MP8860 snubber or other EMI reduction techniques

DC-DC Power Converters
1

Hello,

we are currently using MP8860 regulators for one of our products. We’re very happy with them but we’d also like to improve EMI performance.

Our system includes 4 MP8860 per board, input 12V, output between 9 and 20V, load between 0 and 750mA. Load is composed by multiple elements (30mA each) which switched on/off, sometimes all together.

We’re currently using EV8860-based PCB layout on 4 layers, shielded WE inductors. Analyzing emissions from inductors reveals very steep slopes on the square wave.
Is there any suggestion available about possible snubber circuit for this chip or other techniques?

Thanks in advance.

Diego

2

Hi Diego,

Thanks for reaching out. EMI noise can come from design/layout, switching speed, and ringing at switching transitions. When you mention the square wave, can you clarify? Are you experiencing conductive radiation or emitted radiation?

To reduce EMI at switching transitions, you can increase the switching time by adding an external 1kOhm resistor in series with another switch driven when the gate drive signal goes low to provide some of the power to be released to ground. There should be no issue connecting a snubber to the circuit’s switch node.

Here are some resources regarding EMI optimization and mitigation from MPS:

Optimizing EMI in Switching Regulators for Consumer and RF-Sensitive Applications | Article | MPS (monolithicpower.com)

Using Filter Capacitors and Inductors to Suppress Radiated EMI | Article | MPS (monolithicpower.com)

Hope this helps!

Best,
Stephen

1 Like
3

Hi Stephen,

sorry for the delay.

Thanks for your suggestion, two snubber circuits on both SW1 and SW2 pins mitigated ringing in both buck and boost mode. Unfortunately there is no gate drive signal available in MP8860 thus we cannot add the additional switch.

About square wave: sorry, I was very “criptic”. As we had some issues with emitted radiation (> 10MHz), we tried to connect a small antenna to a scope and spectrum analyzer. That’s where we would get a perfect square wave and we’d like to reduce switching time to reduce HF emissions.

BR,
Diego

4

Hi Diego,

Glad to hear the snubber circuit worked for your SW node ringing. In order to continue reducing HF emissions you can do a few other things.

  1. Use a boot resistor in series with your bootstrap capacitor. This method allows more time for the parasitic network to discharge. Start with a 0-ohm resistor and slowly increase until desired EMI level is achieved (typically 10 ohms).

  2. To add an additional shield to the inductor you can place a piece of copper tape connected to the top of the inductor and connect it to GND. This will serve as a shield and reduce your overall emitted radiation.

  3. Adding a Low Pass Filter and a ferrite bead at the input can also help mitigate the EMC/EMI. We typically place a pi filter on the bottom of the board to keep it away from the noisy SMPS components.

Hope these recommendations help!

Best,
Stephen