Sensors have become ubiquitous in the industrial environment. As they increase in sophistication and shrink in size, sensors are enabling Industry 4.0. In turn, sensor electronics are becoming more complex, requiring on-board voltage regulators to deliver power more efficiently and with minimum heat generation. How do you safely deliver low-voltage power to tiny sensors in high-voltage, industrial environments, while minimizing solution size and maximizing efficiency? In this Design Solution, we will review a typical industrial sensor architecture and provide a simple solution to this challenge.
Industrial Sensor Applications
Industrial end equipment often operates in a harsh electronic environment. Sensors detect and diagnose many parameters and also make decisions. They must be durable and reliable, regardless of the environment. Proximity sensors (Figure 1), temperature sensors and pressure sensors are used in many industries, including food and beverage, chemical processing, oil and gas, pharmaceutical, manufacturing, construction, hydraulic and pneumatic applications, water and wastewater, HVAC, and refrigeration systems, to name just a few!
Figure 1: Proximity Sensors in Action
Sensors may be located anywhere on the factory floor. The control center receives information from the sensor and sends the appropriate action to the actuator via a field bus. The sensor ‘box’ includes a front-end connector/interface handling data and routing the power to a step down voltage regulator which delivers the appropriate voltage to the ASIC/microcontroller/FPGA and the sensing element.
Safe Low Voltage Operation
The sensor is typically powered by an isolated 24V DC power source. However the factory floor can be a very challenging environment, with long cables and strong electromagnetic interference resulting in high voltage transients. Accordingly, the step-down converter inside the sensor must withstand voltage transients of 42V or 60V that are much higher than the sensor operating voltage. According to SELV/FELV regulations, an isolated device handling up to 60V is considered safe to touch. Protection above 60V can be provided with the addition of dedicated TVS devices.
Sensor Current Consumption
Table 1 is a list of the most common sensors and motor encoders and their typical current consumption. Proximity sensors (Table 1a) are by far the most common and can be catagorized as optical, inductive, capacitive, photoelectric and ultrasonic.
Table 1a. Proximity Sensor Typical Current Consumption
Pressure sensors (Table 1b) are based on the piezolelectric effect or on strain gauge. In piezoelectric sensors the crystal produces a voltage proportional to the pressure. In a strain gauge sensor the silicon resistance varies with pressure.
Table 1b. Pressure Sensor Typical Current Consumption
Rotary or linear encoders (Table 1c) are widely used in sensing speed and position of electric motors.
Table 1c. Motor Encoder Typical Current Consumption
Temperature measurements are based on diodes, thermocoupleas or resistors depending on the application temperature range. As an example a typical resitor temperature detector (RTD) is a 100 Ohm platinum resistor.
Powering the Sensing Element
Most industrial sensing elements need an input voltage significantly lower than that supplied by the system. In many systems an LDO is used to step-down a 24V system voltage to 5V to power the microcontroller (3mA) and the sensing element (100mA). This is a lossy process (η=21% in this example) that ends up costing 2.5W of power dissipation. If the voltage stepdown is performed by a buck switching regulator with 85% efficiency the power losses are reduced down to 624mW.
Packing More Punch
As mentioned earlier, sensor electronics are becoming more complex while sensor housings are shrinking to enable intelligent factories. Hence, it is important that the on-board voltage regulator is small in size and delivers power with high efficiency and minimum heat generation. Figure 2 illustrates the size of a Himalaya switching regulator IC (MAX17552) and the companion inductor housed in a small, M8 sized proximity sensor. Clearly the MAX17552 consumes minimal space in this application.
Figure 2: Buck IC, Inductor and Sensor Size Comparison
Digital Sensor System Architecture
Figure 3 illustrates a digital sensor system based on the popular IO–Link® point-to-point serial communication protocol. IO-Link, used for communicating with sensors and actuators, has been adopted as an international standard (IEC 61131-9). The IO-Link bus carries power (24V) and data.
Figure 3. Digital Sensor System
The sensor may be located anywhere on the factory floor. The control center receives information from the sensor and sends the appropriate instruction to the actuator via the standard I/O field bus. The sensor ‘box’ includes an IO-Link transceiver interface which handles data and routes the 24V power to a step down voltage regulator. The regulator delivers 5V to the microcontroller and to the sensing element.
A Tailor-Made Buck Converter Family
The MAX17550-MAX17552 and MAX17530-MAX17532 families of high-efficiency, high-voltage, synchronous step-down DC-DC converters save space with integrated MOSFETs and operate over a 4V-to-60V and 4V-to-42V input voltage range, respectively. Delivering output current up to 25mA, 50mA or 100mA, the devices are ideal for sensor applications. The output voltage is accurate to within ±1.75% over the -40°C to +125°C temperature range. The converters consume only 22μA of no-load supply current in PFM mode. The low-resistance, on-chip MOSFETs ensure high efficiency at full load and simplify PCB layout. The devices offer programmable switching frequency to optimize solution size and efficiency, and are available in compact 10-pin (3mm x 2mm) TDFN and 10-pin (3mm x 3mm) μMAX® packages. Simulation models are also available to simplify design.
The typical application circuits for the 600 kHz configurations — optimized for small size — delivers 5V to a load up to 100mA. This enables a total solution size of only 50mm2. With a 24V input the peak efficiency is 87% for both device families. As discussed earlier, these devices decisively outperform any LDO based solution in terms of power savings.
We discussed common industrial sensor architectures and examined the power needs and challenges of various types of sensors. In addition, we provided a solution to efficiently and safely deliver power in a small form-factor using a switching regulator as an alternative to the lossy and thus, inadequate, LDO. Finally we introduced a new family of Maxim Integrated switching regulators, the MAX17550-MAX17552 and MAX17530-MAX17532 high-efficiency, high-voltage, synchronous step-down DC-DC converters with integrated MOSFETs. These small buck regulators deliver an output current up to 25mA, 50mA, or 100mA and are tailored to the needs of industrial sensor applications.
About the Authors:
Nazzareno (Reno) Rossetti is a seasoned Analog and Power Management professional, a published author and holds several patents in this field. He holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.
Viral Vaidya is an Executive Business Manager for Power Management products at Maxim Inc. He has over 10 years of experience in product management for various power management products. He has been at Maxim for more than five years and has served in a similar role for the previous five years. Vaidya has a MSEE from San Jose State University in California.