Safe Transistor temperature?

[Damo666]

Hello Guys,

I recently got hold of a transmitter that uses an Sd1127 RF transistor, but have a concern; Although it's got an heatsink on it, I can only hold my finger on the actual case of the Transistor for about 8 seconds until it becomes difficult to bear the heat.

I've done some searching around about Junction temperature, Thermal resistance, etc'.. But the more I read the more baffled I become.

Can someone please check the datasheet for this and enlighten me as to whether it's probably operating at an unsafe or safe temperature?

Just a FYI, I measured the Transistor temperature with an IR thermometer and the reading was 59.2C at the hottest part- on the Transistor case itself.

 

[heydonms]

60degC isn't particularly hot for a transistor. A quick glance at the datasheet says the max junction temp is 200, I don't think you will be anywhere near that.

Maybe someone else will do the maths, but I wouldn't bother, if it doesn't burn you when you touch it then it is probably within spec.

 

[ronv]

It is hard to say with absolute certainty without knowing either how much power is being dissipated in the transistor or the thermal resistance of the heat sink, but your temperature is probably ok. The maximum chip temperature is 200C so if it was running at it's 4 watt rating it would still be ok.

 

[dr pepper]

So long as at 60 degrees its been running on a constant load for a while it'll be fine, the junction is probably around 70 - 80 degrees.
If the case gets to 60 degrees a few seconds after switch on then that would be a worry.

 

[Damo666]

Thanks for the replies, Guys.

The case doesn't get to that temperature at switch on, but it does get to that temperature after about 3 or 4 minutes of being switched on.

It's being used in an exciter driver and is giving 1 Watt out and running off 13.8V while the whole circuit is drawing around 600mA including the PLL.

Does this information assist at all?

 

[dr pepper]

Is that device a To5 can, if so you could put a clip on 'sink on it if your worried it'll blow.

 

[Damo666]

Is that device a To5 can, if so you could put a clip on 'sink on it if your worried it'll blow.

It's a TO39 can which is the same as a TO5 as far as I'm aware.

There's already a clip on heatsink on it albeit low profile, so I'll search for a taller one.

So, to summise, would you all say the internal junction temperature is around 80-90C?

 

[MrAl]

Hi,

You look at the thermal resistance junction to case. It will be something like 2 degrees C per watt. You know how much wattage is in the device, then you know the difference between the case and junction temperatures.

For example, with a thermal resistance of 2 degrees C per watt and 10 watts the junction is 20 degrees C hotter than the case.

 

[Damo666]

Hi,

You look at the thermal resistance junction to case. It will be something like 2 degrees C per watt. You know how much wattage is in the device, then you know the difference between the case and junction temperatures.

For example, with a thermal resistance of 2 degrees C per watt and 10 watts the junction is 20 degrees C hotter than the case.

Excellently said in a nutshell.

I've been reading up about thermal resistance, heatsinks, etc' all day - the more I read the more I was getting baffled.

Your explanation has simplified it perfectly for me.
EDIT: I've just found out the Thermal resistance of my SD1127 is 21.9oC/W