Originally Published MEM Fall 2005
POWER SUPPLIES
EMI Considerations in Selecting AC/DC Switching Power SuppliesUnderstanding the electromagnetic interference issues for power supplies helps designers to select the right power supply to integrate into their medical electronics.
Ryan Gordon Benhard
) |
| Ferite core selection is integral to achieving low EMI. Of the three cores shown here (left to right: EER, RM, and E types), RM may be the most efficient. (click to enlarge) |
Design engineers often report system-level electromagnetic interference (EMI) issues. Unfortunately, a system's power supply is often the primary source of unwanted electromagnetic emissions. When EMI is a concern, it is crucial that device manufacturers select a power supply that meets all applicable electromagnetic compatibility (EMC) standards to minimize EMC issues when integrating the power supply into a medical system.
Interference can occur to a power supply (i.e., immunity) as well, so it is essential that the functional performance of the power supply is acceptable to the system it is integrated into. For example, some power supplies shut down their outputs momentarily during fast transient burst tests.1 This may be acceptable for a power supply on its own or when driving an indicator lamp, for example. However, a power supply that is powering programmable circuits (those that run software) is likely to cause a reboot, which may not be acceptable for the system's application.
In other words, a power supply that is fully compliant with all of its EMC immunity tests when tested alone, might cause a system to fail its immunity tests. So, the system integrator needs to know exactly how the power supply performs when it is interfered with to know whether it is suitable for an application. It is also important to note that system interactions and the high-frequency switching within an electronic power supply can cause problems.
It is impossible to guarantee that a compliant power supply, once integrated into a system, will still remain compliant. Therefore it is vital that designers plan for these interactions and ensure proper EMC performance as early in the design stage as possible. This article looks at the extent to which switching power supplies contribute to system-level EMI issues. It also discusses the design considerations that affect EMI in power-supply design.
Contribution to EMI
Because they are so much more efficient, switching power supplies have almost completely replaced linear power supplies. However, this shift comes at the cost of increased EMI. Switching power supplies generate more EMI because they switch large currents at very high frequencies, anywhere from 50 KHz to 1 MHz. At these high frequencies, optimal power efficiencies and smaller components (primarily magnetics) can be achieved.
The theoretically optimal switching waveform in these power supplies approximates a square wave. However, the sharp edges of these waveforms produce considerable noise at the fundamental switching frequency and its harmonic frequencies. When a rectangular waveform is measured in the frequency domain (typical of EMC emissions tests), a comb of frequencies harmonically related to the fundamental is seen. The amplitudes of the higher harmonic frequencies depend upon the switching edge rate of the waveform. Therefore, there is a trade-off between power efficiency and EMI.
The switching edges of the power devices, and perhaps their gate drives as well, should be slowed to reduce high-frequency emissions without reducing efficiency by too much. A cost-effective design may have to be less efficient and have larger heat sinks. This reduces the cost of the filters and shielding that would otherwise be needed for the power supply to comply with EMC requirements.
Although the switching metal-oxide semiconductor field-effect transistors (MOSFETs; and their heat sinks), transformers, and output rectifiers are typically the greatest sources of radiated EMI, at high frequencies even printed circuit board (PCB) traces radiate noise. Inductive radiation (magnetic field emissions) happens at all frequencies. PCB traces are effective radiators of electromagnetic waves when they start to act like antennas.
Transformer Design and Construction
The following section identifies critical factors to consider when selecting a power supply. Because the transformer is a large inductive element subjected to high-frequency currents, it is typically the noisiest component. It is also the component around which the rest of the power supply is designed. EMI reduction should begin in the design of the transformer.
Every facet of the transformer's design and construction can affect EMI, and each aspect can be critical. Approaches that can solve EMI issues in one design will often compound them in others. Below are some transformer design and construction techniques that reduce EMI.
Ferrite Core Selection. Typically, an RM type core is better at containing magnetic fields than an E or EER type core. An RM type core has additional core material on all sides of the center leg that act as an electromagnetic shield. Another important consideration is how the core is gapped. By gapping only the center leg of the core, radiated external magnetic fields are further contained. A gapped outer leg will leak radiated magnetic flux through the gap, possibly requiring additional shielding to contain the stray emissions.
Switching Drain of the MOSFET. This drain will normally be connected directly to the primary winding of the transformer. One design technique that can help reduce emissions in a multilayer winding is to make this drain connection to the innermost winding of the transformer. By doing this, the EMI caused by these high-current, sharp-edged waveforms are shielded to some extent by the other windings and shields in the transformer.
Winding Direction and Orientation. The direction and orientation of one winding relative to another has an effect on EMI and so must be considered in the design.
Shielding. The use of Faraday shields between the primary and secondary winding will typically reduce EMI. These shields can be tied to the primary or secondary to achieve optimal suppression. Applying additional tape insulation between primary and secondary windings can also help reduce EMI. The reduction in interwinding capacitance by the extra thickness of insulator reduces charge injection across the galvanic barrier.
A shield can also be applied directly to the outside of the core (and all of its windings). This shield can be tied to either the output common or the input return for optimal suppression. It is important to note, however, that each shielding layer on a transformer will increase the capacitive coupling, directly increasing the leakage current of the unit. A shield can also be used to create a shorted turn on the outside of the transformer, which further reduces the stray magnetic fields of the transformer.
All of these techniques can be quite effective at reducing EMI, but they can also have undesirable effects. Designing a transformer is a balancing act between EMI suppression and transformer efficiency.
PCB Layout
) |
| Figure 1. A large current loop. (click to enlarge) |
) |
| Figure 2. A small current loop. (click to enlarge) |
The main inductance loops of concern for EMI are the PCB traces and their current return paths, which inevitably enclose an area between them. Otherwise, the signals would all be shorted out, and nothing would work. These PCB traces and their current return paths lead to both conducted and radiated noise. Also, if multiple loops are in close proximity to each other, they can have a coupling effect that increases EMI levels (see Figures 1 and 2).
Stray coupling between loops can cause problems for EMI when these loops allow noise energy to bypass a filter or shield or escape from the boundaries of the switching circuit and its devices. Otherwise, loops simply add noise to the circuit's waveforms. This can cause self-interference with sensitive circuits; however, this is really a signal integrity issue rather than an EMI issue. Careful attention must be given to the size, placement, and alignment of these loops to preserve signal integrity.
Reducing these loop areas minimizes their capacity to radiate and conduct noise. Also, separating noisy loops from each other minimizes their coupling effect, further reducing their contribution to interference. Separating high current traces from low current traces increases system stability and can help reduce EMI.
Also, a linear flow of components on the primary side, from the input line to the transformer, helps reduce coupling of the switching waveforms back to the line voltage, ultimately reducing EMI.
Capacitors
An effective technique for reducing EMI is the use of Y-type and X-type capacitors. Y capacitors are double insulated and are typically used between the ac line and ground, between ac neutral and ground, and between the primary and secondary circuits. X-type capacitors are single-insulated and placed between the ac line and neutral (or between phases or phase-neutral in a three-phase mains supply). However, it is important to note that Y capacitors can contribute directly to increasing the total leakage current in the ground lead. This is critical in medical applications because the leakage current limits are lower than for other applications. Therefore, these capacitors can only be used up to the point where the leakage current requirements are not exceeded. Beyond these limits, other solution techniques must be considered.
Inductors
) |
| Inductors can be used in combination with two capacitors to create a low frequency filter. (click to enlarge) |
Common-mode (CM) noise currents are the main cause of EMI problems above 1 MHz. The CM choke works by creating a high impedance at the frequencies to attenuate. CM currents flow through the line and neutral paths in the same direction, while differential-mode (DM) currents flow in opposite directions and are out of phase with each other. CM and DM chokes are essentially composed of two identical inductors, one on the input line and the other on the input neutral, coupled together by a single core.
In a CM choke, the high impedance path is designed to filter noise currents in the same direction on line and neutral. In a DM choke, the high impedance path filters noise currents in opposite directions on line and neutral. These components can be placed before or after the rectifier, depending on where they achieve optimal EMI suppression. In some designs, as many as three of these components may be used (usually two on the input and one on the output).
) |
| Figure 3. A Pi filter combines both capacitive and inductive filters to remove high-frequency ripple current. (click to enlarge) |
An inductor can also be used in combination with two capacitors in a Pi configuration (see Figure 3). A Pi filter combines both capacitive and inductive filters in a Pi configuration, which can effectively attenuate the high-frequency ripple currents that cause EMI but pass the low-frequency ac waveform.
If the power supply uses power factor correction, a DM choke is often used in a Pi filter configuration with metal-film capacitors to reduce the DM noise. DM chokes with or without metal-film capacitors are widely used in all sorts of filters for many power supplies. These components can be used without contributing to the leakage current of the device. However, these are very expensive components, and budget constraints often preclude their use.
EMI Compliance
A switching power supply is typically designed to meet FCC and CISPR emission standards and IEC immunity standards. Designers should obtain a sample of a candidate power supply and perform a simple, low-cost conducted EMI test to get a preliminary assessment of the manufacturer's compliance.
It is important to remember that these standards are applied by self-declaration. To show compliance, many power-supply manufacturers test only a single unit to demonstrate emission levels below the maximum limits. Unfortunately, this practice fails to account for inherent variation in emission levels from unit to unit. Component tolerances and manufacturing variations introduce deviations in the emission levels during production.
Therefore, a power-supply design that has been qualified by the customer can fail to meet EMC requirements in production. In addition, even if the power supply conforms to the standards, there is no guarantee that when it is used in a larger system, the system will pass EMI tests. The system itself will add to the emissions of the power supply. Furthermore, the load on the power supply can cause system interactions, which can amplify the emissions further at resonant frequencies that are not present when the power supply is tested alone.
Immunity
Immunity in power supplies is not difficult to deal with. To address immunity, however, the power-supply manufacturer must include all of the data on the actual performance of the product—both during and after each of the immunity tests are applied. Such information should be available in a manufacturer's data sheet or at least readily available to potential customers. Typically, such data are presented in terms of output voltage error and error duration.
The best power supplies, suitable for the widest markets, will never deviate from their normal functional specifications during or after any immunity test. It is also important to note that where the correct functioning of a system has implications for human health and safety, for protection of the environment, or that could cause significant financial or project losses, normal EMC immunity testing methods and levels are known to be inadequate. A risk-based approach to immunity such as that referenced in IEC 61000-1-2 should be applied as well.2
Conclusion
A system's power supply is often the primary source of unwanted electromagnetic emissions. When EMI is a concern, it is crucial that device manufacturers select a power supply that meets all applicable EMC standards to minimize issues when integrating the power supply into a system. Interference can occur to a power supply as well, so it is also critical that the functional performance of the power supply is acceptable. Certain techniques can minimize EMI in a power supply. These include the following:
- Selection of transformer core.
- Transformer winding and shielding.
- Component layout and loop size.
- Use of X and Y capacitors.
- Use of CM and DM chokes.
For optimal EMI control at the OEM system level, it is imperative to ensure that all of these points have been considered when selecting an ac/dc switching power supply.
References
Ryan Gordon Benhard is product engineer for Elpac Electronics Inc. (Irvine, CA). He can be reached at rgordon@elpac.com.
Copyright ©2005 Medical Electronics Manufacturing
