Delivered PD Power 60.00 30V 30V 40.00 50.00 40V 40V 40.00 30.00 50V 50V 30.00 20.00 52V 52V 20.00 10.00 10.00 0.00 0.00 6.25 12.5 20 40 6.25 12.5 20 40 Rcable Rcable 90% Efficiency 95% Efficiency 45.00 25.00 12V 12V 40.00 24V 24V 20.00 35.00 Delivered PD Power Delivered PD Power 30V 30V 30.00 15.00 25.00 40V 40V 20.00 50V 50V 10.00 15.00 52V 52V 10.00 5.00 5.00 0.00 0.00 6.25 12.5 20 40 6.25 12.5 20 40 7 Rcable Rcable
Efficiency More realistic: PPD = VPSE2 / 6 * R This yields an efficiency of about 80% and is good simple guide to estimate power delivered to a PD 8
More Realistic Efficiency PD power delivered 70.00 50V 40V 30V 60.00 24V 12V 50.00 40.00 Ppd 30.00 20.00 10.00 0.00 6.25 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Rcable 9
Conclusions Need to keep VPSE higher, RCABLE lower. Target worst case efficiency of about 80%. 12V VPSE is a tough target. Looking at less than 4W PPD. 24V can deliver about 10W over 10 ohms. Can get significant power from 50V with a reasonable RCABLE (10 ohms is about 42W). 10
