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Tips to quite radiated emissions of small 24V DC brush motor

Hello Gentleman (and ladies?),

My company manufactures an automotive assembly that uses a small 24VDC motor to pump fluid. We recently went through a round of EMC testing under SAE J1113-41 guidelines.

To make a long story short our product behaves very well until the motor turns on. At this point emissions go through the roof. The product is well above an acceptable limit.

A little background. The PCB and pump motor are housed in a custom molded plastic enclosure. It is not possible to change the housing at this time in case you were going to recommend metal. The motor sits on top of the PCB and is housed in its own plastic pump sub-assembly (again, cannot change). The circuit board is a 4 layer board with dedicated power and ground planes. The pump is switched on and off with a N-channel MOSFET low side drive transistor. Two wires (approximately 30mm each) go from the circuit board to the pump. I have

1000pf capacitors placed on the PCB from the point of entry of each wire to the PCB and ground (common mode filters I believe). I also have a 100nF SMD capacitor placed between the pump motor wire leads at the point where the wires leave the board. The PCB seems to be immune to the high levels of RE. We have had no problems even considering the fact that this noisy pump sits right on top of the PCB. My big concern is what this motor might do to other electronics. We would like to meet SAE J-1113 guidelines for at least a region IV device, preferably region III or higher.

I tried to attenuate RE by connecting the motors metal body to the ground plane of my PCB. It did not seem to help much.

Do you have any suggestions? Is there something I can do to my assembly to quiet down RE ( like metalized paint on interior of housing)? Is there something that can be changed with the pump design to reduce RE?

Looking for any hints you have to offer.

Thank you,

George

Reply to
Ge0rge Marutz
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"We would like to meet SAE J-1113 guidelines for at least a region IV device, preferably region III or higher."

Sorry, I was confused. I meant to say we would like our product to be considered at least Class 2, prefereably Class 3 or higher.

Ge0rge

Ge0rge Marutz wrote:

24VDC

at

on

have

at

immune

the

concern

preferably

design

Reply to
Ge0rge Marutz

Could you change to BRUSHLESS EC (electronically commutated)?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Don't electrically cummutated motors require much more complex drive circuitry? Could the circuit fit in the same package outline occupied by a SOT-223 transistor and five or six 0603 caps and resistors? PCB real estate is at an extreme premium for me.

Thank you for the feedback

Ge0rge

Reply to
Ge0rge Marutz

Hi.

I once worked on a product which had a transducer within a couple inches of a brushed DC motor where the signal transduced was a few MHz wide and was expected to be noise free down to the approximately

50 Ohm noise floor of the transducer. Needless to say, brush noise was a real issue with that design.

One "fix" that did the most good was to place a small ceramic cap right across the motor leads where they exited the metal housing of the motor itself. From the standpoint of localizing interfering currents, this is about as good as you can do with a brushed motor. For this to be maximally effective, the cap leads must be kept as short as possible. Ideally, the RF current would not have to leave the motor housing at all; the cap would be placed inside across the brushes themselves.

If your motor is not in a metal housing, I doubt that you will have much sucess bringing the brush noise down by much. In that case, Jim's suggestion of a brushless motor would be mine as well.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
Reply to
Larry Brasfield

[Chop]

try a 1nF ceramic cap from each motor terminal to the motor body (short leads!), also a ferrite ring with both motor leads passed thru it a few turns can help.

The caps from the motor terminals is how the radio controlled toy cars manage to work with really NASTY drive motors and cheap radios, so it may solve your problem. Just some random thoughts.

Regards, Dan.

Reply to
Dan Mills

More complex drive circuitry yes, but at least some small burushless motors have the commutating circuitry built-in. Of course that may just mean that the motor is too large for you.

Robert

Reply to
R Adsett

As usual that is only half the answer- DC motor EMI suppression strives to confine RF current circulation to paths of small dimension relative to the frequencies involved- and this means they must be kept off the power lead wires. It also means common mode low impedance shunt at each line to GND as well as the differential shunt you mention. As far as a good GND goes- he must make a high frequency low inductance plane at the motor itself- and this can be a simple band around the housing. This should be joined to the controller with a braided shield- might as well be around the power feed.

Reply to
Fred Bloggs

24VDC

testing

yep, as said a cap as close to the motor terminals as poss, ie definately mounted on the motor itself. Plus ferret beads - not sure which part of the ferret theyre made from.

I dont rememebr trying this, but screened motor leads would likely help with whatevers left after that. Ditto keeping the output traces running side by side, to some extent.

NT

Reply to
bigcat

from.

As long as we're being silly, are those micro ferrets or nano ferrets?

Reply to
JeffM

ferrets?

Micky ferret, Nanna ferret and Millie ferret.

NT

Reply to
bigcat

8-) Even better.
Reply to
JeffM

twist the leads to the motor - about one twist per cm.

As well as FB & LBs suggestions.

Cheers Terry

Reply to
Terry Given

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