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CSD88584Q5DC: TIDA - 00774

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Replies: 3

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Part Number: CSD88584Q5DC

Good Morning I´m doing a version custom to my company and using as reference design TIDA 00774. After study about boards to control BLDC, I have some doubts.

1) Why does the  Board TIDA 00774  has  12 mosfets instead of 6?

2) Also, I thought of changing the capacitors to my custom version, they are in red in img....

2.1) What are these capacitors for in circuits ???

2.2) How do I calculate capacitance according to my custom circuit?

2.3) What is influence ESR from on board?

Best Regards

  • Hi Oliveira,

    Thanks for posting the question. Please find below my comments on your questions.

    1) Why does the  Board TIDA 00774  has  12 mosfets instead of 6?

    This is for increasing the power rating. Here two MOSFETs are used in parallel to enable high current operation, where each MOSFET share half the load current.

    2.1) What are these capacitors for in circuits ???

    In TIDA-00774, C14, C15, C16 are used as the bulk electrolytic capacitors for providing the ripple current and also to provide a stable DC supply. You may also need small ceramic capacitors near each inverter leg to avoid switching voltage spikes.

    2.2) How do I calculate capacitance according to my custom circuit?

    This has to be calculated according to the ripple current in inverter, lead inductance from your DC power source to the board, and also the value should be selected such that during hard braking (turning off all MOSFETs) at maximum current, the regenerative energy from the motor should not create a high voltage build up on the DC bus (The cap value should be such that the voltage increase on the capacitors should be with in a limit, which will not damage the circuit).

    2.3) What is influence ESR from on board?

    You need to calculate the RMS of ripple current in the capacitor and then calculate the loss in the ESR. A high ESR creates high losses and heat up the capacitors.

    Please let me know if you have any further questions.

    Best Regard,

    Manu

  • In reply to Manu Balakrishnan:

    Hello Mr. Manu Balakrishnan, thanks for your answer.

    1) I understanding perfectly. 

    2.1) I don´t know where are located this small capacitors "You may also need small ceramic capacitors near each inverter leg to avoid switching voltage spikes"???

    2.2) I do not know in specific what is the current of the inverter. Whether it is the phase current, or the current measured at the DC link, or the total current of the phases. You could give me an example, please.

    3) And after I finished my custom board I was expecting a current waveform, in the total DC link, from an INDUCTOR charging and discharging. But, I got a very different result, this result in img 03..... And the output from mosfets to motor are OK. Could you help me about this?...

    BEST REGARDS

      

  • In reply to Andre Luiz Silva de Oliveira:

    Hi Oliveira,

    Please find below my comments.

    2.1) You can locate these capacitors from TIDA-00774 schematic. The TIDA-00774 schematic C18-C22 & C42 are these ceramic capacitors,

    2.2) The capacitance at the DC bus (bulk electrolytic capacitor) has to be calculated according to the DC bus current ripple. I assume the waveform shown here is the DC bus waveform (which looks so). I am not able to exactly see the ripple current value. For example, if the DC bus current peak to peak ripple is 10A, and assuming a constant average current from the supply, you can calculate the ripple in the DC bus voltage, using the basic capacitor current voltage relationship (I = cdv/dt). The capacitor has to be designed to minimize the voltage ripple as per your requirement.

    As I mentioned earlier, the capacitor value also depends on:  the hard braking (turning off all MOSFETs) at maximum current, the regenerative energy from the motor should not create a high voltage build up on the DC bus (The cap value should be such that the voltage increase on the capacitors should be with in a limit, which will not damage the circuit). Which means the energy stored in the motor inductance will be transferred back to the DC bus capacitor.

    You can refer the below blogs on the regenerative energy pumping to DC bus:

    https://e2e.ti.com/blogs_/b/motordrivecontrol/archive/2013/10/18/the-art-of-stopping-a-motor

    https://e2e.ti.com/blogs_/b/motordrivecontrol/archive/2015/07/13/motor-drive-forum-top-faqs-part-2-how-to-estimate-motor-regeneration-and-vm-pumping

    3) The waveform looks like the expected DC bus current

    Let me know if you have further questions.

    Thanks & Best Regards,

    Manu

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