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Simple DC-DC Boost Converter 19.50V(dc) to 20.50V(dc) and 8A for charger.

Discussion in 'General Electronics Discussion' started by markcdn, Dec 4, 2018.

  1. markcdn


    Dec 4, 2018
    I bought a DCDC Boost Converter from ebay, it worked well, but the design was flawed for my uses as the input and outputs were on the same side, yes I did short output + and - and I probably messed up the output fet. It worked well, I attached a 12V fan to cool the dcdc down.

    I tried my best to copy the circuit and couldn't, but figured there has got to be a simpler way. The chargers output is 4-8A (measured) 19.50V and I require 20.50V to the batteries.

    I ran into one problem with using the charger by itself. It would shut down when I connected the battery. I had an old coil resistance heating element to calm the rush of current down enough to start the charging process. I would then bypass the resistance after a few moments hooked up to it. Different resistances would give me various current. One inch would be mA. and shorter pieces would achieve higher amps.

    I attached one circuit diagram that perhaps could help me, I have not tried it because its 50W. It brought me back to the same question, there has got to be a simpler way, but if those are the total amount of components I require to achieve my simple goal, then it is no problem for me to buy them and solder them up.

    The wait for the dcdc from China, along with the Canada Post strike will be at a minimum a month or two.

    Any help would be appreciated. I know there are some dcdc on ebay that have cc / cv control, but my laptop chargers achieve those modes. I would prefer each pos and neg from input and output to be on every corner, so my DMM probes dont slip off and break the dcdc.

    I use this charger everyday to charge up my mobility batteries. I can get by with 19.50V from the charger, but that extra 1.00V would help in being able to go longer distances per charge.

    Thanks for your time in reading this.

    My probe touched both inputs, so cap, fet, or smt.
    Both gnds are common.
    It seems so simple, on the top view. Then when I get to around IC chip I get lost trying to find the traces. The cap seems good with my contiuity probes, the Input fet is all open no matter what combo I do.Unlike the output fet. So I will unsolder, look at the markings and buy a few. I am sorry I lost the paper work I have done on my progress for the circuit diagram for this dcdc. To improve on the design of this dcdc, if the fet fix works, is to put the red pos on both in and out right on the + of each cap.

    That 8 pin IC is a UC843A
    Quote - The UCA/UCA are fixed PWM controller for Off-. Line and DC to DC converter applications. The internal cir- cuits include UVLO, low start up current. Optimized for Off-line and DC to DC. Converters. •. Low Start Up Current (

    Input Power Mosfet is 80NF70 G640KTFT



    Just a random circuit I found, has nothing to do with the dcdc above. I was hoping to copy it and apply it, but I think its just better to copy the QSKJ dcdc I bought on ebay and just lay it out differently.

    Last edited: Dec 4, 2018
  2. BobK


    Jan 5, 2010
    164W DC/DC converters are not simple.

    I don't understand why having the inputs and output on the same side makes the board you show flawed for your usage.

  3. markcdn


    Dec 4, 2018
    I was just checking the outputs and inputs with my dmm and one lead slipped off and shorted the output.
    I am going to be using those small cheap digital voltage meters from ebay for input and output voltages.

    ( 80NF70 ) POWER MOSFET N-Channel - Is broken. Heat sink mount body and Source are connected. But negative probe on body and positive probe on either lead comes up OPEN/Infinite. My DMM has continuity check and 200 ohm as one setting, so I moved it to 2000k (the highest) and it was confirmed open. Seems like an easy fix.

    ( 048 Morocco STPS20 ) Schotty Diode - Seems to be ok. Heat sink mount continuity to K, and measure 150 to A1 and A2.

    Been working on the schematic diagram to learn how it works. I find it an interesting device to investigate and more importantly to learn. I have no idea what the other components are labeled as. I ordered a 50X magnifying glass. My assumption is the 8 pin is an OpAmp because I remember getting the lettering and searching for it, but I could be wrong, because I was also working on an opened up laptop charger. I am going to see if I can use the magnifying glass I have now to get the lettering, but its very hard to, its only 10X, my eyes are that great for my age.

    As you can see I am still missing a few connections.

    QSKJ circuit diagram.jpg
  4. markcdn


    Dec 4, 2018
    Here is a more complete picture from my last post.
    Also a simpler diagram for the schematic (Forgot to label the potentiometer bottom left)

    I doubt I could make a copy of this device for the price I can paid for it on ebay which was $4.21(CDN) with free s/h. Also I have no clue what the SMT components values are.

    I was hoping there would be a simpler method of achieve a 1.0V gain.

    Atleast I know the Power MOSFET N-Channel is no good. An easy fix.
    I would like to install some Diodes one all 4 I/O's to prevent my probe from destroying the FET again. This would be problematic, just read they use up 0.5V, so I will figure out a pigtail system where I can use my probes. Not a problem at all.

    I wonder though if I could build something similar, capable of more current and running cooler.
    I also bought some alum heat sink in larger sizes. What I want to do is make a box, with two laptop chargers inside, dc-dc booster and some fans. I find the laptop chargers do get hot, measured 45C at the big coil. The inside is potted quite heavily but still easily removable, the similarly the Dells fets are attached to alum heat sinks. They are quite impressive on the insides. At first I couldnt get them going when I plugged them into my battery because the rush of current. A small power resistor fixed that issue. Just a few seconds and I used a jumper to bypass the pwr resistor. Its all been working great so far, except for the fried FET on the dc-dc when the voltage probe slipped.

    I bought some 20V 170W laptop chargers last night, good for 8.5A.
    The current ones I have are only 130W and 19.50V and 6.70A. So this may help me a little bit.
    The Alienware 240W laptop chargers are 12.80A 19.50V. Those would be ideal, look for them on ebay every now and then.


    QSKJ circuit diagram3.jpg
    Last edited: Dec 5, 2018
  5. CircuitMaster


    Dec 17, 2016
    I don't pretend I read your posts.
    Normally the simpler method is to take it from the input, but it might not be possible in your case.
  6. kellys_eye


    Jun 25, 2010
    What does that mean????

    The UC834 is a 'standard' SMPS controller and as such sets the output voltage by comparison with a reference applied to pin 2 of the device. The reference is typically taken from a voltage divider (pair of resistors) across the PSU output (or other area that has an output-related voltage as part of it) so if you want to adjust the supply by a small amount all you need do is change the value of ONE of those resistors.

    Your requirement for a voltage that is only 5% different (i.e. 20V instead of 19V) isn't easy to understand. Are you sure you require this 'small' increase at all?
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You are so far out of your depth @CircuitMaster...

    Please limit your posts to where you have knowledge to contribute. I have been on the verge of deleting a number of your posts as nonsense. I will be far more active in the future if your posting style does not improve.

    Unlike you, I have read what @markcdn has posted. And I think it's a pretty good idea to do that before you reply to him!
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