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TIDA-00366: Bootstrap Capacitors not charging

Part Number: TIDA-00366
Other Parts Discussed in Thread: UCC21520, TIDA-00199

Hello,

According to the schematic of TIDA-00366, at the starting point it's clear that bootstrap capacitors aren't charged.

So AMC1301 IC's used for three-phase current sensing are not supplied because their VDD1 are supplied from bootstrap circuit (VCC_U,VCC_V,VCC_W).

It is noted in AMC1301 datasheet that when VDD1 supply is missed, there is a fail-safe output strategy that Vout-p goes to about zero and Vout-n about 2.7v. The Amc output is Amplified with a gain of 0.797 through OPA320 and is summed with 1.65V which is (2.7*0.797+1.65=3.8v). so OPA output is saturated in 3V3.

therefore overcurrent protection comparator(TLC372) does it's duty and overload signal (output of TLC372) is produced making the nand-gate output to go high which is Disable signal.

Finally, Disable signal turns off the ucc21520. So bootstrap circuit cannot be charged at all.

Am I right? this is what I am experiencing in my circuit. Is there any solution?

Thanks.

  • Hello,
    Thank you for looking into our TIDA-00366. Note that this design has provision to power the gate drivers with either external isolated supplies or internally using Bootstrap technique. Your analysis on overcurrent protection circuit getting active on power up with bootstrap technique is correct and this phenomenon will not occur while powered with external supplies. So in your design, you can modify the circuit such that MCU SW is disabling the gate drivers instead of HW. Alternatively you can add logic in the "overload" hardware path which will be enabled by MCU after power up sequence. Hope this clarifies.
    regards
    Navaneeth
  • Hi,

    Thanks for your detailed response. You are right, the issue resolved by adding logic in the "overload" hardware path.

    But the thing I don't understand is that in my circuit, DC‌ link voltage is 100V and i used a simple buck converter to generate 15V for gate drive supply. Isn't it like a external supply?

    And another problem by adding logic in the overload hardware path is that whenever motor stops and there is no input pwm for ucc21520, bootstrap caps discharge and i should repeat the start-up procedure.

    Thanks,

    M.A.A

  • Hello MAA,

                               Regarding gate drive supply, you have the choice of using single +15v rail to power all 6 drivers using bootstrap technique. It doesn't matter if this single +15v rail is generated internally on your board or fed from external power supply. Alternative to this is to use isolated power supply like TIDA-00199 which generates 4 isolated rails - 3 for top drivers and 1 common rail for all bottom drivers. If you use the later type with TIDA-00366, there wont be any start up problem.

    On your second query, Yes you need to repeat the start-up procedure if the motor is stopped.

    regards

    Navaneeth

  • Alternatively you can also keep the bottom switches ON while you don't spin the motor. This would keep the bootstrap capacitors charged and you don't need to repeat the start up procedure.
  • Hello Navaneeth,
    Yes, i know the difference between isolated gate drive supply and non-isolated one. But the point is that you said "this phenomenon will not occur while powered with external supplies". In my circuit, gate drive supply is produced from dc link voltage through a buck converter. So why this phenomenon still occurs?
    regards
    M.A.A
  • Hello MAA,

                           Let me rephrase to add clarity : when I meant external supply - I referred to external isolated supply like TIDA-00199. This would ensure all gate drivers have supply including AMC130X at start up. For the bootstrap technique it doesn't matter if the 15v is generated internally on board or externally. The top drivers get supply voltage only when the bottom switches are turned ON during start up. Hope this clarifies.

    regards

    Navaneeth

  • Hello Navaneeth,

    I got my answer. Thank you.
    But as you said for bootstrap technique the problem still exists. Maybe this should be modified for TIDA-00366, since it is clearly based on bootstrap.

    regards,
    M.A.A