Miscellaneous MOSFET notes.

I mentioned early on in this blog that other MOSFETs could work, and they will work in most cases as long as they are in circuits that are within their peak ratings. The same with the Bipolar transistors.

The keys are to make sure when using multiple semiconductors, such as the 10 Amp regulator as one example; make sure they are all the same type. While you might get mixed types to work, the chances of them performing as expected is rather small. The Bipolar types I covered fairly well in the early section of the blog.

For MOSFETs, the Vds rating needs to be adequate for the voltages you expect to see. MOSFETs are also rated for impulse current, which can seem amazingly high for such a plastic package, but this is just impulse use such as you might find in a Pulse Width Modulated circuit for power conversion, or class D type Amplifiers. There will be two ratings for Impulse use in most cases, one at 25 degrees C and one at 100 degrees C. These current ratings are also referenced normally at 10 volts. At higher voltages, the current carrying ability is lower than it is at 10 volts Vds potential.

There is also “Continuous Current,” which can still seem high for a plastic device. This is the value to look at when either building a system with new components, or even with used components that differ from the ones I have used in my examples.

If you can look at the temperature versus Current graph, you can extrapolate from the graph what the maximum current for your input voltage will be for that device in your application. and it may not work sometiimes. The graph would show if you cut it close, or fall below or outside of safe cutoff points.

If you have P Channel Mosfets, In the Asian numbering scheme, these will be denoted by “2SJxxx” or “Jxxx” In this latter case, verify you are indeed using a P- Channel MOSFET, because over the years there have been some “J” marked semiconductors that were not MOSFETs. If you happen to be fortunate in possesing or having accsess to a Transistor Tester like the B&K “510 Transistor Tester,” MOSFETS will test out of circuit in the manner of the JFET testing listed on the back: 2 positions where the transistor tests as one type. In other words, 2 tests that indicate “NPN” with the same Base test lead color, indicates an N type MOSFET. 2 that indicate “PNP” means you have an P Type MOSFET. The 510 tester initially dates from about 1978, so they have been around for a while. Very useful to have, and worth looking for if you do not have one; however any transistor tester that can test JFETS and identify them should work for MOSFETS too.

P Type MOSFETs can be used in the circuits listed so far with one major change in addition to some lesser changes: Instead of regulating the positive rail, P-Channel MOSFETs are regulating the negative rail, and instead of using 78xx regulators or the LM317 type, you would use the 79XX series negative regulators, and the LM337. Otherwise using them in the manner described with a variable resistor will work as well too.

This testing ability of MOSFETs is useful to know when servicing an LCD monitor too- only in that situation you need to remove the transistor from the circuit for an accurate test. The inductances of the step up transformer they drive normally is not high enough fo the tester to indicate accurately. This is also true for those LCD monitors that use Bipolar transistors, usually Darlington types, the inductances are usually too low for accurate measurements. Use care in removal as the corners of the TO252 series carriers as they can crumble easily, and in the act of failure they also pull one of the leads with them. These too have respectable characteristics that will allow for their considerations in the regulator circuits as well.

As an aside, the AP9563 and AP9960 used in monitors are not easy to locate for a repair, I did actually locate their manufacturer and have found some potentially suitable replacements in the AOD4185 and IRLR3114ZPBF/IRLR3114Z respectively. The link to the AP manufacturer list with downloadable PDFs for the AP types is here. While this may be a vendor in Taiwan, from the appearance of the website it seems geared more for the manufacturers in Taiwan and China mainland who would by purchasing by the thousands at a time rather than one or two units.

Heat sinking MOSFETs is essential. If you do try to use the D paks or TO252 series carrier types, try use as much virgin copper board as you can for a heat sink. Using a moto tool or other carving tool carefully, you can create a mounting circuit board simply by carving around two pads for each device to isolate them electrically for the three leads (actually 2 leads and large pad opposite the leads) of each device and then drilled to accept the wire termination. You only need to isolate the 2 small leads. The large pads can be soldered to the same copper board- NO GREASE.

If this does not make sense to you, open up your computer, or take a spare motherboard and look for some 1 centimeter square items soldered with 2 leads on one end and a pad at the other- these would be MOSFETs and they usually can be found behind the cluster of interface connectors.

The pads make heat transfer through the solder junctions so no grease needed, the transfer medium is the solder itself. Tin/Lead solder can work, Silver/Tin solders have a higher melting point and stronger tensile strength. Whichever is available to you, is the one to use.

I prefer to use dielectric heat transfer grease, but clean heat transfer pads can be reused if salvaged, or new ones can be used. It all depends on what you have available to you and your preferences. Lastly, using a fan to move air across any heat sink improves the performance of the heat sink.

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