Electronics > Beginners
Darlington pairs resistors.
sureshot:
Thank you for the replys, yes it all makes sense. As my application is linear, do I really need the resistors ? Looking at the LM723 circuit there is a darlington pair from the IC with no resistors. I'm wandering if there really necessary for a series pass element.
Not sure what kind of leakage current, as in the numbers involved. I'm only asking this as it's not a switching application where completely on to completely off is important. What do you think ?
Thanks again.
exe:
--- Quote from: sureshot on July 26, 2018, 07:14:57 pm ---As my application is linear, do I really need the resistors ?
--- End quote ---
In my opinion they are essential for performance. They help removing stored charge -- making circuit faster. Darlington / Sziklai pairs are very slow without them as power transistor doesn't have path to quickly "discharge" base. Of course there is some power loss on resistors, they all may heat up quite a bit. So, you trade efficiency for speed.
PS I'm no expert, but I had same question in the past. I did some research.
Kleinstein:
The resistor from the larger transistors base to emitter really helps to speed up the transistor. Even if a linear application a power supply also has to handle transients and this can include turning off transistor. The resistor mainly shifts loss from the large transistor to the smaller one - so there is not much disadvantage from the resistor. For a linear PS keep in mind the FBSOA. The actual useful power handling capability can be quite a bit smaller than P_tot.
The other transistor may not be needed or may be part of the circuit anyway.
exe:
--- Quote from: Kleinstein on July 26, 2018, 07:57:57 pm ---For a linear PS keep in mind the FBSOA. The actual useful power handling capability can be quite a bit smaller than P_tot.
--- End quote ---
Yeah, under short-circuit condition instantaneous current can be quite high and power supply may not react fast-enough or current limiting can be slow. Normally there is a separate bjt across shunt resistor that pulls the base current. Like Tr3 here: http://www.learnabout-electronics.org/PSU/images/Series-with-current-limit.jpg .
Also, my observation, pass transistor has much higher rating (2-3x) than expected maximum power dissipation (may be because of SOA). Also good idea to put a reasonably beefy diode (reverse-biased under normal conditions) between collector and emmiter (to protect from input voltage higher than output if power supply charges a battery or big cap).
Simon123:
You can calculate the value for each resistor across base and emitter. Since Ube=0.6V and current leaking from collector into the base (Collector Cutoff Current in datasheets) is about 1mA for 2n3055, the resistor value is about 600 ohms.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version