First, the system calculates the target value of energy efficiency during the entire battery life, helps the designer to select the most efficient DC / DC converter and an inductor.
Second, by using two tools Otii, different inductors to one or more DC / DC converter using a full characterization over the entire operating range. Finally, the designer can select the best combination to achieve optimum battery performance.
Measurement Setup
Case 1
Qoitech AB of Otii-Arc-001 (hereinafter abbreviated as OTII) acts as a cell, a scan voltage range is 1.5 V to 0.9 V. By from the DC / DC converter output power (expansion ports OTII ADC measuring current and voltage) to the input divided by the energy (OTII primary current and voltage) DC / DC converter efficiency obtained. Load DUT (device under test, i.e., the target system). Be sure to note the measurement time should be long enough to ensure proper claim to the average, this will be discussed later.
Figure 1: Case 1 measurement setup. (Source: Qoitech AB)
For the setting, the DUT 1 shown in FIG measured once every 30 seconds the temperature, humidity and light, the use of 10 such cycles mean strike. The total efficiency value is calculated by weighting the battery holding time at a predetermined voltage level, see Figure 2, where the battery voltage is 1.5 V is estimated 9% of the time, 8% of the time the voltage is 1.4 V, and the like. This is not entirely correct, but this case is concerned it is appropriate to estimate.
FIG 2: AAA battery discharge curve. (Source: Qoitech AB)
Case 2
A battery serving as a power Otii, scan voltage from 1.5 V to 0.9 V. The power supply is also responsible for measuring Otii. Another Otii acts as a programmable constant current load, beginning from 1 mA, and then is 3 mA, 5 mA, 10 mA, 30 mA, 50 mA, and finally to 90 mA (DC / DC converter is an upper limit of 100 mA).
Figure 3: Case 2 of the measurement setup. (Source: Qoitech AB)
OTII powered by the energy output (OTII expansion port ADC measuring current and voltage) divided by the input power (current and voltage main OTII) to measure efficiency. Typically the output voltage multiplied by output current divided by the input voltage multiplied by the input current, but the Otii can calculate and display energy, the energy is much easier to use.
Otii tool also supports SENSE + and SENSE- input, input and output voltage measured by the four-terminal detection method. This method is not discussed here, because the current is quite low, and the connection cable used Otii very short, little resistance.
OTII two (or more OTII connected) and all the measurements (the main current, the main voltage, current expansion port ADC, ADC expansion port voltage, SENSE +, SENSE- etc.) are provided in the same window, so the display is very easy the resulting data.
result
Using three different Texas InstrumentsDC / DC converter in these cases.
TPS61097A-33DBVT
TLV61220DBVR
TPS61221DCKT
As described above, measures the DUT 10 cycles, i.e., the battery voltage continues for each 10 x 30 second = 5 minutes. Figure 4 shows TPS91097A-33DVBT DC / DC screenshots.
Figure 4: Case 1 Otii measurement, TPS91097A-33DVBT. (Source: Qoitech AB)
Otii tool allows efficient calculation becomes very easy, simply by dividing the output energy to input energy, and a value weighted according to the Efficiency Case 1 described in the measurement setup. 5 All three DC / DC converter to provide an overview.
FIG 5: different DC / DC efficiency calculation. (Source: Qoitech AB)
This calculation may be used lua script (https://www.lua.org) Otii done automatically, but for more intuitive, FIG. 5 shows a table using Excel.
When using a small 4.7 μH chip inductor, three performance DC / DC converter is substantially the same. In order to continue to study DC / DC, use different inductors to understand whether the efficiency has improved. Selecting three different Bourns Murata inductors and one inductor.
4.7 μH (Murata)
4.7 μH (Bourns)
12 μH (Bourns)
22 μH (Bourns)
22 μH inductor is too large for such applications, but is very interesting to know the corresponding performance.
Before using the same settings, select TPS61097A-33DBVT as DC / DC converters, as a variable inductor (FIG. 6).
Figure 6: Calculation of efficiency of different inductors. (Source: Qoitech AB)
As to the expected results, the larger the inductor and which lower the resistance, the DC / DC solution of higher efficiency. However, 22 μH inductor large is undesirable.
To learn more about the characteristics of the DC / DC converter case 2 is obtained using DC / DC converter input voltage and load a series of more in-depth characterization.
First, FIG. 7 shows the measurement results of 22 μH inductors corresponding large. Figure 8 shows the same analysis of other inductor.
Figure 7: Case 2, a large inductance of 22 μH TPS61097A-33DVBT Otii measurement. (Source: Qoitech AB)
Otii receiving power from the absorber 1 mA begins, then 3 mA, 5 mA, 10 mA, 30 mA, 50 mA, and finally 90 mA. Repeat for all cell voltages.
As can be seen in FIG. 7, for lower input voltages, DC / DC can not handle 90 mA. DC / DC can not adjust for such low voltages, and begins to oscillate.
.Csv data stored in the file for Matlab introduced for analysis and mapping. FIG 8 depicts a relationship between efficiency and output current.
FIG 8: shows the corresponding different inductor DC / DC efficiency Matlab pattern (Source: Qoitech AB)
This method is very good, it is possible to view the DC / DC converter characteristics under different load conditions.
Otii script
Here is the complete Otii script, Otii project files, .csv files, and Matlab code.
Summarize
Otii is a very useful tool, you can easily analyze the efficiency of DC DC converter /, both for use in the target system can also be used to achieve a complete characterization.
In a simple analysis system used in this document, the performance of three kinds TI DC / DC converter is very similar; chose TPS61097A-33DBVT, simply because it uses SOT23-5 package. About inductor selection, 12 μH inductor should be selected, because it has higher efficiency, and there is enough space to use it.
Mentioned in this article DC / DC converter and the number of inductor small, but the design may be based on this analysis extends to their favorite components.
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