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FEATURE EMC & THERMAL MANAGEMENT


Design your power supply to have optimum performance


Shane Callanan, Director of Applications Engineering at Excelsys Technologies looks at the Electro Magnetic Interference (EMI) sources on a power supply, and how to address these issues to design low profile performance leading EMI designs


A


ny source of changing voltage or changing current with respect to


time will result in ringing. Any given switch mode power supply will be full of these events occurring every switching cycle. All tracks, points and components are a potential source of this on a power supply. But the hidden truth is that most EMC sources do not appear on any bill of materials. Within any given design there will be


loops with high di/dt and high dv/dt’s. With such high di/dt’s and dv/dt’s it doesn’t take much interwinding capacitance to generate significant common mode currents and voltage spikes of up to several volts. This will result in ground bounce, and hence EMI.


MINIMISING EMISSIONS Limiting EMI can be accommodated right from the project concept stage. The first focus for any designer is to eliminate or at least minimise the emissions from the source of the generators. Also remember that receivers re-transmit, so minimise the susceptibility of receivers. If this is still not achieving the target


emissions then as a final solution you may need to look at filters, but it should be remembered that filters can result in significant addition of real estate to your final design.


HOW PCB LAYOUT PLAYS IT PART PCB layout is often the least understood when it comes to designing for EMI. But it need not be a difficult thing to if you keep the following items in mind. 1. Minise loop areas 2. Leave no floating parts, ensuring that all loops are brought back to ground


3. Keep signal and power ground connections separate This will add to the complexity of the


24 OCTOBER 2014 | ELECTRONICS Figure 1:


Hi-Reliability XF power supply from Excelsys


PCB design so there are normal compromises in order to save on layers. On one layer you should track the input and output signal grounds with the star points at the IC’s common pins (allow for the isolation barrier between input and output). On a separate layer the ground plane for the power grounds. Input ground plane under the input components only, output ground plane under the output components only, allow for the isolation barrier between input and output ground planes Also, remember that earthing schemes


were designed for safety purposes, not EMI, so use ground planes and short connections. Also there should be only one connection between signal and power grounds, through a via at the signal ground star point.


CONSIDER YOUR COMPONENTS WITH RESPECT TO FREQUENCY The difficulty facing power supply designers is that EMI performance is not usually charactereised by the component manufacturer. Understanding your components is key to knowing how they will behave in terms of emissions. This is especially true at the highest effective frequency. You must also consider that things like capacitors, wire wound resistors and even wire leads will vary in behaviour as the frequency changes. Compliance of the power supply in a test house to various limits does not


necessarily guarantee the user that it will pass when installed into a system. In the same way that the power


supply designer needs to ensure that the sources for radiation are kept to a minimum, the system designer also has a role to play in ensuring that the overall system has the optimum performance. Cabling arrangements and PCB tracking layouts are the greatest contributing factor to system EMC performance. It is important that PCB tracks and power cables are arranged to minimise current carrying loops that can radiate, and to minimise loops that could have noise currents induced into them. All cables and PCB tracks should be treated as radiation sources and antennae and every effort should be made to minimise their interaction. Electro Magnetic Interfernece is


considered by many as being a black art, but trying to address this issue at the back end of a design process can be costly, time consuming, and end up having to use relatively large filters. Power supplies by their nature are noisy environments, but taking every aspect of the design into consideration a market leading performance is achievable.


Excelsys Technologies www.excelsys.com +353 21 4354716


Enter 212 / ELECTRONICS


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