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Thread: Car SMPS Project

  1. #41
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    @badboy

    If the alternator is capable of delivering 90 amps then it should be strong enough to give the full voltage at 70A with adequate revs from the engine. The voltage regulator in the alternator will do this job.

  2. #42
    Quote Originally Posted by Silvio View Post
    @badboy

    If the alternator is capable of delivering 90 amps then it should be strong enough to give the full voltage at 70A with adequate revs from the engine. The voltage regulator in the alternator will do this job.
    thanks man

  3. #43
    Hi again

    I manage to build a full bridge inverter to drive my amplifier in the car
    everything is good except mosfet's overheating under load
    here's a picture of primary transformer without any output load :
    *** Oscilloscope Volt/Div is on 2 (((Channel 2))) and the probe is set to X10


    1.jpg

    and here's a picture of primary transformer with 500W load:
    2.jpg

    Could those sparks be the cause of mosfets overheating ?????
    Last edited by badboy_6120; 09-05-2018 at 05:29 AM.

  4. #44
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    @badboy

    Mosfet current rating should be 3 times the operating current. EX, for 3 amp load max current of mosfet is 9 amp.

    voltage spike will not help either, did you try to suppress it?

  5. #45
    Quote Originally Posted by Silvio View Post
    @badboy

    Mosfet current rating should be 3 times the operating current. EX, for 3 amp load max current of mosfet is 9 amp.

    voltage spike will not help either, did you try to suppress it?
    I use two IRF1404 in parallel at each channel
    at 500W in the output there should be around 40A in input going through the masfets!!!!

    I used snubber as well to controll those sparks and I did eliminate them but the overheating is still there

  6. #46
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    Quote Originally Posted by badboy_6120 View Post
    I use two IRF1404 in parallel at each channel
    at 500W in the output there should be around 40A in input going through the masfets!!!!

    I used snubber as well to controll those sparks and I did eliminate them but the overheating is still there
    Did you buy genuine transistors? Be careful as it happened to me. I bought IRF740 from aliexpress and also E-bay and they where fakes. They could not withstand the current and at only 1/3 the current they where getting very hot.

    I Broke one of them in the vice and saw that the die was half the size than a genuine one.

    Note A mosfet is made from a lot of small mosfets in parallel diffused on the same die. If the die is small then there is not the amount there should be hence the current rating will suffer.

    I have a video on Youtube where I found a way to check them by measuring the internal capacitance. (The more capacitance the more fets there are on the chip)

    Lastly IRF1404 can withstand 115 amps at a case temperature of 100 degrees Celsius. They are suppose to handle the load very easily.

    Here is the link https://www.youtube.com/watch?v=gRRfb5YxvIA

    Regards Silvio

  7. #47
    Quote Originally Posted by Silvio View Post
    Did you buy genuine transistors? Be careful as it happened to me. I bought IRF740 from aliexpress and also E-bay and they where fakes. They could not withstand the current and at only 1/3 the current they where getting very hot.

    I Broke one of them in the vice and saw that the die was half the size than a genuine one.

    Note A mosfet is made from a lot of small mosfets in parallel diffused on the same die. If the die is small then there is not the amount there should be hence the current rating will suffer.

    I have a video on Youtube where I found a way to check them by measuring the internal capacitance. (The more capacitance the more fets there are on the chip)

    Lastly IRF1404 can withstand 115 amps at a case temperature of 100 degrees Celsius. They are suppose to handle the load very easily.

    Here is the link https://www.youtube.com/watch?v=gRRfb5YxvIA

    Regards Silvio
    as you said I checked the output capacitance of my mosfets and I get about 8.7 nf
    I think they are good to go

  8. #48
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    Quote Originally Posted by badboy_6120 View Post
    as you said I checked the output capacitance of my mosfets and I get about 8.7 nf
    I think they are good to go
    You cannot tell exactly until you make some comparisons with a known genuine fet. The input capacitance tend to vary according to frequency and voltage of operation.

    Do they get hot at a lower load say at 20A? Do not get fooled about the parameters as current is always limited in these fets especially if they are in TO220 package. The transistor leads themselves cannot carry more than 60 to 70 amps and they must be kept as short as possible. The traces at the pcb should be wide as possible to eliminate any heating.

    If all seems good and wave form, frequency, flux density etc then it could be the fets themselves not capable for the load.

    I would like you to check the voltage at the gates when operation on high load. See that it is not dropping in any way especially the high side gate. See also that you have adequate dead time.

    Silvio
    Last edited by Silvio; 09-07-2018 at 12:13 PM.

  9. #49
    Thanks Dear Silvio for your helps

    So it turned out that my multi meter was reading incorrectly (Broken) and the temp on the back metal part of mosfets never reach 50 degrees with a low value snubbers (I use another multi meter)
    So now I need to know how exactly I can choose the best values for my snubbers????? (As I see experimentally higher the value of Capacitor in snubbers between Drain @ Source, Lower the amount of Sparks and therefore less heating on mosfets)

    I manage to draw my Schematics and also took some pictures of my project:

    CarSmps.jpg

    IMG_20180907_201708.jpg
    IMG_20180907_201715.jpg
    IMG_20180907_201720.jpg
    IMG_20180907_201726.jpg
    IMG_20180907_202041.jpg


    Could you also look at the schematic and see if there is any problem with it??????
    Thank you

  10. #50
    I also use one 2.7 ohms resistor for every two mosfets parallel in every channel
    should I use a higher resistor value or use a seperate resistor for each one of the 2 parallel mosfets??

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