SMPS Inverter Repair

Discussion in 'General Electronics Chat' started by hazim, Mar 11, 2011.

  1. hazim

    hazim Thread Starter Active Member

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    Hi

    I have a 300W inverter that I use for a desktop computer with LCD monitor (not CRT...) and a router and few insignificant loads. The inverter has an overload protection (and overheat and short-circuit protections).

    I use it with a 12V 200Ah battery. A 20A charger charges the battery when there is electricity. I installed a relay inside the inverter and wired it as a contactor so when electricity is off the computer (+ router...) they switch to the inverter output..

    Everything was working fine till few days ago when something happened when I was outside home but the inverter was ON as always, the inverter failed to start. I tested it and repaired it successfully after replacing about 20 components (power MOSFETs, BJT transistors, diodes, resistors, photocoupler -317 ...)

    I didn't used the same MOSFETs but I chose suitable replacements on hand.
    Usually, I compare most important parameters to choose I good replacement, but this time while I'm using "vrt-dvd 2009" and looking to transistor FTP18N06 which is used in the inverter and was bad, that software says IRF540 is a good replacement so I used it in place of FTP18N06.

    Today the inverter stopped to work. I found that 2 from 4 IRF540 transistors are damaged. I looked for both FTP18N06 and IRF540 parameters and found Id of FTP18N06 is 59A while that of IRF540 is only 30A. Off-course this means that the IRF540 is not a good replacement.

    This time I used IRF2807 which has a much higher Vdss and Id and Power dissipation, and much lower Rds(on). One thing I'm not sure about is about the switching speed. These MOSFETs are on the 12V side. There are other MOSFETs on the 220VAC side which have Vdss=500V (FQP13N50C), I replaced them the next time with IRF840. I found one of them shorted this time and replaced it.

    No after I replaced the bad components, the inverter is failing to start. The big high voltage capacitor has no voltage across, the rectifying diodes are ok. and the high frequency (ferrite) transformer is ok, and the signals on the gates of the MOSFETs are oscillating at 30KHz (I thought they usually oscillate at a frequency between 50KHz to 100KHz).

    The only thing I doubt with is the IRF2807's Gates' and switching characteristics.

    Any help or idea is greatly appreciated.

    Regards,
    Hazim
  2. SgtWookie

    SgtWookie Expert

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    You also need to look at the total gate charge, usually specified as Qg.

    FTP18N06 - N-ch power MOSFET (Vdss=60,Rds(on)=18m,Id=59,Qg=40)
    IRF2807 - N-ch power MOSFET (Vdss=75,Rds(on)=13m,Id=82,Qg=160)

    The IRF2807 has four times the gate charge as the FTP18N06. Unless you have upgraded the gate driver(s) to handle 4x as much gate charge, your MOSFETs will be turning on and off much more slowly than the originals, and will be dissipating far more power as heat. Using logic-level MOSFETs as replacements can get tricky, as Qg will be specified at 4.5v or 5v instead of 10v; it will be much higher if you're charging them up to 10v.

    Vdss, Rds(on), Id and Qg are all related. Any increase in Vdss, Id or decrease in Rds(on) will result in an increase of Qg; and increasing Qg will make your turn-on and turn-off times worse, resulting in greater power dissipation in the MOSFETs, burning them up. Of course, you also need to match the package type, so that's five parameters you need to match as closely as possible.

    Here are some possible substitutes for you to try for the 12v side:
    FDP65N06, MOSFET N-CH 60V 65A TO-220, Fairchild
    BUK7511-55B,127, MOSFET N-CH 55V 75A TO220AB, NXP
    AOT462, MOSFET N-CH 60V 70A TO-220, Alpha & Omega Semiconductor Inc
    IXTP90N055T2, MOSFET N-CH 55V 90A TO-220, IXYS
    FDP13AN06A0, MOSFET N-CH 60V 62A TO-220AB, Fairchild

    I found the above parts on Digikey using their parametric search engine; Vdss from 55v to 75v, Id from 57a to 90a, all TO220-x package variants, gate charge near 40nC, etc. I have no idea if you can obtain any of those in Lebanon.
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  3. hazim

    hazim Thread Starter Active Member

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    Thank you very much SgtWookie. Actually it's hard to find these parts in Lebanon, and even I'm far from Beirut where I get parts, so I try to substitute from on-hand parts (I have a "good variety" :) )
    I understand what you said but I had tried IRF2807 and they didn't heat up, maybe there is another problem but I thought the problem is with the transistors' switching as said before. I'll try to fix the main problem first and see.

    Regards,
    Hazim
  4. hazim

    hazim Thread Starter Active Member

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    I found the problem, I was wrong. There was two shorted transistors (2PN733) in the oscillator circuit that drives the four low voltage transistors' gates (FTP18N06). I replaced them with PN2907A regarding the pin-out incongruity by rolling up a pin.
    I kept on the IRF2807 and tried the inverter on its rated power capability and it seems to be working fine.
  5. hazim

    hazim Thread Starter Active Member

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    Back to this thread. This is the third time I repair this inverter... After I replaced the low voltage MOSFETs, the inverter worked fine for two days only, and for 4 continuous hours on a real load.. Now I found the high voltage MOSFETs bad, initially they were FTP06N40, the first time I replaced them with IRF840. Now I have FQPF10N60 and replaced the bad IRF840 with FQPF10N60.

    FTP06N40: Vdss=400V(shouldn't it be 450V or more since the output voltage is 220V?); Rds(on)=1Ohm; Id=5.5, Vgs=+/-30V; P=95W

    IRF840: Vdss=500V; Rds(on)=0.85Ohm; Id=8A; Vgs=+/-20V; P=125W

    FQPF10N60: Vdss=600V; Rds(on)=0.73Ohm; Id=9.5A; Vgs=+/-30V; P=50W

    Note that in the last transistor, Pd is only 50W because it is insulated but Rds(on) is lower then the original transistor (50W<95W & 1Ohm>0.73Ohm) so this one 'may' fit as a replacement.

    any idea?
  6. SgtWookie

    SgtWookie Expert

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    The original HV side MOSFETs were:
    FQP13N50C - N-ch power MOSFET (Vdss=500,Rdson=0.48,Id=13,Qg=43)
    This is what you stated in your original post. Now you're saying they were FTP06N40 MOSFETs, so I don't know what they were.
    FTP06N40 - N-ch power MOSFET (Vdss=400,Rdson=1,Id=5.5,Qg=18.6)
    You need to match or better the specifications of the original MOSFETs used, along with the power dissipation rating. You might be able to use slightly lower Vdss ratings.

    220VAC would have around 312v peak to peak.

    I don't think any of the MOSFETs you found will work for very long.

    You might be able to use one of these if you can find them:
    IXTP16N50P
    STP19NM50N

    You should really use a parametric search engine like on Fairchildsemi.com, Digikey.com, and other sites to narrow down what MOSFETs you might be able to use as a replacement. I can't do that for you, as I have no idea what you might be able to find in your area, or be able to order.
  7. hazim

    hazim Thread Starter Active Member

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    I made a mistake. The HV MOSFETs are FTP06N40.

    This in a (small) 300W inverter. I have two similar 600W inverters from the same brand of the 300W one, one of the two 600W inverters uses FQP13N50C HV MOSFETs, this made me mistaken. And by the way, the other 600W inverter uses FTP10N40 insteed of FQP13N50C. The low voltage MOSFETs are not the same too.

    I usually use a software I have and sometimes use digikey to search for replacement transistors, but I give periority to transistors I have and look if any works fine as a replacement, if not I compare the parametric search results with parts available in Lebanon (I look herefor parts in Lebanon.. http://www.ekt2.com/ekt/prodList.asp?idCategory=4)

    Anyway, I'll get STP20NM50N (most probably available) as a replacement for FTP06N40.
  8. SgtWookie

    SgtWookie Expert

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    For this MOSFET:
    FTP06N40 - N-ch power MOSFET (Vdss=400,Rdson=1,Id=5.5,Qg=18.6)
    Try an IRF730; that supplier carries them.
    The IRF730 Qg (gate charge) is somewhat higher @ 22nC than the original part's 18.6nC, but the other specifications are virtually identical. This means turn-on and turn-off times will be slower, and more heating. You may need to improve the heat sinking and/or airflow through the converter, or de-rate the converter output by about 15%.
    hazim likes this.
  9. hazim

    hazim Thread Starter Active Member

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    In post no. 5 I underlined the Vgs of IRF840 which is +/-20V and which is the same for IRF740, while it is +/-30V for FTP06N40. I think IRF740A (with suffix A) has Vgs=+/-30V, but don't think it's available in Lebanon.
    I belief using STP20NM50 should be fine especially that the load is not more than 200W while the inverter is 300W rated. Also I may use FQP11N40, it depends on availability since this straitens choices.

    When looking for a MOSFET replacement, in addition to current, voltage, Rds(on),power, I used to look for switching speed parameters and specifically on td(on) and not the total gate charge. I learned about it from this thread so thank you, and thanks for helping :).

    Regards,
    Hazim
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