Darlington transistor hfe.

Discussion in 'General Electronics Discussion' started by sureshot, Jan 16, 2018.

  1. sureshot

    sureshot

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    Looking for some advice on transistor hfe of bipolar juction transistors. I have some TIP36C transistors and wanted to pair two as a darlington pair. The application would be as a linear voltage regulator pass transistor. I have built circuits with a single darlington package in the past, that worked well. My uncertainty is the maximum hfe of a paired TIP36C transistors. Typically possible oscillation of such an arrangement. I know there is a forward voltage drop larger than a single transistor. But the hfe of the TIP36C is 10 25 and maximum 50 hfe, I'm looking to minimise voltage drop under load for a single boost transistor circuit, using the two TIP36C transistors. Input voltage would be around 18 to 22 volts from the rectified and filtered input voltage. Would it be viable to pair two TIP36C transistors.
    Any thoughts appreciated. My maximum final output voltage would be 12 to 13.8 Volts. Thanks for reading.
     
    sureshot, Jan 16, 2018
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  2. sureshot

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Generally speaking the hfe of a Darlington is the product of the hfe's of each individual transistor.

    A common pairing is a TIP31 and a 2N3055.

    A TIP36 is a PNP transistor. For positive linear regulators, NPN transistors are more common.

    PNP transistors are used in low dropout regulators, but you need to be careful to maintain stability.

    Do you have a circuit you want to use?
     
    (*steve*), Jan 16, 2018
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  3. sureshot

    sureshot

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    Thanks for the reply Steve. Yes stability is the reason i wandered if a pair of TIP36C transistors is to high in hfe of a final darlington pair. The circuit is the simple boost linear regulator current circuit. I know its a simple circuit, I've done it with a single MJ11015 power transistor. That worked great. I was wanting to try a pair of TIP36C transistors as a darlington pair. I posted a long time ago now on an unstable MJ11015 boost circuit that was unstable.

    I since found the reason to be my input to that circuit from a 24 Volt smps just for testing. Once i used a chassis transformer it was stable. I was going to try the TIP36C transistors as a darlington pair, as its something I've never tried before.
     

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    sureshot, Jan 16, 2018
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  4. sureshot

    sureshot

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    Having had time to think it over, and looking at your NPN suggestion, its looks like the first transistor could be lower rating. As the current passes from emitter to collector, the final transistor is doing all the work. I noted the TIP31 was rated for a maximum collector current rating of 3 Amps. The hfe of the TIP31 looked the same as the 2N3055 or there abouts. I've only gone with a PNP as its using a 3 pin linear regulator. As if i went NPN on the output side of the regulator, i would drop some voltage on the output. Couple of pictures of what i think works well in a relatively small footprint, size for output power.
     

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    sureshot, Jan 16, 2018
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  5. sureshot

    AnalogKid

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    The recommendation to use an NPN transistor came before you decided to share your schematic. Now that I wee that you meant *current* boost, not voltage boost, stick with PNP transistors. They are the standard part for your circuit. Suggestion: add reference designators to all components; int makes discussions mych less confusing.

    There is no reason for the 10 ohm base resistor. Also, look at the datasheets for darlington transistors and you will see that often there are two resistors built in. I recommend adding them to your discrete darlington circuit. Then guarantee that the transistor will turn off about as fast as it turns on, and immportant thing when dealing with transient load currents through the regulator circuit.

    ak
     
    AnalogKid, Jan 16, 2018
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  6. sureshot

    sureshot

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    Yes thank you, indeed i posted the schematic after Steve made his suggestion, not knowing what i was using as a circuit. I did note these resistors in the MJ11015 data sheet, the description explained it. Thanks for the reply. I'm just playing about with a few different transistors at the moment. The linear voltage regulator current boost circuit has fascinated since i first found it. The MJ11015 MOT based power supply is in the picture below. If you might remeber from way back you helped me out with transistor specs and base resistor scenarios.

    I'm still playing about now and then, i just hadn't tried a darlington built from two separate transistors yet.

    Should point out the 4mm posts have clear heatshrink on them to avoid shorting out.
     

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    sureshot, Jan 16, 2018
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  7. sureshot

    sureshot

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    I ended up using 90000uf of capacitance and a low drop out regulator do avoid to much voltage drop underload.
     

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    sureshot, Jan 16, 2018
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  8. sureshot

    Ratch

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    You might want to read the posting I made on another site concerning pass transistors. It uses a big resistor to take most of the heat instead of dissipating it with the pass transistors. https://www.electro-tech-online.com/threads/bullet-proof.146490/#post-1240701

    Ratch
     
    Ratch, Jan 17, 2018
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  9. sureshot

    sureshot

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    Hi, the capacitance of 90000uf is filtering on the input from the bridge rectifier. Its only that large because i only had 14.70 Volts for that input. To avoid voltage drops under load. After rectification and filtering i ended up with just over 20.00 Volts. If i would have gone with less capacitance, say 10000uf the voltage would almost certainly would have dropped into the headway room. Then regulation might have suffered. I'm not entirely sure what you mean by dumping the heat on the pass transistors. Under a 12 Amp load those aluminium plates only reach 38°C the MOT itself runs quite warm at 76°C but i expected that. I think i referred to that power supply with friends, as a coal fired boiler. In other words its brute force current, but far from an efficient unit in final. It was just something i had been wanting to try for a while. I'm still tinkering with power transistors, hence the discrete darlington pair i would like to give a go with.
     
    sureshot, Jan 17, 2018
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