How much effect will linear power supplies’ EMI have on machine controls?

A machine builder asks about the trade-offs between a switched mode power supply and linear regulated.

By Control Design

We’re trying to save cabinet space by changing from a linear regulated power supply to a switched mode power supply. From what I understand, we’ll get a higher power density with a smaller footprint, while the power factor is about the same. My biggest concern is, with so many devices offering built-in intelligence these days, will a switching power supply create so much electromagnetic interference that it will cause a controls problem on our form, fill and seal machines?

Any mounting recommendations to reduce EMI? What about unregulated? And how close are power supplies to offering their own control capabilities, or is that already available?

 

 

Show Comments
Hide Comments

Join the discussion

We welcome your thoughtful comments.
All comments will display your user name.

Want to participate in the discussion?

Register for free

Log in for complete access.

Comments

  • <p>EMI or Electromagnetic Interference is a very interesting subject, especially when it comes to power supplies. There is no doubt EMI can be a control systems' worst enemy. However, there are many common practices to avoid electrical noise issues within a control cabinet, the most important being proper "earth" grounding of the box. Of course additional LC Com filters or RC snubbers can also be used to reduce EMI. </p> <p>With regards to your specific question on SMPS' (switched mode power supplies) and its impact in a control system with regards to EMI, it is important you select a properly rated SMPS. Such ratings include the NEMA standard ICS-230 for noise immunity. Another rating to consider is IEEE's standard "ANSI C37.90" for surge withstand capability. Many power supplies including the EZAutomation Smart Programmable power supply undergo what is called "Level C Chattering Relay Tests" which basically induce a lot of EMI around the SMPS to ensure that the integrity of the signal is not deteriorated. All of EZAutomation's power supplies when initially designed go through extensive noise testing in a isolated noise chamber. </p> <p>With regards to power supplies offering "their own control capability" can you please elaborate on this statement? We here at EZAutomation have come out with a "Smart Power Supply" unlike anything else on the market that has the following characteristics:</p> <p>a) 3 Digit LED Display for Output Voltage</p> <p>b) 2 Digit LED Display for Output current (no more need for a meter)</p> <p>c) Built-in Maintenance Alert that shows what maximum current has been achieved by the SMPS, and how many hours remaining in the power supply (MTBF) before the electrolytic capacitor goes out. (Preventive maintenance feature to protect entire controls system shutting down due to power supply)</p> <p>d) Programmable Current Limit with Dry-Contact so a signal can be sent to PLC or other device indicating when there is an in-rush of current </p> <p>-Vikram Kumar -EZAutomation</p>

    Reply

  • <p>Switched mode power supplies are tested to many standards, including EMI. The control devices are also tested for EMC (immunity) against external noise usually to the IEC61000-4 standards. Linear power supplies are no longer allowed to be used because of their poor efficiency in many consumer applications (chargers for phones, tablets &amp; computers to name a few). To best manage EMI from a switcher, make sure it is earthed properly and that the grounding wiring is short. If it is a open frame product, always use metal standoffs to ensure connection is made to the noise decoupling capacitors. Keep output wiring bundled together. Any reputable power supply manufacturer will have phone support or application engineers to assist you.</p>

    Reply

  • <p>If radiated emissions are a concern in regards to a primary switched mode regulated power supply, the compliance to “Emitted interference" in accordance to 61000-6-3 is normally published and should be confirmed. This testing is required to make a CE declaration. CE is a requirement in order for a device to be sold on the European market and many other countries around the world that adopt European standards.</p> <p>Radiated noise can certainly be an application problem however if the concern is based on interrupting the normal operation of sensitive, intelligent device the DC output ripple voltage should additionally be considered. Extremely low ripple voltage is normally a benefit for regulated linear power supply. If there is a specific device that is of a major concern the sensitivity to DC voltage fluctuation should be noted (power supply connection voltage tolerance). An example of a concern might be an analog device. </p> <p>For your consideration, to change to a switch mode power supply you can easily find a switch mode power supply specified with a ripple voltage under 50 mV. It’s been my experience that at this ripple level, even analog applications are reliable. In my opinion, it would be risky to consider an unregulated solution, as the ripple can be measured in volts and not millivolts.</p> <p>To your interest in a power supply with control capabilities: I'm not aware of a standard industrial power supply with configurable internal logic with I/O; however the feature/benefit set is rapidly expanding. Power supplies can now monitor current and provide a status output to a control system (via a solid-state output or relay contact) as an early warning indication that the power supply is starting to draw current close to the rated DC output value. This indication signals the need for maintenance to be conveniently scheduled while the application is still up and running. With this indication, the control system can send a text, set an output for online monitoring solution, or set a more basic alarm to be seen on a HMI. A reason for the additional current draw could be an aging device. Know that if the current draw continues to increase, the output voltage will start to load down as the DC power limit is reached. To this situation, a power supply can also monitor voltage. At 10% down from the adjusted DC voltage, a second and final warning is sent to the control system. If this alarm is ignored, any further voltage drop could potentially bring down the system</p> <p>Lastly, I agree that grounding key for both noise immunity and radiated emissions.</p> <p>Mike Garrick, Power Products, Marketing Manager, Phoenix Contact USA </p>

    Reply

  • <p>SMPSs are all tested and regulated to the according standards to prevent EMI. Of course proper installation techniques such as a consistent "Earth" ground and proper cable separation and runs will also help to reduce noise in the cabinet.</p> <p>As far as SMPSs that offer "control capabilities," there are a few products on the market such as the WAGO PRO Series SMPSs that provide interactive display capabilities and "smart" programming options. To my knowledge no product with internal programmable logic is available on the market yet.</p>

    Reply

  • <p>Several commenters have posted regarding how to address EMI concerns when using a switch mode power supply. I'd like to approach the question from a different perspective. Is a high frequency switching power supply the right solution? When selecting a power supply, consideration needs to be given to the required power level, the harshness of the environment, the sensitivity of the load to variations in output voltage, the required output voltages, and the annual volume of the equipment.</p> <p>In terms of simplicity and reliability, an unregulated linear power supply is difficult to beat and may be the best option in an industrial setting. Low component count assures a high degree of reliability, and if the load is relatively constant, or can accept a range of voltages a regulated supply may not be needed. If power levels are relatively low, a regulated linear power supply remains a viable option although heat dissipation and efficiency become concerns as the power levels rise. </p> <p>Another option that is frequently overlooked is a ferroresonant power supply. In power ranges of hundreds to thousands of watts, this time tested design provides extreme reliability, protection from transient voltages, regulation against variations in line voltage, and can be designed to provide any output voltage. </p> <p>There is no simple answer to the question. While a high frequency switch mode power supply does offer the benefit of reduced size and higher efficiency, in most cases, there are tradeoffs. Higher complexity, EMI/RFI issues, cost (for non-standard voltages), leakage current, susceptibility to damage from transient voltages, protection from dirt, dust, etc. Each type of power supply has its advantages and disadvantages and needs to be judged in context with the application.</p>

    Reply

  • <p>With power supplies being the heart and soul of most modern control panels, there are reasons to be concerned with EMI and their effects on the control system. Since the power supply is producing the control voltage, they are often blamed when things don’t always go right with the connected loads. Sometimes it is other contributing factors in the application like VFDs, other motor types or electronics mounted in the vicinity. There are two types of Electromagnetic Interference emissions, Conductive and Radiated. Conductive emissions are the EMI that travels via the wires and Radiated emissions are the EMI that travels through free air. A datasheet or a CE declaration for a power supply will detail the compliance standards for “Generic emissions and immunity for industrial environments”. There are ways to minimize EMI through such methods as shielded cable, twisted pair cables and grounding. Depending on the application, shielded or twisted pair cables can become very expensive so grounding is the best option for most installations. Many current switch mode power supply designs offer galvanic isolation, where there is no electrical association between input and output. This method leaves the output floating with no ohmic connection to ground, allowing for any electrical noise in the cables to bounce back and forth between the power supply and the load. By tying the output negative terminal to ground, the noise has a path to bleed off. The output ground should be tied to the exact same “earth” ground as the AC side to prevent ground loops from occurring. Grounding provides the most practical protection from EMI.</p> <p>A quality switch mode supply will offer the best all-around benefits such as efficiency, size, overload capabilities, operating current, heat loss, mounting, weight, modest ripple &amp; noise, etc. as compared to a linear or unregulated supply.</p> <p>With regards to control capabilities, this would be more in line with a programmable lab style power supply where voltage and current can be controlled by external means. These features are not traditionally offered in an industrial grade switch mode DIN-rail supply. However, specific manufacturers offer power supplies that are microprocessor controlled which are factory programmed during the design process to handle multiple complex functions including output control. With microprocessors, the power supply can become smaller, more efficient and more reliable due to fewer components. This method also allows design improvements without a hardware change and specific unique design techniques can be protected.</p> <p>Ed Merkle - Engineering Manager - PULS, L.P. – www.pulspower.us </p>

    Reply

  • <p>Thirty years ago, linear power supplies (LPS) were the norm, and switch mode power supplies (SMPS) were just starting to become more available. Early SMPS were highly unreliable and generated much more electrical noise (radiated and conducted) than an equivalent LPS. Today, SMPS dominate the market and perform excellently, in all but the most sensitive applications. Regulatory requirements have tightened over the years, more performance specs have been added, and diverse industries globally have adopted the technology. And overall, this has lessened issues with EMI. The higher power density allows the SMPS to be significantly smaller than an equivalent LPS and provide an incredible increase in efficiency, which minimizes the heat generated by the power supply. The top industrial SMPS are fully regulated, with wide-ranging input voltages and controls, such as active power factor correction, over-voltage protection and output short circuit protection. They are also still able to start difficult loads, such as motors and contactors with proprietary power-boosting technologies. </p> <p>Response provided by James Furukawa, engineer, Rockwell Automation </p>

    Reply

  • <p>Not necessarily. There are many switched mode power supplies (SMPS) on the market today that have implemented circuitry internal to the devices that limit EMI and problems associated with EMI generated by SMPSs. Most devices sold in the US have to comply with FCC limits for EMI. However, the FCC does make an exception for industrial power supplies. Therefore, as a consumer, your best bet to ensure low EMI from your power supply is to find one with a CE mark. In order for a manufacturer to put the CE mark on their power supply, they need to test to EMI limits set by an IEC specification. </p> <p>It is also noteworthy that 61000-6-3, 55022 Class B, and CISPR22 Class B are all specs for EMI emissions. These 3 specifications are basically harmonized and are considered ok for residential use. (There are a few minor spots where the specs differ, but they are basically the same.) 61000-6-4, 55022 Class A, and CISPR22 Class A are also harmonized. These are the higher EMI limits for industrial environments.</p> <p>Randy Durick, Vice President, Network &amp; Interface Division, TURCK</p>

    Reply

  • <p>In General, the most EMC producing devices are drives and e-motors. EMC is divided into three main parts; 1: EMC in general, 2: EMC as Emission, 3: EMC as Immunity. From the approval side, just a few chapters of the IEC specification are really related to Switch Mode Power Supplies. From the approval ratings for switch mode power supplies there is the Class B which is related to emission and the effect on other devices. And there is EMI class A immunity which applies to the effects from other devices to the PS. Class A is always worse than B. So, an industrial environment has a higher EMC pollution than a household. The Emission class of a power supply should be a “B” rating, except for medical applications, then it should be a C rating. On the other hand, the PS should be able to handle the worse industrial environment of Class A without any failures. If you have a real fear of EMC pollution, analyze the whole application and the approval ratings of all components in the cabinet. Regarding design and the desire to eliminate EMC emissions you have to rely on the appropriate standards and design criteria of the device and the norms that are applied to the products. Murrelektronik builds to very stringent design criteria with CE rated products and with a desire to not affect or be affected by cabinet devices when possible. Furthermore, watch the use of grounding and bonding. Bonding the cabinet together with the installed devices and applying proper grounding principles (by DIN-Rail or common ground) you can set the basics for reducing the EMC pollution. When it comes to the topic of diagnostics: Does it make sense to have a LED display on the Power Supply? Is the customer willing or the OEM desiring the customer to open the cabinet to view status of the power supply? At Murrelektronik, we believe in diagnostics sent through a PLC or HMI are better and offer products with alarm contacts, feedback, and other measures to allow the user to better troubleshoot and diagnose problems both inside and outside the panel reducing their exposure to cabinet devices and related voltage concerns. </p> <p>Aaron Henry, Marketing Manager, Murrelektronik, Inc. Michael Boehmerle, Product Manager </p>

    Reply

  • <p>Posted on behalf of Tom Edwards, Opto 22</p> <p>Modern switching power supplies implement effective noise filtering that minimizes EFI, reducing or eliminating EFI concerns for most applications. Compared with a similarly rated linear power supply, a switching power supply has the smaller physical size the reader mentions, plus advantages of lower cost, lower weight and greater efficiency (generating less heat). Getting FCC- or CE-certified switching power supplies from reputable manufacturers virtually eliminates reliability concerns.</p> <p>Tom Edwards, senior engineer, Opto 22</p>

    Reply

RSS feed for comments on this page | RSS feed for all comments