82v 2a-3a power supply using tl783

Discussion in 'General Electronics Chat' started by auraslip, Oct 17, 2011.

  1. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    I'm looking to make a cc-cv power supply. A very simple one at that! Basically I just want an easy way to get 80v-100v. The tl783 is a linear regulator and can handle that. The data sheet even shows how to use it with some power transistors for higher power. The following is something I made in ltspice, and is copied directly from the datasheet.
    [​IMG]

    I'm looking for reliability at 2a-3a of power. Hopefully it won't be a huge package either. Can anyone advise me on my drawing if it looks good? thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Q1 and Q2 must be PNP transistors, not NPN.
    With 110V in, 80v out, and a 3A load, Q2 will dissipate 2.7 Watts, the regulator ~9.2 Watts, and Q3 about 70 Watts.

    Do you have a plan for getting rid of all that heat? You'll need a big heat sink for Q3, and even then it'll be hard to keep it from melting.

    Datasheet for the BUH100:
    http://www.onsemi.com/pub_link/Collateral/BUH100-D.PDF
    THermal resistance junction to case is 1.25°/Watt, so that's 87.5 degrees above the temperature of the heat sink.

    If the temperature of the case can be maintained at 25°C, then the maximum power dissipation of the transistor is 100W; but you derate 0.8W/°C above 25°C

    When you are using linear regulation, the only way to reduce the voltage is by burning off the excess power as waste heat. If you live in a cold location, that's not all bad.
     
    Last edited: Oct 17, 2011
  3. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    Thanks for the thought out reply. I'm coming to terms with this, despite spending a fair bit of time on this project. It was fun, but I'm afraid you're right.

    [​IMG]

    Here is a current limited, voltage adjustable power supply I made in LTspice. It seems to work.

    However, I understand that even if I take care of the heat problem, I would still need a massive transformer. And since this is supposed to be a mobile power supply, I'm not sure I an handle that. And large transformers are expensive as hell. I don't know what to do next to buying a commercial SMPS unit from china of unknown quality :(
     
  4. SgtWookie

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    Jul 17, 2007
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    Where were you going to get the 110V DC from?
     
  5. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    A 1:1 transformer and rectifier.

    Actually, here is an interesting idea; what if I just rectified the AC line, regulated it down to 83v, and THEN used a 1:1 isolation transformer and a rectifier on the output? This way I could use a much smaller transformer but still retain the safety benefits of a having a transformer.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The transformer really should come first.

    There are ways to use TRIACs (a type of thyristor) as a pre-regulator to get the voltage on the filter cap within range of a linear regulator; I don't happen to have a schematic for one at the voltage you're talking about.

    But that's something to look at.
     
  7. #12

    Expert

    Nov 30, 2010
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    That schematic in post #3 is a disaster waiting to happen. 2N3055 transistors can not be trusted to share the load like that. You need most of an ohm in each emitter circuit to make them behave properly because their Vbe will not match and the best transistor will go into thermal runaway as it heats up and becomes even more conductive.
     
  8. colinb

    Active Member

    Jun 15, 2011
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    This won't give you 110 V dc, but will give you pulsed dc with a peak value of 170 V. You'll also need some big capacitors to filter the pulsed dc voltage if you want clean output.
    OK, so you rectify the ac to dc and regulate it. But then you can't use an isolation transformer on that dc.
     
  9. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    My last post is pending because I included a link I guess. Anyways, thanks for the help. It should be clear I have no clue what I'm doing :D

    I thought this would be as easy as making a 5v regulator, but I guess not.

    The TL783 might still be an option? It seems easy. I'm just trying to wrap my head around how large a transformer I would need to supply even just 1a, let alone 2a. The issues with running multiple voltage follower transistors doesn't seem that big, nor does current limiting. I just don't have a clue where to start on the AC side. I understand the efficiency of linear regulators is Vout/Vin, so with 110vdc input I'm looking at ~30% of the power at 2a (82v * 2a * .30 = 50w) worth of heat. Which isn't a huge deal spread out over a few transistors. I'm not looking for efficiency. I just need to get the job done.
     
  10. colinb

    Active Member

    Jun 15, 2011
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    You better run the numbers. That 50 watts of heat will certainly require some massive heat sinks to keep the transistors from overheating. Possibly some fans would be helpful. You'll need to evaluate the power dissipation in the transistors, thermal resistance from junction to ambient (probably including a heat sink), and maximum temperature above ambient allowed for the transistors.
     
  11. #12

    Expert

    Nov 30, 2010
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    The proper way to do this is to get a transformer that will give you the right voltage after rectification. The formula for rectification is: Volts peak = (1.414 times Volts RMS) minus volts lost in the rectifiers. The most efficient rectifier is the 4 diode type and that will cost about 1.4 volts off your peak voltage.

    Then you need a filter capacitor and the formula is: I = radical 2 Eripple (peak to peak) x frequency x C.

    That's: Amps = .707 times the sag between power pulses times the frequency after rectification times the capacitance in farads. You seem to speak American, so the frequency will be 120 Hz.

    This will give you a mostly DC voltage (with a little bit of ripple) to feed the transistors. Make sure the lowest DC voltage after diode losses and sag is about 3 volts higher than the output voltage. That will keep the transistors from running out of voltage.

    Be sure to calculate the power dissipation on your zener diode. It looks a bit high from here.

    edit: colinb poster while I was typing, in case it matters.
     
  12. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    50 watts is a lot of heat? lol, guess I'm used to dumping 3kw into my hub motor at 20% efficiency at starts :D of course it's ventilated and I monitor it with a temp sensor.

    Thanks for the help ya'll. I'll figure out the math tomorrow and try to find a suitable transistor. I just put together my new ebikes battery pack (20s = 78v nominal) and need to do a test ride. Should be rolling at 40mph!
     
  13. colinb

    Active Member

    Jun 15, 2011
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    It's all relative... but think about an Intel Core 2 Duo desktop processor, which has a thermal design power of 65 W. This requires a fan-cooled heat sink, which I would call “massive” in the context of common transistor circuits... not to mention the CPU has a big, flat surface area for thermal path to the heat sink.
     
  14. auraslip

    Thread Starter New Member

    Oct 17, 2011
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  15. colinb

    Active Member

    Jun 15, 2011
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    Variable linear power supplies are MASSIVE. Some of the bench supplies we have around here weigh about 40 pounds. For efficiency I think they usually have multiple “ranges” with different transformer ratios from mains power so that if you want 30 V, you can have it, and if you want 1.5 V, you aren't burning 50 watts of heat to do so.

    In your case, you could implement a switch-mode converter instead of linear regulation. It would certainly be MUCH smaller and more efficient, and possibly cheaper. The main disadvantage would probably be in electrical noise on the output due to the switching converter. However, with adequate filtering and/or inserting a low-dropout linear regulator after the switching converter to further reduce noise output I think you could get good results. But I've never made something like this so I may be wrong.
     
  16. colinb

    Active Member

    Jun 15, 2011
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    Here's a thread you might find interesting: Variable Output SMPS. Someone wants to design a variable output switching power supply circuit.

    Also, Roman Black's “Black Regulator” is a nifty two-transistor buck regulator. Check out the section “High Voltage Use” at the bottom of the page. (Roman Black is an AAC member and goes by THE_RB.)
     
  17. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    I thought ya'll might like to see my @ home charger
    [​IMG]

    I got 6 12v50a server power supplies for $5 each shipped. On the top I'm using a meanwell switching supply to limit current and fine tune the output. Here is the build thread.

    Others use the variable lab supplies mention to for voltage and current control. This setup is overkill for my 800Wh pack, but I wanted something that'd be easy to scale up for whatever EV project I build.

    Anyways, I was quite happy with the performance I was getting at 20s. I wanted wheelies, and only kinda got them :D so I'm going to be running 22s now which should have a final charge voltage of 22s * 4.15v = 91v. Still not sure how I'm going to achieve this. Too busy with school right now to do anything complicated :/
     
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