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Tokin SIT DEFiSIT musings
Tokin SIT DEFiSIT musings
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Old 15th December 2019, 05:03 PM   #11
Nelson Pass is offline Nelson Pass  United States
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Tokin SIT DEFiSIT musings
Easier to work with parallel resistors when developing the right values. Start high
and add test parallel resistances w/o soldering.
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Old 15th December 2019, 05:36 PM   #12
woofertester is offline woofertester  United States
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All of the measurements above are without heatsink for the Tokin SIT. With a heatsink for the IXYS hockey puck P-FET.

Today I drilled and tapped a heatsink for the Tokin SIT.

One of the tapped holes is to mount a Vishay threaded head 5k ohm thermistor.

Vgs measurements over temperature ongoing now.

From initial observation, the Tokin SIT Vgs moves 12mV between 35 deg C and 50 deg C at 24V Vds at 1.5A Ids.

My guess is that the hockey puck P-FET Vgs moves a lot more over that temp range since.

It would make sense for the bottom bias voltage to be adjustable and make the spread between the two gates adjustable.

Last edited by woofertester; 15th December 2019 at 05:41 PM.
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Old 15th December 2019, 05:47 PM   #13
woofertester is offline woofertester  United States
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Quote:
Originally Posted by Nelson Pass View Post
Easier to work with parallel resistors when developing the right values. Start high
and add test parallel resistances w/o soldering.
Thank you for the suggestion. I will do that.
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Old 15th December 2019, 08:49 PM   #14
woofertester is offline woofertester  United States
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Temp vs Vgs for the DEFiSET pair at 1.5A Ids at 24V Vds

The Vgs temp coefficient of the P-FET puck is about 10X compared to the Tokin SIT at 50 deg C.
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File Type: jpg DEFiSET_pair_temp_vs_Vgs_24V_Vds.jpg (43.0 KB, 465 views)
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Old 16th December 2019, 09:24 PM   #15
woofertester is offline woofertester  United States
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Running these devices in tandem as a follower pair on the bench now. Using + / - supplies to allow me to easily see balance of the two halves.

The bias current and offset voltage is very temperature dependent.

At 50 deg C and 1.5A bias and as close to zero offset as possible, the Vgs values are -3.26v for the Tokin SIT and -3.568v for the IXYS puck.

The transconductance of the Tokin SIT at temp at 1.5A Ids is ~2.8 and the transconductance of the IXYS puck is ~12.

The tempco of the IXYS puck is ~10x the tempco of the Tokin SIT

I am wondering if degenerating the P-FET puck would help with matching the tempco of the parts better?

.... or the bias circuit needs one or more thermistors.
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Old 16th December 2019, 10:23 PM   #16
Nelson Pass is offline Nelson Pass  United States
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Tokin SIT DEFiSIT musings
Any degeneration will improve the temperature dependency, and a small amount
on the more offending part might do the trick.
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Old 17th December 2019, 02:35 AM   #17
woofertester is offline woofertester  United States
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Quote:
Originally Posted by Nelson Pass View Post
Any degeneration will improve the temperature dependency, and a small amount
on the more offending part might do the trick.
Thanks. I just inserted a ceramic 0.22R multi-watt resistor and re-adjusted the bias. -3.2v on the Tokin SIT and -3.9v on the IXYS puck.

The inserted resistor increases the spread of the gate voltage differential.

The behavior of the Vgs vs temperature is now much slower and easier to dial-in.

I adjusted the bias at 1.5A and offset for 0 for 45 deg C or so.

On a really large heatsink, at room temp, the initial offset is 4.5V and is slowly drifting down. The initial bias is 1A.

I will see how things look after an hour of warm-up.

Thanks again for for sharing your knowledge.
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Old 17th December 2019, 09:46 AM   #18
Zen Mod is online now Zen Mod  Serbia
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Tokin SIT DEFiSIT musings
Use Papa's amp schmtc and you'll have several things solved at once
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Old 17th December 2019, 03:26 PM   #19
woofertester is offline woofertester  United States
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Quote:
Originally Posted by Zen Mod View Post
Use Papa's amp schmtc and you'll have several things solved at once
Hello ZM

As you suggest, I re-read the DEFiSIT schematic by Nelson and listened to his BAF 2016 lecture.

I am playing with only the output stage and using the Tokin SIT and the IXTN210P10T.

I used independent voltage sources for the two gates and determined the setting for Vgs at 1.5A for + / - 24V Vds.

I tried with and without degen resistor.

The temp coefficients of the transistors biased independently causes lots of bias instability without degen. With degen, the bias is more stable but the spread of the gate voltages approaches 1V.

Next I removed the degen resistor, tied the gates together and floated them with a 220K resistor returning current to common(gnd).

As an aside, I measured gate current at 50 deg C. The Tokin SIT draws 200nA at room temp and draws 800nA at 50 deg C. The Puck P-FET gate draws less than 1nA.

I raised Vds until I measure close to 1.5A at 50 deg C. That happens at +/- 29V Vds. The actual current was 1.45A.

For the time being, I conclude that it will make more sense to source a special transformer to use the Tokin SIT and the IXYS puck rather than obtain and sort through an ocean of P-FETs to match the Tokin SIT at + / 24V.

It was fun to characterize the DEFiSIT pair over temp at different biasing arrangements. I ordered some more P-FETs and will sort them for the best matched pair. They are expensive little buggers at USD $36/each.
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Old 18th December 2019, 09:41 AM   #20
woofertester is offline woofertester  United States
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A picture is worth......
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File Type: jpg DEFiSIT_Pair.jpg (781.0 KB, 295 views)
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