TPS40170: Failed radiated emission test

Hakan Wahlnas

Part Number: TPS40170

Hi, I used Webench to create a DC/DC design with TPS401070 that could output 18V 8A.

To be able to rapidly test this, I bought the TPS40170EVM-578, modified it according to the Webench results (with a larger coil and capacitors with higher voltage). This design worked well with appropriate load when looking at oscilloscope. The goal is creating a custom design but based on TPS40170 and Webench results.

Then I sent the whole thing to an accredited EMC lab and it failed completely in radiated emission. Conducted emission was not measured. See table below with columns: Frequence, Quasi-peak, Limit and Margin.

39.390000 42.47 30.00 -12.47

39.420000 42.65 30.00 -12.65

98.100000 36.87 30.00 -6.87

165.870000 37.47 30.00 -7.47

218.880000 33.80 30.00 -3.80

What modifications are needed in the design to get lower radiated emission in the range 30 - 200 MHz?

Any help is appreciated

Håkan

39.390000	42.47	30.00	-12.47
39.420000	42.65	30.00	-12.65
98.100000	36.87	30.00	-6.87
165.870000	37.47	30.00	-7.47
218.880000	33.80	30.00	-3.80

over 8 years ago

0 Piotr F. over 8 years ago

TI__Intellectual 2910 points

Håkan,

can you provide the PCB layout of TPS40170 that you are using in your design for the reference? Is the DCDC output inductor shielded?

There are some ways to improve EMI performance of DCDC supplies. First of all you can look at EMI app note that discusses frequency components of EMI that arise from switching:
www.ti.com/.../slyt682.pdf

Have in mind that conducted emissions are tightly connected with radiated emissions - good conducted performance most of the time results in good radiated EMI. Below few ides for EMI performance improvements:

1. Use shielded output inductor
2. Add an RC snubber from SW to GND
3. Keep SW plane copper area as small as possible.
4. Bring the GND plane closer to the power traces by reducing the PCB intra-layer spacing (this is described further in the app note above).
Ensure the dotted terminal of the inductor is tied to SW (such that the winding end that’s tied to SW is short and nearest the GND plane).
3. Add small resistors between HDRV/LDRV and transistors' gates, e.g. 10 Ohms (to slow down the switching a little)

Best regards,

Piotr

0 Hakan Wahlnas over 8 years ago in reply to Piotr F.

Prodigy 60 points

Hi Piotr,

thank you for your reply. As I posted initially, I use the TPS40170EVM-578 so all pcb layout will be found there. The coil is shielded Coilcraft XAL1010-153MED.

Thank you for the app note. I did not see that before.

BR

Håkan