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Part Number: LM5107
Dear e2e support,
My customer is facing some noise issue with his design incuding our LM5107.
It's driven through a 25Khz PWM.
Has you can see below, we don't see any over/undershoot which could cause some EMI.
Most of the noise is seen through the different cables which are populating the board, around 40MHz and 70MHz.
Could you give us some advices to locate the noise source?
Here is a short view of the schematic. I can send the full schematic and layout in private.
PWM inputs: PWM_1H (yellow) PWM_1L (blue)
PWM_1H (yellow) H-bridge middle point (purple)
Iout (pump/motor current)
Vout (yellow) (pump/motor volt), Iout (green)
Thanks for reaching out, Im an apps eng with this device. Let me try to help out with this noise issue.
RF noise can be tricky to locate, you have already found the frequency which is a great start. The next step is to find where in the system the RF noise is an issue. Look for signals with ringing and spikes that correspond with the frequencies that have EMI issues. Experiment with filtering, or snubbing those signals near the noise source, to reduce the noise so its no longer an issue.
The source of the noise can be located by keeping an eye on the noise in your circuit with a probe then start to change parameters in the system for example move the cable around to see if the source starts to get bigger as you move closer to it. You can also shut off power to things one by one to find the culprit. Its possible that the noise may be coming from outside the system. If the noise is coming into the cables its possible that the cable is acting like an antenna in which proper shielding and feed-through methods should be used.
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In reply to Jeffrey Mueller:
Gate Driver Applications
Dallas, TX USA
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With a H feild probe, we found the noisiest part in the board. It's in the middle of 1 half-H bridges ($779 in the schematic ):
Green => Write, Yellow => Max Hold.
And here, an exemple of CM noise in cables around the board ( measured with a 10R current probe):
If the H_bridge is not ussed, there is no noise from 55 to 80MHz.
In reply to Don Dapkus:
Hi Don, You are totally right, that why there is already 510R as Rgate in my design. And as you can see switching edges are already "very slow" and without ringing. Maybe 510R it's enought ? with regards, PAM
In reply to PA M:
Since most of the noise is coming from the full bridge transformer then its possible that this near field noise is the true source, by rotating the probe to be in parallel with the offending trace you can narrow down the exact trace thats having emissions problems then filter from here. It might be that with the high dv/dt switch node is causing the H field probe to show lots of di/dt. You can use an E field probe in the same spot to determine if there is high dv/dt emissions. Is the noise coming from the primary or secondary?
Is the transformer shielding connected to GND through a capacitor? it looks like C5 and C6 are trying to do this.
A certain capacitance value will yield Icm = 0 where the Icm is fully canceled between the primary and secondary sides. The value of this capacitor is dependent on the parasitic capacitance.
Hi Jeff,I double checked; the noisiest part in my design is here.
By rotating the H-field probe there is no noice.And in the same place, there is also E-field noise.For me it seems to bo logic be cause a 24Vdc is switched (at 25kHz) here. And 4.5Arms.Green => H-field (Write), Yellow => E-field (Write).L2 nanocrystalline common mode choke. But L2 is shorted now.( if L2 is not shorted, there is non common mode noise in the motor's cable. But noise in cables around the board still the same ).
C5 and C6 are not fitted.
More over, I also tried with small ferrite bead as Rgate, without convincing result
If you probe the noise in the cables around the board while bringing the cables closer to the suspected logic trace, does the signal grow? Go idea, I will check ASAP. First I need to extend my cables.Is the logic signal interface termination impedance matched? H-bridge drive a vacuum pump. I also have an USB ( matched ) a CAN bus ( matched ), a FAN and some power for actuators ( ON/OFF or 80kHz PWM ).are there any stubs on the transmission line? No.sometimes we cant reduce the noise source, will using a shielded cable or ferrite bead at the interference spot work? If I use ferrite on my cables, cable’s decries. But I got the same H-field noise in the middle of my H-brige. ( Maybe it’s the same for E-field but I did not try it in those condition ). Moreover, if I probe noise in a cable, I get noise >30dBµV only when the H-bridge is ON
Green => H bridge OFF, Yellow => H bridge ON ( noise <20MHz comes from my power supply )
Thank you for your help.On Monday, I will check my device in an EMC room. I hope to give you some good news after
Thanks for this update what do you mean by the cable decries with a ferrite bead?
It seems like E field noise and H field noise are present so shielding wont fully solve the problem. A filter solution will likely be needed. Check out the first page of this well pictured app note from Murata. Since this noise is most likely common mode noise (since line/return H field noise is picked up with a current probe loop) and radiated from the far field (more than 1 wavelength away) we want to filter the noise before it hits the load (at the termination of the cable). The image at the bottom of page 1 shows a bypass cap from the line to the metal chassis as well as a bypass cap from the return to the chassis to filter the noise to large heat sink and away from the signal. The location is important as we need to shunt these frequencies as close as possible to where they are picked up.
Keep me posted, Im hoping for good news.
Hi Jeff and everyoneMy bad, I wanted to say:If I use ferrite on my cables, cable’s noise decries.
I think; my bridge creates noise; to his load and also, to other outputs on my board. So I have to filter this noise to each outputs.My goal was to reduce this noise form his source to get “smaller” outputs filters.Maybe a common mode filter in the input of my H-bridge could be a good solution.But anyway, even if I see noise >30dBµV with a 10Ohms current probe in cables, I don’t see this noise in the EMC Chamber. So,… I think there is no problem anymore !
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