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Selecting a suitable component

Finding a component that suits your needs at the right price

  1. (*steve*)
    0 - Contents
    1. A common question
    2. How Common?
    3. What's here?
    4. Finding that MOSFET
    5. General principles
    1 - A common question

    One of the common queries we get is:

    How do I find a cheap <component type> for <use case> with <specifications>.​

    This may be stated like "I need the best MOSFET to switch a string of LEDs operating from a 24V supply from my 3.3V microcontroller. The LEDs draw 1A."

    2 - How Common?

    Here is a list of some of the times I've been asked this question (some of them may even be helpful to you). Note that the amount of detail diven varies from detailed to very terse:
    3 - What's here?

    This resource will first answer the specific question given above, then expanding on this method to assist you in doing it for any other component.

    The key is to use a parametric search. This allows you to specify the required parameters, showing you only those components that match. There are many parametric search engines around. Many parts manufacturers have them as to parts retailers. I use Digikey's site because they list a huge range of products from a variety of manufacturers. Additionally they list many obsolete parts that may not be listed on the manufacturer's sites.

    4 - Finding that MOSFET

    What we're going to find is a suitable component to: "switch a string of LEDs operating from a 24V supply from my 3.3V microcontroller. The LEDs draw 1A."

    The first step is to go to digikey.com.

    1.jpg

    To start searching, go to the search bar and type in something that generically describes what you are looking for. In this case, "MOSFET" would be an appropriate thing to enter.

    2.jpg

    Then press return or click on the magnifying glass.

    This normally takes you to a page showing matching categories. Typically the one you want is near the top of the list, and frequently has the largest number of matching items.

    3.jpg

    In this case, we are looking for a mosfet. The first three results listed are for mosfets, but the second is for arrays of mosfets (and we don't want that), annf the third is for RF mosfets (that we don't want either). Later items in the list are even less like what we want.

    So we can click on the first entry -- "Transistors FETS, MOSFETs - Single". This takes us to the first page of the parametric search.

    4.jpg

    This page shows that we have over 48,000 items to select from.

    The scroll boxes going across the screen are the parameters we can search by. There are more than can be seen on one screen. In the top right corner there is an option to show them stacked or scrolling. On a desktop platform I would choose scrolling.

    Changing that to scrolling allows us to see all the parameters at once.

    5.jpg

    The scroll bar that has appeared allows you to scroll through all the parameters.

    The most important parameter for us is that the MOSFET has to be N Channel. So the first thing to do is to select this from the "FET Type" by clicking on it. When you do this, a "Clear" option appears below the box. Use that if you select something wrong.

    6.jpg

    Once you have selected some parameters (you can make selections from several at once) then you can apply the filter.

    Note that next to the "Apply Filters" button, the number of matching parts is shown. Beware that if this number falls to a very low number, you may have been too specific.

    7.jpg

    After applying the filters, a filtered list appears.

    8.jpg

    As well as the updated list, you will notice we cannot select the "FET Type" any more. However, the "FET Type" is shown as a filter. We can remove this if we desire (but we don't).

    The next most important parameter is the Drain to Source voltage for the MOSFET. In this case the supply voltage is 24V, so it needs to be greater than 24V. Let's select 30V. This can be done by scrolling through the appropriate parameter and clicking on a value.

    9.jpg

    But wait! What if there;s a better component with a Vdss of 50V? The simple answer (and this is always good practice) is to select multiple values that will be equally suitable. In this case we need a Vdss greater than 24V, so we can select everything from 30V to 100V.

    You can either click on every individual value, or click on one, scroll to the end on, and hold shift while clicking on it.

    10.jpg

    That looks better (you'll have to trust me that all the required options are selected).

    You could select all the values up to 1000V (or however high it goes) but because (generally speaking) parts differing in maximum voltage tend to get more expensive with that voltage rating, you're unlikely to find a better part with extreme ratings.

    With this option, because it's numeric, it was also possible to select the minimum and maximum values.

    Now we can click on the "Apply Filters" button again and see what we get.

    11.jpg

    Note that this time the selection box for Drain to Source Voltage still appears, but it is limited to the values we've selected. We can select from this again if we want to narrow the range. This list will also only contain values that are available in the current subset, so you might notice the range reduce as you apply more filters.

    We still have over 22,000 MOSFETs in the list, and we need to select some other important parameters.

    One important parameter is the maximum current. From the "Current - Continuous Drain (Id) @ 25C" box, select values between 2A and 100A.

    We also need this to operate from 3.3V, so we need to select a device with a low "ON" voltage. The correct column is the "Vgs(th)" column. We need to be careful with this, because this voltage specifies when the device starts to turn on -- typically allowing 240uA to flow. You must select Vgs(th) that allow several volts headroom, but do not select 0V! In this case I'll select everything from above 0V to 1.8V.

    In the image below you will note that I've scrolled the parameters across to the right a little. You will also note the significant reduction in the number of matching items.

    12.jpg

    Pressing "Apply Filters" will give us that shorter list:

    13.jpg

    The next thing we need to do is to specify how many of these parts we want, and sort them in ascending order of price.

    The reason we specify the number of items we require is that the cheapest parts are often a price break for purchasing several thousand -- if we only want one or two, this information isn't useful.

    So, enter the number you want, press "Apply Filters", then click on the sort button for ascending order of price.

    I will enter a quantity of 4.

    14.jpg

    As you can see, the first item on the list costs 44c from Digikey.

    However, that item is in a package which may be hard to solder. At this point you can either do a further selection on package type (it is one of the options at the far right), or just scroll down the list until you find something that you can deal with.

    15.jpg

    As I scroll down, I find a part in an SOT-23 package (you can scroll to the right to see that information). It's 54c each.

    So I can just buy that?

    Well, maybe.

    You need to check:
    1. Whether it can do what we need it to do.
    2. Whether it is available (either at Digikey or wherever we buy our components)
    And just in case, it's always best to get a few options.

    To check if this will do what I need it to, I will select that component (by clicking on the part number) and find the datasheet.

    16.jpg

    Here's the part. Digikey have over 300,000 of them -- so it's definitely available from here. And there's a datasheet.

    If we look at the datasheet we can immediately see that the device works with gate voltages in the range we need, and that it allows modest power dissipation. Since we're switching, that modest dissipation means we don't need to be too demanding with our gate drive.

    17.jpg

    If this information was not available, we would look for a graph showing Drain current against drain-source voltage at various gate-source voltages.

    18.jpg

    This graph shows that with a Vgs of 2.5V (and we should be able to manage closer to 3.3) the MOSFET will drop just a little over 0.1V at 1A. This will cause a dissipation of 0.1W, which is well within the 1W limit for this device.

    Armed with this information, I could buy the component from Digikey, or perhaps I might take the part number and see if it available from my favourite retailer. In this case I looked on Mouser and found the same part.

    19.jpg

    Good luck, they also sell it in small quantities, and they have plenty in stock.

    I've found a viable part!

    5 - General Principles

    Generally, a search follows these steps:
    1. Determine the type of component and the required specifications
    2. Go to a Digikey.com
    3. Search for a class of component
    4. Select the best matching category
    5. Narrow down the options by filtering by your required specifications
    6. Specify your desired quantity
    7. Sort by price
    8. Locate suitable part(s)
    9. Check the datasheet to make sure the part is OK
    10. Check for availability at your favourite vendor
    The steps in red are not easy for beginners.
    5.1 Determine the type of component and the required specifications

    This can be difficult for a beginner. You may not know what type of component you need, or what specifications are required. In addition, there are a huge number of parameters, and different ones could be important even where the same part is used in different circuits.

    If you are replacing a part and can find its datasheet, the first step is to search for parts having the same or better specs. (Do you know what "better" means n all cases?)

    If you have any doubts, don't be afraid to ask. If you ask for suggested specifications for a device you are likely to get a more useful response than if you ask for a replacement part number. What good is it for you if you ask for a replacement part number and I offer a part that is not available to you?

    5.2 Go to Digikey.com

    I've used Digikey in my example and I've provided reasons why. However, many other resellers have parametric searches (e.g. Mouser, and RS), as do most manufacturers. If you find another site better suits your needs, use it!

    Note that the exact method to use may vary at other sites.

    5.3 Search for a class of component

    Different components have different sets of parameters. Using this method at Digikey starts you at a high level for a particular set of specifications. It is possible to start with something more specific (like "N Channel MOSFET") but the more specific you are here, the higher the risk that the search engine will filter too tightly (or perhaps not understand you) .

    5.4 Search for a class of component

    Beware classes with very few items. These may include the search term, but not be directly related to it.

    5.5 Narrow down the options by filtering by your required specifications

    This is the heart of the process. You need to know what your desired specifications are (see 5.1). You must keep an eye on the number of items remaining. If it drops too far, you may need to loosen your filter a little.

    5.6 Specify your desired quantity

    This is important to remove rows which offer discounts for quantities in excess of your requirements. Conversely, if you require a large quantity, it will show you appropriate price breaks.

    5.7 Sort by price

    Normally you want the cheapest matching part. If you're interested in something else you can sort by that -- but that's pretty rare.

    5.8 Locate suitable parts

    If you haven't sorted by package, then typically you'll be looking for a suitable package here. Beware that some packages look almost exactly like others but are WAAAAAY smaller.

    If you want through-hole parts, you can filter by this. It will save you the effort of scrolling past them (frequently these are cheaper, so they sort to the top).

    You can also filter by package type, but beware that the same package can have multiple names, so this can exclude parts by mistake if you're not careful.

    It's best to select a couple of possible parts in case you need to eliminate them later. Be aware that some otherwise identical parts have slightly different part numbers due to their package or other differences.

    5.9 Check the datasheet to make sure the part is OK

    This is another area where beginners might seek help. As for step 5.1, if you need help, you must be able to impart sufficient information about the application to allow others to help you.

    This step is important because the basic specifications of parts do not always tell the whole story. The datasheet will contain information that allows a more detailed assessment of the capabilities of the part.

    5.10 Check for availability at your favourite vendor

    Even if a part is not available from the site where you search, it may be available elsewhere. The reverse is also true. Prices may also vary between vendors. This is another good reason to have several options up your sleeve.




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