10M45s: strange behaviour @ 100mA

[pauldune]

There's something strange when using a 10M45s. It cost me about 4 hours work to figure out why my amplifier wasnt working. I used it as a CCS on top of a 2A3. I chose the 100mA current setting resistor according to the diagram (22 ohm). However after searching for about 2-3 hours I found that the CCS only delivered 60mA. So I put in a 10 turn potmeter, and adjusted till it delivered 100mA. Then I measured the potmeter, only to find it at 0 ohms

I confirmed it after that: with a wire-bridge between G and K it delivers exacly 100mA. So it seems it has already a resistor build in, so it can never deliver more then 100mA. Safety? Ixys made a design change? Did everybody know this already?

Anyway; if you want more then 60mA out of your 10m45s, the diagram isnt accurate anymore, and you can best use a potmeter, and replace it with a fixed resistor representing the same value.


gr. Paul

[tomchr]

The IXCP10M45 data sheets that I have seen have not been real detailed. I.e. they have not included any information about the performance of the part versus temperature. The max output current of the 10M45 is supposed to be 100 mA so that you are seeing 100 mA with G-K shorted does not chock me, although, the data sheet shows that you should have 20 ohms between G and K. But if the device is warm, that will slide, and Vth increases with increasing temperature so it makes sense that you get less current if the device is warm.

Rather than operating the device at the limit, how about connecting two devices in parallel? Just build them as two independent 50 mA sources and connect both to the 2A3.

~Tom

[pauldune]

Thanx for your reply Tom,

I thought that too, but even with 10 ohms between g-k it was about 75 mA and with 5 ohms about 90 mA. Now with the g-k shorted, it drifts between 99 and 103 mA. 99 when cold, 103 when hot. I'm dropping 180v/ 100mA: thats 18watt dissipation, well below max spec. (it sits on a pentium 4 cooler and even at full 18 watt doesnt get hotter then 40-50 degrees C, see pic)

Dont know about the parallelling of two devices, doesnt it give some unwanted behavior?

Oh and it wasnt a broken chip, because I swapped it, and the new one gives exactly the same behaviour...

[Salas]

Move to Supertex DN2540 when you can. Has more IDSS and better high frequency. The 10M45S loses high frequency rejection when pushed.

[pauldune]

Originally Posted by Salas

Move to Supertex DN2540 when you can. Has more IDSS and better high frequency. The 10M45S loses high frequency rejection when pushed.

I just bought 25 of the 10M45s... And now you want me to replace them?
I thought the 10M45 had better specs? I remember a comparison (tubelab?) between both, and only cascaded did the dn 2540 win.

But isnt it better to build a discrete ccs for these currents?

I was planning that IF the proof of concept amp worked and sounded good. (this still is a prototype. (nr 4) The final one gets a CNC milled case.)

[pauldune]

Originally Posted by Salas

Move to Supertex DN2540 when you can. Has more IDSS and better high frequency. The 10M45S loses high frequency rejection when pushed.

Sorry Salas,

Not very good advice: the Idss of a 2540 is higher, but the Pmax is only 15Watt. It will die very soon in my application. The 10M45S can have 40 watts if cooled accordingly. Vmax of the 2540 is also only 400v, but I need only 360v, so that would just be enough...

[Salas]

18W on a TO-220? I would cascode to halve it anyway. 10M45S has more gate capacitance, not better when single. Also tends to be noisier than the 2540 when in cascode too.

[tomchr]

Originally Posted by pauldune

I thought that too, but even with 10 ohms between g-k it was about 75 mA and with 5 ohms about 90 mA. Now with the g-k shorted, it drifts between 99 and 103 mA. 99 when cold, 103 when hot. I'm dropping 180v/ 100mA: thats 18watt dissipation, well below max spec. (it sits on a pentium 4 cooler and even at full 18 watt doesnt get hotter then 40-50 degrees C, see pic).

Maybe IXYS doesn't test them very much before they ship the parts. Their data sheet hardly guarantees anything. Most of the specs are typical specs. Graphs are always "typical, not guaranteed". The max output current of 100 mA is not guaranteed as it isn't listed as "output current, min = 100 mA" it's only listed in the marketing blurb at the top of the data sheet.

Because IXYS doesn't actually guarantee anything in the spec sheet, the part you are using does meet the spec.

Get over it. You're not doing anything wrong. Use two in parallel... In the future, if you want predictable performance go with a part that is specified on the key parameters you need for your circuit.

~Tom

[pauldune]

according to specs 10m45s in a to220 should be able to handle 40 watts...
I wonder if anybody ever used it at that much dissipation...

[pauldune]

Originally Posted by tomchr

Get over it. You're not doing anything wrong.....

~Tom

I wasnt really upset or something...Did the tone of my story sounded like that? That must be some dutch influences, always a problem for me in finding the right "nuances" in english...

I only wanted to inform other people if they are using the part in high current setting, to expect some problems