Products > Test Equipment
Fool for the 8656A Sig Gen
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Brumby:

--- Quote from: Swainster on March 02, 2023, 03:10:15 am ---The other day I tried to list all the projects and repair candidates on my to-do list ...

--- End quote ---

I'm too scared to even consider that.   :scared:
jrharley:
Thanks swainster! I would never have thunk someone would use a device on a DC load, that wasn't rated for DC loads!  :-//
The kits generally do seem to use those devices though.
How do you tell on the spec sheets whether they will have a DC operating zone?

Thanks again for all the comments

JRH
Swainster:
Hi JRH,

For DC operation I just quickly scan through the datasheet looking for the graph with a 'house' shape - typically titled something like "Safe Operating Area". If it doesn't have a DC line/contour then it pretty much means that the manufacturer has optimised the silicon for switching performance so much so that the DC performance is not worth even worth specifying. Not all switching MOSFETs are this highly optimised so its worthing checking for this graph even if the MOSFET has "switching" in the description.

Note that SOA is not just a MOSFET thing - it is fully applicable to power BJTs as well.

At the opposite end of the scale are so-called linear MOSFETs. As linear operation is not so popular, these tend to be rather pricey. I've attached a quick comparison of the 110N8F6 with the same package similar voltage ixys linear MOSFET to show the difference.

Just to be clear, you can use non DC specified MOSFETs in a DC application, but if you do, then it is up to you to characterise the safe operating conditions (and they may not have much relationship to the datasheet numbers)

jrharley:
Great info swainster, thanks!

I had a few minutes to study the charts and it's much clearer what you mean now. :-+

Tony_G is right about the philosophy of the journey for sure, I'll have to start planning the next one! Even though like you, brumby, and I suspect everyone else, there's no shortage of projects on the spreadsheet!

Hope to put in a new 5V regulator in the 8656A tomorrow night, so more to come.


JRH
Tony_G:
Looking forward to what you find when you swap out the regulator.

TonyG
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