DCDC converter efficiency measurement - Page 1
Forum section: electronics / general

author: 3x8rga
author_rank: Newbie
timestamp: December 09, 2021, 04:45:27 pm
content: Hey everyone,I have built a DCDC converter and I'd like to do an efficiency measurement to approximate losses and maybe do some optimization. Here are the specifications:Input Voltage: 20-36VInput Current: limited to 70ANominal Output Voltage: 48VIt's a four-switch buck-boost topology with 6 phases in interleaved mode, so for high loads, the output voltage can go below the input voltage.I can' really say what the efficiency is but I guess something around 95% is reasonable to assume (so around 50W losses). In order to get the losses, I'll need to measure input voltage, output voltage, input current, and output current. Since I am not aware of a current meter that can directly measure currents in this range (I have current clamps available but those have accuracies around a few %) I'll need to use two shunts and measure four voltages in total. I did a first accuracy calculation assuming 1mR 1% tolerance shunts and accuracy specs of a decent hand multimeter (Fluke 87V) for all four measurements and quickly realized that this won't lead anywhere (Measurement values for 30V,70A in, 48V,42A out with uncertainties: input shunt voltage: 70mV±0.17mV, input shunt current: 70A±870mA, input voltage: 28V±25mV =>input power: 2100W±28W, output shunt voltage: 42mV±0.142mV, output shunt current: 42A±562mA, output voltage: 48V±34mV => output power 2016W±28W => efficiency between 93.4% and 98.7% => losses between 28W and 140W).I got a quote for precision resistors with 0.02% accuracy and I guess with those I'd get a decent result but I am asking myself if this is really necessary. Additionally I have a more accurate bench multimeter (Keithley 2000) but I do not have enough calibrated multimeters to measure everything at the same time using calibrated equipment. So what I am thinking about is a procedure like this: put two decent (1%, <50ppm/°C, four terminal) shunts in series, use the force terminals of the Keithley in four wire mode or an external power supply (in this case I could use a high current like 50A) to supply a current, and measure both shunt voltages one after another (maybe back and forth a few times and average). Afterwards I put the shunts on the input and output of my DCDC converter, turn it on and measure all four values one after another using the Keithley.My questions are: Is that a reasonable way to do this? What would the accuracy calculation for this procedure look like? How do others obtain efficiency numbers, it does not seem to be that straight forward?Thanks for your help!

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2021, 05:31:02 pm
content: Using 2 4 Wire shunts in series is tricky, as this defeats the 4 wire conection for the center part.The weak part is very likely the current measurement. So one would need a good meter for small votlages. One may get away with just a custom (DIY) build amplier for the shunt(s), to get good resolution there too.The Current for the input an output side is not that much different. So one could use the same type of shunt for both currents. When repeating the measurement with the shunts swapped one could average out the errors from the shunt absolute value to a large part.Similar swaping the meters for the voltage also can resuce some error.Chances are one would need 4 meters to do all the 4 measurements at the same time. One may do fast swaping for the voltage measurement.With swaping the shunts and averaging one does not need accurate shunts : even 1% arruracy would be OK than - they should still be high power and low thermat EMF to avoid errors.For just the efficiency measurement the calcibration of the meters is not even that important. Most the scale factor errors can be eliminated by swaping input and ouput side. The meters still need to be stable and linear.Simple DC meters would not include a possible superimposed AC ripple part, with correlation between voltage und current ripple. So one may still have to use a scope for that part.

author: chris_11
author_rank: Contributor
timestamp: December 09, 2021, 07:35:03 pm
content: It is less complicated than you might think. First good thing is, that the inpout and output voltages are in the same range which make the currents in the same range. So you can use the same shunt resistor type and value for input and output. You can use a 4 terminal shunts in the sub mOhm range. Voltage measurement of the 4 wire shunts either with good meters or direct with separate 24+ bit sigma delta converters.Since efficiency is a quotient, there is no need for absolute accuracy, only for relative. So you can calibrate both current shunt measurements in series and calibrate for the same reading. So you run 0A, 50A through both current shunts and calibrate for the same reading. Since you run DC-DC converters you only need averaged values. Average is better than RMS which would give false results for DC-DC converter measurements. The VIN and VOUT voltages can be measured direct. Either you have enough meters to make current and voltage measurements at about the same time, or you have some trustable mux. Typical the shunts make some dissipation, so cooling/ temperature management is mandatory. If you go for cheapo Chinese brass shunts, you might consider oil bath cooling for them. You can easy check your measurement without a DC-DC with input and output connected. Your efficiency measurement should read 100% for your relevant current ranges.

author: 3x8rga
author_rank: Newbie
timestamp: December 10, 2021, 08:33:26 am
content: Quote from: Kleinstein on December 09, 2021, 05:31:02 pmSorry, I didn't say clearly what I want to do: I plan to put the shunts in series with respect to the force leads but attach the sense leads only to the sense terminals of one of them (so only one of them is measured at a time). Then I'd attach the sense leads to the other and measure that one. The idea is that, since they are in series, the exact same current is flowing through both of them and I get I an accurate relative value for both. But I guess this does not work if the reading is not taken at the exact same time, since the current can fluctuate.Quote from: Kleinstein on December 09, 2021, 05:31:02 pmI understand but unfortunately, I think am not allowed to take any measurement for this purpose with uncalibrated equipment (except for equipment that is sufficiently characterized using other calibrated equipment). And using the effects above I'd need to present an uncertainty calculation and the uncertainty of the final number.For my uncertainty numbers above, the majority of uncertainty comes from the tolerance of the two shunts. So the question here mostly is how I can measure those to less than 1%. I indeed use two shunts of the exact same model and value. The Keithley has rather high uncertainty measuring low resistances because its lowest range is the 100R range. What assumptions on uncertainty could I make here if I measure the two shunts of the same nominal value with the same meter (and doing zero offset correction before)? Since I'll do the measurement within a minute or so I'd guess that I could at least assume that the relative uncertainty is better than that of the "24h hours after last calibration" from the datasheet, right?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 10, 2021, 09:28:27 am
content: One can measure the ratio of the 2 shunts with a test like planed, with the 2 shunts in series. Just make sure the current source is relatively stable. This does not have to be the SMPS, but should better be a low EMI current source like a linear power supply, maybe a battery with resistor / lamp. If needed repeat the measurement (swap meter(s) for the voltages) to suppress variations in the current. So the error would be the short time uncertainty of the voltage meter(s) and the measured variations in the current. I may take a bit math to get a good estimate for the uncertainty due to current variations. With more repeats this error parts would go down, so it would not be the dominant uncertainty.There is no need to know the absolute value of the shunts, just the ratio is that matters.Another way to reduce the effect of the shunts is to average over 2 tests with the shunts (and meters) for the input and ouput side swapped. An error in the shunt and meters would lead to opposite direction errors for both cases. One can see this as also calculating the shunt ratio form the same data and than use that result implicitely.The efficiency is a ratio measurment. So this is to a large part depending on the offsets and linearity of the meters, not on the scale factors. The calibration of modern meters is usually mainly about the scale factors. The linearity is rarely strictly checked in the calibrations.An additional tricky part in this test can be EMI: the meters are usually tested and calibrated for a low EMI environment. The measurement at the SMPs would likely be with quite some superimposed RF background. So ideally one would have to add some extra filtering for the meters and check if EMI has an effect (e.g. by swapping meters, swapping the polarity of the meters). The extra uncertainty due to EMI is usually not in the specs for the meters and not tested during calibration. The EMI part is another reason to repeat the measurement with different / swaped meters.

author: mzzj
author_rank: Super Contributor
timestamp: December 10, 2021, 04:52:56 pm
content: OP's case looks still "reasonably" easy but sometimes it might be easier to measure the losses.For example 1000W 98% efficient dc-dc power supply: Input-output method needs something like around 0.05% relative accuracy for current and voltage measurements to give meaningful numbers.Losses measurement&estimate is really inaccurate (maybe 5-10% with reasonable effort) but calculation is really insensitive for it when the efficiency is high.10% error on losses is only 2W and would skew the efficiency calculation by 0.2 percentage point. (97.8...98.2% efficiency)APT had once nice appnote about 3kW PFC converter efficiency measurement via losses but good luck finding it from Microchip website.