Achieving EMC for DC-DC Converters
By Chris Likely, Field Applications Engineer, Cooper Electronic Technologies
DC-DC converters, whether they be an off the shelf brick design or a discrete equivalent, are a source of EMI (Electromagnetic Interference). EMI is unwanted electromagnetic energy that propagates by radiation and conduction over system signal and power lines. EMC (Electromagnetic Compatibility) is the ability of a system to function reliably in the presence of significant levels of EMI and, at the same time, to limit its internally generated EMI to avoid interference with the operation of other systems around it. Whichever approach is taken to meet system DC-DC converter requirements it is certain that some filtering will be required on the input power lines.
There are a number of EMC regulations concerned with all aspects of EMC whether it is conducted or radiated emissions, ESD, electromagnetic field or surge immunity. Looking primarily at emissions requirements, international standards are prepared by CISPR (Comité international spécial des perturbations radioélectriques – International special committee on radio interference) and adopted by regional authorities. In Europe the generic EMC emissions standard is EN61000-4-3 and more specifically for DC-DC converters the standard that should be applied is EN55022. In the USA FCC part 15 would be the applicable standard, figure 1 shows the emissions limits for EN55011 and EN55022.
Figure 1 - EN55022 Limits for Conducted & Radiated Emissions
The conducted emissions are measured at frequencies between 150kHz and 30MHz, the curve A limit is the for EN55011 which applies to industrial environments and curve B is for EN55022 which applies to residential, commercial and light industrial environments. Radiated emissions are measured over the frequency range 30MHz to 1GHz.
Conducted emissions are usually comprised of two types of noise, these are common mode and differential or series mode. Common mode noise appears as a voltage on both supply lines with respect to earth while differential noise appears between the supply lines. The approach taken for suppressing these emissions depends up on the DC-DC converter solution, where off the shelf bricks power supplies are used then only external filtering is required. Where a discrete solution is being employed the first step it to suppress the noise at it source.
There are two main areas of noise generation in a DC-DC converter, the first is associated with the switching frequency of the power supply. Obviously the power supply switching in an integral part of its operation and there are limited steps that can be taken to suppress noise caused at this frequency and it’s harmonics. Switch frequency noise is both common mode and differential, the differential noise can be reduced by placing a de-coupling capacitor across the DC line local to the main switching element.
Common mode noise is injected into the earth of the power supply via the parasitic capacitance between switching devices, such as transistors and diodes, and the chassis to which they are mounted. Using an electrostatic screen between the device and chassis can reduce this but this tends to hinder the thermal performance. In practice a filter on the input line is generally used to deal with the common mode element of this noise. Implementation of modern circuit designs that use ‘soft switching’ techniques can greatly reduce both the common and differential mode noise associated with the switching frequency. This in turn allows the use of simpler and cheaper input filters designs.
The second aspect of DC-DC converter noise is associated with the fast switching edges, these edges will tend to over-shoot and ring causing high frequency noise. The normal way of dealing with this type of noise is to fit a snubber circuit, typically a RCD configuration, in parallel with the switching device in order to damp any ringing.
EMI filters have two functions, they attenuate noise generated within the power supply preventing it getting on to the supply lines. They also prevent noise on the supply lines interfering with the power supply operation. The filter circuit shown in figure 2 is the basic design required to attenuate differential mode noise.
Figure 2 - Differential Mode Noise Filter
At the power supply switching frequency, which can be anything from 50kHz to 1MHz, the inductance in the line appears as high impedance. As a result most of the noise current will flow through the low impedance of the capacitor fitted across the lines, this capacitor is referred to as an X capacitor. The inductor can be two separate parts, one in each line, or alternatively a couple inductor can be used. The advantage of couple inductors being that two series connected windings on the same core will give four times the inductance of the single winding. This means that, in general, a coupled inductor will be cheaper than two single inductors and require less PCB space while achieving the same results.
Figure 3 shows the basic filter circuit required to attenuate common mode noise, again the inductor presents a high HF impedance in the line and the capacitors, which are referred to as Y capacitors, provide line and the capacitors, which are referred to as Y capacitors, provide a low impedance path for the noise current to earth.
Figure 3 - Common Mode Noise Filter
Common mode inductors are constructed with two windings on one core, these windings are connected in the circuit in anti-phase one in each line. The result of this is that the current in each winding is equal and opposite resulting in no peak flux in the core allowing common mode chokes to be constructed using relatively small core sizes for their rated current.
In practice a combination of both the common mode and differential mode filters is required. For a modern DC-DC brick power supply design that uses soft switching the filter will look something like the example in figure 4.
Figure 4 - Simple Filter
The inductance Lx is the leakage inductance of the common mode inductor. In many applications this is large enough to filter the differential mode noise when used in conjunction with a relatively large value of capacitance across the line. C4 is fitted to filter any noise on the supply line and prevent it from interfering with the power supply operation. Typical components for a nominal 48V input, 30W output DC-DC converter with a 250kHz switching frequency might be;
L1 – Coiltronics 780µH 1.2A, CMS3-11, Lx – 5.1µH
C1 & C4 – AVX 1uF, 100V, MR081C105J
C2 & C3 – AVX 4.7nF, 1.5kV, 1812SC472KA1
With these values the common mode switch frequency noise will be attenuated by –20bB and the differential by –28dB.
For applications that require greater attenuation multistage filters can be used, this can involve adding a differential mode inductor or extra common mode choke. Figure 5 shows a three-stage filter that employs both these additional components.
Figure 5 - Three-Stage Filter
Typical components for a 48V input 15W output DC-DC converter with a 250kHz switching frequency might be;
L1 - Coiltronics 15µH 1A coupled inductor, CTX15-1A (effective circuit inductance – 60µH)
L2 - Coiltronics 1.6mH 0.75A, CMS3-14, Lx – 9.6µH
L3 - Coiltronics 840µH 0.8A, CMS2-11, Lx - 5.0µH
C1-C3 – AVX 1uF, 100V, MR081C105J
C3-C5 – AVX 2.2nF, 1.5kV, 1812SC222KA1
With these values the common mode switch frequency noise will be attenuated by –35bB and the differential by –90dB.
Layout and Positioning
The layout and positioning of the filter is critical to achieving optimum performance, figure 6 shows an example layout for a simple filter. The X capacitors are fitted directly across the input power lines with the common mode choke directly in line. Similarly the Y capacitors are fitter directly from each of the power lines to ground.
Figure 6 - Input Filter Layout
The filter should be positioned as close to the power inlet as possible, ideally mounted on the wall of the housing, this will prevent noise ‘pick-up’ in the power lines between the inlet and the filter. It is also common practice, although not always essential, to screen the filter and any cabling there may be between the inlet and the filter. Screened filters are often used to prevent high frequency noise, above 30MHz, getting on to the power lines and being radiated. For this sort of screen to be effective it must be connected to earth.
Whatever the approach to solving DC-DC power requirements it is certain that DC line filtering will be required. Whether it is a modern converter topology utilising soft switching or a more traditional design the basic requirements are the same. Selection of the correct parts to give the required attenuation along with a good layout will produce switch frequency and switch frequency harmonic noise levels within regulatory limits. When used with an off the shelf DC-DC brick solution the addition of a filter should be enough to achieve regulatory EMC. However discrete solutions will require more work, the filter will be relatively ineffective against noise at higher frequencies and a lot of work will be required to resolve these issues for both the conducted and radiated noise.
Chris Likely is based at Cooper Electronic Technologies, Burton-on-the-Wolds, Leicestershire and can be contacted via email: firstname.lastname@example.org