DRV425: reference design information

Jim.McDonald

Part Number: DRV425

Hi Javier

Could I request the FEMM models along with any supplementary information for TIDUAZ4 reference design please? Both the standard +/- 100A as well as the modified version boosting the current rating.

I'm especially interested in knowing how low the magnetic fields are for very low currents and any thoughts on how to improve sensitivity at this very low end whilst still maintaining good accuracy at the top end. I'm assuming the wider the busbar the better... perhaps using a 1.0mm PCB rather than a standard 1.6mm might help (there is reference to "mounting, back to back, as close as possible" but I don't know if 0.6mm is going to matter).

Whilst I've never used the FEMM tool I'm hoping that with both the stock & modified model files (and any matching supplementary info) I can work out the best shape/size of busbar to suit the current range and modify accordingly. I don't know if the FEMM models you have to hand show a variety of busbar sizes and it's possible to switch between them to see the performance differences?

Any recommendations for the ADC (24bit delta sigma perhaps, but any other characteristics that would be beneficial?) that would well match this reference design?

Many thanks.

Jim.

over 8 years ago

0 Javier Contreras4 over 8 years ago

TI__Mastermind 32670 points

Hello Jim,

We have two items released on the TI.com to assist you with your design. I should have another tool releasing next week for FEMM.

DRV425_Bus_Bar_Application_Magnetic_Field_Calculator will be a good tool to design the bus bar and get a very good estimate of the magnetic field you will see. As soon as the other tool is released I will add it to this thread.
We also have a our Bus Bar Theory of Operation application note which goes through how the TIDUAZ4 works.

I don't see there being to much issues with the high current but to get a dynamic range you are looking for may not be possible. The challenge is the lower currents. When you have a bus bar that can handle the 600A a lower current running through the same conductor will not generate the same level of magnetic field. Another thing at the lower current is the influence of stray fields will have a greater impact on the measurement. In the Theory of Operations document you can see how you can limit the effects of stray fields.

Low Current Measurement

The equation below can be used to predict the field from a given current along a wire. This will change depending on the size and shape of the conductor but this is a good starting point. The closest you can place the top of DRV425 is 0.4mm but that is for a single DRV425. The TIDUAZ4 bus bar was designed to limit the magnetic field inside the hole. The calculator is also a good tool to see the field seen inside the whole.

B = u0*I / (2*π*r).

u0 = 4πe-7
I= current on wire
r = distance away from wire

ADC selection

A differential ADC is a good option to measure the ref and the output. Depending on the choice here you could also separate the analog portion of the two DRV425 and have two outputs and apply post processing to calculate the difference. Section 4.1.5 of the Bus Bar Theory of Operations covers the limitations of the current circuit. The benefit in the current circuit is you have only one output.

Please let me know if you need any assistance with the document or the tool or anything else.

0 Jim.McDonald over 8 years ago in reply to Javier Contreras4

Prodigy 35 points

Thanks Javier

I'd seen the application note you linked but not seen the Excel tool before... very useful!

The Excel was exactly what I was hoping for... seeing what the impact of size/shape/etc was on small currents.

Clearly a small round hole, small busbar and widely spaced sensors increases the field in the hole - which is what I need.

What will the new FEMM tool do/achieve?

Best regards, Jim.

0 Javier Contreras4 over 8 years ago in reply to Jim.McDonald

TI__Mastermind 32670 points

Jim,

The FEMM tool is like the excel tool in that it will draw the cross section on the bus bar and simulate the magnetic field for a specified current. Since the FEMM is a 2D simulator it can easily simulate the slot configuration and not a hole. This tool will give you the basic number you will get from the Excel calculator. The benefit is after you draw additional noise sources and see the magnetic field calculated. Please keep me updated and ask for any other assistance you require.

Regards,

Javier