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TIDA-00366: Replacing the Bootstrap by Isolated DC/DC

Part Number: TIDA-00366
Other Parts Discussed in Thread: TIDA-010006, LM3480, TIDA-01420, AMC1311, AMC1301, , UCC21530, ISO224, ISO122


I am trying to design a 15-20kW AFE motor drive, which consists of an active rectifier and an inverter. I may or may not use the bootstrap DC for the inverter part. But, when the bootstrap is used in the rectifier's gate driver, the modulation index range will be limited. Due to this limited Duty Cycle, for the rectifier part I have to replace it with the Isolated DC/DC converters. I need to use 15/15 DC/DC for the gate driver and 15/5 DC/DC for the ADC current sensing part. Do you have any suggestions from TI products, which is compatible in this application design and works good with the other parts? Also, I need to add three voltage sensors to the rectifier board for the PLL implementation. Do you have any suggestions for that part as well?

Thank you

  • Hello Mohsen, You can refer to TIDA-010006 for generating isolated supply rails for the gate drivers. For 15v to 5v conversion, please refer to TIDA-01420 on using LM3480.

    For grid voltage sense you can consider using AMC1311.

  • Navaneeth,

    Thank you for your useful suggestions. I was investigating those application notes. For my rectifier part, I need to use the isolated dc/dc supplies instead of the bootstrap. However, since myrectifier power is not that high, I prefer to not use TIDA-010006 as a solution. I prefer to use three isolated 2W 15/15V DC/DC, one for each phase of the rectifier. Please see the above picture. My question is:

    1) is that fine to use only three isolated dc/dc converters, one for each phase? I mean is it fine to supply the low side gate drivers directly without any isolations? I guess i should be fine.

    2) is it fine to supply the high side gate driver (VDDA) and VDD1 of AMC1301 with only one 2W isolated dc/dc converter?

    Thank you for your time.

  • Mohsen, 

                           See below for recommendations to your query:

    1) is that fine to use only three isolated dc/dc converters, one for each phase? I mean is it fine to supply the low side gate drivers directly without any isolations? I guess i should be fine.

    Yes, you could use three isolated DC/DC - one for each TOP side gate drivers.  Since you are also using isolated current / Voltage measurement, i would recommend you to isolate the low side driver power supply as well. This would simply your inverter meeting high voltage requirements. You can also explore

    2) is it fine to supply the high side gate driver (VDDA) and VDD1 of AMC1301 with only one 2W isolated dc/dc converter?

    Power requirement depends on the IGBT gate charge and the switching frequency.  Please refer to equation 1 of design guide at For Inverter power levels of <22KW, 2W gate drive is more than adequate.



  • Navaneeth,

    Thanks for your detailed response.

    1) I agree that by using isolated dc/dc for the Bottom side gate driver, our rectifier better meets the high voltage requirements. But, I wanted to minimize the PCB layout changes compared to the TIDA-00366 to take the advantages of that great design. Replacing the bootstrap circuits by three isolated dc/dc converters would cause minimum changes in the design, however, adding a fourth dc/dc for the Bottom side it is not that straight forward and may affects the original design. However, when you recommend using the isolated dc/dc for the Bottom side, I would add it as well.

    2) I had calculated the average current for the dc/dc converter before I contacted you.

    a) the bias current for gate driver + Qq*fsw < 20 mA
    b) the current passing through R9 is equal to (15V-5V)/R9=10/820 < 2mH

    for a 1W 15/15 dc/dc converter the average is 67 mA which is clearly safe for a+b. However, for the instantaneous current drawn by UCC21530, I preferred to use a 2 W dc/dc converter, which is able to provide higher currents.
    Please let me know what you think about my third question about this instantaneous current which charges the gate capacitor:

    3) I understand how to calculate the average currents for the UCC21530, however,
    Isource=16/(16.5+4)=780 mA, this current must be supplied by the gate driver and consequently the dc/dc converter during the turn on delay time. I cannot find any details about the instantaneous current ability of the dc/dc converters in their datasheets. Do you have any advice on how to take this Isource of UCC21530 into account for designing the dc/dc converter?

    Best Regards
  • 4) For the grid voltage sense you suggested using AMC1311? Is it a good approach for AC voltage sensing as well?

    5) which one do you recommend for more accurate measurment? AMC1311 or AMC1301? I know AMC1311 is smaller. maybe cheaper and easier to work with, but which is more accurate for AC sensing?

  • Mohsen, If you are looking for analog based interface with MCU, i would recommend AMC1311 for voltage measurement and AMC1301 for current measurement .



  • Navaneeth,

    Yes, I'm looking for analog based interface with MCU, but as I mentioned before, the grid voltage is AC. AMC1311 is unipolar, input can be 0 to 2V, so, we cannot use AMC1311 for three phase grid voltage sensing purposes. Am I right? 

  • Thats right. AMC1311 input can be level shifted with external components , meaning 1v input may correspond to zero grid voltage. 

  • That's not a practical solution, the external components need isolated power supply to level up the input.  

  • That's not a practical solution, the external components need isolated power supply to level up the input. You suggested AMC1311 for AC voltage sensing over AMC1301. I understand the voltage sensing by using AMC1301 needs careful design due to higher input impedance, but, AMC1311 will be more expensive compared to AMC1301 even more expensive than ISO224.
  • Hi Mohsen,

    Have you decided to go with the AMC1301 or the ISO224?  Without level shifting as Navaneeth suggested, we do not have other high input impedance devices other than the ISO122 type devices which require a bi-polar power supply on both the high and low side of the device. 

  • Hi Tom,

    Thank you for your reply. I have not decided yet, I am thinking more about having a nice design. For level shifting with AM1311, we loose the isolation between hot and cold sides. ISO224 is not a good choice for me, because for mass production I need to minimize the cost. In addition, for my cold side controller layout I have two layers for ground and +3.3 V. Thus, it is so much better for me to use amplifier that uses 3.3 V for the cold side. Now, I am trying to find which of these options works better for me:

    A) Isolated solution: TI AMC1301 with a low shunt resistor to minimize the gain and offset error. I am trying to use 50 to 100 ohms resistor for the shunt resistor. The advantage is the isolation, the drawbacks are higher cost and increased gain error.

    B) Non-Isolated solution: TI OPA4305 by using this non isolated op amp, I can measure three AC voltages with only one op amp. The advantage is the lower cost, but the drawback is not providing the isolation for my cold side design. My problem is that, here in the U.S. we have no access to the neutral of the grid. We have three phases and the ground. If I use the following design (please see the picture below), the common mode voltage in the ground will be transferred to the cold side. I don't know if this cause any problems for me or not.

    Here are my two questions I need your advice on:

    1) If I use the first solution (A), is using 50 ohm resistor with compensation components reliable enough?

    2) If I use the second solution, using a virtual neutral helps to lower the common mode voltage effect? Is it a reliable approach for industrial drives?

    Thank you for your time and help.


  • PS1- The reason for having a layer of GND and a layer of 3.3 is interleaved design. I don't wanna loose it by using ISO122 type devices which require a bi-polar power supply.

    PS2- In the above figure, GND in purple is cold side ground, GND in red is grid ground.

  • Hi Moshen,

    You've lost me a little on why you would need to forgo the isolation on the AMC1311 using a level shifting approach on the high side.  For the OPA approach, is your idea to use a summing amplifier configuration?

  • Hi Tom,

    The AMC1301's isolation is still good, but our control side is not isolated from low side anymore. As Navaneeth suggested, we should pull up the input from (-1 to 1) to (0 to 2). For the pulling up purposes, we should use some components, these components need supply, if we use isolated supply then it is a too expensive solution for me, if we use the non-isolated supply, then the ground of control side is electrically connected to the high side and let the CM and EMI noises in the controller side. 

    About the OPA, yes, please see the below:

  • Hi Mohsen,

    OK - I see, thanks for the clarification.  Also, what's your error budget?  The AMC1301 with 50 ohms might be fine - put in a place holder for the R3', but you may not need it in the end.

  • Thank you for your reply. I appreciate it.