Hello,
In a scenario where a darlington pair is built using a high-current low-beta device driven by a signal high-beta transistor, what is the recommendation for placement of the driver, close to the output device's heatsink or on the contrary farther away so it doesn't pick up heat from the high-current pass transistor? Logically the driver should thermally track the output device, since in a monolithic Darlington they are on the same die no? I also have observed that the darlington input drivers tend to be next to their output transistors in power amplifiers. Thermal issues causes and effects are still a bit fuzzy to me, I would appreciate an explanation of the thermal effects at play in such a situation.
Below is a tentative pcb layout where I put the driver (Q1, TO-92) inside the output's transistor (Q2) heatsink fins, the output device being on the other side of that heatsink. Note that this circuit is high voltage (200-400V).
As always, thanks for any insights. Regards,
- Joris
In a scenario where a darlington pair is built using a high-current low-beta device driven by a signal high-beta transistor, what is the recommendation for placement of the driver, close to the output device's heatsink or on the contrary farther away so it doesn't pick up heat from the high-current pass transistor? Logically the driver should thermally track the output device, since in a monolithic Darlington they are on the same die no? I also have observed that the darlington input drivers tend to be next to their output transistors in power amplifiers. Thermal issues causes and effects are still a bit fuzzy to me, I would appreciate an explanation of the thermal effects at play in such a situation.
Below is a tentative pcb layout where I put the driver (Q1, TO-92) inside the output's transistor (Q2) heatsink fins, the output device being on the other side of that heatsink. Note that this circuit is high voltage (200-400V).
As always, thanks for any insights. Regards,
- Joris
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Depends. With a Darlington, other sections will (should) take car of the thermal stability, as bjt's can run off rapidly.
With Sziklai's, thermal separation is a rock solid law.
I would not recommend thermal bonding with Darlingtons for the above mentioned reason. Every stage of the amplifier involved (signal level, low power, high power) has its own merits and shortcomings, so this is another fine balancing act.
In short: no fixed rules here...
What are those odd thin black lines?
With Sziklai's, thermal separation is a rock solid law.
I would not recommend thermal bonding with Darlingtons for the above mentioned reason. Every stage of the amplifier involved (signal level, low power, high power) has its own merits and shortcomings, so this is another fine balancing act.
In short: no fixed rules here...
What are those odd thin black lines?
Lol just saw that, these are the cursor crosshair in Kicad that got captured in screenshot. I corrected the image, thanks.
Sziklai pair... Never used it. In another circuit where its single Vbe drop could have been an advantage I compared both in a current buffer simulation and the Darlington had better ripple performance. In this circuit here the darlington is a pass device in a Maida regulator, I guess I could compare both configurations again, the following LM317 would take care of any excess ripple. But to be honest I have trouble figuring where a Sziklai pair would be necessary except where low voltage drops are needed. The Maida is not a low drop-out regulator in any ways.