Simon Duggleby, Mouser Electronics
Factories in particular and industry in general are changing beyond recognition. Filled with sensing devices and driven by the Industrial Internet of Things (IIoT) and programs such as Industry 4.0, modern industrial organizations are relying more and more on the data available to them. The primary driver for this shift is operational effectiveness, but there are many other benefits that can be derived from the available data.
For example, maintenance, repair and operations (MRO) are able to be more proactive. By placing sensors on machines, such as a vibration sensor close to an important bearing, they are able to monitor wear in real time. This enables them to obtain parts and schedule maintenance before an expensive breakdown, which saves costs and, ultimately, increases efficiency too.
Making the shift to being a data-led organization requires a lot of hardware, encompassing sensors, communication gateways and programmable controllers. This all carries a significant cost, and, equally, requires a lot of space – which is generally at a premium in modern densely packed factories.
Normally, there is a single control cabinet for this equipment. In larger factories, there may be several cabinets but, nevertheless, a lot of electronic equipment must be crammed into a small space. Achieving this depends on a combination of designers using as few components as possible, selecting the smallest components available and designing efficiently to remove the need for thermal management measures.
The Benefits of Electrical Isolation
The industrial environment brings additional challenges, including the presence of large amounts of electrical noise from the motors and actuators that create high voltage transients. As automation increases, so these devices become more prevalent and designers have to address associated challenges.
Sensors use analog signals extensively and these signals are particularly susceptible to the transients and spikes present in the industrial environment. Most applications perform an analog-to-digital conversion as soon as possible in the signal chain but, even with this approach, susceptibility to transients remains a challenge. The effects can be mitigated, to an extent, by adding galvanic isolation to the signal path. This technique will also provide protection for peripheral components and, in some cases, it is needed to meet standards for electrical safety and similar.
Placing isolation between a sensor and the analog-to-digital converter (ADC) as well as between the ADC and microcontroller is considered good design practice. This is especially true in applications such as the monitoring of motors, where line voltages may be present. Here, isolation will ensure that dangerous voltage spikes and/or leakage currents that can be found at the sensor or ADC are not able to get as far as the microcontroller.
As well as isolating the signal path, the power will require isolation as well, especially the supply for the ADC. This can be achieved with isolated DC-DC converters that are available as ICs for discrete solutions or as complete modules.
Figure 1: Functional block diagram of a condition-monitoring analog front end attached to a host microcontroller. (Source: Mouser)
Reducing Size and Cost
Integration of multiple functions into one device/package is one way of reducing size and potentially reducing cost as well. For example, the power conversion/management, power isolation and digital signal isolation can be aggregated into a single device such as the ADP1031 from Analog Devices, which brings all of these functions together into a single compact LFCSP 7mm x 9mm package that can operate across a wide supply voltage range, from 4.5V to 60V.
The single-chip solution includes three micropower DC-DC converters – an isolated flyback converter, an inverting regulator and a buck converter within its micropower management unit (PMU). Input-to-output isolation across all three power domains is 300V. The ADP1031 can deliver up to 2.0W of power at efficiency levels as high as 90%. The device also includes four high-speed isolation channels that can transmit data bidirectionally via a serial peripheral interface (SPI), and three general-purpose input/output channels (GPIOs) that use Analog Devices’ patented iCoupler technology to provide the isolation.
Figure 2: Key features of the Analog Devices ADP1031 isolated PMU with external components including the flyback converter transformer. (Source: Analog Devices)
Included within the PMU are the ability to power up with a soft-start as well as protections for input over-current and output over-voltage. Various options within the flyback and inverting converters are also programmable by the user. The ADP1031 provides for control of the converter slew rate to minimize EMI to allow for conformance with CISPR11 (EN 55011) Class B radiated emission levels.
The MOSFET for the flyback converter is included within the ADP1031 package, thereby reducing the external component needs to a simple 1:1 isolation transformer and a few passives.
The ADP1031 is intended to be used in conjunction with the Analog Devices AD5758, a single-channel 16-bit resolution digital-to-analog converter (DAC) – see Figure 3. When used as a pair, the ADP1031 and the AD5758 reduce design time and complexity as they meet basic 300V isolation requirements. This provides a low-risk route for engineers to take to achieving necessary safety approvals.
Figure 3: Analog output application powering an Analog Devices AD5758 DAC. (Source: Analog Devices)
Selecting Suitable Magnetic Components
One of the key decisions when configuring the flyback converter is the choice of the external transformer. The transformer ratio will be defined by the input and output voltages needed within the design. As the feedback scheme used does not require a feedback winding, the complexity (and, therefore, cost) of the transformer is reduced. It also means that the DC resistance (DCR) of the windings is lower, and leakage inductance is also improved.
From a performance perspective, selecting an optimum transformer with low leakage inductance and DCR will improve the overall efficiency as well as EMI performance. Perhaps the greatest benefit for the designer is the ability to select a simple off-the-shelf transformer.
Transformer specialist Coilcraft has worked with the engineering team at Analog Devices to develop two transformers that are specifically for use with the Flyback converter in the ADP1031. The Coilcraft WA8478 and the Coilcraft YA9293 both offer a 1:1 ratio and are able to accommodate an input voltage in the range 4.5V to 60V. They provide 2250Vrms of isolation and are configured to provide the necessary creepage and clearance for basic insulation. The transformers are qualified to AEC-Q200 Grade 1, confirming their suitability for use in rugged automotive applications where the ambient temperature can be between -40°C and +125°C. The WA8478 has nominal leakage inductance of 1.2µH and the YA9293 has 1.62µH.
Coilcraft has also developed the PA6594-AE, a 47µH inductor that is specifically designed for use with the integrated buck converter in Analog Devices’ AD5758 16-bit DAC IC. This inductor has a compact footprint and is only 1.8mm in height, making it easy to design into compact modern applications.
To meet the challenges of modern ultra-compact designs, it is common for designers to select ICs that integrate multiple functions within a single low-footprint package. This, with the addition of just a few external passive components, can dramatically reduce the PCB real estate needed for designs.
The Analog Devices ADP1031 is a good example of this, as it combines power management and isolated serial communications in a package measuring just 7mm x 9mm. Matching the IC with the dedicated Coilcraft magnetics ensures a small, high-performance design is easily realized.