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Deceptively simple USB charger

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    #1

    Deceptively simple USB charger

    I have a tiny USB charger that decided to stop working completely, so I opened it up and found and fixed a loose supply wire and it works now, but one thing that caught my attention was how simple the SMPS circuit looked --- no ICs at all, only common discretes. I decided to trace the PCB to see if I could figure out how it worked, but the circuit (attached) was a little too difficult for me, someone with moderate electronics knowledge, to understand; I can see the basic parts like the primary rectifier, reservoir cap, transformer/feedback optocoupler, and secondary rectifier (half-bridge only, but not too surprising as the supply is only rated for 300mA), but how the oscillator/feedback actually works is totally not obvious. Noticing how small the reservoir is (2.2uF), does it somehow cleverly use the ripple current to start the oscillation? It sure starts up and gives a pretty clean and steady 5V at the output.

    Can anyone here give their thoughts on this circuit, especially the primary oscillator? The secondary side is rather straightforward. Here's what I've figured out so far:

    Q1 is the switching transistor; the top coil on the primary is the main switching coil, the second coil on the primary provides a low voltage (<25V) rectified through D8 and filtered by C4 to power the feedback loop
    Q2 somehow regulates the oscillator via the feedback loop to control the output voltage
    C2, R3, and D6 are the transient suppressor
    C3 might be the oscillator cap
    DZ1, a 4.3V(?) zener, sets the output voltage
    Q3 and Q4 control the charging status LEDs
    Attached Files
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    #2
    Re: Deceptively simple USB charger

    look like 2-transistors 5VSB circuit in ATX PSU.
    Q1 Base-Emitter might be swap to circuit below, I guess.

    Attached Files
    Last edited by POM_MJ; 10-28-2010, 10:35 PM.
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    Comment

      #3
      Re: Deceptively simple USB charger

      I agree. Looks like a typical 2 transistor 5vsb circuit. Capacitor C4 is most likely the "critical capacitor". If it goes bad with age, the output voltage will probably increase beyond spec and maybe damage your equipment. It may be a cheap solution, but I prefer circuits that use a pwm chip for voltage regulation.

      By the way, you did a great job on making the schematic, and your analysis was also good. Keep up the great work.
      Last edited by everell; 10-30-2010, 09:28 PM. Reason: added comment
      Old proverb say.........If you shoot at nothing, you will hit nothing (George Henry 10-14-11)

      Comment

        #4
        Re: Deceptively simple USB charger

        It has an opto and a voltage reference. It's quite unlikely that a dried up C4 will make the voltage go up. Or am i missing something?
        Originally posted by PeteS in CA
        Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
        A working TV? How boring!

        Comment

          #5
          Re: Deceptively simple USB charger

          High ESR on C4 means fluctuating voltage (not good DC) feeding the opto and the two transistors. This is what causes the two transistors to drive the transformer harder. It is popularly called "getting Bestec'ed"
          Old proverb say.........If you shoot at nothing, you will hit nothing (George Henry 10-14-11)

          Comment

            #6
            Re: Deceptively simple USB charger

            Here is my analysis of the problem

            https://www.badcaps.net/forum/showthread.php?t=10301
            Old proverb say.........If you shoot at nothing, you will hit nothing (George Henry 10-14-11)

            Comment

              #7
              Re: Deceptively simple USB charger

              Thanks, i get it now.
              Originally posted by PeteS in CA
              Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
              A working TV? How boring!

              Comment

                #8
                Re: Deceptively simple USB charger

                Thanks for the analysis, and the circuit does make a lot more sense with Q1 connected as POM_MJ showed, but unless the pinout on Fairchild's site for the E13003 is wrong (http://www.fairchildsemi.com/pf/KS/KSE13003.html), it really is connected as shown. I've attached the actual PCB layout with the components identified.
                Attached Files

                Comment

                  #9
                  Re: Deceptively simple USB charger

                  Check that Q1 one more time - I would have to agree with pom-mj correction. The R1 is a startup resistor. I don't think it would work properly the way you originally drew the schematic.
                  Old proverb say.........If you shoot at nothing, you will hit nothing (George Henry 10-14-11)

                  Comment

                    #10
                    Re: Deceptively simple USB charger

                    Finally found the time to check Q1 more thoroughly, and indeed their datasheet was wrong on the pinout.

                    Also replaced the "critical capacitor" with a new bigger one (Nichicon 63uF/50V), just in case. Output voltage didn't change, but this PSU doesn't seem to regulate well with no load --- at less than ~2mA the voltage hovers around 5.5V, but with just a little bit more current draw, around 2.5mA, the output drops to 4.98 and stays there almost perfectly all the way to 300mA. Is this normal for simple switchmode supplies like this?

                    Comment

                      #11
                      Re: Deceptively simple USB charger

                      because of this circuit isn't just 5V power supply, it's output has config to current source for charging purpose.

                      D8.2,R8.2 and Q4 are current sense.
                      Q4 turn ON, LED11 lit while charging.

                      generic SMPS always need a load resister to maintain voltage regulation.
                      | AMD Phenom II X2 550BE | GIGABYTE GA-MA790FXT-UD5P | GeIL DDR3 Ultra 2x2GB 1600C7 |
                      | XFX GTS250 DDR3 512MB | Dell H525EF-00 | Lancool PC-K62 Black | Samsung 2232GW |
                      | 2xWD7500AYYS | 2xHD322GJ Raid0 |

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