The design of PLC DCS analog input module breaks the barriers of channel isolation and high density

In high-end factory automation applications such as natural gas and oil plants, as well as power plants, the requirements of low EMI, small size, high reliability, and low cost are particularly challenging for inter channel isolation design. Therefore, the channel density implemented by standard modules is usually limited to only four or eight channels, and the isolation between channels is only a few hundred volts.

 

This article will briefly discuss isolation in process control analog input modules and traditional methods for achieving this goal. Then an alternative high-density, easy to design inter channel isolated analog input module architecture was outlined. The test results show that the 16 channel, 2.5 kV rms inter channel isolation demonstration module easily passed the EN55022 Class B isolation standard.

 

Isolation in Process Control Analog Input Module

Current isolation is the principle of separating physical and electrical circuits, so there is no direct conduction path, but data and power can still be exchanged. This is usually achieved using transformers, optocouplers, or capacitors. Isolation is used to protect circuits and the human body, disconnect the grounding loop, and improve common mode voltage and noise suppression performance.

 

Usually, process control inputs are either group isolated or inter channel isolated (see Figure 1). For group isolation, multiple input channels are combined to share a single isolation barrier, including power isolation and signal isolation. This saves costs compared to inter channel isolation, but it limits the common mode voltage difference between channels in the group, which means they should all be placed in the same area. As shown on the right side of Figure 1, inter channel isolation is always beneficial for improving its robustness. That is to say, the cost of each channel is much higher, so factory manufacturers must carefully evaluate this trade-off.

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Figure 1. Group isolation and inter channel isolation.

Adopting inter channel isolation design, each channel requires dedicated power isolation and signal isolation. Isolation is one of the main limitations on input module channel density, EMI, cost, and reliability. In modern design, each channel uses a digital isolator for data isolation. Typical digital isolators such as ADuM141E have four isolated data channels in a 16 pin SOIC (6.2 mm x 10 mm) package. However, each channel still requires power isolation, so let’s discuss three traditional power isolation methods: multi tap transformers, push-pull designs, and isolated DC-DC modules.

 

Figure 2 shows the flyback isolated DC-DC architecture using a multi tap transformer. The flyback converter drives the transformer to produce multiple outputs on the tap. It is a mature power architecture, but there are six main drawbacks for process control applications, which are:

It requires a custom transformer with multiple taps and shielding to control EMI. This is difficult to achieve in a small size with sufficient reliability.

Only one channel can be used for feedback control loop, which means that the adjustment of other channels is looser. This requires careful evaluation to ensure reliable operation.

The channel density is limited by the placement of specific transformers. For the power output from each tap, the transformer is placed as the center of the analog input module, and each input channel is arranged in the fanout sector around the transformer, limiting the analog input module card channels to four or eight.

Interference from one channel can be coupled to other channels through the coupling capacitance between transformer taps.

Isolation voltage level. Unless special insulation materials or designs are used, Multitap transformers can only achieve inter channel isolation of hundreds of volts, which greatly increases the cost of transformers.

The high cost of obtaining UL/CSA certification for customized transformers.

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Figure 2. Power isolation design for multi tap transformers.

Another method is to use a separate transformer for each channel and use a push-pull method to isolate each channel. In this method, feedback is not used. Instead, use a well regulated power supply (such as 7 V) to drive each transformer, and then use LDO to further regulate on the secondary side. This method is feasible because the current consumption on the secondary side is relatively low, which makes appropriate adjustment possible.

Some drawbacks of this method are the need for pre adjustment and additional components for each channel. The selected transformer must meet the required isolation rating. Pre voltage stabilization, as well as transformers, switches, and LDOs for each channel, occupy circuit board space and increase costs. A significant amount of evaluation work is needed to ensure that the regulation is sufficient under all conditions.

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Figure 3. Push-pull isolation design.

The use of UL/CSA certified surface mount isolated DC-DC modules makes isolated power supply design easier, increases channel density, and increases isolation voltage to several thousand volts. That is to say, the cost is relatively high and can usually only be achieved through EN55022 Class A. These modules may also have conducted electromagnetic interference issues, as most modules have PWM frequencies below 1 MHz to minimize electromagnetic radiation interference. In addition, most process control analog input modules consume less than 10 mA of current, which is much lower than most isolated power supply modules on the market.

All three traditional methods discussed are difficult to meet the required isolation performance and cost. These methods still require separate data isolators for each channel, which increases additional space and cost. What if power isolation can be used as part of a data isolator? It can, and indeed it is.

ADI i coupler technology and iso power supply technology are widely used in the industrial and automotive markets, and these two technologies can be integrated into a single package. Taking ADuM5411 as an example, according to the block diagram shown in Figure 4, the device adopts a 7.8mm x 8.2mm, 24 pin TSSOP package, including complete power isolation and 150 channel data isolation. It provides an output of up to 2500 mW, sufficient for analog input signal conditioning and digitization, and meets the 1577 V rms UL75 isolation standard. In addition, CMTI (Common Mode Transient Immunity) is greater than<>kV/ μ s. Very suitable for harsh industrial environments with high transient voltage and current, such as power plants. ®®

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Figure 4. Principle Block Diagram of ADuM5411

Due to the high integration of data and power isolation, the design of analog input modules is greatly simplified and can achieve higher channel density. It allows for the use of older isolation methods to provide<>channels or more within the same space as 16 channels.

A 16 channel, inter channel isolated temperature input module was designed and tested using this isolation method (see Figure 5). The ADuM5411 device in the module provides isolated power and data for each of the 16 temperature input channels. Thermocouple and/or RTD measurements are performed by highly integrated temperature front-end ICs (AD7124 or AD7792), which can save more space compared to split design. ADP2441 converts the 24V backplane power supply to 3.3V, providing power to the MCU, touch screen, and ADuM5411. Each input channel only requires an area of 63.5 mm x 17.9 mm.

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Figure 5.16 Block diagram of isolation input module between channel temperature channels

Layout design of ADuM5411

The switching frequency of ADuM5411 is 125 MHz. Due to the large number of channels, special attention should be paid to ensuring that the circuit board passes the EN55022 Class B electromagnetic radiation interference test.

The principle used to minimize radiation emissions is to minimize power consumption and minimize the current loop return path. By using low-power integrated temperature front-end ICs, power consumption is minimized. This means that less power is consumed through isolation barriers, which means less energy is radiated. When AD7124 is fully effective, only 0.9 mA of current is consumed. To minimize the current return loop, two ferrite magnetic beads and a small amount of bypass capacitors were used.

Ferrite magnetic beads are a useful method for controlling radiation signal sources by providing much higher impedance than PCB wiring. Refer to Figure 6, the ferrite magnetic beads are placed in series with the pins of ADuM5411. The frequency response of ferrite magnetic beads is a very important consideration factor. The ferrite magnetic beads used are BLM15HD182SN1, providing resistance greater than 1 k Ω in the frequency range of 2 MHz and 100 GHz. Ferrite magnetic beads should be placed as close as possible to the solder pads of ADuM5411. The E9 on the VISO path and the E10 on the GNDISO path are the most critical ferrite magnetic beads.

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Figure 6. Schematic diagram of ADuM5411

Capacitors can also be used to provide a low impedance return path, thereby reducing radiation. One method is to use surface mounted safety rated capacitors to pass through potential barriers, ensuring compliance with creepage distance, electrical clearance, and withstand voltage standards. These capacitors can be obtained from suppliers such as Murata Manufacturing or Vishay. However, due to the introduction of inductance when installing capacitors, this method is only effective up to around 200 MHz. Therefore, a more effective technique is to construct bypass capacitors inside the PCB under ADuM5411. This can be a floating bypass capacitor or an overlapping bypass capacitor, as shown in Figure 7.

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Figure 7. Floating bypass capacitors and overlapping bypass capacitors.

For floating bypass capacitors, two series capacitors C1 and C2 are built-in. The total capacitance is calculated by formula 1.

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Among them:
ε It is the dielectric constant of PCB insulation material, and the dielectric constant of FR4 material is 5.4

For overlapping bypass capacitors, the capacitance is calculated by formula 2.

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Among them:
ε It is the dielectric constant of PCB insulation material, 4 × 10-11 F/m for FR4 material

Under the same material, area, and distance, the total capacitance value of floating splicing is half of that of overlapping splicing, but the thickness of the insulation material has doubled. According to IEC60950 2.10.6.4, the minimum insulation material thickness for the inner layer of reinforced insulation is 0.4 millimeters (15.74 mils), but there is no such requirement for basic insulation. Due to the fact that ADuM5411 only provides basic isolation at 2.5 kV rms, overlapping bypass capacitors were chosen to maximize capacitance. For the same reason, the thickness of the inner layer is also controlled at 5 mils.

The 16 channel and inter channel temperature input module PCB uses a 6-layer board. To maintain mechanical and EMI performance, the top and bottom layers are controlled at 20 mils, and the inner layer is controlled at 5 mils, as shown in Figure 8.

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Figure 8. Six layer PCB stacking allocation.

As shown in Figure 9, the plane of overlapping bypass capacitors is embedded in GND1, SIG, PWR, and GND2. The planes on GND1 and PWR are connected to the secondary side of ADuM5411, while the planes on SIG and GND2 are connected to the primary side of ADuM5411. This means that three parallel bypass capacitors are formed between GND1 and SIG, SIG and PWR, PWR and GND2. The width of the overlapping area is 4.5 mm and the length is 17 mm, which means the total bypass capacitance is 72 pF.

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Figure 9. Six layer PCB layout in the ADuM5411 area.

Test results in compliance with EN55022 specification

According to EN10 specifications, two sets of EMI tests were conducted at 55022 meters. In the first test, a circuit board with bypass capacitance was used, as shown in Figure 10. Figure 11 shows the results, which meet the EN55022 Class B standard with a margin of approximately 11.59 dB. In the second test, a circuit board without bypass capacitors was used, but an external safety capacitor KEMET C1812C102KHRACTU 3 kV, 150 pF was installed on the circuit board. Figure 12 shows the result – it passed the EN55022 Class B standard with a margin of 0.82 dB.

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Figure 10. The bypass capacitor embedded in the PCB does not require a safety capacitor.

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Figure 11. The built-in splicing capacitor in the PCB EN55022 Class B test results.

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Figure 12. There is no bypass capacitor, but the safety capacitor PCB and EN55022 Class B test results are used.

The results indicate that stitched capacitors under IC are a more effective decoupling method than safety capacitors.

 

conclusion

Inter channel isolation is often seen as a design challenge for high-end process control systems. Compared with traditional digital and power isolation methods, ADI’s iso power technology and i-coupler technology can significantly improve channel density. They also greatly simplify design tasks and can improve the robustness and reliability of channels. By using bypass capacitors built-in in the PCB or safety capacitors installed next to the PCB, EMI radiation can be easily controlled to pass through EN55022 Class B or A. This is a technological breakthrough.