How to measure temperature of SMPS FET?

[eem2am]

Hello,

I am doing a 230VAC, offline, 21W flyback with TOP264VG (eDIP-12 package)
The SMPS will be in an adapter (i.e. totally enclosed)

This PWM controller has an on-chip FET

TOP264VG datasheet:
http://www.powerint.com/products/topswitch-family/topswitch-jx

I wish to measure the temperature of the case of my TOPSwitch whilst it is running on full load, Minimum Vin and inside its enclosure with maximum ambient temperature outside the enclosure.
(-when I have the case temperature of the TOP264VG, I will then work out the junction temperature using the datasheet provided Rth(JC) value.

I am presuming that I must measure the TOP264VG case temperature by glueing a thermocouple to the case of TOP264?

Do you know what type of glue I should use to glue the thermocouple to the case of the TOP264VG?
I presume it must be thermal glue?…Please could you recommend a part number for this glue, or give the “specification” of this glue?

Also, I assume that it will not be enough to merely glue the end of the thermocouple to the case of the TOP264VG(?)……and that in order to provide some strain relief (so that the thermocouple does not accidentally get pulled off) I will have to glue the thermocouple wire itself to the PCB , close to the TOP264VG (?)
-If so, then what kind of glue should I use to glue the thermocouple wire to the PCB?

Also, to what part of the case of the TOP264VG should I glue the end of the thermocouple? (right in the middle? , as close to the source pins as possible? Or as close to the drain pin as possible?)

Sorry for long post…thankyou for reading.

 

[Mr.Cool]

you need thermally conductive RTV silicon adhesive.

http://www.acc-silicones.com/products/adhesives/thermallyconductiveadhesive.ashx

but to be fair, i'm not sure if this is the best solution. glueing a thermal couple onto the case. maybe if you bolted the switch the PCB you could measure the temperature *near* the mosfet.

?
Mr.Cool

 

[keith1200rs]

I don't know the device you are talking about but be careful you don't electricute yourself - if it has a metal case it may be live.

I usually use either a thermocouple surface probe (it has a handle and thermocouple bonded to a small disc and is spring loaded) or non-contact infra-red thermometer.

Keith

 

[FvM]

Using a selfmade thermocouple from NiCr/Ni wires with a diameter of 0.1 mm or even below, thermal conductivity of the glue effectively doesn't matter any more.

 

[karesz]

Hi,
Masterbond has (I think) the worlds biggest sortimente on epoxy & other thermally (& electrical too) conductive glues_most are Space & Mil qualifyed too... -)
https://www.masterbond.com/
K.

 

[eem2am]

hello, if i glue the thermocouple to the PCB metal of the drain pin, then that will mean its got the switching voltage on it, (switching between 0V and 330V)............won't that be a problem for the thermocouple, and the accuracy of the temperatur reading?

 

[keith1200rs]

hello, if i glue the thermocouple to the PCB metal of the drain pin, then that will mean its got the switching voltage on it, (switching between 0V and 330V)............won't that be a problem for the thermocouple, and the accuracy of the temperatur reading?

... and your lifetime! It is one of the reasons I bought a non-contact IR probe (that and the risk of blowing up devices by shorting legs with a surface probe).

Keith.

 

[dick_freebird]

Another possibility is to sample the negative pedestal voltage of
the output switch. If you know load current then the freewheel
pedestal voltage (diode) will give you a temperature reading
if you calibrate it temp:V.

Crude, but something you could make a part of the supply with
maybe a blocking diode, filter network and comparator.