L297 with MOSFET H-Bridge

Discussion in 'General Electronics Chat' started by joprinz, Oct 7, 2008.

  1. joprinz

    Thread Starter New Member

    Oct 7, 2008
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    Hello Everyone,

    I have been using the L297/L298 based stepper drivers for driving stepper motors for my hobby CNC Router. I was using 2A 155 Oz.In Portescap steppers earlier. But the feed as well as the Jog rate was very low. So I wanted to upgrade the steppers to a higher torque one. But most of the steppers i found are rated 5A and more. So I cannot be using the L297/L298 combination anymore. I did some reading over here and found a few people using the L297 to drive a MOSFET H-Bridge and thus providing Higher currents to the motor. However I am not able to find a scematic for the same.

    I hope someone can provide me guidance for the same. Any Schematic/Circuit idea/suggestions/opinions regarding the same s highly appreciated.

    Please Advice!

    Thank You,

    Joe!
     
  2. mik3

    Senior Member

    Feb 4, 2008
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  3. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    mik3,

    He is asking how to use the driver to drive MOSFETs, he already has those circuits in use. He knows he needs MOSFETs to drive the motor already, that is his question.

    I'm looking into it.

    Steve
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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  5. joprinz

    Thread Starter New Member

    Oct 7, 2008
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    Thank You Mik3, Steve.

    Steve, well I have a couple of A3986 with me. But most people who have used the A3986 complain about its inherent step inaccuracies. In micro-stepping mode, using voltages above 30 it seems the issue gets further amplified. I'm using 48V supply for my steppers and I was advised to not use the A3986 for CNC applications in that voltage range. I believe you have experience using the A3986. Please let me know if you have experienced such issues.

    Please Advice!

    Thank You.

    Joe!
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Hi Joe,

    No problem for the response. I actually own a CNC machine, so it is of interest to me. Although, I used brushless servos instead of steppers.

    Unfortunately, stepper motors cannot be trusted for precision in the sub-step. That's their intrinsic nature and cannot be changed. Of course, you may overkill the amount of current through the coils in order to maintain some sort of precision via brute force. Do you really need better than 1 step resolution anyways? What is the lead of your screws? What kind of precision are you trying to achieve?

    I'm guessing that the A3986's gate drive is fairly weak and people are not picking suitable FETs. Then, they drive to step them to the limits and are losing steps because of this. The datasheet eludes that the designer should use about 1000pF gate capacitance FETs. The DIY CNC crowd probably get the ultra heavy duty FETs that won't go well with these.

    There are some modern FETs that may still work well with the allegro part mentioned.
    http://www.st.com/stonline/books/pdf/docs/10907.pdf
    It will need a heatsink for anything over 4A. You can run this to 10A without an issue with a modest heatsink.

    I haven't used this specific driver before, but I have used others in the past with success. They make a good product as far as high integration and simplicity goes.

    Steve
     
  7. joprinz

    Thread Starter New Member

    Oct 7, 2008
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    Thank You Steve.

    Well to be honest I do not need anything more than single step or half step. But Microstepping would be great as it helps with reducing the resonance n also might help in running the steppers smoother. There is a LSI/CSI chip LS7290 that can be used as a direct replacement for L297. Only difference is that, the LS7290 can Microstep. I was planning to replace the L297 with the LS7290 once I have come up with Higher Ampere (>5A) L297/MOSFET H-Bridge schematic. This way we cud get a Microstepping Bipolar Driver that can provide more than 5A.

    But I'm not familiar with Discrete MOSFET based H Bridges. I hope someone can help me with a schematic/Circuit Idea/Advices for the MOSFET H-Bridge section of the L297 Driver with PWM current control.

    Please Advice!

    Thank You,

    Joe!

    PS: Mean while I'l also check with the availability of the STF40NF06 MOSFET You (Steve) suggested.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Hi Joe,

    No problem :)

    Driving H-bridges can be tricky, this is why I advice using a pre-made solution rather than trying to modify something for which it is not intended. You can use good FETs and still use that driver, it shouldn't be a problem. As I mentioned, it was probably because the guys that are modding the electronics aren't understanding the circuitry and thus getting poor performance.

    If you are planning on using a higher power driver for the h-bridge, then use better FETs. The one I suggested is optimal for the allegro driver and your application. If you have more gain drive, then you can get lower RDSon (lower power losses) from your fets. Once, I did an H-bridge driver for a 30A load at 12V and didn't use a single heatsink on a small SMT device. This is because I had a huge gate drive with FETs that can take advantage of it. This is because they parallel many FETs on the same die, which lowers their on-resistance at the expense of higher input gate charge.

    I would really need to look into modifying the LS7290/L297 for operation. It seems to me that you may be able to drive a complimentary-FET mosfet bridge (PFET top NFET bottom), but I would be worried about shoot-through.. This isn't an ideal driver. Sure, it can provide a lot of current, but I do not think the response is fast enough. Check out the slew rates of the outputs, which would negate the ability to produce current for the FET gate. Hence, keeping the FETs in their linear region.

    I recommend sticking to the allegro part and using the indicated MOSFET. Do you know anyone that can do the PCB layout for it? It may be a bit tricky to do and you should get it done professionally. You will need a minimum of 2-layers, perhaps 4.

    Steve
     
  9. joprinz

    Thread Starter New Member

    Oct 7, 2008
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    Thanks again Steve... Infact i'm heading in the direction U suggested. So I've started a parallel project to design a A3986 board. Biggest Hiccup is the A3986 TSSOP package... Man THEY ARE TINY :eek: ...:D... But I guess I'l manage :)

    And about the MOSFET U suggested. I checked its availability but it seems to be not available. But I found IRF540 which has similar Gate Capacitance and about 49mΩ Rds(On). Some A3986 Schematics I found online seem to be using IRFZ24N or IRFZ44N. I was wondering if any of these would suit for my application.

    DataSheets

    IRF540

    IRFZ24N

    IRFZ44N


    if not, can U please suggest other commonly available MOSFETS that can be used?

    Thank You,

    Joe!

    PS: I still have the "L297 with MOSFET H-Bridge" Project open. If someone has experience/ideas/suggestions about the same plz advice!
     
    Last edited: Oct 8, 2008
  10. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Joe,

    I hear ya, I just did a brushless driver with an allegro IC and it wasn't pleasant. Fortunately, I went 4-layers because it is a one-off prototype, so it was fairly easy to do. I wouldn't want to do it on 2-layers. Allegro suggests a lot of kelvin connections, multiple grounds, etc., which are hard to do on a 2-layer PCB.

    I was actually considering switching over my CNC machine to steppers because my encoders are a pain, along with the PID controller I am using. I would rather run openloop with much higher torque than I would need. Steppers are horribly inefficient, but remarkably simple to use..

    IRF540 Cin is about 2000pF, too high for reliable switching, try to keep it under 1000pF
    IRFZ(2/4)4N - VDS cutting it too close, 55V VDS on a 48V supply is asking for trouble, especially because of the inductive nature of what you're driving.

    You caught me on my knowledge of what VDS should be. I cannot think of a good calculation from a model that will give you a rule of thumb about what the VDS spec should be. In the past, to be honest, I usually just double the supply voltage.

    Can anyone enlighten us ??

    Steve
     
    Last edited: Oct 10, 2008
  11. joprinz

    Thread Starter New Member

    Oct 7, 2008
    9
    0
    Steve...

    Finally found the STF40NF06 you suggested. It was on the Farnell Site. I be getting them by Monday. Will post updates sooner.

    Thank You,

    Joe!
     
  12. yakkov

    New Member

    Feb 21, 2009
    1
    0
    Joe,
    I found your thread while digging the internet. My requirement is very similar to yours.
    Found the correspondence in this thread highly interesting and enlightening.
    What happened ultimately. What is your final solution?
    I am curious.
    It is surprising that there is not a single reference design in the internet, using L297 and an external MOSFET H-Bridge.

    yakkov
     
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