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N-MOSFET drain to source leakage current?

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Astrid

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I have 8ohm heating element in series with N Type thermocouple. This is powered +30V over N-Mosfet transistor on high side.
Why high side, because thermocouple voltage is measured when heating element not powered.

Zero gate voltage leakage drain current make on 8ohm any error voltage.
For example IRF3205 have max leakage 25uA for 25C and 250uA for 150C. Typical value is not published. It is too much
IPP057N06N3 G have max leakage 1uA - 100uA for 25 - 125C. Typical 01 - 10uA.
That's a bit better.
My question
1. Leakage current is for Vds 60V, At half voltage (30V) drop to one half or more?
2. Leakage current value is typ. 10uA for 125C What is likely to be at 85 C?
3. There is a transistor with a significantly smaller than IPP057N06N3 G ?
 

I guess that just driving the gate at the OFF condition with a negative voltage a few beyond 0v would probably close a bit more the chanel, increasing its resistve characteristic, therefore decreasing the leakage current.
 

It is curious that you apply such a voltage (or any but
a trivial sense-bias) to the thermocouple at all. Why is the
thermocouple electrically tied to the heater rather than
insulated from it?
 

>Why is the thermocouple electrically tied to the heater rather than insulated from it?
Click to expand...
Because it normally produces :sad:
For examples Hakko solder tips T15 or T12 have thermocouple in sesies with heater.

Interesting thead for me, I wonder how it is with dependencies of leakage currnet on temperature.
 
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    Astrid

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Yes, heater with series N-thermocouple is commercially produced thing.
Preferably only two-pole connection.
The question is how drain to source leakage current depends on D-S voltage.
Infineon 60V IPP100N08N3G and 100V transistor IPP086N10N3G have 1uA for T25C and 10uA for T120C for DS Voltave 60 /100V if leakage current is function of DS voltage and dimension DS may be 100V transistor have on 30V lower leakage current or not :evil:
 

Typical leakage current is much lower than the specified values. But to make a design that still works with worst case parameters, you'll probably want to use two switch transistors with a pull-down resistor in the middle.
 

Astrid said:
That's a bit better.
My question
1. Leakage current is for Vds 60V, At half voltage (30V) drop to one half or more?
2. Leakage current value is typ. 10uA for 125C What is likely to be at 85 C?
Click to expand...

Leakage will drop significantly with a lower Vds/Vds(max) ratio and a Tj/Tj(max) ratio.
It will not be linear, but some exponential power.

My guess would be that leakage would halve for each 10-C temp reduction.
 

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