Affordable and fast! Is this the IoT DC/DC measurement solution you're looking for?

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 utilization of the battery. A key part of this is designing a high-efficiency DC/DC converter to boost the voltage from the battery to the powered device.

In the following example, a 1.5V alkaline battery is used to generate a 3.3V output. Achieving a high-efficiency design requires extensive knowledge and measurement. Small IoT companies often lack access to expensive measurement equipment, so this article introduces two practical measurement methods that can help users achieve a cost-effective and rapid design.

Case 1: Calculating the energy efficiency of the target system over the entire battery life helps 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 across the entire operating range. Ultimately, designers can select the best combination for optimal battery performance .

Measurement plan settings

Case 1

The Qoitech AB Otii -Arc-001 (hereafter referred to as the Otii) acts as the battery, sweeping the voltage range from 1.5V to 0.9V. Efficiency is calculated by dividing the output energy from the DC/DC converter (current and voltage measured by the Otii expansion port ADC) 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 the correct average value is calculated, as 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, with the average value calculated over 10 such cycles. The overall efficiency value is calculated by weighting the time the battery will remain at a given voltage level, as shown in Figure 2. Here, the battery voltage is estimated to be at 1.5V 9% of the time, 1.4V 8% of the time, and so on. This isn't entirely correct, but it's a good estimate for this case.

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

Case 2

One power supply Otii acts as a battery, sweeping the voltage from 1.5V to 0.9V. This power supply Otii also performs the measurement. 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)

The Otii measures power efficiency by dividing the output energy (current and voltage measured by the Otii expansion port ADC) by the input energy (Otii main current and voltage). This is usually done by multiplying the output voltage by the output current and dividing it by the input voltage by the input current, but since the Otii calculates and displays energy, using energy is much simpler.

The Otii tool also supports a four-terminal sensing method for measuring input and output voltages 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 measurement results (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 display the generated data.

Measurement results analysis

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

As mentioned previously, the measurement is for 10 cycles of the DUT, meaning each cell voltage lasts 10 x 30 seconds = 5 minutes. Figure 4 shows a screenshot of the TPS91097A-33DVBTDC/DC .

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

The Otii tool makes efficiency calculations simple: simply divide the output energy by the input energy and then weight 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 calculations for different DC/DC converters. (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 approach, Figure 5 uses an Excel table to show it.

When using a small 4.7μH chip inductor, the performance of the three DC/DC converters is nearly identical. To continue the DC/DC study, different inductors were used to see if efficiency 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 gain a deeper understanding of the characteristics of the DC/DC converter over a range of input voltages and loads.

First, the measurement results for a large 22μH inductor are shown in Figure 7. 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 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 plot 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 can easily analyze the efficiency of DC/DC converters, both for use in the 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, a 12μH inductor was 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 upon this analysis to their favorite components.

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