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I'm not ruling out the Supertex DNxxxx FETs

Posted by VoltSecond on May 23, 2004 at 22:42:58:
I'm not ruling out the Supertex DN FETs. You don't need me to design a circuit to use them. However, I'll politely point out some of their short comings.
If you are running less than 20 mA, less than 400V B+ and don't mind hand tuning each CCS, the DN2540 may be your beast of choice. From the data sheets I'd actually go for the DN35xx instead if it was actually offered in a TO-220 package.
With the DN series, like all FETs, I'd recommend a resistor in series with the drain to act both as a drain stop resistor and a current limiting resistor under fast rise time fault conditions. Drop as much voltage across this resistor as you can afford and still have the CCS operate at maximum output power at 104V ac low line.
Here we go:
A comparison of a text book FET CCS vs. DN2540 CCS assuming the similar protection measures on both.
1. The DN2540 is sole sourced. There are widely available drop in replacements from different manufacture's for the FETs version. Strangely enough, this is the biggest reason why drew up the FET version.
The IXYS CCS parts aren't drop in replacements.
The curves on the IXYS depletion mode IXTP02N50D and IXTP01N100D aren't given on the IXYS site. So I can't tell if they'll drop in with or without tweaks.
Future-Active has the IXTP01N100D in stock, but I don't think they sell to hobbyists and I think the minimum order is 50 pcs.
The IXTP01N100D isn't in stock at Digikey and NuHorizons, but shows up in the product search.
The IXTP01N100D isn't carried by All American.
2. Let's look at the allowed temperature rise.
A K10 thermal insulators runs 1C/W Rcs.
A DN2545N3 TO-92 runs 170C/W Rja
A DN2540N5 TO-220 runs 8.3 C/W Rjc, 9.3 C/W Rjs with insulator
A TO-220 FETs run 1.0 to 3.5 C/W Rjc, 2.0 to 4.5 C/W Rjs with insulator.
A IRFBC30 FET is 1.7 C/W Rjc, 2.7 C/W Rjs with insulator
With a 30C ambient, 30C sink rise and a 100C junction
A DN2545N3 TO-92 is good for 0.41W..(9 mA at 45V)
A DN2540N5 TO-220 is good for 4.3W..(95 mA at 45V)
A FET is good for 8.9 to 20WW..............(198 mA to 444 mA at 45V)
A IRFBC30 FET is good for 14.8W.........(325 mA at 45V)
2.1 With the FET version, I can easily divert power loss to a cheap resistor. Strangely enough, heatsinks often cost more than the FETs on them. I don't know if I can pull this off with the DN FETs, it may actually work.
3. At currents over 20 mA, the FET current source still swings below 2V drop, the DN2540 can not.
You can see this from the saturation characteristics on the datasheet. For a MOSFET to be a CCS, it needs to be in the "Flat Horizontal Line" area of the curve. The DN2540's -1V curve cuts out at 20 mA with a 0.75V drop. Which is the same voltage drop where the FET CCS cuts out if you eliminate the 0.7V drop from the R2 protection resistor.
At 100 mA, you'll need >5V across the DN2540 for it to operate as a CCS, at 100 mA the DN2530 looks good down to about 2V.
4. With B+ over 400V, the FET current source will have a good chance of surviving a tube arc. The DN2540 doesn't have much of a chance. Below 400V the DN has a chance. We can buy FETs to 1000 to 1200V now.
5. The FET CCS uses an external part (transistor) to set the bias current. This transistor will cause the CCS to have a consistent -0.3%/C temperature coefficient independent at the current set point. This transistor will be thermally coupled to the ambient temperature, not the heatsink temperature.
With 10V drop across the part, the DN2540's temp co will vary from zero to -0.3%/C depending on the current set point. However, the DN2540 gets hot and will see a bigger swing in temperature so it will be easier for the DN2540's to be mismatched at higher current levels. However, the DN2540 will perform better at lower current levels.
Because of the FET version is coupled to the ambient temperature instead of the heatsink, a pair of CCS's using the FET version are likely to track better than a pair of DN CCSs.
6. I normally see about +/- 0.5V change in the gate voltage thresholds over different lots of parts. The data sheets usually allow +/- 1.0V change. This means at some current levels, you may not be able to adjust in a DN2540. The FET version will always work and will work the same.
Look at the DN curves and go up 0.5V or down 0.5V. If you can adjust it in to your operating point with this variation and keep the junction temperature less than 100C in a 30C ambient, I'd consider using the DN part.
7. After the FET version is tuned up once, you can step and repeat to make multiples of it for years. The DN2540 probably will require adjustments between production lots of parts.
What I've seen with FETs is parts made within 6 months of each other tend to work the same. When they are separated by years or come from different factories, they start to see more variation. I expect the DN's to behave the same as standard FETs in this regard.

Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warrantee.