Connect with us

IGBT on resistance - conduction loss

Discussion in 'General Electronics Discussion' started by BlinkingLeds, Aug 22, 2013.

Scroll to continue with content
  1. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    Hi, with MOSFETs the power losses are calculated P.loss=R(ds-on) * I^2 (I think)
    how can i calculate the power losses of an IGBT if i can't find Rds (or Rce ?) in the datasheet?

    Suppose we have a MOSFET 600v 20A and an IGBT also 600v 20A which will be better (less losses?) ?

    Thanks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,489
    2,830
    Jan 21, 2010
    IGBT's don't have an Rds. They have a saturation Vce.

    Here is an IGBT datasheet (I just googled for IGBT datasheet and took the first hit)

    On the third page there's a graph of the saturation characteristics. There are several lines, each showing the characteristics for a different gate voltage.

    This is a 1A device, so we go across at 1A and we find that for pretty much ang gate voltage higher than 5V, the voltage drop is 2.5V.

    So this device will dissipate about 2.5W at 1A.

    You don't say what your possible IGBT's are, but you'll find that some voltage drop between 2V and 4V is not unusual.

    This might sound a lot, but the dissipation is linear with current. In a mosfet is it related to the square of the current. That's why you do see IGBT's in very high current applications.

    Imagine a current of 200A. Across 4V, this is 800W. If you wanted to use a mosfet, you would require one with a channel resistance of 20 milliohms or less or it would dissipate more power.

    If you think that's a bit fanciful, here is a datasheet for a 1700V 2400A IGBT. It drops 2.6V at 2400A (that's 6240W dissipated). Imagine trying to find a 1700V mosfet with a channel resistance of 1 milliohm or less?

    At any current though, you will need to do the math for both and compare. (I^2R for the mosfet and IV for the IGBT).

    Note that switching losses may need to be calculated for both as well since a difference in switching speed could easily swing the equation the other way.
     
  3. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    Ok thanks now i know what Vce sat is for.



    So about the switching part... what if we have a 300vdc 20A heater switching at 5khz and duty cycle from 1% to 99%.

    MOSET:
    IPW60R075CP
    Rds-on = 0.075ohm
    max amps = 39A

    and

    IGBT:
    IXYP15N65C3
    Vce sat = 2.5v
    max amps = 38A

    So, which one would you choose without even calculating. Is there an easy answer?

    Thanks
     
  4. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    What is wrong with calculating?

    Mosfet
    P = 20 8* 20 * 0.075 = 30
    IGBT
    P = I * Vce = 20 * 2.5 = 50
     
  5. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    Nothing wrong with calculating conduction losses but i meant switching losses :) . I know it's difficult to calculate.
     
  6. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Why are you using a 5KHz PWM for a heater? Lower switching frequency = less loss, and a heater is not going to be responding at 5 KHz.

    Bob
     
  7. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    Well it doesn't has to be 5khz i am just trying to "dim" it make it consume less power when i want.

    So let's say 500hz. Another question i have is will the switching losses be lower at 1% duty cycle or at 99%?
    Thanks
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,489
    2,830
    Jan 21, 2010
    The switching losses are identical at 1% and 99%.

    The switching losses are proportional to the frequency and the time taken to switch the load.

    In general, the mosfet will be able to switch faster than the IGBT

    Look at the switching speeds for both devices. (typically a rise time and a fall time)

    Say they're 17ns and 7 ns.

    Add these together (24nS of switching time each cycle).

    Calculate the power being switched (V * A) = 300 * 20 = 6000W. That's 6000J/s

    Calculate how many joules are dissipated during each switching event. the rough approximation is 1/2 * switching time * power switched

    1/2 * 24x10-9 * 6000 = 0.072 mJ

    Now we multiply that by the number of times the load is switched per second (let's take 2 cases)

    at 500Hz -- 500 * 0.000072 = .036J
    at 5000Hz -- 5000 * 0.000072 = 0.36J

    Since this energy is dissipated over a second, the power is 0.036W for 500Hz, or 0.36W for 5 kHz.

    This is very small compared to the total dissipation.

    Those figures just happened to be for your mosfet (assuming the ideal gate drive).

    Repeating them for the IGBT, you get something like

    63ns spent switching
    189mJ per cycle
    0.0945W at 500Hz, 0.945W at 5000Hz

    These losses are small, and represent the minimum obtainable losses -- you may not do so well if you do not have a good gate driver.

    Note that there are other losses as well. In this case I believe these are the most significant. If you were driving a highly capacitive or inductive load, you would have to consider them.
     
  9. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    So what's the ideal gate drive? :)

    so switching losses aren't that bad compared to conduction losses at least with resistive loads. I was trying to calculate the switching losses using those huge equations but couldn't yours simplifies it A LOT.
     
  10. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    PWM seems to be well over the top for a heater.
    You could use phase angle control or a solid state relay switched by a timer circuit.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,489
    2,830
    Jan 21, 2010
    The ideal drive is generally applied using a gate driver IC.

    In your case, it's probably not that important and I question (along the lines of what Duke37 is saying) why you need to switch it so fast (for light, 100/120 Hz is fine. For heaters 0.1Hz may be fine.
     
  12. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    Well... I don't deed to switch it that fast i was just using it as an example :) .
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-