Go with the original part, since it is available when I checked again.

My original post regarding a replacement was made before the original part had been identified.

My defense of my alternative was made when the part was identified but not in stock.

I have seen that the AOT430 part is available. As much as I would like to prove that eblc and I are both right I can't justify using your plane for my experiment.

Half the threshold and 1/25 the Gate charge for the part I suggested so roughly 50 to 100 times more efficient at switching. Worst case is it blows immediately and you can tell that by running the engine on the ground and if it gets hot (or very warm) then the switching efficiency was likely not enough to save the transistor. I was counting on the transistor staying at under a watt of dissipation through efficiency

 

There seems to be room so why not add a heat sink thats made for the 220 style mosfet? Such as; http://www.radioshack.com/product/index.jsp?productId=2102856 They are available at most other suppliers too.

 

Hi PP,

What is the part number for the original part you say is available? And, where is it available? If you can give me the original part number (as you will recall, throughout the various posts in this thread, 3 different part numbers have been suggested, so which one do I use?). You say in your most recent post, "I have seen that the AOT430 part is available." Where is it available and who is the supplier so I can proceed with ordering it.

Also, again in your recent post, what does this mean, "Half the threshold and 1/25 the Gate charge for the part I suggested so roughly 50 to 100 times more efficient at switching. Worst case is it blows immediately and you can tell that by running the engine on the ground and if it gets hot (or very warm) then the switching efficiency was likely not enough to save the transistor. I was counting on the transistor staying at under a watt of dissipation through efficiency."

 

what does this mean,

I was talking about the switching efficiency of the part I suuggested as a substitute compared the the original part.

Look at the datasheets.
The newer ST Fet
The AOT430 at Digikey. They had 791 available.
The AOT430 datasheet


The original part has total gate charge of 114 nC
The part I suggested has total gate charge 4.6 nC
Over 20 x less gate charge.

Gate charge directly effects the time required for the gate to rise and fall. Which means it sets how long the part spends in the active area where it dissipates energy. At the on or off position the transistor will not normally dissipate much energy. While switching it has a large energy spike. Fast switching times make that spike narrower.

The Threshold Voltages are also important.
AOT430 Gate Threshold Voltage is 2 to 4 Volts with 2.7 typical.
The ST FET only lists the minimum but that is 1 Volt.

It turns on with less Voltage. That could turn out to be problem if the gate doesn't drop low enough, but probably only if it has a very old design of gate drive IC.

So the rise times would be about 20 times faster and the level they have to rise to would be less to get the FET turned on.
But the AOT has lower Rds so it still looks like it is turned on just as much or more.

The FET I suggested.

The Original Fet.

The original part is a good FET.

It is designed so that it conducts very little at 4.5 Volts and then conducts all you need at 5.5 Volts. Thermal steering will change that some but it is good. That one Volt break is nice and means it should switch with a fast and clean. If there were anything seriously wrong with it I would not agree that you stick with the original. That clean transition in a good position is important with its high gate charge because that high gate charge gives a much lazier capacitive charging curve to the switching drive voltage. You want to get a good middle section like 4.5 to 5.5 Volts where the current slams on and off.

The ST Fet looks less crisp in comparison. But keep in mind it can afford to since its switch gate drive voltage changes over 20 times faster, because it has a much lower gate charge. The difference in spread is there but divide that gap between its gate charge lines by 20 to see the time modified comparison image. That means imagine the whole image for a ST image no higher than 2 lines of text and that is the real comparison you can place side by side with the AOT. It switches much faster. That should mean it wastes less power.

The lower thresholds would be a bigger advantage at the lower voltages of a draining battery except the total specs for the AOT for Current are high enough that it is still good enough in limp mode. It is basically so far over what I expect is required that the AOT is not half way to its top range when the ST tops out.

The part I suggested would not be a sure thing and the Original type of component should be. Things like avalanche energy are also important to the FETs ability to survive and the ST is not nearly as rugged compared to the AOT.

 

PP,
The data sheet you cite in your most recent post describes the following MOSFET: MOSFET N-CH 75V 80A TO-220 - AOT430 (DigiKey Number: 785-1145-5-ND). So, given this is the one you are suggesting, I will proceed in trying to purchase it somewhere. Again, thank you for your help. Once I get the part and install it, I will let you and eblc know the outcome.

 

Hi PP and eblc,

I just installed the AOT430 MOSFET and it works like a charm. I guess it's back to me now not to over heat it. Thanks for your help and suggestions. I couldn't have made this fix without you both. Thanks again.
Dan

 

Check post #22 for help in over heating.

 

Just in case you could always have bought spares.

It might be years before it smokes again but it would be that much harder to find the original FET in a few years time.

Glad that it is working for you.

 

I just installed the AOT430 MOSFET and it works like a charm.

It should because it was the same part as the original design.

I guess it's back to me now not to over heat it. Thanks for your help and suggestions.

It should not cause you extra concern as long as you fitted the replacement MOSFET to the same condition as it was fitted in the controller.

The failure of the first is overcurrent only after the RC plane crashed and has nothing to do with normal use.