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Anyone here using Aliexpress ?

Discussion in 'Buy, Sell and Trade' started by BGAmodz, May 23, 2016.

  1. BGAmodz

    BGAmodz Member

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    Hello everybody .

    I am doing electronics repair , and i am mainly changing components like IGBTs , Mosfets and ultrafast diodes like the STTA 1206.

    If anyone here is using the popular aliexpress website , i am looking for a store that can be trusted when it come to the quality of components

    I appreciate your time .
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    There is a thread here "where to buy parts".
    It would help to know where you are.
    -----edited----
    I can't find the thread now.
    digikey.com
    mouser.com
     
    Last edited: May 23, 2016
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  3. spec

    spec Well-Known Member Most Helpful Member

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    Aliexpress, is part of the Alibaba group, a huge oriental distributor. They have some incredible bargains, but for critical components like BJTs MOSFET IGBTs etc I would only buy from the mainline Western distributors: Mouser, DigiKey, Farnell, Radio Spares etc.

    spec
     
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  4. dave

    Dave New Member

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  5. BGAmodz

    BGAmodz Member

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    I appreciate that , but how about the shipping prices ? are they the same as alibaba group ?
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    You can sort of think that alibaba is wholesale and aliexpress is retail. Alibaba's quantities are usually higher, but not for large ticket items. In general, Alibaba acts as a broker. Money apparently doesn't get released to the vendor until your satisfied. i.e. You tell Alibaba when the goods are received.

    For a big ticket item, like $2500 USD, I have dealt directly with a vendor. Expensive DHL shipping and delivery in a few days. I had it delivered to a "UPS Store". II have ordered Semiconductors. I had one small item that wasn't received and they refunded everything.

    No PayPal. Wire transfers for me are like $75.00 USD now. For that transaction, credit cards would not work either and the didn't want Western Union.
     
    Last edited: May 23, 2016
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  7. BGAmodz

    BGAmodz Member

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    Thanks a lot KISS . I am right now trying to contact Farnell .

    What would be the price for like 30 igbts from mouser or farnell without taking shippings costs in consideration ?
     
  8. spec

    spec Well-Known Member Most Helpful Member

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    I don't know what the shipping costs are. I have exchanged emails with Alibaba but, while they were very polite and helpful, I could not get any sense out of them. :grumpy:

    Shipping costs from the main-line western distributors vary, but in general they tend to be free over a certain value of order- typically £40UK for the UK and similar value for the USA.

    Where are you located?

    spec
     
    Last edited: May 23, 2016
  9. BGAmodz

    BGAmodz Member

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    Norrh africa
     
  10. Nikolai Petrenko

    Nikolai Petrenko Member

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    Don't buy any semiconductors or capacitors at Aliexpress for special applications like audio, lighting, high voltages,........ But will acceptable for normal applications. Many semiconductors devices they showed are counterfeits. Such as genuine 2SC5200 are Japanese transistors for audio with price nearly equal to ONsemi TIP35C, but many of them in Aliexpress are only have price like TIP41C. (In some store which got parts from Ali in Vietnam, 2SC5200 only 1$, 2N3055 TO-3 only 0,5-0,7$.). So it is not neccesary to talk about quality. only try if you lucky enough :D:D:D.
     
  11. BGAmodz

    BGAmodz Member

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    I see .
    I appreciate your time , but just one question ; how to tell the difference between a fake IGBT and good one even from a usa or uk distrubitor ?
    for example hgtg30n60a4
     
  12. spec

    spec Well-Known Member Most Helpful Member

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    You can't always tell fakes. The only way you can get a feel for it is by reading the exposures on the net and by experience. You could test the parameters but for many devices that would be a lengthy and complex task requiring specialist test equipment. But even if the device passes all of its parametric tests you will have no idea about reliability. Proper manufacturers spend a lot of money to make sure that their components are reliable and electronic devices, especially semiconductors, are made to incredibly tight tolerances.

    As we have said, only buy from mainline distributors for your critical components, especially IGBTs, which are relatively expensive and thus worth while counterfeiting.

    Even something as basic as resistors can be substandard. There have been quite a few cases where people have bought a batch of 1% resistors only to find that they are as much as 10% out.

    The number one rip off items are batteries- the net is full of them and even your local store sells them.

    The irony of it is that if you buy wisely you can often get components from the main line distributors at prices not much more than the counterfeit stuff. If you are a volume used the best bet is to get a trade account.

    spec

    http://sound.westhost.com/fake/counterfeit-p3.htm

    2016_05_24_eto_FAKE_IGBT.png [​IMG]

    [​IMG][​IMG][​IMG]
     
    Last edited: May 24, 2016
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  13. BGAmodz

    BGAmodz Member

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    I will use farnell as it has a store in morocco called MEST . I hope it will be a good trade .
     
  14. spec

    spec Well-Known Member Most Helpful Member

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    I have been using Farnell, both at work and privately, for 46 years. I am still evaluating them, but I think they will be alright.:p Radio Spares the same.

    spec
     
    Last edited: May 24, 2016
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Showing your age now, they changed their name to RS Components many decades ago :D
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    :p They will always be Radio Spares to me. :rolleyes:

    spec
     
  17. fezder

    fezder Well-Known Member

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    It's like local electronics store changed their name from vekoy to startelec, still I talk about vekoy :D
     
  18. fezder

    fezder Well-Known Member

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    About IGBT's, I'd love to learn how to use them, but they're quite expensive just to blow up during learning -_-. I got 4pcs, irf gp4068d
     
  19. spec

    spec Well-Known Member Most Helpful Member

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    IGBTs are not difficult fezder- just a small MOSFET driving the base of a power Bipolar Junction Transistor (BJT). That way you get the high input impedance of a FET with the conductivity of a BJT. Because the MOSFET is only required to drive the base voltage/current of the BJT, its gate area can be relatively small and thus the input capacitance is much lower than an equivalent power MOSFET. Also, for the same reason, the gate drive voltage is relatively low.

    You can make a IGBT from discrete components. That way you can make a P type version too. Only N type IGBTs are available.

    spec
     
    Last edited: May 26, 2016
  20. fezder

    fezder Well-Known Member

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    So basicly IGBT is transitor but controlled like FET? What about thermal runaway?
     
  21. spec

    spec Well-Known Member Most Helpful Member

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    Hy Fezder,

    That is exactly correct; an IGBT is only a MOSFET driving the base of a power BJT in a common case. IGBTs also have a diode connected between the collector and emitter (just like the intrinsic Schotkky diode in a MOSFET). The fabrication process is complicated and hence the relatively high price of IBBTs. Because the two transistors are fabricated at the same time, the characteristics of the MOSFET can be optimized to match the base drive characteristics of the power transistor. There are other advantages too, but they are only secondary.

    IGBT have a major shortcoming compared to BJTs and MOSFETS for general purpose applications: IGBT collector saturation voltage (Vsat) cannot be any lower than the band gap voltage of the bipolar power transistor emitter/base, which is typically 600mV at low currents. This coupled with higher cost and the lack of a P type IGBTs means that IGBTs are not widely used.

    The three main practical limits of a power transistor, BJT, MOSFET, IGBT, are maximum junction temperature (TJmax), thermal resistance junction to case (ThRJC), and Safe Operating Area (SOA). The maximum power dissipation figure for a transistor is meaningless and is only a marketing ploy (not the case for some specialist transistors like RF types).

    Thermal runaway is no more of an issue with IGBTs than a normal power BJT, like a 2N3055 for example. Thermal runaway was a major problem with germanium transistors with a maximum junction temperatures limited to around 100 deg C, but not really with silicon which can stand a junction temperature of either 150 deg C or 175 deg C, depending on manufacturer.

    If you stay within the transistor's SOA, defined by the graph of Fig 2 in the attached 2N3055 data sheet for example, you will not damage a transistor or cause thermal runaway. The term thermal runaway is not used much there days; it has effectively been replaced by secondary breakdown. For example the 2N3055 is a 60V, 15A, 115W transistor but it can only stand 800ma with a collector/emitter voltage (VCE) of 60V. And at 15A collector current it can only stand 8V. In fact, even this would not be possible because 8V * 15V= 120W which far exceeds the practical dissipation for a 2N3055).

    The Thermal Resistance Junction to Case (ThRJC) allows you to calculate how much power the transistor can dissipate, and not exceed the TJmax, with or without a heat sink, in a given ambient temperature. The ThRJC for the 2N3055 is 1.52 degrees Centgrade Watt (DCW) which is pretty poor: 0.5DCW would be more reasonable for a TO3 can power transistor. In practical terms the maximum safe power dissipation for a 2N3055 would be around 20W. As a general rule of thumb, 30W is the maximum safe dissipation for any TO3 case transistor without liquid cooling.

    The IRGP4068D IGBT ThRJC is 0.5DCW which is good for a TO247 plastic case power semiconductor. The TJmax is also good at 175 deg C. The SOA is a bit complicated because it is specified in terms of switching performance but as it is a 600V part, unless you are doing some high voltage work, secondary breakdown will not be a problem. Of course, you must not exceed the IGBT maximum junction temperature of 175 deg C. This means that without a heatsink the IRGP4068D can only dissipate 3.4W and then only providing the case is in free air with a temperature not exceeding 40 deg C.

    To make the IRGP4068D conduct collector current, just place a voltage of 4V to 6.5V (Gate Threshold Voltage) between the gate and emitter. At DC the gate current will be low: 100nA maximum. To increase the collector current make the gate voltage more positive, just like for a MOSFET.

    2N3055 Data Sheet

    http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF

    IRGP4068D-EPbF Data Sheet
    http://www.irf.com/product-info/datasheets/data/irgp4068dpbf.pdf
     
    Last edited: May 26, 2016
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