DRV421: Question about use case condition

Hello Satoshi,

Thanks for reaching out.

1) I would not say the fluxgate or analog circuitry is completely separable in this case. The DRV421 fluxgate senses a magnetic field emanating from a primary current passing through the system’s toroidal-like core. Thereupon some conditioning is done to this sensed field signal, to produce a secondary current in another wire wrapped around the core (compensation coil). If the magnetic core gain, coil windings, and compensation coil inductance (magnetic field conditioning-fluxgate related properties) are properly designed for along with proper device gain setting (precision analog property), the secondary current will have a linear response to the primary current. However, the linearity of the DRV421 output still also relies on analog circuitry that amplifies a differential voltage produced across the sense resistor that is place on the compensation coil line between pins ICOMP1 and ICOMP2.

2)We do have some application notes regarding the DRV421 device here: www.ti.com/.../technicaldocuments. However, I would recommend using a simulation tool to help with your design as there are several competing design parameters that will affect the measurement performance. One free software package is FEMM. This software is great for illustrating the effects the magnetic core has on the primary as well as nearby stray fields. If you provide us with the primary current range you are intending to measure along with the stray field sources and expected proximity, we can put together a simulation to help you with your design and illustrate the behavior mentioned in that paragraph you posted.

3) Sumida has halted production on the SC2912 now and for the foreseeable future. We are currently working with some core vendors for a solution to complement our device, but these are still in development. Yet, if you do give us your system conditions, through simulation we can help guide you toward cores that may be available from various vendors. Although, you may need to modify the air gap and compensation coil windings.

Hello Satoshi-san,

Before we can proceed with helping on the design, I need to clarify a few things. I have figures below that illustrate the core parameters as I understand them.  With the figures I also have few questions over requirements that did not make sense.  Also, there are two different core examples below.  Please let me know if your customer plans to use one of these or a different shape.

2)  Yes, you have the correct link.  How close will your customer place their cores next to each other?  Or if they are only using one core, one DRV421 will have to be switched off while the other is operating.

3) We are working with vendors right now.  Yet those cores are still in the development phase.  However, development should finish soon.  Once they release their part, we will provide their name.

4) The test condition is DC current with the full-scale error plotted.  For frequencies above 1KHz the core and transformer will determine the primary to secondary gain.  This can be observed in the DRV421 System Parameter calculator listed on this page: http://www.ti.com/product/DRV421/toolssoftware.

Based on user inputs, this calculator can generate the transfer function curve, such as in the figure below.

Patrick-san

Sorry for my reply delay,

I summarized customer information below.

・About core examples, customer is not fixed and no strong request.

　If Example B is affect by external magnetic field, customer request is Example A.

・Current by primary is 0~327.68A. (Over 20A is very short time)

・Distance for neighboring current is 2~3cm.

・About 300A/m and 1000A/m, customer need both condition. (It is depend on End environment)

Additional question about ④, is there test data for ratio error?

Customer is expected for good ratio error spec by DRV4xx series, because AC input is important spec for ratio error.

Best regards,

Satoshi

Hello Satoshi-san,

My answers to your questions are below in blue.

If there need additional customer spec/application, I will confirm soon.
And, I get additional customer request spec(transient current), please see below; 
・When max current is 5Arms ⇒ 1250A @ 10ms
・When max current is 1Arms ⇒ 250A @ 10ms

These high currents will saturate your core and create an offset which will need to be adjusted for.

About datasheet P30 Figure-67;
Is there real test data version(condition) of AC(~3kHz) and large current range? You mean test data for conditions like those posted above? No we do not have.
※Figure 67 is DC and 100A_max. Yes, Figure 67 shows DC measurements with 100A being the max primary current.
And if there detailed scale for current error, it is more great helpful.
(Customer want to confirm the condition for 0.001% current error. 

The total current error depends on internal device sources of error as well as the design of the core and the layout of the device. If you look at Figure 69 of the datasheet, the error is significantly higher than 0.001%. The linearity of your core can potentially have a very strong influence on the total error. A core linearity error of 0.1% would be normal. For more information on the errors you may encounter as well as tips on how to design with the DRV421, I would recommend looking at: www.ti.com/.../sloa223.pdf