Biasing/thermal compensation of Thermal Trak transistors

[dadod]

Hi Bob,
This tread was very interesting but it is dead now.
Your ThermalTrak bias circuit, in the begining of the tread, is really good one and I am going to construct an amp as I have got some pairs of njl1302/3281.
Do you have any news about TT bias stability and how good was circuit diagram you proposed. Do I have to change something to get better thermal stability?
Any help will be appreciated, and thank you in advance.
dado

[Bob Cordell]

Originally Posted by dadod

Hi Bob,
This tread was very interesting but it is dead now.
Your ThermalTrak bias circuit, in the begining of the tread, is really good one and I am going to construct an amp as I have got some pairs of njl1302/3281.
Do you have any news about TT bias stability and how good was circuit diagram you proposed. Do I have to change something to get better thermal stability?
Any help will be appreciated, and thank you in advance.
dado

Hi dadod,

Yes, this was a good thread, and my participation in it helped me formulate some of my thinking for the ThermalTrak section of the thermal stability chapter in my book. I have learned quite a bit more about ThermalTrak biasing while writing my book, so I recommend you rely more heavily on what I wrote there.

One approach that I have liked, and which is not too complex, is to have one Vbe multiplier that uses the ThermalTrack diodes to manage the bias of the output transistors, and to have a second Vbe multiplier in series to manage the temperature compensation of all of the other transistors in the Locanthi output triple (i.e., the pre-drivers and drivers). I then mount the second Vbe multiplier on a largely isothermal bar to which the pre-drivers and drivers are also mounted. By keeping all of these transitors at essentially the same temperature, and recognizing that their power dissipation is not much of a function of program material, good thermal stability of the Triple can be achieved. Independent adjustment of the thermal multiplication coefficients of the two Vbe multipliers provides freedom in achieving good overall thermal tracking. After initial design centering, only one of the Vbe multipliers need have a pot for adjusting the bias of the amplifier.

Cheers,
Bob

[dadod]

Thank you very much Bob,
could you post example circuit diagram of the OPS?
dado

[dadod]

Hi Bob,
Sorry to bother you, just some morre questions here.
<to have one Vbe multiplier that uses Tthe RhermalTrak diodes to manage the bias of the output transistors>
How to connect two TTdiodes (I suppose two are enough), in series with the Vbe multiplier or into base - collector connection of the Vbe multiplier?
Does this multiplier has to be mounted on the same heathsink as the output TT transistors?
dado

[Edmond Stuart]

Hi Bob,

Regarding thermal attenuation, would you please have a look at this post: Rebirth of the Phoenix
Am I right that we have to take this matter into account and make some provisions to compensate for thermal resistances by means of some (say 20%) electrical overcompensation?

Cheers,
E.

PS: Great book. Congratulations!

[JPV]

I have read your suggestion of using the left over TT diodes to detect the temp of the junction and protect the transistor before reaching let say 150°C. If we have such a detection coupled with a maximum current detector we are save with respect to short circuit protetcion.
The question remaining is secondary breakdown. Do you believe that monitoring the power max temp is enough. Secondary breakdown will happen with some reactive loads but it has been shown elsewhere that derating for temp during secondary breakdown should less stringeant than the derating of power dissipation because the effect of increase of emitter resistance with temperature. In other words, with increase in temperature, the normal power limit takes over the secondary breakdown limit for higher Vce than at lower temperature. If you couple this with the 10 ms SOA limit, is it not reasonnable to say that monitoring the 150°C crossing is totally safe for normall ( less than 45° phase) loads.

Then a totally non intrusive SOA protection device can be designed.

What do you think. The secondary breakdown is the problem.



JPV

[Bob Cordell]

Originally Posted by Edmond Stuart

Hi Bob,

Regarding thermal attenuation, would you please have a look at this post: Rebirth of the Phoenix
Am I right that we have to take this matter into account and make some provisions to compensate for thermal resistances by means of some (say 20%) electrical overcompensation?

Cheers,
E.

PS: Great book. Congratulations!

Hi Edmond,

I'm sorry I'm late getting back to this post - I just lost track of this thread.

You are right about thermal attenuation within the ThermalTrak devices between the transistor junction and the tracking diode. This is certainly non-ideal, but the ThermalTrak arrangement is still far superior to the conventional arrangement, which suffers even more thermal attenuationa and far more thermal delay.

In general, some experimentation is needed in setting up the optimum compensation for a ThermalTrak amplifier. Some form of electrical multiplication usually has to take place in the Vbe multiplier associated with the ThermalTrak devices. Indeed, some multiplication is often needed anyway because the temperature coefficient of the ThermalTrak diodes is not quite the same as that of the BJT. So the needed amount of multiplication is all wrapped together the best we can.

In many cases I'll use two Vbe multipliers, one working with and incorporating the ThermalTrak tracking diodes and the other providing bias spreading for the remaining devices of the output Triple. That way, the amount of multiplication allocated to each process can be set somewhat independently. The transistor of the second Vbe multiplier usually wants to track the temperature of the pre-driver and driver transistors. Indeed, if the driver transistors happen to be mounted on the heat sink, then the transistor of the second Vbe multiplier may also be mounted on the heatsink.

It was a two Vbe multiplier arrangement like this that I used for the test amplifier whose bias behavior is shown in Figure 14.23 in the book. In that case, the second Vbe multiplier along with the pre-drivers and drivers were mounted on an isothermal bar/heatsink on the circuit board.

Cheers,
Bob