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LM74670-Q1: Design for Full-bridge rectifier for resonant converter secondary stage with LM74670-Q1

Sidd Bhattacharya
Prodigy 145 points
Part Number: LM74670-Q1
Other Parts Discussed in Thread: TIDA-00858, CSD18532KCS, , LM74700-Q1, TLV1805

Hello, 

I am designing a LLC resonant converter with output rated at 3.5kW/ 28V. Since the current drawn from load is around 120A, even the rectifier stage with SiC Schottky diodes causes a lot of conduction losses and makes the converter efficiency really poor. 

I came across LM74670 to use MOSFETs as smart diodes and find it very interesting.  I looked into TIDA-00858 reference design and planning to test out similar design to see if it fits with the customer requirement. 

I have a couple of questions and would be awesome if you can clarify these: 

1. Do you think the Full bridge rectifier technology can be used in resonant converter (like LLC) secondary stage to rectify square waves signals? My switching frequency is around 150 kHz.  I am just planning to use them as just for rectification stage without any smart-logic. Do you see any potential problem with this?  

2. My load current is around 120A, hence I think the MOSFETs (CSD18532KCS) used in TIDA-00858 will not cause thermal problems (ofc with good heatsink combination).  Do you agree with this or should I look for MOSFETs with higher current rating. Awesome would be if you can recommend me something with very low Rds,on   that is good compatible with the IC.  What additional recommendation you suggest to the ref design to accommodate high current. 

3. Do you have any literature to calculate conduction losses in such Smart diodes in bridge configuration? Some clarification or hints on that would be nice. 

Thank you and looking froward to hear from you. 

Regards,

Sidd 

P.s We hope you and yours are staying healthy and this corona situation will be over soon. 

  • 0
    TI__Genius 16305 points

    Hi

    This is an interesting problem, I am looking into your question and will get back in more detail sooner today.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 145 points

    Sure. Looking forward to hear from you. 

  • 0 in reply to Sidd Bhattacharya
    TI__Genius 16305 points

    Hello Sidd,

    Firstly, the reverse voltage of LM74670 is -42V (ANODE-CATHODE). Need to check if there is any condition where this will be violated for 28V secondary side of the LLC. 

    Regarding the TI Design TIDA-00858, need to mount the bulk capacitors on the output for the design to work properly (diode rectification)

    For a 150KHz design, the gate drive current of LM74670-Q1 is not enough to continuously turn ON big MOSFET quickly. 120A is lot of current and the selected MOSFET power dissipation capability would not be good enough. Need to add more of the in parallel and heat sinks.4 of them in parallel would be good.

    But nevertheless, LM74670 controller is not capable to drive such big MOSFETs at 150Khz.

    Is there any possibility of using LM74700-Q1 which has 10x more gate drive to drive MOSFETs.

    Only issue here is that this has the GND pin and need to find a way to bias this IC to work in LLC secondary path.

    Can you let us know if LM7470-Q1 GND connection and supply bias is possible, I am not an expert on LLC.

    Regards,
    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 145 points

    Hello Kari, 

    Thank you for your kind reply. Yes I feared the same due to huge amount of current. Since in the manual of the reference design it is stated that the bridge can handle up to 100A mad me a bit optimistic. So my questions to you now: 

    1. If I understand correctly if LM74670  is unable do drive parallel high power MOSFETs. In that case would it be wise to drive 4 parallel MOSFET for each leg with a dedicated LM74670  for each? In that case there would be 4X4 Mosfets for 4 legs and 16 LM74670s in total. 

    Being said that does LM74670  support parallel operations? Would it cause current sharing problem considering the are not SiC body diodes? 

    2. Another possibility I see is to use a dedicated gate driver which would utilize the LM7460 signals as PWM input and provide output gate signals accordingly. Her it would also enable me to use SiC MOSFTETs with high Vgs. (hence lower Rds,on) . What do you think? 

    2. I find LM7470 also interesting. However I am not sure about the EN pin + GND pin connection and about functionality. Can you clarify this further? If I use LM7470 for each leg of the full bridge ( in a similar fashion like LM74670 ref design)  and connect the EN and GND pin of each of LM7470 to an external bias ( with a galvanic isolation from the rectified output) will it still do the job? Or must I connect the EN and GND to the anode and cathode pins of the corresponding switch. Arranging an external power source will not be a problem but it won't have a common ground as the ground of the full bridge rectified output. 

    Looking forward to hear from you. 

  • 0 in reply to Sidd Bhattacharya
    TI__Genius 16305 points

    Hello Sidd,

    Thank for quick response.

    But at this point due increased number of questions, i would need some time to get back.

    Can I check this out on Monday 6 April?

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 145 points

    Sure thing Kari. Wish you a nice weekend! 

  • 0 in reply to Sidd Bhattacharya
    TI__Genius 16305 points

    Ok. Will get back as posted to you earlier.

    Regards,
    kari.

  • 0 in reply to Karikalan Selvaraj
    TI__Genius 16305 points

    Hi 

    Using 4x LM74670 for each of the 4 parallel MOSFETs is not a cost effecitve solution.

    We are investigating using 2x LM74700 for driving the full bridge, let me get back to you soon on this.

    I would require some more time to check and get back to you.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    TI__Genius 16305 points

    Hi

    I will require time till thursday or friday this week worst case, but will try to get back earliest.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    TI__Genius 16305 points

    Hi

    I have looked into this thread in more detail.

    1. Using 4x LM74670-Q1 to drive 4x MOSFETs would possibly work, but we have not tested them in parallel in bridge recitfier combinaiton. Running simulation would help here. Current sharing with diode is not a big concern, but once all the MOSFET are on, the PCB trace impedance needs to be matched and thermals taken care. 120A is huge and PCB design plays a big role.

    2. Using Gate drivers: LM74670 is not referenced to GND and translating the gate ON/OFF to gate drivers would be tough. Additionally you can also look at using comparators such as TLV1805 to sense reverse current and turn of and another to turn on in forward conduction. 

    3. Next on the use of LM74700-Q1 EN and GND connection can not be isolated because internal comparators are biased with reference GND. Ideally two high side ORing with high gate driver is required for to drive the TOP MOSFETs (High side or +ve polarity) and two low side ORing drivers with high gate drives currents are required. 150KHz and 120A would require very high power charge pump strength to continuously turn ON/OFF the large gate charge of MOSFETs.

    4. Using external comparator circuits will enable to provide sufficient gate drive current to drive the big MOSFETs.

    Please let us know your comments.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 145 points

    Dear Kari, 

    Thanks for your detailed reply. 

    Since the other options now seems very tricky, I would like to focus mainly on point one since this look a viable option for me. I will try to do a couple of simulations in TINA to check if i get some meaningful results.

    I will try to make a new PCB 4x4 controller on board. Is it possible to send the designed PCB to you for an initial review? if yes please send me if possible your email address, That would really help me out  a lot. I am planning to use high thickness Cu PCB to address the high current issue.  I am happy to share my test results to you if you are interested. I would definitely require a week or so to finish sims and design. 

    Look

    Cheeres! 

    Sidd

  • 0 in reply to Sidd Bhattacharya
    TI__Genius 16305 points

    Hello Sidd,

    Yes we can support to review the simulation and PCB review.

    I have sent a friendship request, to share the email id.

    Regards,
    Kari.