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Original question:

UCC21520: Recommendation for gate driver power supply

UCC21520: Same power supply for VCCI, VDDA and VDDB and Grounds

Sidd Bhattacharya
Prodigy 145 points
Part Number: UCC21520
Other Parts Discussed in Thread: UCC21530, TIDA-01605, UCC5350

Hello, 

I am planning to use UCC21520 for SiC MOSFET based full bridge  design. My operation frequency would be around 100kHz.  I selected UCC21520 in order to achieve the isolation between the power and signal side. However, during design I am stuck at deciding how to select power supply for the VCC and VDDA/B.

My questions are following: 

1. If I use same power supply for VCC and VDDA/VDDB ( say 15V) would that work? I would use a laboratory 15V power supply ( isolated) and connect the ground of the power supply to the ground pin of the IC. 

2. If I indeed need to use two power supplies say 3,3V +GND1  for VCC and 15V +GND2 for both VDDs, how do I deal with two new ground points? and how would they connected to the GND pin of the IC. 

3. Speaking of grounds, I am planning to carry over the PWM signal to H-Bridge board from another PCB that has its own auxiliary power supply. I will carry over the PWM signals and and respective ground ( SGND) to the H_bridge PCB. How should I deal with this SGND with respect to aforementioned GND1 (and /or GND2). 

Since, aim of using the IC is to achieve isolation from SGND and PGND, I am really confused how to deal with other grounds ? Your kind support is solicited. 

  • 0
    TI__Expert 3355 points

    Hi Sidd,

    Thank you for your question. I work on the applications team in the high power drivers group.

    If your goal is to gain full isolation between primary and secondary, you will need to use separate isolated power supplies for input and outputs respectively. The output side will need separate isolated bias supplies for low-side and high-side respectively, since the high-side driver VSS is referenced to the switch node.

    I recommend taking a look at the reference design linked below. This reference design shows how to use isolated supplies to gain full galvanic (high impedance) isolation between primary and secondary sides of your isolated gate driver.

    https://www.ti.com/tool/PMP21553

    Since you are planning to drive SiC FETs, I would recommend taking a look at UCC21530. This device should perform the same as UCC21520, but has a higher creepage distance from CHA to CHB and higher UVLO to protect your SiC FETs from being underdriven. The UCC21530 datasheet also contains a good typical application schematic in figure 38. This should give you a good reference for what I am recommending.

    If you have more questions, can you please help draw how your circuit currently looks, so I can understand it better? It is hard to keep track of net names when there is no reference schematic.

    If this answered your question, could you please press the green button?

    Thanks and best regards,

    John

  • 0 in reply to John Geiger
    Prodigy 145 points

    HI John, 

    Thanks for your reply. I found a reference design of TIDA-01605 which also gives a overview how to connect them properly. 

    One question, do this gate driver chip works well with a PWM signal of 3.3V (high) - 0 (low)? A colleague of mine was telling me that ´he had encountered some problems with low voltage PWMs and would love to hear your insight on this matter.  

    Best,

    Sidd

  • 0 in reply to Sidd Bhattacharya
    TI__Expert 3355 points

    Hi Sidd,

    This device has TTL input thresholds, which are 1.8V rising, and 1V falling respectively – fully compatible with 3.3V PWM.

    We have occasionally seen this type of confusion when using other drivers (such as UCC5350) with CMOS based input thresholds. CMOS based thresholds have input rising and falling levels that are based on VCCI voltage, and can potentially require signal level shifters if using a higher VCCI voltage with a low voltage PWM signal. That is not the case for this driver.

    If this answered your question, could you please press the green button? If not, feel free to ask more questions.

    Thanks and best regards,

    John