Inductive load current sensing with CS30.

[Yosmany Hernández Sánchez]

Posted on June 21, 2018 at 21:44

Hello everyone. I would like to know if is possible to use the CS30 current sense amplifier with inductive loads, i.e. something similar to the coils of solenoids used in automatic car gearboxes. Thanks in advance.

The operating conditions are 12V and about 1 to 2A.

[AUPETIT nicolas]

Posted on June 28, 2018 at 14:21

Hello Yosmany

I just would like to clarify some points regarding your application. Actually you would like to sense the current flowing into an inductive load right? and thus by using a shunt in high side current sensing with a common mode of 12V, am I right?

If it is the case, the CS30 can be effectively a good candiate to sense the current.

But if the current  flowing into the coil is regulate by a transistor like the picture described below, unfortunately the CS30 won't work anymore.

Effectively in the below case, when the Mos turns on, the diode is reversed biased off, and current flows through Rshunt to actuate the solenoid. If the Mos is turned off, the current in the Mos is interrupted, but the energy stored in the solenoid causes the diode to turn on and current to freewheel in the loop consisting of the diode, RShunt and the solenoid, and the input common mode voltage will fall down one diode below ground. 

And unfortunately the operating Vicm range of the CS30 is from 2.8V to 30V.  The CS30 won't be damaged but won't be able to measure something if the Vicm is outside of 2.8V to 30V.

Hope it can answer to your question, if not don't hesitate to ask

BR

nicolas

[Yosmany Hernández Sánchez]

Posted on June 28, 2018 at 21:44

Hello Nicolas, thank you very much for your detailed answer, very informative also. In this case there is another not-expensive solution I can take?. Let me describe my final application:

In essence I would like to be able to precisely control the current flowing through a solenoid valve using a microcontroller (specifically from ST Microelectronics, STM32F103C8) with PWM and ADC feedback. Right now I’m trying to design the interface between sensing resistor and ADC input, in order to use a PID control loop.

Best regards and thank you very much for all your help and time. Yosmany.

De: AUPETIT nicolas

Enviado el: jueves, 28 de junio de 2018 08:22 a.m.

Para: Yosmany Hernández Sánchez

Asunto: Re: - Re: Inductive load current sensing with CS

<https://community.st.com/?et=watches.email.thread> STMicroelectronics Community

Re: Inductive load current sensing with CS

reply from AUPETIT nicolas <https://community.st.com/people/AUPETIT.Nicolas?et=watches.email.thread> in Analog and Audio - View the full discussion <https://community.st.com/message/203969-re-inductive-load-current-sensing-with-cs30?commentID=203969&et=watches.email.thread#comment-203969>

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Attachments :

image001.jpg : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006HxSY&d=%2Fa%2F0X0000000ayJ%2FVcNkZMSfQsYmy9w9cnZgGk26iPYjFYQbUbIGOBPoRsw&asPdf=false

[AUPETIT nicolas]

Posted on June 29, 2018 at 13:28

Hello Yosmany

Your application is more clear, and the previous schematic shown the limitation of the CS30 use . And to bypass this limitation we can imagine to change a little bit the topology of the High side current sensing, like the schematic below:

In this case when the MOS is turned OFF the input common mode voltage of the CS30 remains 12V and works anytime.

Or if it is possible, you can also choose a low side current sensing approach as described by the schematic below:

in this case a simple Opamp can be chosen, the TSZ121 is a good candidate as it is an extremely precise Opamp (Vio=8µV over temperature and Dvio/dt=30nV/°C).

The main draw back of this kind of architecture are .

• you have to fix the gain with 2 resistors.
• due to low side, a short cut of the load to gnd cannot be detected
• when no current flown through the shunt the output of the opamp is saturated, and need almost 25µs to exist saturation when a current flow again into the shunt.

Just for information and maybe it cannot really match with your application, but in the past year I have experimented the following schematic.

The main goal of this application is to control the current into the load and switch off the system in case of over current event. The current measurement is extremely important, firstly to control the current flowing into the load and also to be able to take a decision in case of a default like a short circuit or over current event. This function is realized thanks to the high side current sensing CS30  which allow a precise measurement. The Power N-MOS is the key function in this application as it helps to control the current into the load and also serves to protect the application when default appears.

When a short circuit or over current occurs, the application must be switched off as quickly as possible. And moreover after such event, the application must not restart by itself and must stay switched off until a manual reload is applied. The TS3021Y is a high speed comparator, which combined with D flip-flop HCF4013, is used to realize a latch function in case of an over current event. As soon as an over current event occurs into the load, it takes less than 9µs to switch-off the power NMOS. The MCU generates a configurable PWM in order to drive the gate of the power NMOS by analyzing the data coming from the current sensor.

Hope it will help you in the design of your application

BR

Nicolas