Help with driver stage heatsinking

[edsays73]

Hi everyone,

I'm in the process of starting a new amp project that i have a schematic for but i have a doubt:

how do i know if i need to include the driver trannies on the heatsink of the output fets or not?

I have done several projects succesfully, some with construction details and others without, but this is going to be my most complicated one yet. I have gotten this far in the planning and have got stuck on this detail. I suppose i don't have to say that i don't have construction details for this and am on my own. If you guys need more specifics just let me know.

thanks a bunch!!

 

[Jan Dupont]

A schematic posted here would had helped, however in general terms the drivers should should be fitted on the heatsink together with the output devices, to ensure temperature stabillity.

 

[AndrewT]

it depends on whether the drivers are influencing the bias current stability with changes in ambient and device temperatures.
Vfets behave quite differently from Lfets and both are different from BJTs.

CFP is completely different from EF or SF output stage in their bias stability requirement.

Look at your schematic and decide if Vbe (Vgs) value will significantly alter the bias current and thus heatsink temperature.
Then decide what range of temperatures the drivers and the outputs will have to tolerate for a range of different operational circumstances and occasional abuse.

 

[Jan Dupont]

Andrew;
You are right 😉

 

[edsays73]

Thanks for the quick reply!!

I thought about posting the schematic but i didn't want to violate any copyright laws as this is a commercial amp.

The output stage fets are hitachi 2sk1058 and 2sj162s which will be run at around 56 volts and the drivers are mpsa56 and mpsa06s which will be run at 24 volts regulated...

The bias will set at around 300ma...

I really appreciate the advice given so and i'm sure it will be really helpfull

Thanks again

 

[AndrewT]

Hi,
Your 1058/162 are Lateral Fets.
Don't bother putting the drivers or the bias arrangement on the output stage heatsink.

Is the bias a plain adjustable resistor?

 

[edsays73]

Hi Andrew,

Thanks for the reply... yeah the bias will be set by a 1k trimpot to around 300ma

 

[nigelwright7557]

A schematic posted here would had helped, however in general terms the drivers should should be fitted on the heatsink together with the output devices, to ensure temperature stabillity.

This is Ok so long as the driver is mounted very close to the output transistors.
Even then there could be considerable difference in temperature between the output transistor and the driver.
Especially if the amp is heating up quickly, I have seen the output transistor hot while the heatsink is still cold if the heatsink is large. So there can be considerable thermal lag between the output transistor and the driver transistor.