SMPS current sense resistor made of PCB traces

[ysba]

Hi,

I'm working on a SMPS design, and I need a current sense resistor about 100 mOhm / 3W.

I'd like to know if there are any problems if I make this resistor using a long enough PCB trace.

The reason I have to do so is to reduce BOM and manufacture costs, since SMD resistors are expensive, and THT resistors need to be formated and manually inserted in PCB.

I already made some testing, it works. But I want to know if this is not a good practice for EMI reasons, etc.

The attached image shows the track length I'm talking about.

Thanks.

 

[KlausST]

Hi,

it is as unprecise as cheap.
Because the thermal dirft of copper is relatively high and the precision depends on etching and galvanic tolerance.

Also you have to take care about stray inductance. With about 250mm trace length this is not negligible. Not good for peak currents and high frequency signals.
As i can see it is about 100mOhm with 1oz / 35um copper thickness as you wanted.

* The price of PCB area may be more than you safe by a extra resistor. But the resistor is way more precise.
* to calibrate your resistor may cost more than a resistor.

You could even use a 20mohms resistor with 0.2W and amplify the signal by 5 and get more reliable results.

Usually my goal is precision, therfore it is no option for me, but it can be a solution for you. It depends on your needs/specifications.


Klaus

 

[Ishaan Karnik]

I want to know how you would compensate for manufacturing drift/tolerance in mass production? Have you given a thought on how to tackle electro-migration?

 

[andre_luis]

I want to know how you would compensate for manufacturing drift/tolerance in mass production?

It is possible identify some components at the bottom side of the drawing, like pads shunt distributed throughout the structure, which can be used for a calibration during assembly time.

 

[Ishaan Karnik]

It is possible identify some components at the bottom side of the drawing, like pads shunt distributed throughout the structure, which can be used for a calibration during assembly time.

Sir can you enlighten me what will be the effects of aging or heating on such system. and how to tackle them? As per my understanding current sense resistors offered by Vishay or Rohm tend to be very stable.
Thank You in advance.

 

[andre_luis]

Sir can you enlighten me what will be the effects of aging or heating on such system. and how to tackle them?

As Klaus said before, this is not a precise solution, however if you want to do anyway, should avoid use if the current flowing heats the copper track enough to change its resistivity significantly.