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Class D Power Amp Output Current Sensing

Other Parts Discussed in Thread: TPA3255, INA240, INA240EVM, TPA3255EVM

We make vibration control equipment and would like to use the Class D TPA3255 Amplifier in Parallel Bridge Tied Load mode to drive a 300+ watt dynamic shaker and would like to be able to sense the output current being driven to the shaker which will be in the 0 to 12 Amp range.  We are designing a new PC board for the power amp and then there will be discrete wires connected to the shaker.  We are looking for the best way to monitor the current to the shaker, preferably through a PC mounted sensor or via the inter-connecting wires.  Any thoughts, comments or suggestions on what TI sensors might best fit this application would be greatly appreciated.

Best regards,

Randy Mitchell

Design Engineer

Spectral Dynamics 

  • Hello  Randy,

    Thank you for considering Texas Instruments in your design.

    First off I would like to clarify what you are trying to do.  Would the circuit/system below accurately reflect what you are wanting to do?  If so, I am calculating that you would have ~50 Common-mode voltage for your current monitor.  My calculation was done as seen below.

    Figure 1: Possible Shaker Circuit

    Equation 1: Current Monitor Common-mode Max

    Assuming this is the max common-mode voltage you expect for your circuit, then we do offer several current monitor options that can be found on the following link: http://www.ti.com/amplifier-circuit/current-sense/analog-output/products.html. You can further tweak your desired design parameters as seen below in Figure 2.  There is one caveat though.  Not all of the listed devices will be able to measure down to 0 Amps.  One that should meet your criteria would be the INA240.  However, if this one does meet your expectations.  Perhaps you can give us a few additional details on what you need from your current monitor.

    Figure 2: Current Monitor Search Options

    Also if you want to know specifically which devices will not measure down to 0A so you will not make the mistake of choosing one, you may want to look at this FAQ link: https://e2e.ti.com/support/amplifiers/current-shunt-monitors/w/faq/3588.measuring-vsense-20mv-with-the-ina193-ina198-products

  • Thank you Patrick,

     

    Yes, your circuit shown below does indeed reflect what we are trying to do and based on the information that you provided, I have to get up the learning curve for current monitors.  We are going to use the TPA3255 in Parallel Bridge Tied Mode which supposedly allows it to output up to 600W when used with a 50V DC power supply with 12 amps of output current.  With that configuration, the output voltage swing of the Class D amp is 0 to 50V peak to Peak as I can see on my TPA3255 EVM.  So I’m still a little confused on the Common Mode voltage calculation and read through the information you sent a couple more times before I can ask anymore reasonably intelligent questions.

     

    Randy Mitchell

     

    Spectral Dynamics

  • Patrick,

    On the front page of the INA240, second paragraph, it states:

    "The low offset of the zero-drift architecture enables current sensing with maximum drops across the shunt as low as 10-mV full-scale."

    I was not able to find a spec for the V Sense Range anywhere, but does this say that the V Sense is good down to 10-mv and un-stable below that?  Or is my understanding of the V Sense Range incorrect?

     

    Thanks,

    Randy

  • Hello Randy,

    That line alludes to what we think is a good minimum max value for Vsense.   Which means the Vsense can be smaller than that and will be stable.  We say this because, some customers  may choose to use a very small shunt resistor to limit power dissipation.  In those cases they may have a vsense that ranges from 0 to 1mV.  With such a small range, it is possible to see larger percent error.  Larger Vsense values which a larger Vsense range permits will have smaller percent error.  On any of our current monitor's product page you should be able find a calculator that will tell you the percent error.  Below is an example with the INA240 with 1mohm shunt based off some of the conditions you have provided in your system.  One caveat to note with this though is that as you approach 0, the percent error will go towards infinity: (1- measured value/ideal value)*100 = (1-measured value/0V)*100.  However, the error voltage will be very small.

    Figure 3: Percent Error Calculator

  • Randy,

    While you are ramping up on current monitors, I figure we can explore your setup a little more.  Based off the details you have now provided, I would do my calculations a bit differently.  In figures 4 and 5 I show the two boundary states of operation where both the voltage and current values are max.  In these conditions, I would calculate that your instantaneous power is 50V*12A=600W.  However, because you are switching between these two states, I would expect that at either the Output A or Output B pin you will see a sinusoid, square wave, or something in between those two.  In the case of a simple sinusoid I would calculate the average power over one period as in the equations provided below.  With the sinusoid, I also assume that the wave is centered on 25V with a 25V amplitude letting the wave swing from 0 to 50V.

    Figure 4.  Output_A shorted to PVDD (50V)

    Figure 5: Output_B shorted to PVDD (50V)

    Based on the information you provided.  I would presume that per cycle one of your current monitor’s pins will see 50V with respect to ground.  This actually will be the max common-mode voltage your current monitor will see.  So you will need to choose a current monitor that can handle that.  Since the direction of your current is changing, you will need a bidirectional current monitor.  Bidirectional current monitors include a reference which acts as the origin of the measurement output.  For instance, if you use current monitor that has a 2.5V voltage reference, zero amps detected will result in a 2.5V output from the monitor.  If this same device has 20V/V gain and a positive current generates 10mV across the shunt, you will see 2.5V+20*.001=2.7V at the output of the monitor, while an equal current flowing in the opposite direction will result  in 2.5-20*.001=2.3V at the output of the monitor.  The ADC range as well as your min max current conditions will determine what gain option and current shunt resistor you can use.

  • Patrick,
    Based on the specs for the TPA3255 PWM Power Amp, it was decided that we were going to power the AMP with 50 volts to get max power. But finding an off the shelf 50V power supply at 12 amps is easier said than done. In talking to the Power Amp folks at E2E Community, they said that a standard off the shelf LED 48V power supply at 12.5A worked quite well and recommended it for our application. Which now brings the common mode voltage down to 48 volts, which is within the 49.5 common mode voltage range that you had calculated for the INA240 current sensor that you originally suggested. The INA240 is also bidirectional so the circuit configuration that you suggested looks like it will drop straight into our design. The only monkey wrench is someone at the last minute suggested AC coupling the output so the output would be +/-24V. But I told them that the current sensor common mode voltage can not go any further than -4V, so we needed to stay with DC coupling. In your learned opinion, is that true?
  • Hello Randy,

    You are correct, the INA240's minimum common mode voltage is -4V and would definitely not work in the +/-24V case. Additionally, we do not currently have any devices that will take such a negative voltage directly. You can potentially put resistors in series with the AC coupling, but that will increase your gain error. From my point of view it makes sense to go with the DC coupling. However, what is the reasoning behind the AC coupling suggested by your coworker?
  • Patrick,


    Thank you for your quick response and I believe you have answered all of my "known" questions. They would like for me to get in the INA240EVM board and try to hook it up with our TPA3255EVM. If you have any thoughts or comments on that, it would be appreciated. As far as your question as to what the reasoning behind a co-workers suggestion of AC coupling the TPA3255 output, I chalk that up do someone in a meeting who really don't know what they are talking about saying what ever comes into their head to try so show how smart they are and try to come up with things that no one else had thought about or considered. I think every one has at least one of those type people around, we call them "speed bumps." Just so I cover all the bases, I had to ask about the AC coupling, even though I was already pretty sure it wasn't going to work.