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    Modifying SMPS input voltage range

    Good day folks. Long time no see. It's been a while since I've tinkered with any new projects, despite one thinking this would be the perfect time, with the pandemic and the quarantine, people tend to stay indoors....it's not my case anyway: I go to work like normal, but it's here where I'm bored out of my mind with nothing to do, so here's something.

    A while back, I got myself this adjustable load kit form off Ali. I put it together and it works. It's powered by an AC or DC input and I've been running it off a 12v wall adapter on the bench. What good is such a device if it doesn't have a built-in voltmeter/ammeter to display the status of the DUT ? So I also got myself one of those, with intent of building a case for the whole thing. This is where I ran into a bit of a problem which I didn't foresee: the panel meter requires a separate power supply, since otherwise the GND of the meter would be common to the DUT and I'm not sure how good that is (there must be a reason why the manufacturer specifies this on the back of the meter !). Instead of having two wall adapters, I was thinking of using a dual output transformer. I dug through a box of junk at work and found one. It's unnecessarily large, the size of a cube measuring around 5cm all around. There's a bit of an issue though: it outputs 9v on one set of windings, which is OK, but 32v on the other one ! This is way too high, especially since it jumps to 43v when I add a cap to it...how can I step this down to 12v ? An off the shelf buck converter doesn't go any higher than 30v. I was thinking of a zener+transistor regulator, which would be highly inefficient but doable I think....we're talking a drop of around 30v after all ! At a current draw of 1a, that's already 30w of wasted power if my theory is correct ! ! (though the load most certainly doesn't come close to that). I don't know the current draw of the adjustable load unfortunately (the circuit itself, not the DUT).

    Still, one other thought that crossed my mind was to feed the 43v into the input of a wall-powered SMPS and modify that to work with a low input voltage like this....what is the limiting factor here ? I see most wall adapters taking anywhere from 90 to 240v and above, so what's stopping them from going lower ? There's probably something complicated involved with the transformer itself, like saturation or other factors which go a bit over my head. I know the output can be hacked to modify the output voltage (done it before successfully on my own), but what about the input voltage ? I couldn't find anything about this. I know there DO exist SMPSs which take lower input voltages DC or AC (I've seen them for sale), but those are specifically engineered for this purpose. I was thinking a regular-old power brick or phone charger or something readily available. One thing I know for sure from TV sets is that the control IC on the primary of some has a brown-out protection and actually senses the if the input voltage is withing range, so those would require a lot more hacking. The switching frequency would also probably need tuning to cater for the much lower input voltage. Cheers guys. Thank you.
    Last edited by Dannyx; 03-30-2020, 06:47 AM.
    Wattevah...

    #2
    Re: Modifying SMPS input voltage range

    There are plenty of 40V buck converters on Ali.

    Comment


      #3
      Re: Modifying SMPS input voltage range

      48v convertors are common - they are used in small aircraft.
      i doubt they are cheap though,
      as soon as you say something is for a boat or plane the fuckers add a zero to the price - kind of like medical equipment!

      Comment


        #4
        Re: Modifying SMPS input voltage range

        Yeah, I can probably build something with junk I find around the store, so price is a factor here. I'm not willing to spend a lot on this. This listing also shows the schematic (not sure if the original post did). All it is it's an LM324 and some passives....not sure how much power this would use up, so the zener regulator idea might be the way to go after all...or I could try stripping some windings off the transformer...has anyone tried that with small ones before ?
        Wattevah...

        Comment


          #5
          Re: Modifying SMPS input voltage range

          Changing the input voltage is likewise dependent on how the circuit is designed much like the output. The wide range SMPS likely is either running too much current in the primary at low voltage, or exceeding limits the control circuit. You'll likely need to reduce and thicken windings in primary and/or increase windings in secondary -- as long as bootstrap will still work at lower voltages and not lose too much in the switch which is even more critical at low voltage.

          That being said, what is the requirement of the panel meter power, is there a reason why it can't be powered after the bridge rectifier on the load board? Or is the panel meter not isolated 4 wire? Link to specific panel meter?

          Comment


            #6
            Re: Modifying SMPS input voltage range

            The panel meter will run quite happily on the 9v winding. It will go up to more than that once I drop a cap on it after the bridge rectifier, but the meter can handle up to 30v so we should be fine. It's the other winding (for the board) that I'm trying to step down, since I need 12-something volts to power the kit I linked to.

            THIS is the panel meter, in case you're curious. The schematic shows it using an individual power supply. If I were to power it from the same adapter as the load, the GND of the DUT would become the same as the meter's GND.....not entirely sure if that's a good idea...
            Wattevah...

            Comment


              #7
              Re: Modifying SMPS input voltage range

              Not clear but have to assume the panel meter is not isolated.

              The load board probably can take a wide range of voltage input, with a minimum of probably 11-12V to be able to saturate the MOSFETs. Can you use the 30V to power the load board? It shouldn't use much power, probably can use a LM317 or something to get the voltage down to 12V (or higher, probably fine) without too much dissipation.

              I think whatever you can do to get the load board to run with the 32V supply would be your cheapest option. A regulator to 12V like a LM7812 is enough, and if that's too much dissipation (it shouldn't use that much power, I'd imagine it only should draw no more than 100mA or so) you can use a LM7815. You may even be able to go higher with a LM7818 or more, but looking at the schematic you should not go past around 20V. I would suspect the PSRR of the load is fairly high so you don't even need to regulate that much, just need to make sure you don't go past 20V.

              As far as I can tell:
              - (Vin²/10KΩ) - TL431 power draw will be minor, R7 will be dissipating most of the power - ~ 40mW
              - Voh²/(1KΩ+4.7kΩ) - Dissipation from voltage dividers - ~ 200mW - depends on setpoint. These resistors probably would take the brunt of increasing voltage input.
              - Vin * about 5mA - draw of the LM324 - ~ 100mW + a bit from the above.
              - frequency dependent portion... not easy to calculate...

              So I'd say this thing should be around a half a watt or so just from its own power consumption, not including power dissipation by the load... so a linear dissipative probably won't hurt much here.
              Last edited by eccerr0r; 03-31-2020, 01:20 PM.

              Comment


                #8
                Re: Modifying SMPS input voltage range

                I opted for a different solution: the meter's range is larger than the load's, so I'll use the "high" winding to power the meter and the "low" winding to power the load. I can then step down the 43v to something more sensible, around 20-24v, hence the dissipation will be lower. According to the specs, the panel meter only draws 20mA.

                Come to think of it, yes, a plain ol' LM7812 would probably do...to be fair, it didn't even cross my mind and I just went straight for the more complicated version, like I often tend to do...
                Wattevah...

                Comment


                  #9
                  Re: Modifying SMPS input voltage range

                  I have the red/blue LED version of that panel meter. It uses HM5333B LDO for 3.3V and draws around 15mA. It's fine with 5V input power.
                  The current-sense is not isolated from the voltage-sense or from LED power, there is a common ground node.
                  Internally it is like this other OLED panel meter.
                  Attached Files

                  Comment


                    #10
                    Re: Modifying SMPS input voltage range

                    I'm surprised those meters only draw 15mA, LEDs are usually more hungry than this. You can bet an ICL7107 and 4x7-seg uses quite a bit more power.

                    Also the isolation that's necessary is between the measurement jig and the measurement electronics - between the measurement modes is not absolutely necessary.

                    Also keep in mind there is a maximum voltage for the 7812 (35V?), you may need to use a LM317 instead, as it doesn't care about the absolute input voltage. You still need to worry about its drop, dropping 20 volts at 100 mA still is two watts and that will still get quite warm.

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                      #11
                      Re: Modifying SMPS input voltage range

                      For one power supply project, I wrapped around 10 turns on the power transformer to make an aux winding for the panel meter. If there is room, just try add a winding, thin wire. Then you just need a bridge rectifier and filter cap.

                      I think the current drain is so low at 15mA because the displays are multiplexed, so only one digit (out of eight) is on at a time.

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                        #12
                        Re: Modifying SMPS input voltage range

                        However, multiplexing reduces brightness because it would be ⅛ what it would have been at 100% duty cycle. Only way is if the LED elements are normally much brighter than regular LEDs, and now because of multiplexing are just usably bright...

                        (Traditionally LEDs each need about 2mA or so average current to be usably bright. 7 segments = 14mA. However with multiplexing they need to be driven 8x as much current for ⅛ the amount of time, times the 8 digits so it still comes out to be over 100mA ...) If the LEDs individually needed only 500µA then...that's a different story...

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                          #13
                          Re: Modifying SMPS input voltage range

                          Right, I didn't account for the maximum Vin of such a regulator. Are you sure the LM317 can take it though ? The datasheet lists a maximum of 37v for it, hence why I went straight for the transistor-zener idea in the first place...I'm a mess lately, who can't piece his thoughts back together

                          The right transistor will probably have a much higher maximum C-E voltage to take those 40+ volts.
                          Wattevah...

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                            #14
                            Re: Modifying SMPS input voltage range

                            LM317 the maximum input to output voltage is ~37V -- since your output is 12V, you can be almost 50V in, tops. Just don't short the output, its self preservation won't kick in.

                            You can add a whole bunch of junk 1N4002's in series if you're worried...

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                              #15
                              Re: Modifying SMPS input voltage range

                              I actually plan to have the output at around 24v, to minimize "stress" on the LM317, since the meter can handle it.
                              Wattevah...

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                                #16
                                Re: Modifying SMPS input voltage range

                                Went with the LM317 and it works. Set it to 24v and based my circuit on the schematic 8.3.2 on page 12. Not sure whether I should stick with that 10uF electrolytic for C2 the way it is now or go with a ceramic one like on page 10 where it's given as CAdj, though that's probably splitting hairs for a low power application like this.

                                Side note: just watching some videos of Dave on this newer PSU module and was curious/worried about the noise on the output of these and how well these would stack against a linear PSU of the same power (5-10A). Those can get quite expensive, so I can see why one might go with an Aliexpress DC-DC converter like that, the way I did. I have two of the earlier models at home, a DPS5020 and a DPS5015 - basically the same thing but the former can supposedly put out more amps and has communication to hook up to a PC, which I haven't tried yet. I wonder how "bad" these really are for your everyday troubleshooting of electronics like laptops and phones and whether I should suggest a bunch of these newer "RIDENs" to our shop to build and use.....probably not - there's better alternatives out there, albeit expensive ones....
                                Last edited by Dannyx; 04-11-2020, 03:42 AM.
                                Wattevah...

                                Comment


                                  #17
                                  Re: Modifying SMPS input voltage range

                                  why dont you just use a pre-regulator before the main one?
                                  a big resistor followed by a 5w zener to get about 5v above your target voltage followed by your 317

                                  Comment


                                    #18
                                    Re: Modifying SMPS input voltage range

                                    TBH, I don't have such a zener on hand, so I just pieced it together with what I had available
                                    Wattevah...

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