How to Select and Use an Isolated DC/DC Converter for an Industrial IoT Sensor

Powering wireless Industrial Internet of Things (IIoT) sensors for applications such as machine conditioning monitoring is challenging. The sensors must be compact, robust, easy to deploy, and inexpensive, yet work reliably for long periods with little or no maintenance. Sensor failure implications range from missing vital data about the machine’s condition to expensive repairs to catastrophic failure of the system or production line.

This article details the challenges designers face when building power supplies for battery-powered IIoT sensors for condition monitoring applications. It then describes how high-energy-density, isolated, DC/DC converters from RECOM Power can be used as the basis for power supplies that address these challenges, without resorting to expensive and bulky heatsinks.

What is condition monitoring?

Condition monitoring overcomes the problems of preventive maintenance schedules for large and complex machinery and processes. The technique relies on knowing the status of a machine component such that it can be serviced or replaced well before failure occurs. For example, by constantly monitoring the vibration signature of a motor, software can determine the wear state of a bearing and extrapolate to determine when the current rate of wear would cause a breakdown. Such information allows engineers to extend service intervals while avoiding unplanned downtime (Figure 1).

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT SensorFigure 1: Without maintenance, complex industrial processes and machines eventually fail resulting in extended downtimes. Even after repair, repeated failures of worn-out equipment continue to occur (bottom). Preventative maintenance schemes schedule frequent service intervals to ensure processes and machines run for long periods and machines don’t wear out, but are resource intensive (middle). Condition monitoring allows for extended service intervals without risk of failure while lowering maintenance costs (top). (Image source: RECOM Power)

IIoT sensors are a good option for condition monitoring applications. These compact devices can be mechanically attached close to known machine or process failure points to enhance measurement precision. Wireless connectivity allows for regular condition updates without the need for expensive communication wiring.

The power supply design challenge for IIoT sensors is tough. The typical application environment is dirty, there can be a lot of vibration, temperatures can be very high, and hazardous voltages are commonplace. Space is often at a premium, and the sensitive electronics demand a continuous, clean, and precisely regulated DC voltage.

A new generation of isolated DC/DC converters such as the RxxCTExx Series from RECOM Power provide a solution. These compact devices provide the high energy density, compact size, durability, and efficiency needed for IIoT sensor applications. The converters are supplied in surface-mount packages capable of delivering up to 1 watt while taking up minimal space on the pc board.

Rugged commercial IIoT sensor power supplies

Packaging advances, such as incorporating power and control elements on the same piece of silicon and employing low-profile transformers, enable manufacturers to offer high-specification isolated DC/DC converters for IIoT sensor applications. The RECOM Power DC/DC converters, for example, use design elements such as planar-type transformers to shrink chip heights to less than three millimeters (mm) (Figure 2).

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT SensorFigure 2: RECOM’s RxxCTExx Series are supplied in compact surface-mount SOIC-16 packages with a profile less than 3 mm. (Image source: RECOM Power)

The use of standard SOIC-16 packages enables handling and assembly using automated equipment. Finally, the chips’ compact size allows the power regulation to be placed much closer to the load, simplifying and shrinking the design.

The low-cost RECOM Power DC/DC converters provide 0.5 (R05C05TE05S-CT) or 1 watt (R05CTE05S-CT) at 5 volts output (output voltage ripple is a maximum of 50 millivolts p-p (mVp-p)) from a nominal 4.5 to 5.5-volt input. The converters’ output voltage is compatible with popular families of active sensors and the microcontroller or DSP front-ends commonly used for data analysis. The R05C05TE05S-CT 0.5 watt device has an input current of 240 milliamps (mA), while the R05CTE05S-CT 1 watt version has an input current of 370 mA. The converters are equipped with short-circuit, overcurrent, and overtemperature protection for high reliability in IIoT applications.

The 0.5-watt version can operate in ambient temperatures up to 100°C without derating, while the 1-watt product can be used up to 72°C. Both devices are IEC 62368-1 compliant (Information Technology Equipment, General Requirements for Safety).

The DC/DC converters have no minimum load requirement which makes them suitable for applications which often switch into very light load operation modes to conserve energy. This is a common mode of operation for IIoT sensors. The R05C05TE05S-CT can output 0.6 watts (with an input current rising to 255 mA) for up to 60 seconds (s). A recovery period of three times the peak power duration is required before peak power can be accessed again (Figure 3).

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT SensorFigure 3: The RECOM Power R05C05TE05S-CT 0.5 watt DC/DC converter can deliver a peak output power of 0.6 watts for up to 60 s. A recovery period of three times the peak power duration is required before peak power can be drawn again. (Image source: RECOM Power)

Meeting isolation demands

The environment around the IIoT node is subject to high power surges whenever heavy machinery is started or stopped. For safety reasons and to protect fragile electronics, the sensor’s DC power supplies require isolation from the main supply.

The RECOM Power DC/DC converters use an internal transformer to isolate the output from the input. The devices feature a 3 kV DC isolation voltage (rated for 60 s) and are tested for 1 s to a maximum isolation voltage of 3.6 kV DC. The insulation resistance (500 volts DC, 25°C) is 50 ohms (Ω) and the external clearance is >8 mm. Figure 4 shows an application circuit for the isolated DC/DC converter.

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT SensorFigure 4: Application circuit for RECOM Power RxxC05TExxS isolated DC/DC converter. (Image source: RECOM Power)

The importance of thermal management

The energy density of a DC/DC converter is measured in watts per cubic centimeter (W/cm3). Higher power density allows the designer to increase the power available for the application without using a larger component size, or to maintain the power output while shrinking the overall dimensions of the product.

For the DC/DC converter supplier, the key to offering high energy density is to boost the chip’s efficiency and/or enhance its thermal performance – enabling the use of a smaller package and a higher maximum operational temperature.

The RECOM Power DC/DC converters offer good efficiency for inexpensive, isolated, semi-regulated switching devices. A key feature that marks them out from competitive devices is that the efficiency curve is relatively flat across the 20 percent to full output load range (Figure 5). Competitive devices often exhibit poor efficiency at low and medium output loads.

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT SensorFigure 5: Shown is a graph of efficiency vs. percent output load for the R05C05TE05S-CT. The isolated switching converters deliver good efficiency over a wide load range. (Image source: RECOM Power)

The maximum junction temperature (Tjmax) of a component (measured center top of the silicon die) is typically detailed in the datasheet. Provided the device does not exceed this limit, the manufacturer guarantees performance. Operation above this temperature can change the conductance of the semiconductor such that it no longer performs as intended and can even cause permanent damage.

Tj for a fixed power dissipation device such as a DC/DC regulator is largely dependent on the internal multipath thermal resistance (Ψjt), and the effectiveness of heat transfer to the immediate environment. Ψjt considers all the means by which heat can escape from the component, including through the bottom of the chip via the pc board. This parameter is difficult to measure outside the lab and is often not included in the data sheet. A good proxy for Ψjt is θja; this is a measure of the thermal impedance (Rθja) of a single heat path from the silicon die directly to the ambient environment that is simpler to measure. The units of Rθja are degrees centigrade (or Kelvin (K)) per W (°C/W). Tj can be estimated from the equation:

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT Sensor

Component designers aim to minimize internal thermal impedance and to maximize conductive and convective heat transfer to keep component temperatures down and provide satisfactory “overhead” between Tj and Tjmax (Figure 6).

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT SensorFigure 6: Component makers specify a maximum junction temperature (Tjmax) for an active device to ensure proper operation. For a given power dissipation, Tj is largely determined by the component’s total thermal impedance and the ambient temperature (Ta). (Image source: RECOM Power)

IIoT sensors often operate in confined environments with little ventilation. This can cause the ambient temperature to rise, and it can easily approach 70°C in industrial environments. This high Ta impacts the temperature overhead of the component.

Consider the following example for a typical DC/DC converter:

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT Sensor

Without adding cost and volume through the use of a heatsink, this device would be unsuitable for the application because of the very limited temperature overhead.

A better solution would be to select a device with an extended temperature range. There are many commercial DC/DC regulators that offer a Tjmax of 125°C, and a few, such as the RECOM Power solution, that extend that to 150°C. Second, the input and output voltages could be more closely matched (which raises the efficiency of a linear regulator and hence lowers power dissipation). And third, the designer should look to select the device with the lowest thermal impedance.

Consider a second example for a DC/DC converter selected with these criteria in mind:

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT Sensor

This option provides considerable temperature overhead which helps to extend product life.

RECOM Power’s RxxCTExx Series employ 3D power packaging (3DPP) to lower thermal impedance. 3DPP takes advantage of materials optimization, manufacturing techniques, and a variety of junction-to-ambient heat transfer methods such as flip-chip-on-lead (FCOL), embedded ICs, and thermal vias to lower thermal impedance. These techniques enable the manufacture of SOIC-16 sized DC/DC converters that can power high loads without the complication and costs of active cooling methods or large passive heatsinks. The RxxC05TExxS products have an Rθja of 63.8°C/W, compared to around 90°C/W for conventional products.

In certain circumstances, for example in enclosed spaces near machines powered by large electric motors that radiate a lot of heat, the ambient temperature can rise even higher. In these situations, chip makers recommend derating (i.e., limiting the output power of the device to lower power dissipation and in turn Tj). For example, consider the second DC/DC converter detailed above; a rise in temperature to 110°C would leave only around 38°C of temperature overhead, which is less than recommended for extended product life. Figure 7 shows the thermal derating curve for the RECOM Power RxxC05TExxS.

How to Select and Use an Isolated DC/DC Converter for an Industrial IoT SensorFigure 7: Thermal derating curve for RECOM Power RxxC05TExxS DC/DC converter. The manufacturer recommends decreasing output power above Ta = 104°C to avoid long-term damage to the component. (Image source: RECOM Power)

Conclusion

Powering low-power wireless IIoT sensors for applications, such as machine conditioning monitoring, is a considerable task as the operational environment is hot and dirty. Sensitive monitoring devices must be supplied with a steady, clean DC voltage, and be protected against the high voltage surges common to industrial equipment. In addition, space is typically at a premium and costs need to be kept down.

A new generation of isolated DC/DC converters now helps designers meet these challenges. Compact surface-mount solutions allow for easy assembly and offer the high energy-density, space-savings, durability and efficacy demanded. Additionally, novel packaging and manufacturing techniques have lowered thermal impedance, allowing the devices to operate in enclosed, high-temperature environments without the need for expensive and bulky heatsinks.