Newbie question about MOSFET

Discussion in 'General Electronics Chat' started by urbaud1, Oct 17, 2010.

  1. urbaud1

    Thread Starter New Member

    Oct 17, 2010
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    I am new to this forum and I am not an engineer. I hope this is the right forum for this question. I’m posting this question with the hope that someone out there can give me a bit of guidance to my problem. Here it is: I have an rc (radio control) receiver (for a model plane) that has a MOSFET as part of the circuit board. I THINK it’s a MOSFET because I have done some searching on the internet. The characters on the component are as follows: T430 BL7F1L. I’d like to get another one to replace the one on the circuit board. I had a friend look at the board and he believes that it is burnt out. The receiver has a motor speed function in it and when the motor on the plane overheated he thinks it also blew the MOSFET. Hence the reason for wanting to replace it. Because this component is not working correctly, I can’t vary the speed of the new motor (I replaced the burnt out motor). The receiver is powered by a 11.1V, 1300mAh 14.4Wh Lithium Polymer battery; 15C continuous discharge, 20C burst. It would be relatively easy to un-solder it and replace it. Does any one know where I could purchase the MOSFET?
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
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    You will need to post a picture of the component to be absolutely sure.

    The size and shape of a component tells a lot more besides the part number.
     
  3. tom66

    Senior Member

    May 9, 2009
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    Unfortunately you may not be able to get a direct replacement...

    However, if you post a picture of the circuit board we can see the configuration of gate, drain and source and thus find an appropriate replacement.
     
  4. urbaud1

    Thread Starter New Member

    Oct 17, 2010
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    Hi eblc1388 and Tom66,

    Thank you so much for replying to my post. I have included two photos of the circuit board. I hope they will help you determine what kind of MOSFET I need. Also, eblc1388, you suggested that knowing the shape and size would be helpful. I believe you'll be able to see the shape from the photos. As for the size it is: 10mm wide and 15mm long or 5/8inches long and 13/32inches wide. I hope the photos and info on each photo is useful. I feel hopeful that you will be able to help me find the right MOSFET. Again, thanks for the reply to my post.
     
  5. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    If you told us the maker and model for the receiver they would or should have a schematic and a parts list, and that makes everything much easier.

    Any Manufacturer really should have that information - so it is just a matter of how they want their references on the internet to turn out that will decide for them whether that info is worth providing.

    It is my experience that asked nicely and with the correct approach you can get the information you need and the trick is just finding out who to ask.

    Assuming you email the manufacturer, don't expect the first person to read your request to be the right person, so you ask them if they can forward it to their designers and manufacturers, or service center. They may actually want to ask you questions about the failure because that might have relevance to how they design and manufacture newer versions. As a minimum they should tell you what the part you need is - in terms of what will work for a replacement if your current FET is not likely to be available. Expect a warning about the maximum ratings for their equipment, questions about the motor you are using, and a suggestion of another module they make that would perform better with your motor.

    It is normal for the people in this type of business to be avidly involved in the hobby. That should work in your favor.
     
  6. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Or you can just reverse engineer to which is the gate, drain, and source.

    A hint here, is that the burnt trace is connected to either the Drain or Source.

    The other of the Drain or Source should be connected to either the motor power lead or the battery ground.

    The gate is the remaining pin and will connect to a signal source- likely an IC (Integrated Circuit.)

    Your FET is probably NMOS.

    It might require a logic level threshold.

    http://www.st.com/stonline/books/pdf/docs/15617.pdf

    That is a part with a Gate Drain Source pin out that you should be able to pick up for a dollar or two.
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Also make certain you never use any extension wiring between the motor and the controller in the receiver. The battery should a little more tolerant of extension wiring but still better to avoid them.

    You can always consider an extended prop drive shaft so that you can put the motor back closer to the center of gravity.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
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    The T430 MOSFET is probably the AOT430, 75V, 80A as this is the sort of MOSFETs commonly used in power controller. The price is reasonable too so you might consider ordering more than one. Make sure the one you ordered has a body/package shape of TO-220.

    As the stock quantity shown is ZERO, you would need to search around to see if you can locate this part somewhere else. I offer you a close alternative STP75NF75 as a last resort which is more commonly accessible should you fail to locate the original part.

    One note. The burnt copper trace on the board that your friend has to patch up had acted somewhat like a fuse. Now the trace had been bridged with a solid copper wire and you will lost that function. But hey you have rescued the controller once so that's OK unless you are so unfortunate as to have another motor failure.

    [​IMG]

    [​IMG]
     
  9. urbaud1

    Thread Starter New Member

    Oct 17, 2010
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    Hi Potato Pudding(PP) and eblc1388,

    Again, thanks for responding. PP I don't know who the manufacturer is. eblc1388 thanks for the data sheets. I have attached a hand drawing of the bottom of the board and identified the wiring. Hopefully, this will confirm for you regarding the suggested replacement MOSFET. If after reviewing the wiring you determine that a different MOSFET is required please advise. I have to thank you both again for your assistance in this problem of mine.
     
  10. tom66

    Senior Member

    May 9, 2009
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    It's apparent then that from the top on your diagram, pin 1 is the SOURCE, pin 2 is the DRAIN and pin 3 the GATE.

    The MOSFET is configured on the low side and grounds or disconnects the motor negative to turn the motor on or off.

    What is the battery voltage?
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Actually I am thinking that the pin out is gate, drain, source.

    It seems that way from the pictures (look at the photos) that is how the pins are running, after looking at the description of the wiring.

    I suspected that pin 1 was the gate before.

    I stick with recommending a STD27N3LH5 unless your batteries and motors are likely to be running more than 15Amps average, and 50 Amps pulse I think it will work.

    It should be more efficient and cook less.

    It is cheaper.

    The higher voltage and current ratings of the original are not needed unless there are major flaws in the design which is a possibility.
     
  12. urbaud1

    Thread Starter New Member

    Oct 17, 2010
    13
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    Hi tom66,
    The battery voltage, as mentioned above, is: The receiver is powered by a 11.1V, 1300mAh 14.4Wh Lithium Polymer battery; 15C continuous discharge, 20C burst. I have sent a new diagram, outlining that the top pin on my diagram is pin 1, the middle pin is 2 and the bottom pin is 3. I entered the pin numbers where you said you thought they were. Pin 1 = source; pin 2 = drain; pin 3 = gate. As you know, eblc1388 is suggesting that I use: AOT430, 75V, 80A, T0-220 or STP75NF75 as a close alternative while Potato Pudding (PP) is suggesting that I use: STD27N3LH5.

    Also, I think PP is saying that he thinks pin 1 is the gate, whereas you're saying you think pin 1 is the source. So, my question to all of you is, which one do I use?
     
  13. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    [​IMG]

    Keeping in mind that we are looking at the back of the TO220 package in the photo of the board, PIN 1 is the pin next to the edge of the board and PIN 3 is the pin towards the middle of the board.

    That is how I chose my Gate, Drain, Source Pin out.

    Source to the black battery cable and gate is the pin you didn't see a connection to.

    It will be much smaller than the other two traces because it needs to carry a tiny amount of currrent. Drain is a connected to the motor.
     
  14. urbaud1

    Thread Starter New Member

    Oct 17, 2010
    13
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    Hi PP,

    Thanks for your latest reply. I've included another photo, this one shows the other side of the MOSFET. Don't manufacturers of these components usually put some sort of marking on them indicating which pin is #1? I looked at it from as many angles as I could, but could not see any markings, except for the symbol on the component above the "B", which according to your recent reply is above pin 1 ('gate' - nearest the edge of the board; or is pin 1 always on the left side when looking at the front of the component?). So, should I use the MOSFET that you are suggesting: STD27N3LH5? I do want to thank you for your continued help.
     
  15. eblc1388

    Senior Member

    Nov 28, 2008
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    Are you looking at a different MOSFET as I do? If so, you'll need to post a link to that STD27N3LH5.

    The STD27N3LH5 is a surface mounted DPAK MOSFET rated at 30V and 27A. Its max. power is only 30W compares to nearly 300W that of the original MOSFET and has a 19mΩ on resistance, also nearly twice that of original.

    DPAK package also requires the soldering to the copper foil for heat dissipation.

    A MOSFET with nearly double the Rds ON will cook less? :mad:

    Manufacturer deliberately use over-rated component when it is not called for? :confused:

    Sadly these comments are so far from the truth. I'm speechless. :eek:
     
  16. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Urbaud yes you go left to right for the pins of a TO220 while looking at the front.

    At ELBC - I was trying to put the part in the circuit.

    I have gone out too far on the branch of a shaky tree but I will try to explain how I made that mistake.

    With a 1300mAH battery which is about what I expected, if you drain the battery in 5 minutes which some RC vehicles will do, you get about 15 amps from the battery, in a pattern that will start around that high and drop as it drains, reflecting the inefficiency of such a fast discharge. I took that as the max that would be likely.

    I also picked a FET that will switch faster and at a lower threshold.

    Especially as the battery drains I think that the lower threshold would help the FET survive better.

    I see the switching speed as a likely problem of the other FET. Compare their gate charges. I think the old FET was spending too much time in its active region and then it failed.

    Rdson means nothing in any situation where the transistor is not switching efficiently. A better switching transistor will not get even close to 1W, if it is always working near the ideal switching regime. Lower gate charge = Faster rise times and a lower threshold should also help that.

    That is still a big if.

    I also missed a tradeoff. In a model airplane there is very unlikely to be a large heatsink or any heatsink. That does make 300W make more sense without even considering other factors.

    A few seconds of stalling the motor by holding onto the prop and the 30W FET could be toast where the other FET would have a chance.

    I had seen the Rdson and Wattages that you mentioned. I am actually surprised that you are the only one that called me on them. Yes I took chance but I have done enough repairs to know that they are always taking a chance.

    He might put the exact same transistor as he had before back in - fresh and new - and have it blow at the first application of power.
     
  17. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    That transistor does come in a TO220 package.

    I posted the link to the datasheet when I first suggested it.
     
  18. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I stick with that - but I want to emphasize the qualifier statement, since you emphasized. I don't believe 50+ volts and hundreds of amps of rating are needed where they should only see 12 volts, 50 Amps momentary and 15 Amps average.

    If the design is somehow expected to be driving higher currents and voltages, maybe by motor/generator action with the plane in a power dive, then my advice would be wrong but I don't think I was wrong.

    Find the original part would be the first option. Other options can follow that.
     
    Last edited: Oct 20, 2010
  19. urbaud1

    Thread Starter New Member

    Oct 17, 2010
    13
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    PP and eblc,

    I realize you disagree about the type of MOSFET to use. I don't know anything about electronics and loads and gates and drain, but remember it's just a small rc plane. Wingspan of 40". The reason the motor burned out is because I forgot to turn it off after the plane crashed. The propeller wanted to continue turning, but the crash immobilized it, so the motor overheated, burnt the circuit board and the MOSFET. The lesson for me is that I will have to remember to throttle down immediately if it crashes. I don't know how to help you guys figure out which MOSFET I should try to get. I have to absolutely go with whatever you suggest. If there is any more information about the circuit board, motor etc that I can gather to try to help you decide which type of MOSFET is the best pick, please let me know. And, if you think it can't be replaced then tell me that. I want to believe it can be replaced, but if it can't, then so be it. Please let me know what you think.

    I must also say that I am heartened by the amount of time and thought that you guys have put into this problem of mine. When I originally logged on to this forum I had no expectations that I would even get a reply. So, I guess the best way to say this is you have both "made my day"-the fact that you have responded this way is amazing. It's amazing even if it turns out that the MOSFET can't be replaced.
     
  20. eblc1388

    Senior Member

    Nov 28, 2008
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    My position is clear.

    I found out the original manufacturer of the MOSFET and its full part number so you can replace it.

    If you can't find the original then I suggested a similar replacement with the same rating.

    That's all.

    This is public forum, others can have their say and its your judgment to weed out what to believe and what not. Usually more comments are helpful to aid you in deciding but in this case you are limited to two posters so far, which is a bit unfortunate. :confused:
     
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