+ Post New Thread
Results 1 to 9 of 9
  1. #1
    Junior Member level 1
    Points: 1,108, Level: 7

    Join Date
    Feb 2008
    Posts
    17
    Helped
    0 / 0
    Points
    1,108
    Level
    7

    MOSFET burning when a BLDC changes its direction

    Hi;

    I'm trying to control a BLDC motor using the attached circuit. However, MOSFETS burn when I apply full reverse voltage to brake the motor.
    The bus voltage is 48V. The inductance of the motor is 0.06mH.
    Can you help me figure out what's the problem? Does it have anything to do with TVS?

    Thanks in advance
    Gorkem

  2. #2
    Member level 5
    Points: 938, Level: 6

    Join Date
    Jun 2012
    Location
    Bayreuth, Germany
    Posts
    82
    Helped
    24 / 24
    Points
    938
    Level
    6

    Re: MOSFET burning when a BLDC changes its direction

    1. What is the dead time between switching of High Mosfet and Low Mosfet?
    2. Put snubber capacitance between Drain of High Mosfet and Source of Low Mosfet. About 1uF. I suggest to measure bus voltage during switching and post result here.
    3. What is the reason for TVS. Limit voltage?



    •   AltAdvertisment

        
       

  3. #3
    Full Member level 6
    Points: 2,743, Level: 12
    zsolt1's Avatar
    Join Date
    Aug 2012
    Location
    Cluj-Napoca, Romania
    Posts
    367
    Helped
    50 / 50
    Points
    2,743
    Level
    12

    Re: MOSFET burning when a BLDC changes its direction

    voltage may be OK but you may exceed drain current ... not necessarily only auto induction is producing trouble , you may also consider some delay times between commutations as a cumulative cause ... I suggest to experiment on virtual models of the system (pspice....)



    •   AltAdvertisment

        
       

  4. #4
    Member level 5
    Points: 938, Level: 6

    Join Date
    Jun 2012
    Location
    Bayreuth, Germany
    Posts
    82
    Helped
    24 / 24
    Points
    938
    Level
    6

    Re: MOSFET burning when a BLDC changes its direction

    Quote Originally Posted by zsolt1 View Post
    voltage may be OK but you may exceed drain current
    I have big doubts that without some extra circuit design bus voltage will be OK. At least during switching period induction current will flow to power bus increasing voltage. And I need notice that it is very rough approximation. In real situation parasitic inductions of wires must be considered. Here is good explanation of switching process http://www.mitsubishielectric.com/se...owermos3_0.pdf

    To gsecer. TVS doesn't help you. When load voltage is more than +Vbus, built-in diode of HIGH MOSFET will be conduct, when voltage is low then -Vbus, built-in diode of LOW MOSFET will be conduct. So you should restrict voltage of bus line in switching period.



    •   AltAdvertisment

        
       

  5. #5
    Super Moderator
    Points: 251,556, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    43,804
    Helped
    13320 / 13320
    Points
    251,556
    Level
    100

    Re: MOSFET burning when a BLDC changes its direction

    A possible reason is that the bus voltage is driven above safe limits by the energy recuperation.



  6. #6
    Full Member level 2
    Points: 1,792, Level: 9

    Join Date
    Mar 2012
    Posts
    145
    Helped
    28 / 28
    Points
    1,792
    Level
    9

    Re: MOSFET burning when a BLDC changes its direction

    What is motor resistance? When apply reverse voltage on motor runing, current will be i=(Vdc + Em)/(Rm+Rc), where Vdc is bus voltage, Em is electromotive induced voltage (may be slighty below Vdc at nominal speed and no load, say 45V), Rm is motor resistance and Rc, current sensing resistor. Assuming Rm=1ohm, at first time of full reverse voltage applied, current is aprox. 92A, beyond maxim MOSFET current (72A); check with real values. Also at reversing voltage, MOSFET must support a voltage = Vdc+Em, which mean about 93-96V or even more; maxim Vds of MOSFET is 100V, so very probably that conduct to MOSFET burn.


    1 members found this post helpful.

  7. #7
    Member level 5
    Points: 938, Level: 6

    Join Date
    Jun 2012
    Location
    Bayreuth, Germany
    Posts
    82
    Helped
    24 / 24
    Points
    938
    Level
    6

    Re: MOSFET burning when a BLDC changes its direction

    Quote Originally Posted by iop95 View Post
    What is motor resistance? When apply reverse voltage on motor runing, current will be i=(Vdc + Em)/(Rm+Rc), where Vdc is bus voltage, Em is electromotive induced voltage (may be slighty below Vdc at nominal speed and no load, say 45V), Rm is motor resistance and Rc, current sensing resistor.
    You are right. There will be high current. I think that gsecer can look on the torque-speed characteristic of BLDC and determine maximum current during braking mode.
    Click image for larger version. 

Name:	BLDC.PNG 
Views:	13 
Size:	27.1 KB 
ID:	79196



  8. #8
    Advanced Member level 5
    Points: 38,224, Level: 47

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,181
    Helped
    1792 / 1792
    Points
    38,224
    Level
    47

    Re: MOSFET burning when a BLDC changes its direction

    If the motor is still spinning you might see double the net EMF
    at the FET, on a hard reversal, that you'd see in normal
    start-from-zero or free running.

    Might try a controlled ramp-to-zero, then ramp-to-reverse
    rather than an abrupt full power direction change.



    •   AltAdvertisment

        
       

  9. #9
    Advanced Member level 5
    Points: 34,760, Level: 45
    Achievements:
    7 years registered Created Blog entry Created Album pictures
    SunnySkyguy's Avatar
    Join Date
    Sep 2007
    Location
    Richmond Hill, ON, Canada
    Posts
    6,698
    Helped
    1671 / 1671
    Points
    34,760
    Level
    45
    Blog Entries
    7

    Re: MOSFET burning when a BLDC changes its direction

    You are best to not exceed the motor current rating as well..
    Can you drive with zero Volt by shorting Motor with no Voltage applied then remove short then apply reverse voltage in rapid sequence?

    The FETs ON resistance ~5 mOhm, if much less than ESR of the motor then most of the energy will be dissipated in the motor., but that may still cause over current in either device, so acceleration/braking control is better to limit average current with PWM.
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975



--[[ ]]--