For electronic engineers EMC is often a mystical thing with a weird test in a metal box they have to pass with their design. Its true, EMC can be a tricky thing but with the right tricks in your design its much less tricky then before. Here I show you some of this tricks for buck converter : 

1. Get input filtering to your Buck Converter! 

Buck converter produce tons of noise on the input, that means you need that input filter! It always depends on operation mode and frequency of the buck but in the datasheets are usually recommendations for the filtering take them serious.

2. Gate resistors at the gates of your mosfets

The buck converter control IC’s you use do have mosfet drivers but that doesn’t mean that you can conntect them directly to the gate of the mosfet. Sure, the faster you switch the more efficient you are but that also means a lot of noise because of fast rising and falling edges at your mosfets – that harms the EMC a lot. There are many ways to adress this and the most simple way is just putting a resistor between the pin of the IC and the mosfet gate, for low voltages this should be enough. If you switch higher voltages it may help to get diodes within the game. With two diodes and two resistors you can control the rising and falling edge at your mosfet independently this can help you a lot with improving the EMC but not ruining your efficiency.

3. Get your boost cap a series resistor

A very common thing that is ignored. In a buck converter you need a bosst cap to get the voltage of the gate from the high side mosfet above input voltage level. The circuit to get this done is always in the IC itself there is often just the need of pluging an external capacitor to the IC and it really seems that easy but it isnt. Lots of IC vendors dont get their layout and bonding properly done. This means that you create a resonance circuit with the cap, IC bonding, IC layout , mosfet bonding and mosfet gate cap. I’ve seen mosfet drivers myself where the overshoot was about 80% of the acutal switching level which caused a lot of damage to the EMC. With the small resistor in series of the boost – cap you can damp this resonance circuit – rule of thump: values between 5 – 25 Ohms schould be enough.

4. Get a fixed frequency switcher

There are several switching and control approaches on the market and some are variabel frequency like constant on or off time and some are fixed frequency like voltage mode control and peak mode control. With constant on or off time it means that your overall switching frequency changes with the load. If you have a circuit with different load situations like a µC controlled board with different peripherals on the µC your board behaves different dependending on what your software is doing with the µC and the peripherals. That often causes a lot of headaches during the EMC testing and may cause beside layout changes also tons of software changes which adds more work. With fixed frequency converters you switch at one frequency and optimize the board for that – much easier to do! BTW: when using µCs at your buck converters output you just should use peak current controllers – much faster and stable control loop. It ensures that the supply voltage is stable when changing from sleep mode to normal operation at your µC

5. Use fixed frequency switchers with spread spectrum

When having a constant frequency switcher you can optimize your board to one frequency but there is another trick you can add to this design: spread spectrum. With constant frequency you have all the noise energy you radiate is fixed in one place at the frequency spectrum, that creates a peak at one frequency. With spread spectrum the IC modulates the switching frequency in a controlled way. This can happen for example with a sine wave modulation like FM modulated radio. This spreads then the radiated energy over a certain band of frequencies which damps the peak. Here is for example a list of switchers with that technology from LT: Spread Spectrum Switchers

6. Take the layout recommendations seriously

Every vendor of power supply IC’s which understands this topic puts layout recommendations in his datasheet – read them! It also makes lots of sense to analyse the layout if its very complex it may be that the manufacturer wasn’t paying a lot of attention to the pinout of the IC, this takes revenge at the layout and higher EMC later so they have to do a more complex layout. Linear recently introduced their silent switcher series (another big fanboy moment for me!) where they have put some extra attention to the IC, the bonding and the layout which ensures in very calm EMC even at high switching frequencies and integrated mosfets with up to 6 amps. This should also help in your design!
BTW: Some vendors still recommend 2 layer boards. Do yourself a favor and use 4 layers. This makes your life easier a lot!

7. Get control about the current flow

Often there is the rule in layouts: There is a lots of cupper on the board, leave it there! Well that helps but sometimes it can make things worse. In layout it is common to pay a lots of attention to the positive power rail but not to the negative. There is often just a ground plane left with hope that its working properly – Do not use ground planes for your power needs! Ground planes do not work for transporting your energy, the current always takes the most convenient way to flow back and this is often the wrong way for your EMC! Make proper tracks even for the ground all the way back to the connector or cable! This will help you to not create ground plane antennas.
BTW: I personally use ground planes to get analog tracks shielded from the digital tracks.

Happy engineering!

PS: if you can add a tipp just comment below!