Heat Sink for Power Mosfet

Discussion in 'The Projects Forum' started by ignite, Feb 12, 2010.

  1. ignite

    Thread Starter Member

    Jan 31, 2008
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    Hi

    Im controlling a 12V motor with a stall current of 18A ( Normal Operation ~ 6.5A) with a standard H-bridge. Im switching the MOSFETs at 20kHz. The application is outdoor. Heat-skink will be in a sealed box i.e. no forced convection. How can I determine if the heatsink ive selected will be suitable??
    Space is critical for this design. The plan is to mount two high side fets on one heat-sink and the other2 on another heat sink.
     
  2. ignite

    Thread Starter Member

    Jan 31, 2008
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    Note motor will only run for a max. five min at a time. Normally operate at ~70% speed.
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When the H-bridge is designed properly and the switching of the fet's is fast enough, almost no heat will be generated.
    Most heat will be generated on the switching moment.

    Greetings,
    Bertus
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    As Bertus said, your heatsink requirements are 99% dependent on how you are driving the gates of the FETs. If you do it wrong, they vaporize. If you do it right, they get moderately hot. Without seeing a circuit a some waveforms, there is no way to tell...
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Taking Bertus' suggestion a bit further, if you lowered your PWM rate to a few hundred Hz, the MOSFETs would spend proportionately much less time in the linear region generating heat.
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    Are the MOSFETs all n-channel?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Nope.
    In our OP's initial post, they included the datasheets for the MOSFETS.
    IRF4905 - P-ch Power MOSFET (Vdss=-55V,Rdson=0.02,Id=-74A),Tgatechg=180nC,Rt=99nS,Ft=96nS

    STP80NF03L - N-ch power MOSFET (Vdss=30,Rdson=4m,Id=80),Tgatechg=110nC,Rt=270nS,Ft=125nS
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    Usually, the p-channel ones are used for the high side and the n-channel ones for the low side.

    Is that the case?
     
  9. russpatterson

    Member

    Feb 1, 2010
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    16
    For what it's worth I had a similar issue controlling a 12V 1.5A pump motor. At a PWM at 20Khz I had a heat problem on the MOSFET and did not at 5Khz. I suppose the downsides to lower frequency are the noticable ringing noise and that you get less resolution on the variable current delivered to your motor.
     
  10. mik3

    Senior Member

    Feb 4, 2008
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    Lower the PWM frequency reduces the switching losses of the MOSFETs. However, the ripple current through the motor increases for a given winding inductance and cause additional losses in the motor winding. Conduction losses of the MOSFETs increase too.

    The acoustic noise when the switching frequency is in the audible range is due to vibrating wires/metals and to magnetostriction.
     
  11. ignite

    Thread Starter Member

    Jan 31, 2008
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    The high side P-channel mosfets are driven directly by 4.5A inverting driver. Both the input and output signal of the driver are pulled low with a 1kohm to ground.
    The low side n-channel are driven similiarly with the non-inverting 4.5A driver, I/P & O/P are pulled to ground again.
    Design also features external IN4007 diodes in parallel with each MOSFET (anode facing +12V).
    Both gate signals are PWM. No series resistors between driver O/P and gate. No capactiors.
    Is this the correct design set-up??? As the circuite currently gets very hot?? Is it possible to calculate optimum PWM frequency??

    [​IMG]
     
  12. Wendy

    Moderator

    Mar 24, 2008
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  13. SgtWookie

    Expert

    Jul 17, 2007
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    Interesting, because your schematic does not show it, nor does it show P-channel MOSFETs (they are all N-channel).

    The INPUT of the driver could be pulled low, but the gates should be pulled high, as the source terminals of P-ch MOSFETs go to the most positive power source (Vdd/Vcc/+V), and they are off when Vgs=0.

    The 1N4007 diodes won't hurt anything, but are essentially useless, as the MOSFETs all have integral body diodes that are capable of quite a bit more current than the 1N4007s are.

    It would help a great deal if you would post both your schematic and your board layout. Layout is critical; if you have long runs, they will have a good deal of parasitic L and when combined with the C of the gate, you will wind up with "ringing", causing the MOSFET gates to rapidly turn on and off.

    I am confused as to why you didn't simply use four N-channel MOSFETs with a high-side driver appropriate for the task?

    While it may seem that the high and low side MOSFETs are fairly closely matched for Id (-74A vs 80A), the Vdss is quite different (-55 vs 30), which is one of the reasons why Rds(on) is so different (20m vs 4m) and total gate charge differs (180nC vs 110nC). Additionally, there is quite a difference in the rise and fall times (Rt=99nS,Ft=96nS vs Rt=270nS,F t=125nS).

    You haven't said which side (high or low) that you are PWMing. If you are monitoring current using a low-side Rsense, you're probably PWMing the low side. If you are PWMing both the high and low side, I'll be wondering why.

    It would help to know more about the motor you're driving (stall current, rated voltage) and your supply voltage; with the low side MOSFET rated Vdss=30, I'm assuming you're below 20v.
     
  14. mik3

    Senior Member

    Feb 4, 2008
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    Ignite,

    In your first post you said you want to mount both th.e high side MOSFETs on the same heatsink. This is not possible because the heatsink will be in contact the drain of the MOSFETs. The drains of the p-channel MOSFETs are not connected together and thus they will be shorted if you put them on the same heatsink. You can use an isolating thermal pad as not to short them. Another idea, is to use one heatsink for the left side n and p MOSFETs and another one for the right side n and p MOSFETs.
     
  15. ignite

    Thread Starter Member

    Jan 31, 2008
    18
    0
    Motor is rated for 12V with a 18A stall current, its driven off a 12V 21Ah re-chargeable battery.
    Currently im PWM both high and low side mosfets! Is this a bad idea?? I do not need current sense feedback. Main issue at moment is thermal build up when operating at 20kHz.
     
  16. mik3

    Senior Member

    Feb 4, 2008
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    Do you want the motor to rotate forward and backwards?
     
  17. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    To determine the temperature rise you need to calculate the junction
    to ambient thermal resistance (Tja) ---

    Tja = Tjc + Tcs + Tsa all the dimensions are DegC/W

    Tjc thermal resistance between the IC junction and IC case. This is
    from the IC datasheet. You need to adjust this number using
    the data in the transient thermal response curves. For short
    on times and low duty cycle there is a reduction in thermal
    resistance.

    Tcs thermal resistance between the IC case and your heatsink. This
    should also be in the IC datasheet. Usually you place a thermal
    compound between the IC case and the heat sink to decrease the
    thermal resistance. The decrease occurs because the thermal
    compound fills the air gaps between the two surfaces and conducts
    heat better than the air. Very thin layer just enough to fill
    the gaps.

    Tsa thermal resistance between the heatsink and ambient air. This is
    in the heatsink datasheet. Most heatsink datasheets specify Tsa
    with and without airflow. You can determine if you need a fan by
    reviewing this number.

    For a single heatsource the junction temperature would be

    Tj = Tja * P + Tamb (DegC)

    where P is the power dissipated in the IC and Ta is the ambient
    temperature. For a MOSFET P = rds(on) * Id (NB: use the rds(on)
    value at the actually operating temperature). For BJT
    P = Vce(sat) * Ic (NB: use the Vce(sat) value at the operating
    temperature).

    The maximum junction temperature is listed in the datasheet (usually
    in the "Absolute Maximum" section). I would not run the device at
    a temperature greater than 80% of the the absolute maximum rating.

    (* jcl *)
     
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