VAS power dissipation

[danny92]

Hello everyone,

I've made this design that is based on one MOSFET amplifier by Bob Cordell, but I've a question to do, the transistors Q7 and Q4, seem to have a high power dissipation, this type of package (TO92) are intended to operate with powers near 0.6 W, but the dissipation required for Q7 and Q4 is about 0.45 W which is near to 0.6 W, this could damage the transistors, I should replace those transistors or there is some measure to prevent transistor overheating?

Thank you very much for your attention,
Best regards,
Daniel Almeida

 

[nigelwright7557]

Its good practice to run components at 50% of their capacity.

I use mje340/350 in my VAS/CCS and they just get warm but they are a slightly bigger package than your transistors.

 

[danny92]

Thank you very much for your repply,
I can use MJE243?

Best regards,
Daniel Almeida

 

[nigelwright7557]

Thank you very much for your repply,
I can use MJE243?

Best regards,
Daniel Almeida

It depends on your power supply volts.
The mje243 is only 100 volts Vce.
The mje340/350 are 300 volts.

 

[danny92]

In my application I will use +/- 40-45 V so I think that the mje243 are okay, right?
😉
Best regards,
Daniel Almeida

 

[nigelwright7557]

It should be ok.

 

[Rafael L]

Really not exceed 50% of power dissipation is a good safety margin.

mje340/350 is very slow and low gain for use in VAS, would very THD. KSC3503/KSA1381 would be a better option or decrease bias in VAS for use To-92.

 

[danny92]

I've changed the design, thank you for your help,

Best regards,
Daniel Almeida

 

[danny92]

This transistors seem fine in the simulation,

Best regards,
Daniel Almeida

 

[AndrewT]

Read David Eather.
He explains the method and why you need to do a temperature de-rated SOAR for both the output devices and the driver devices.

 

[jcx]

you can just parallel a couple of to-92 - you have emitter R already so current sharing is easy

but do chek SOA as well as power under likely faults like when stuck at rails