Interference emission from a battery charge controller

Introduction

To reduce the risk during EMC testing for the approval of new medical devices, we are happy to conduct preliminary tests with devices in the prototype phase. We are currently supporting the development of a device for recording vital signs. The device can be powered by a power supply or battery.
Sometimes these preliminary tests are flawless even during the first measurement, but this device had problems with high-frequency interference. What should I do now?

A good way to narrow down the cause of the error is to deactivate individual functions of the device (if possible) or unplug cables. In this case, you'll notice that the interference decreases somewhat when all sensors are no longer connected, but it's only significantly reduced when the power supply is unplugged.

Hence, the suspicion that the problem is coming from the battery charge controller. In our case, a synchronous step-down converter (buck converter) with two external MOSFETs is used.

Sources of interference Buck Converter

The main cause of electromagnetic interference in buck regulators is high-frequency currents in small current loops. In our case, there are two main loops, which can be seen in the following image.

Because currents I1 and I2 are discontinuous, both have steep edges when switching on and off. This causes high-frequency interference. The critical path here is area A between Cin, Q1, and Q2, because this is where the highest current flows. This also causes the most high-frequency interference.

So we have the following starting points:

• Reduction of current loops through the layout
• Reduction of switching speed
• Damping slew rates
• Limiting the slope, e.g. by snubber

We will look at these below.

Optimizing the layout

When designing the layout, the specifications from the datasheets should always be observed. Unfortunately, this wasn't possible in this project due to the specified dimensions. This resulted in larger current loops in the layout.
With the experience from the first measurements and the following tips, this could be significantly improved in the redesign.
To minimize high-frequency interference, the area A should be kept as small as possible. This can be achieved through clever placement of the components.

Here are a few tips regarding the layout of buck converters:

1. Place Q_HIGH and Q_LOW opposite, so that drain and source are opposite each other, which reduces the loop considerably.
2. Use input capacitors of different sizes and place them as close as possible between Q_HIGH Drain and Q_LOW Source.
3. To connect decoupling capacitors to the GND area, multiple vias should be used to reduce the impedance of the connection.
4. Place GND of the output capacitors outside the switching loop of the input capacitors.
5. The circuit traces of the switching point and boost pins should be kept short and small.
6. Signal path and power path should be separated
7. Do not use a heat trap in the critical path, as this creates additional inductance.
8. The GND surface for shielding should be directly below the surface containing the interference sources. It should be as continuous as possible beneath the input circuit.

Old	New

The loops of the current paths could be significantly reduced by implementing the tips, and the IC was also placed outside the current paths.

Old layout	New optimized layout

A direct comparison shows that the improved layout reduced the interference. However, it's not enough to pass the test.

Switching speed

One way to reduce emissions is to slow the switching speed. A series resistor at the gate of the MOSFET increases its on and off time, thus reducing high-frequency interference. The resistance cannot be chosen arbitrarily, as increasing the series resistance also increases the power dissipation in the MOSFET, causing it to heat up. Typical values in such applications are 5 to 10 Ω.

0 ohms	10 ohms

The images clearly show the relationship between emissions and power dissipation. Using a 10 ohm series resistor, the value is below the limit. However, this also significantly increases the temperature of the MOSFETs, in this case by about 40°C. This significantly reduces the service life of the components.

RC Snubber

Snubber circuits play a key role in reducing EMI by dampening the voltage spikes that occur during switching due to parasitic inductances. This can reduce high-frequency components in the switching signal. The disadvantage of snubber circuits is the additional loss they introduce.
A typical snubber consists of a resistor and a capacitor, which are placed directly next to the MOSFET, in our case the low-side MOSFET.
The resistance value depends on the desired damping (ξ) and the mostly unknown parasitic inductances and capacitances.

The damping value chosen here is 0.4.

The following procedure can be used to determine the parasitic inductance and capacitance.

1. Measuring the oscillation frequency (f_RING) in the rising flank
2. Increase the capacitance between the switching node and GND until the oscillation frequency is halved
3. If the frequency is halved, the total capacitance is 4 times higher than at the beginning of the measurement. The parasitic capacitance (C_p) corresponds to one third of the added capacity
4. The parasitic inductance can be calculated.
5. The snubber resistor (R_S) can be calculated using the formula above and the snubber capacity (C_S) is usually chosen to be 3-4 times larger than the parasitic capacitance

1. Measurement fRING	2. Additional capacity 837 pF
fRING = 138.8 MHz	fRING = 69.4 MHz

3.

4.

5.

For C_S 1nF is selected.
Measurements with RC snubber (4.7Ω and 1nF):

The snubber can significantly reduce interference in the 100MHz to 300MHz range.

Input filter

The ESR and ESL of the capacitors increase their impedance, resulting in high-frequency voltage drops. This voltage induces currents into the circuit's power supply. The intermittent input currents and the length of the power line can also lead to radiated emissions.
To reduce voltage drops, multiple low-ESR MLCCs of different designs should be used. An LC filter can also be used to keep the noise localized.

The LC filter significantly attenuates interference at all frequencies.

Result

To resolve the EMC issues, all discussed measures were implemented. Their effectiveness has already been discussed in the respective chapters. Here is a final comparison of the measures:

Previously	Old layout with all measures	New layout with all measures

This clearly shows how important the layout is for a buck regulator. At first glance, the difference doesn't seem particularly significant, especially since the radiation actually deteriorates slightly in the range above 300 MHz.
However, if you compare the subsequent measures, you can see that these, together with the improved layout, reduce the interference emissions significantly more.

I hope you enjoyed this article. It clearly shows that the layout can have a significant impact on the EMC of devices. And that with the right measures, these problems can be easily addressed.
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