Monthly Archives: January 2023

Driving LED Arrays

Digital signage, area illumination, and display back-lighting require large numbers of LEDs, especially of the high-brightness types. A simple arrangement is adequate for driving a single or a few LEDs. However, driving large numbers of LEDs presents a different challenge. There are issues like the optimum overall interconnection topology, the options for powering them, and then the control of the array.

A drive current from 20 to 60 mA is adequate for most single LEDs with a voltage drop of 1.8 to 4 VDC. For instance, a red LED has a nominal voltage drop of 1.8 VDC, and a white LED as high as 4.0 VDC, with other colors in between. Therefore, driving LEDs means changing over to constant-current supplies or drivers rather than using the more common constant-voltage supplies. Most designers are often less familiar with constant-current modes of supplies and their implications.

In concept, the power supply and the driver chain for LEDs are fairly simple. However, driving an array of LEDs is both simple and difficult. While driving a single LED is simple as it is a low voltage and low current load, and the driver has to operate at an efficient dc-to-dc conversion frequency of between 100 kHz and 1.5 MHz. The difficult part is the driver has to supply a constant current, which places new and difficult demands on the design of the circuitry.

Typically, the LED driver for a multi-LED array is the final stage in the power-supply chain, beginning with the AC input. There are optional low-voltage dc-to-dc conversion stages between the two, and ultimately the final DC voltage-to-current conversion immediately before the actual LED drive.

For creating an array of multiple LEDs, it is necessary for the designer to first decide the optimum combination of series, parallel, and series-parallel topology they must use. It is possible to wire up arrays of multiple LEDs as series-only, parallel-only, or as a combination of series and parallel. Each combination brings its own trade-offs for deciding the driver, its cost, reliability, physical layout options, and failure/fault handling. They must also consider the unavoidable thermal considerations and heat dissipation.

In a series-only configuration, a single power rail of the power supply supplies all the LEDs. Therefore, the current flowing through each LED is the same, allowing more or less equal brightness from the LEDs. However, an open-mode failure in any LED serves to shut down the operation for the entire chain. To prevent this from happening, most designers add a small value resistor across each LED, thereby adding to cost and space requirements.

The series-only combination has another issue, that of compliance voltage. As the chain grows longer, so must the voltage go up to deliver the current to the chain, as this voltage is the sum of the drops across all the LEDs. Regulating 20 to 60 mA at a high voltage of 150 VDC is difficult.

A parallel combination of LEDs means the driver must supply only a low compliance voltage, equal to the voltage drop across one LED. However, the power supply now has to supply a fairly large amount of current, equal to the sum total of currents of all the LEDs.

Back-Lighting with LEDs

Liquid crystal displays require back-lighting. This is because liquid crystal displays do not generate any light, and their visibility depends on light passing through the display. Opaque sections in the display become visible when they block light from behind the display. To make the display readable, manufacturers resort to providing them with light from behind the display.

The back-lighting in liquid crystal display panels may come from sources like incandescent, fluorescence, electroluminescence, woven fiber optics, or LEDs. Appearance, cost, and features consideration decide the ultimate choice for the selection of source for the back-lighting. The most popular is solid-state lighting using LEDs, as these devices offer better luminance and power efficiency, as compared to all other sources. Another advantage of LED back-lighting for LCD panels is the long life of LEDs.

Earlier, LED back-lighting typically used direct lighting, with large numbers of LEDs mounted behind the display. This arrangement provided excellent image quality along with the ability of local dimming. However, the high cost of this method did not allow it to gain market share. Rather, back-lighting technology changed over to edge-mounted LED back-lighting. An added advantage of edge-mounted LED back-lighting was that the edge-lit LCD panels could be made in extremely thin designs.

By using edge-lit LED back-lighting, manufacturers could reduce the number of LEDs necessary, by concentrating them along the edges. Initially, manufacturers used LEDs on all four edges. Very soon, they placed the LEDs along two shorter edges only, and eventually, they were placing the LEDs on only a single short edge of the LCD panel.

LEDs are a good choice for back-lighting. They are compact, operate in a wide temperature range, offer a good color selection, have a low operating voltage, and have a long operating life of at least 50 thousand hours. Over a specified range of drive current levels, LEDs offer a predominantly fixed voltage drop. However, LED back-lighting also offers some challenges. For instance, the light provided by the LEDs is uneven, which improves with a suitable light pipe or diffuser. Another challenge is the current through the LEDs depends on the ambient temperature, and requires close monitoring to allow safe operation over a wide range of temperatures.

The driver for such constant-current devices requires building up the drive voltage until it is supplying the desired current level. It reaches a stabilization point when the drive voltage equals the sum of the forward drops of all the LEDs in series. The maximum voltage of the driver limits the number of LEDs in series that it can drive at a time. However, even the simplest of drivers requires holding some voltage in reserve, for dropping across a current limiting resistor. This means a driver will never be able to apply the entire power supply voltage across a chain of LEDs.

The number of LEDs required depends on the size of the LCD panel, and its brightness. High-brightness and ultra-high-brightness LCD panels require a larger number of LEDs. Driving large numbers of LEDs requires sophisticated constant current drivers and high-efficiency power supplies.

Controlling BLDC Motors in Trapezoidal Form

One of the easiest motor control methods for brushless DC motors is the trapezoidal, six-step, or 120° block commutation control. Optimum torque generation requires applying square-wave currents to motor phases in alignment with the trapezoidal back-EMF profile of BLDC motor. MOSFETs of the inverter drive can exhibit only six combinations of on/off states. Therefore, this method has another name—the six-step—resulting in six possible orientations of the stator field within the plane of rotation of the magnetic field of the rotor.

Depending on the desired direction of rotation of the motor, the six possible inverter states must follow a specific sequence. This is necessary so that the orientation of the stator and rotor magnetic fields produces the maximum torque. There are two ways of sensing the rotor position for determining proper commutation timing—sensing through Hall-effect sensors on the motor, or a sensorless way of back-EMF sensing of the rotating motor phases.

Of the two, using sensors requires no voltage or current feedback signals for proper operation. Rather, the position feedback from the Hall sensors is adequate to determine the proper sequence for energizing the motor phases. Hall sensors in strategic positions in the motor can sense rotor position as a result of the rotating magnetic field of the permanent magnets in the rotor. Trapezoidal control using sensors is easier to implement, as it allows for proper commutation even during startups—the information about the rotor position is available even at zero speed.

For trapezoidal control without sensors, the proper motor commutation sequence depends on the back-EMF that the motor’s rotation generates. Such trapezoidal control requires energizing only two motor phases at a time. As the non-energized phases have no current flowing through them, it is possible to sense the back-EMF they are producing during the non-energized times. Typically, such back-EMF positional feedback in BLDC motors is trapezoidal and is either linearly increasing or decreasing. Therefore, most positional feedback techniques using back-EMF use a zero-crossing detection for determining the moment when it crosses a reference point. This can be either half the DC bus voltage or the neutral motor voltage.

Sensorless control has a major drawback. As the magnitude of the back-EMF is proportional to the rotational speed, the rotor must be rotating at a minimum speed to generate a back-EMF of adequate magnitude for sensing the rotor position properly. Therefore, it is necessary to use a startup mechanism for kick-starting the motor until it reaches an adequate rotational speed.

Although it is easier to implement a trapezoidal control with sensors, the Hall sensors add an increased cost. Additionally, signals from Hall sensors may be noisy and may require hardware or software filtering. The motor also requires more wiring, which in some environments, may be a challenge. On the other hand, sensorless control is more complex. It is necessary to tune it to meet specific loads or operating conditions and may face difficulties in starting up under heavy loads. That makes sensorless control well-suited for applications with a well-known load profile that increases with speed, such as for a fan.

What are RTDs?

RTDs or Resistance Temperature Detectors are the simplest way to measure temperature. RTD sensors work on the principle that a metal’s electrical resistance changes with temperature. For instance, the electrical resistance of pure metals typically increases with an increase in temperature, that is, they exhibit a positive temperature coefficient. RTDs operate over a huge temperature range, starting from -200 °C, right up to +850 °C. They offer excellent long-term stability, high accuracy, and repeatability.

An RTD sensor, being a passive device, does not produce a signal by itself. An electronic circuit is necessary to send an excitation current through the sensor. This produces a voltage across the RTD, proportional to the excitation current and the resistance of the RTD. Further electronic circuitry amplifies the voltage across the RTD and delivers it to an analog-to-digital converter, whose output produces a digital output, a representative of the temperature of the RTD.

The electronics in an RTD circuit have some basic trade-offs. For instance, the excitation current must be small enough to prevent self-heating in the RTD element. Any excitation current produces Joule or I2R heating in the RTD. This self-heating effect can raise the sensor’s temperature to a value higher than that of the environment that the RTD is measuring. By keeping the excitation current low, it is possible to keep the self-heating low to a great extent. Moreover, the amount of self-heating also depends on the medium surrounding the RTD sensor, and how effectively it allows heat to accumulate. For instance, placing the RTD element in still air produces a more pronounced self-heating effect than immersing it in moving water.

System noise, offsets, and drift of different system parameters also affect the minimum detectable change in temperature. Therefore, the RTD voltage must be large enough to overcome them. As the excitation current must be low enough to prevent self-heating, it is necessary to use an RTD sensor with sufficiently large resistance, so that it will produce a relatively large voltage. Although it is necessary to use a large RTD resistance to reduce measurement errors, it is not advisable to arbitrarily increase the resistance. This is because a large RTD resistance leads to an increase in the response time.

Theoretically, any metal should work for constructing an RTD. In fact, Siemens used copper wire for constructing the first RTD in 1860. However, he soon discovered that by using platinum, he could produce RTDs that were more accurate over a wider temperature range.

Precision thermometry typically uses platinum RTDs as the temperature sensor. This is because platinum has a linear resistance-temperature relationship, higher repeatability, and a wider temperature range. Moreover, platinum does not react with most of the contaminant gases in the environment. However, the industry also uses two other materials for making RTDs: nickel and copper. Among the three metals, copper offers the highest linearity and the lowest cost. However, as copper has the highest conductivity of the three, it offers a lower resistance. A copper RTD, therefore, produces a relatively lower voltage and can be difficult to use for measuring small temperature changes.

What is Thermorite?

Kemet has developed and patented a type of ferrite material that is sensitive to temperature. They use this material to make various types of thermal sensors. The electronic industry uses thermal sensors and switches for monitoring temperature and maintaining various applications in stable modes of operation. A reed switch inside the thermal sensor makes or breaks the current flow.

Kemet offers three basic types of thermal sensors. The first type is a Kemet sensor using a reed switch inside a thermos-ferrite body. The second type is a thermal sensor using a bi-metal switch. The third type is thermistors.

The Kemet thermos-ferrite and reed switch combination works by sensing the temperature of the surrounding environment. The thermos-ferrite has a curie point and as the temperature crosses this curie point, the magnetic reed switch switches on or off.

The bi-metal switch contains two metal plates that have different thermal expansion coefficients. As the temperature changes, the two metals deform and disconnect at a particular temperature.

Thermistors are semiconductor sensors and may have a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). They change their resistance in accordance with changes in temperature.

Thermorite is a ferromagnetic material with soft magnetic characteristics when under curie temperature. As the temperature goes up, the saturated magnetic flux density of the material decreases, and the material becomes paramagnetic. That is, the material loses its magnetic property rapidly as its temperature reaches the curie point, becoming close to zero. Time does not affect curie temperature as it is a function of the compounding ratio of the material. That means Kemet can make Thermorites with different curie points by changing the material’s permeability.

Kemet offers two types of thermos-ferrite thermal sensors—the Break type and the Make type. The Break type thermal sensors consist of a reed switch surrounded with a Thermorite jacket around the switch part and two permanent magnet cylinders on each end. The Break type switch remains ON when the operating temperature is lower than the trigger temperature. The switch opens as the temperature rises and reaches the trigger temperature or crosses it. When the temperature goes down, the switch closes again only when the temperature goes below the recovery temperature.

When the temperature is below the trigger temperature, the Thermorite jacket is magnetic, and it generates an annular magnetic field. The magnetic field induces the N tip and the S tip of the reed switch to touch due to magnetic attraction. This turns the switch to its ON position. As the temperature rises and reaches the trigger temperature, the Thermorite jacket loses its magnetic flux. This allows the tips of the reed switch to pull apart, and the switch turns OFF.

Construction of the Make type thermal switch is similar to the Break type. The only difference is the former has two Thermorite jackets with a gap in between. The two Thermorite jackets create annular magnetic fields when the temperature is below curie point. This causes the reed switch poles to stay apart, and the switch remains OFF. As the temperature rises to the curie point, the Thermorite jackets lose their magnetic flux. This allows the two reed switch tips to close, and the switch turns ON.

What is the Pyroelectric Effect?

With the electronic industry trending more toward automated devices, their safety and reliability are assuming the utmost importance. Pyroelectric sensors help to make these devices work properly, by indicating changes that require specific types of reactions. Many types of ceramic materials can absorb infrared rays and generate an electrical signal in response.

Certain crystalline materials demonstrate Pyroelectricity. These materials, which are electrically polarized, demonstrate a change in their polarization when they undergo a change in temperature. The change in polarization of the crystal material generates a temporary but detectable voltage across it. Different materials exhibit differences in pyroelectric coefficients that show their sensitivity to temperature.

Infrared radiation heats pyroelectric ceramic crystals to generate a detectable voltage. It is possible to detect the infrared rays the object is generating by using passive infrared sensors. The sensor can detect the wavelengths that the pyroelectric ceramic crystal absorbed when it is in position between the hot object and the sensor. Pyroelectricity has several applications.

Motion Sensors—Typically, there are two types of infrared motion sensors, active and passive. Active infrared sensors have a long range of operation, and the emitter and sensor can be far apart. A garage door safety sensor is a good example of an active sensor. Anything blocking the infrared beam across the opening of the garage door generates a signal to prevent the garage door from moving.

Passive infrared sensors can also detect motion by sensing infrared radiation or heat direct from a source. Such sensors can detect the presence, or absence, of an object emitting heat, such as a human body.

Pyroelectric motion sensors can be surface-mount devices and are highly sensitive. Manufacturers offer them in single-pixel configuration or as a 2×2 pixel configuration, allowing users to determine the direction of the motion it has detected. The sensors have a high dynamic range and a fast response time that ensures rapid and accurate motion detection.

Gas Sensors—Infrared pyroelectric sensors can detect and monitor gases. In fact, this is one of their most popular applications. The sensors operate by directing infrared radiation from an emitter through a sample of the gas. The detector senses if a certain IR wavelength is present on the other side. If the sensor does not detect that wavelength, it means the gas that absorbs this wavelength is present in the sample. Optical IR filters allow fine-tuning the sensor to a specific wavelength, thereby permitting only the desired wavelength to pass through to the sensing element.

Pyroelectric gas sensors are available in small SMD packages and most have a digital I2C output, although analog outputs are also available. The sensor consumes very low power but offers high sensitivity and extremely fast response times.

Food Sensors—Similar to gas sensors, infrared pyroelectric food sensors can detect food-related substances like sugar, lactose, or fat. These are typically general IR spectroscopy sensors for monitoring commercial, medical, or industrial substances or processes.

Flame Sensors—With pyroelectric elements, it is easy to construct sensors for detecting flames. As flames are strong, flame sensors, apart from detecting the presence of the flame, can also discriminate the source of the flame. Typically, they compare three specific IR wavelengths and their interrelated ratios. This allows them to detect flames with a high degree of accuracy.

MEMS Vibration Sensor

Analog Devices Inc. has unveiled their MEMS or Micro-Electro-Mechanical System-based accelerometer technology at the Sensors + Test Conference in Nuremberg, Germany. The MEMS vibration sensor can track vibrations at frequencies of 22 kHz. This is especially helpful for sensing high-frequency vibrations in industrial equipment.

The MEMS technology from Analog Devices is unique in the sense that it uses two MEMS mechanisms placed beside each other. The arrangement helps to cancel out common mode noise, favoring only the differential mode noise. Vibration and shock sensors from ADI are small format sensors that enable equipment designers to build vibration-detection chips within devices for industrial process-control, rather than as add-on modules.

Most vibration sensors today are piezo-electric-based modules. They have two disadvantages—it is not possible to mass-produce them, and their range is limited to 5-kHz frequencies. On the other hand, ADI makes their accelerometers in CMOS processing lines, and they can mass produce them easily. Additionally, ADI can undercut the prices of piezo-based vibration sensors by about 50 percent. For instance, the prices of ADI vibration sensors are around $35 as opposed to piezo-based sensors at $70.

With manufacturers looking for whatever they can get for improving the production and efficiency of their equipment, MEMS vibration sensors from ADI are the right products in the right place and at the right time. Although, when comparing unit shipments, the industrial market is small compared to the consumer electronics market, revenue-wise, the former is incredibly important and offers better margins. MEMS sensors address identified needs within the industrial market sector, and therefore, provide tangible value.

In the case of piezo-based vibration sensor modules, the standard practice is to bolt them onto the side of vibrating industrial equipment. However, using the chip-based accelerometer sensor from ADI is simpler, as it is possible to integrate it right within the circuit board of the device when assembling. ADI is of the opinion that some piezo-based vibration sensor manufacturers may retrofit MEMS chips into their bolt-on modules. However, ADI also expects OEMs of industrial equipment to stop using modules and rather start integrating ADI MEMS chips directly into their pumps, motors, gearboxes, and other pieces of industrial equipment.

Industrial equipment manufacturers are increasingly using vibration sensors as these can sense on-coming failure before it happens. For instance, a deteriorating bearing will vibrate at high frequencies before it fails. As it nears failure, its vibrating frequencies will drop until it finally disintegrates totally, possibly causing damage to the rotor. This is why predictive maintenance is increasingly becoming popular.

The present trend in the industry is to move towards predictive maintenance from preventive maintenance. Early detection through predictive maintenance can cut down repair costs by as much as 25 percent. Waiting to repair equipment until something fails can push up the maintenance costs more than ten times. Compared to other predictive maintenance techniques such as ultrasonic analysis or infrared thermography, vibration analysis offers better return-on-investments, by as much as three times.

ADI offers its vibration and shock sensors in ceramic packages, available in 70g, 250g, and 500g ranges.

Types of EV Connectivity

Technologies related to EVs or electric vehicles are undergoing enormous research and development efforts with the ultimate aim of achieving widespread EV adoption. Although at present, extending the driving range is occupying much of the direction of this effort, future benefits will ultimately extend beyond progressive battery and charging technologies.

For instance, for future EVs, there are exciting value propositions like the number of different connectivity technologies they will be featuring. This is the V2X or vehicle-to-everything connectivity that includes in-use technology like V2G or vehicle-to-grid, V2N or vehicle-to-network connectivity, and the emerging technology like V2V or vehicle-to-vehicle, which engineers expect will change the future working of EVs.

The recent production of EVs includes V2G or vehicle-to-grid connectivity. This refers to the EV’s ability to allow electricity to flow bidirectionally from the vehicle to the grid and back. The concept is that the batteries in the EV, being relatively large, can not only act as energy storage for the vehicle but also as energy storage for the grid and as V2H, energy storage for the home.

V2G, therefore, relies on a power electronics technology, bidirectional charging. Such an EV requires a versatile power conversion and control circuit, allowing conversion between the AC of the grid and the DC of the battery. There are innumerable benefits of V2G for both the vehicle owner and the grid.

The owner can use the EV not only as a vehicle but also as a backup generator for home use in case of a disaster like a blackout. The vehicle owner can offset their cost by selling excess energy in their EV to the grid.

For the infrastructure of the grid, V2G technology can supplement the grid stress when the demand is at its peak. During low demands, or when the energy generation is higher, the grid can recharge the EV.

V2N is another type of EV connectivity, and it refers to the ability of the vehicle to connect to the Internet and communicate with anything else on the network. This mostly refers to the vehicle connecting to the internal network and cloud service of its manufacturer. This allows the manufacturer to closely monitor the vehicle, update it dynamically, and thereby, ensure maximum performance.

Companies use V2N connectivity for extracting information related to performance from their vehicles. They gather metrics such as battery charge cycles, energy throughput, and range. With such feedback information from all vehicles connected to the V2N network, EV manufacturers conduct statistical analysis for understanding the real-time operating conditions of their vehicles and improve their performance. V2N-connected vehicles can also receive necessary updates for their software and firmware for introducing performance improvements.

However, V2V connectivity will bring the biggest impact of all these, although, currently it is far from being a reality. This connectivity is the interconnection of all connected vehicles on the road. V2V allows all vehicles to wirelessly communicate between themselves, information like position, speed, road conditions, and other important driving information. V2V-enabled vehicles can also share real-time road and traffic condition information for achieving the optimal path to their destination.