Electronics > Projects, Designs, and Technical Stuff
Looking for good simple DIY project of Electronic Load
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001:

--- Quote from: aheid on November 08, 2018, 07:26:30 pm ---
--- Quote from: 001 on November 08, 2018, 05:05:31 pm ---The main problem is MOSFET SOA
so I need only 0.2-0.4A but 400V

Can I use some oldy but goldy BJTs like BU508A?

--- End quote ---

You say "only", but 0.4A * 400V = 160W that the load needs to dissipate, that's not "only"...

Looking at the BU508A datasheet, at 400V the SOA rates it at about 0.0175A max, or 7W. So doable if you have about 30-40 of them in parallel.


--- End quote ---

Yea
I can use oldstyle welding gear for parts  ;)
Jay_Diddy_B:
Hi,
Making a high voltage load has its challenges, because of the Safe Operating Area, SOA, of the MOSFETs in the linear region.
Bipolar transistors do not solve the problem. BJTs suffer from something called second breakdown, if operated at high voltage and low current.

Here is a concept for putting MOSFETs in series to make a high voltage load. I would use IRFP450 transistors in this circuit. I didn't have a SPICE model handy.



I have put three MOSFETs in series in each position. This technique can be used for any number greater than 2.

The main downsides to this technique:

1) The minimum input voltage to the load is increased by one gate threshold voltage for each MOSFET. As shown the minimum input voltage is about 10V.
2) The MOSFETs have to insulated from the heatsink.

The upside:

1) It really helps with the SOA problem.

I would keep each MOSFET dissipation to be under 40W.

In the example shown I have 6 MOSFETs so 240W with suitable (very large) heatsinks.


I have attached the LTspice model.

Regards,

Jay_Diddy_B
001:

--- Quote from: Jay_Diddy_B on November 12, 2018, 12:42:18 am ---
--- End quote ---

awesome idea!

thank You! best regards  :-+
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