Getting the Best Performance from AC to DC Power Supplies in Abusive Environments
This article describes some common pitfalls of AC to DC Power Supplies and provides insight how to get maximum performance for specific applications.
AC to DC power supplies can be found in applications where power needs to be processed from the ac power mains to loads requiring a fixed or variable dc voltage or current. While such equipment has few input and output connections, engineers frequently struggle to obtain reliable performance in their particular environments. Troublesome issues can range from the quality of the ac source, cooling constraints, control wiring, air quality, or user understanding of the power conversion product. This article describes some common pitfalls and provides insight how to get maximum performance for specific applications.
POWER LINE QUALITY
Connecting an AC to DC power supply to the power mains is oddly a common source of problems. Worldwide mains voltages vary in different parts of the word ranging from 200 Vac in Japan to 690 Vac in Europe. Line frequency also varies between 50 and 60 Hz, but with today’s switching power supplies, frequency generally has little effect on performance.
Every year Magna-Power Electronics receives support calls that a customer’s power supply has failed because of connection to the wrong AC mains voltage. Reading the specification label on the rear cover of the power supply and measuring the applied voltage can prevent catastrophic and costly failures.
Power quality, or the purity of voltage applied to the power supply, can be the source of some surprising behavior. Power distribution systems, with associated transformers and distribution impedances, can produce voltage drops or surges with other loads on the power network; these loads can circulate harmonic currents and exciting resonances between inductive and capacitive components. Industrial power supplies with 6-pulse waveforms have strong 5th and 7th harmonic components. Renewable energy sources with their associated power conversion equipment can also affect the voltage applied to a power supply.
Harmonics, as described above, and voltage transients on the ac power mains can damage the front end of the power conversion circuitry. Voltage transients can be suppressed with varistors or other voltage clamping devices, but these devices also have their limitations; they can only absorb limited amounts of the energy. Power line harmonics can be more destructive because these voltage excursions occur for longer time periods. To get past these types of problems, Magna-Power Electronics use front end components rated at 1600V. This voltage rating is sufficient to get past most of the power line conditions except for lightning strikes.
Phase rotation is the line voltage phase relationship of a three-phase power source. While there are standards, phase relationships in industrial facilities can vary. With incorrect phasing, motors can run backwards and power supplies using SCR’s can misfire. Modern SCR power processing equipment circumvents SCR firing circuit issues by sensing and correcting for phase rotation variations.
Grounding issues are frequently encountered in industrial installations. Proper grounding is poorly understood by many electrical contractors and in many cases, noncontiguous ground connections can frequently be found. The primary purpose for power supply grounding is for safety and for EMI suppression. Grounding places the protective enclosure at a safe, or near zero voltage differential from any surrounding equipment. Internal to the power supply, a ground connection is used with EMI filters to steer high-frequency components of current away from the input and output connections and stay within the confines of the power supply enclosure.
By electrical code and from a safety viewpoint, there should only be one connection to earth ground; the ground connection should be made at the electrical entrance of the building, the location of the metering equipment. It is at this point where ground and neutral are connected together and a ground rod is driven into the earth. If the facility’s equipment is properly wired, there should only be a small current flowing in the ground path. In the event of a lightning strike, the entire facility rises to the same voltage potential thereby protecting objects or personnel from dangerous voltage differentials.
Unfortunately, not all power systems are wired to code and a common problem is that grounds used for computers and instrumentation equipment are not at the same voltage potential as the power equipment. While Magna-Power Electronics’ power supplies attempt to adjust for such conditions, sometimes a poor ground connection between user and power equipment can cause strange power supply behavior. The most common problem is loss of communication between the power supply and computer equipment. In most cases, bonding grounds between user interface equipment and the power supply corrects this problem.
Some applications require connection to external monitoring or control circuitry. Many, if not most, power supplies have error and feedback circuitry referenced to the output terminals. Without suitable isolation, like optical isolators, ground loops can develop if external circuitry and the power supply load are grounded. Control errors can result if the external circuitry is grounded and the power supply load is left floating. In this case, conducted EMI is directed to the grounding leads of the external circuitry.
Magna-Power Electronics has circumvented many grounding issues by placing all of its control at near ground potential. Ground reference is established through a connection of a resistor and parallel connected capacitor. These components allow the power supply and external connected circuitry to be protected against poor grounding environments yet provide a suitable impedance for EMI suppression.
Even with a power system properly grounded, problems can develop from an EMI producing source creating a voltage potential in the grounding circuit. The impedance of the grounding circuit increases with frequency and the EMI source, depending on its location in the power system, can introduce voltages between the external monitoring and control circuitry. Like poor grounding conditions, bonding the external equipment to the power supply mitigates such electrical noise issues.
Power supplies contain heat producing components: transformers, inductors, power semiconductors, and the like. No matter how efficient, all of these components require cooling. Smaller power supplies sometimes rely on natural convection, but larger equipment requires forced air or water cooling. Water-cooled units are ideally suited for applications with poor air quality or for higher density rack mount installations that cannot meet airflow requirements. User introduced cooling issues is the dominate cause for field failure returns at Magna-Power Electronics.
For power supplies requiring forced air cooling, thermal issues can result from blocking ventilation openings, poor air quality, and air restriction in cabinet enclosures. Blocking ventilation openings can obviously cause equipment failure. Placing thermal sensors on critical components can help detect this abuse, but there is a limit what is practically possible. Avoiding blockages of enclosure ventilation ensures life of the equipment as anticipated by the manufacturer.
Placing a power supply in an equipment enclosure can also lead to thermal problems. Air flow internal to the power supply requires the same air flow inside the enclosure. Self heating of equipment enclosures is a common problem. Poor location of intake and exhaust vents can cause warm air to be reheated and never be exhausted to the exterior. A conservative approach to equipment enclosure cooling is to place intake vents at the bottom of the enclosure and place fans, rated at the same cubic feet per minute, at the top of the enclosure. To minimize fan pressure and air restriction, the vent openings at the bottom of the enclosure should equal the vent openings at the top.
An environment with poor air quality usually finds it way to the interior of the power supply enclosure. Printed circuit boards are designed to support voltages sometimes in the order of several thousand volts. Layers of dust, paint, and other particulates can cause electrical breakdown. Placing air filters within the enclosure to purify incoming air can minimize this problem, but improper cleaning of these filters presents another. There is virtually no good tradeoff between poor air quality and filtration issues. With extremely poor environmental conditions, sealing the power supply and utilizing water cooling is the best alternative for heat management and obtaining reliable operation.
Water cooling in abusive environments can solve many application problems. MagnaPower Electronics uses thermal sensors to control water flow to prevent condensation in heat sink assemblies. Following manufacturer specifications for water temperature, flow rate, and pressure are critical to making water-cooled equipment operate correctly. Exiting heated water can be cooled with heat exchangers, water-to-air or water-to-water, in a closed loop system or disposed in an open loop system.
Control and Monitoring Connections
Many applications require external equipment for monitoring and control of power supply parameters. Besides making sure that electrical connections do not exceed manufacturers’ ratings, placement of cables can be critical. Voltages and currents, present at the input and output terminals of ac to dc power supplies, contain higher frequency components in the form of transients, EMI, and harmonics. Placing control and monitoring cables parallel with power carrying cables can produce unpredictable results. It is recommended that any control or monitoring cables be routed separately, in its own metal conduit, if possible.
Remote Sense Connections
Regulation of output voltage or current is dependent on sampling of the desired output parameter and adjusting it to a comparative reference. Both reference and output sampling parameters can be external to the power supply. Remote sensing of output voltage is commonly deployed to minimize voltage drop in the leads connected to the load. Properly used, remote sensing provides superior regulation at the point of load.
Switching remote sense connections or configuring the power supply for remote sensing and not connecting the remote sense leads is a common, but wrongly applied, configuration. A power supply operated without sampling an output parameter can either damage output components in the power supply or damage the load. Without an output parameter to control, feedback circuitry drives output voltage or current to its maximum. The maximum, non-regulated output can exceed the safe output rating of power supply components.
A common method to address this potential problem is to add resistors between the output terminals and remote sense terminals. Configuring a power supply for remote sensing and removing remote sense leads causes the output voltage to rise slightly above nominal conditions. The deviation above nominal conditions is a function of local sense resistors internal to the power supply.
Complications of remote sensing can arise when remote sense and power leads are switched. Figure 1 shows a common and wrongly configured system application; output terminals are defined as VO+ and VO and voltage sense terminals are defined as VS+ and VS-. This configuration is deployed to switch power and remote sense leads to different loads using the same power supply. Electronic feedback circuitry is usually faster than the switching of mechanical relays and contactors and during the switching instant, the power supply is operated without sensing the output. Another issue with this configuration is operating the power supply with only the sense circuitry connected, relay K2 on and relay K1 off. This will virtually short the sense lead connections through the load. This causes protection resistors, R1 and R2, to be placed in series with the load when the power supply is operating at maximum.
Figure 1 Remote sense protection with internal resistors
Magna-Power Electronics uses alternate approach for remote sense protection, but it too has some drawbacks. As shown in figure 2, the remote sense voltage, VSX+ minus VSX- , is tested at the beginning of the power-on cycle through electronic switching internal to the power supply. The power supply uses local sense during the beginning of the power-on cycle. It is then quickly switched, faster than the response of the feedback system, to the remote sense terminals to determine if the remote sense leads are connected to the load. If there is voltage present, the power supply remains in the remote sense configuration, if not, the local sense connection is reestablished. The scheme works well except for a user switching or removing remote sense connections after the power-on cycle.
Figure 2 Remote sense protection with internal voltage sensing
ABUSIVE LOAD CONDITIONS
Output Current Ripple
Ac to dc power supplies normally have capacitors connected between the output terminals of the power supply. These capacitors provide a shunt path for reducing unwanted ac currents produced during the power conversion process. These capacitors have an internal series resistance, and when subjected to ac currents, produce power loss resulting in heat.
Maintaining capacitor currents within tolerable limits can become an issue, if ac currents from the load add to those generated by the power supply. Such conditions can be created with a switching type load, like a buck converter, connected to the output terminals of the power supply. As shown in figure 3, the power supply will sink a component ac load current depending on the ratio of internal series resistance, R1 and R2, of capacitor C1 and C2.
Figure 3 Sink load current ripple current
Repetitive Short Circuit Operation
Like excessive output current ripple, output capacitors, especially aluminum electrolytic type, can be damaged by shorting the power supply’s output terminals. Peak current is limited only by the output capacitors’ internal series resistance plus the lead impedance of the connecting cables. Energy stored in the capacitor is released as heat in the capacitor; repetitively shorting the output terminals can cause degradation or catastrophic failure. Film capacitors, such as those employing polypropylene film, have lower dissipation factors and can tolerate more abuse than aluminum electric capacitors, but these capacitors have lower capacitance ratings for a given size, which compromise filtering performance. The tradeoff between output ripple performance and reliable, repetitive short circuit operation is a design constraint.
Back Fed Voltage
Dc power supplies are frequently connected to loads that have their own source of energy or to loads that produce voltages and currents that exceed the ratings of the power supply. Typical examples are battery loads, dc motors, and motor controllers; these loads are capable of bidirectional flow.
Figure 4 Back fed voltage protection with diode
Connecting a battery to the output terminals of the power supply can cause rapid charging of the output capacitors and produce excessive output current. As shown in figure 4, placing a series diode, D1, between the output of the power supply and the battery prevents voltage from being back fed to the power supply’s output terminals. Configuring the power supply for remote sense at the load, eliminates the diode voltage offset. Also, the diode prevents the discharge of the battery through the power supply when the power supply is off. (Ac to dc power supplies typically have bleed resistors across output capacitors to discharge any stored charge when the power supply is off.)
Dc motors and motor controller combinations can back feed voltages while attempting to regenerate energy. If the power supply cannot dissipate energy, its output voltage floats at the voltage produced by the motor or controller. Placing a diode, as previously described, protects the power supply’s output from exceeding its voltage rating.
Most ac to dc power supplies utilize a diode or a synchronous rectifier circuit configuration in the final output power processing stage. These components clamp the output voltage to several volts in the reverse direction. Loading a power supply to produce a reverse voltage generally does not present any reliability issues to the output stage, including aluminum electrolytic capacitors, as long as output currents stay within the ratings of the power supply. Applying a reverse voltage source, such as a battery, can damage output power semiconductors if currents are allowed to exceed ratings. As shown in figure 5, protection of reverse voltage can be accomplished with a series-connected, fast-acting, dc fuse, F1, and a diode, D1, with a surge rating beyond the i2t of the fuse. With this protection scheme, a reverse voltage connection will clear the fuse by forcing current through the protection diode.
Figure 5 Reverse voltage protection with diode and fuse
About the Author
Ira J. Pitel is the founder and Engineer at Magna-Power Electronics. He received his B.S. degree from Rutgers The State University of New Jersey, the M.S. degree from Bucknell University, and the Ph.D. Degree from Carnegie-Mellon University. He is a fellow of the IEEE and has served in many IEEE capacities including Society President of the Industry Applications Society.
This article originally appeared in the Bodo’s Power Systems magazine.