Cheap and fast! Is this the IoT DC/DC measurement solution you want?

Latest update time：2021-08-31 01:26

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Keywords : measurement , tools , Qoitech, Otii, power management , IoT , DC/DC converter , battery

In IoT devices, it is crucial to have an efficient power management system to maximize the energy of the battery. An important part of this is to design a high-efficiency DC/DC converter to boost the voltage from the battery to the device.

In the following example, we use a 1.5V alkaline battery to get a 3.3V output. To achieve a high-efficiency design, a lot of knowledge and measurements are required. Small IoT companies often have difficulty obtaining expensive measurement equipment, so this article introduces two practical measurement methods that can help users achieve cheap and fast design.

Case 1: Calculate the energy efficiency value of the target system over the entire battery life to help designers select the most efficient DC/DC converter and inductor.
Case 2: By using two Otii tools, one or more DC/DC converters are fully characterized with different inductors over the entire operating range. Ultimately, designers can choose the best combination to achieve the best battery performance .

Measurement plan settings

Case number one

The Otii-Arc-001 (hereafter referred to as Otii) from Qoitech AB acts as the battery, and the voltage is swept from 1.5V to 0.9V. The efficiency is obtained by dividing the output energy from the DC/DC converter (Otii expansion port ADC measurement current and voltage) by the input energy to the DC/DC converter (Otii main current and voltage). The load is the DUT (device under test, i.e. the target system). It is important to note that the measurement time should be long enough to ensure that the correct average value is calculated, which will be discussed later.

Figure 1: Measurement setup for Case 1. (Image source: Qoitech AB)

For the setup shown in Figure 1, the DUT measures temperature, humidity, and light every 30 seconds, and 10 such cycles are averaged. The overall efficiency value is calculated by weighting the time the battery will remain at a given voltage level, see Figure 2. Here, the battery voltage is estimated to be at 1.5V 9% of the time, 1.4V 8% of the time, etc. This is not completely correct, but it is a good estimate for this case.

Figure 2: AAA battery discharge curve. (Image source: QoitechAB)

Case 2

One power supply Otii acts as a battery, sweeping the voltage from 1.5V to 0.9V. This power supply Otii also takes the measurements. The other Otii acts as a programmable constant current load, starting at 1mA, then 3mA, 5mA, 10mA, 30mA, 50mA, and finally 90mA (the DC/DC converter is capped at 100mA).

Figure 3: Measurement setup for Case 2. (Image source: Qoitech AB)

Power Otii measures efficiency by dividing the output energy (the Otii expansion port ADC measures current and voltage) by the input energy (the Otii main current and voltage). Normally this is output voltage times output current divided by input voltage times input current, but since Otii calculates and displays energy, using energy is much simpler.

The Otii tool also supports measuring the input and output voltages using a four-terminal sensing method using the SENSE+ and SENSE- inputs. This method is not discussed here because the currents are quite low and the cables used to connect to the Otii are short and have low resistance.

Both Otiis (or multiple Otiis connected) and all measurements (Main Current, Main Voltage, Expansion Port ADC Current, Expansion Port ADC Voltage, SENSE+, SENSE-, etc.) are available in the same window, making it very convenient to visualize the resulting data.

Measurement results analysis

Three different Texas Instruments DC/DC converters are used in these cases.

As mentioned earlier, 10 cycles of the DUT are measured , which means each cell voltage lasts 10 x 30 seconds = 5 minutes. Figure 4 shows a screenshot of the TPS91097A-33DVBTDC/DC .

Figure 4: Case 1 Otii measurement, TPS91097A-33DVBT. (Image source: Qoitech AB)

The Otii tool makes efficiency calculations as simple as dividing the output energy by the input energy, and then weighting that efficiency value as described in the measurement setup in Example 1. Figure 5 provides an overview of all three DC/DC converters.

Figure 5: Efficiency calculation of different DC/DCs. (Image source: Qoitech AB)

This calculation can also be done automatically in Otii using a lua script ( https://www.lua.org ), but for a more intuitive look, Figure 5 uses an Excel table to show it.

The performance of the three DC/DC converters is almost identical when using a small 4.7μH chip inductor. To continue the DC/DC study, different inductors were used to see if efficiency was improved. Three different Bourns inductors and one Murata inductor were selected for the test.

4.7 µH (Murata)
4.7 µH (Bourns)
12 µH (Bourns)
22 µH (Bourns)

The 22μH inductor is too large for this application, but it is interesting to see the corresponding performance.

Using the same setup as before, select the TPS61097A-33DBVT as the DC/DC converter and the inductor as the variable (Figure 6).

Figure 6: Efficiency calculations for different inductors. (Image source: Qoitech AB)

As expected, the larger the inductor and the lower its resistance, the more efficient the DC/DC solution. However, a large inductor of 22μH is not desirable.

To learn more about the characteristics of a DC/DC converter, use Case 2 to get a deeper understanding of the characteristics of a DC/DC converter over a range of input voltages and loads.

First, Figure 7 shows the measurement results for a large inductor of 22μH. Figure 8 shows the same analysis for other inductors.

Figure 7: Case 2, TPS61097A-33DVBT Otii measurement using a large 22μH inductor. (Image source: Qoitech AB)

The receiving Otii starts by absorbing 1mA, then 3mA, 5mA, 10mA, 30mA, 50mA, and finally 90mA. Repeat this for all battery voltages.

As can be seen in Figure 7, for the lower input voltages, the DC/DC cannot handle the 90mA. The DC/DC cannot regulate for these low voltages and starts to oscillate.

The data is stored in a .csv file and can be imported into Matlab for analysis and plotting. Figure 8 plots the efficiency versus output current.

Figure 8: Matlab graph showing DC/DC efficiency for different inductors (Image source: Qoitech AB)

This is a great way to see the characteristics of a DC/DC converter under different load conditions.

Test case summary

Otii is a very useful tool that makes it easy to analyze the efficiency of a DC/DC converter, both for use in a target system and for full characterization.

In the simple system analyzed in this document, the performance of the three TI DC/DC converters is very similar; the TPS61097A-33DBVT was chosen simply because it comes in a SOT23-5 package. Regarding inductor selection, the 12μH inductor should be chosen because it has higher efficiency and there is enough space to use it.

The number of DC/DC converters and inductors mentioned in this article is small, but designers can expand to their favorite components based on this analysis.

Some technical resources related to the above test cases are as follows: