150W Boost Converter Schematic

In a recent project, I needed a boost converter to step up 5V to about 8V at a few amps. A few different Chinese-made boost converter modules are available from various sources: I’ve seen them on eBay and Amazon. One very common one is known as the ‘150W Boost Converter’. I believe it’s intended for charging laptops from car batteries. It’s specified to take an input of 10-32V and output 12-35V, which isn’t quite what I was looking for, but the price was right so I thought I’d take a chance. This is what I found.

boost

I had a good look at the circuit board. It’s based on the UC3843 chip, which is a pretty old device (I think it dates back to 1984) and is often found in PC power supplies. However, its age and ubiquity means that documentation on it is readily available. I traced out the circuit, so here’s the schematic diagram:

150W_ boost

You can also have it as a PDF file: 150W_ boost.

It’s a pretty straightforward boost converter topology with a MOSFET switching transistor and a variable resistor in the feedback loop to set the output voltage. There is no over-current, over-voltage or reverse polarity protection at all, and the chip isn’t designed for low power consumption so this module wouldn’t be suitable where very low standby power is a requirement. There are a couple of interesting features, though.

The circuit includes an arrangement with an NPN transistor which feeds some bias to the current sense feedback loop. According to the UC3843 datasheet, this improves the stability of the converter at duty cycles higher than 50%.

The control supply for the UC3843 is derived from a 9V regulator, so it’s independent of the input or output voltage. This is convenient.

The UC3843 is designed to operate from fairly high supply voltages, and won’t start up until its supply voltage reaches 8.4V. That was a bit of a problem for my application, where the input voltage was only 5V. However, there’s nothing to say that the chip power supply has to be the same as the power input. In fact, the module already has a handy 9V regulator which feeds the control chip. Looking at the circuit diagram, there are even a pair of resistors (I’ve labelled them R1 and R2) which select whether that regulator is fed from the input or the output. As supplied, R2 was fitted, so the control chip was fed from the output. Here’s a closeup of the relevant part of the board showing R1 and R2.

DSC_0310crop

My application happened to have a low-current 12V supply available, which would be perfect for powering the UC3843. I simply removed R2 and connected my 12V supply to the point where the black arrow is in the photograph. The boost converter now worked perfectly with a 5V input.

I also had to modify it a little to be able to reduce the output voltage below about 11V. R3, labelled in the photo, is part of the feedback network. I simply removed it and replaced it with a piece of wire. Now the output voltage was variable down to 5V, and I was able to set it to the 8V I wanted.

The module seemed very comfortable delivering 3.3A at around 8V, and drew about 5A from the the 5V input. The heatsinks only got slightly warm.

Unfortunately, the power supply I wanted to run the converter and its load from didn’t like starting up with it all connected. This is quite often a problem with boost converters, since the inrush current at startup can be very large as the controller tries to bring the output up to voltage as quickly as possible. I solved this by adding a soft-start circuit to the module. More on that later.

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13 thoughts on “150W Boost Converter Schematic

    1. martinjonestechnology Post author

      It might work, but you couldn’t be sure of getting the load balanced between the two converters. I wouldn’t rely on getting double the output, that’s for sure. A slightly more reliable way of paralleling them might be to remove the controller chip from one of them and just connect the gates of the MOSFETs on each converter together, so they switch at the same time. Then at least you won’t have two feedback loops fighting each other. The UC3843 and its brethren are not really designed to be paralleled, though.

      Reply
  1. Thomas

    Hello, i have nearly same module: it is only 120W but it have boost up to 60V. I see diference only in toroidal coil… So I think, if I use your schematic but i replace toroidal inductor by another one with lower max. current, and higher inductance, i will reach effect of higher voltage on output… Am I right?

    thank you for the schematic… there ist that module http://dx.com/p/120w-dc-dc-10-32v-to-35-60v-boost-converter-charger-module-red-156701#.UuWz7hBjWHs

    Reply
    1. martinjonestechnology Post author

      That module looks very similar. To get a higher output voltage from the ‘standard’ 150W module, you won’t need to change the inductor, since this is a flyback-type converter and the properties of the inductor are defined mostly by the input voltage, switching frequency and output power required. You will, however, need to make sure that the switching transistors and output capacitors are rated for the full output voltage, and modify the feedback resistor network so that the output regulates at the new, higher voltage. Increase the value of R3 (try 10k) or reduce the value of the 1K resistor below R3 (try 470R). I hope this helps you.

      Chris

      Reply
  2. Andrew

    Hi,

    Thanks for this write up. I’m trying to use this same converter to run a 100W LED. I will run from a 12V lithium battery, to the converter, then the LED runs on 35Volts.

    Will this booster be suitable, or will the LED draw too much current? I’m not sure,

    Thanks,

    Reply
    1. martinjonestechnology Post author

      I think the converter should be able to handle the load, though it’ll probably get quite hot. I would be more worried about limiting the current to the LED. LEDs can’t usefully be driven from constant-voltage sources like this boost converter: they have a negative temperature coefficient of resistance, so the current will increase rapidly as the LED warms up. You’ll need a constant-current supply. This boost converter could be turned into one but you’ll need a current sense resistor and a an amplifier to boost the voltage across the current sense resistor to the feedback voltage that the control chip needs.

      Reply
  3. vignesh

    dear sir , i am doing my project with this converter so could u explain me the working principle of this model

    Reply
  4. Christian

    Deaar Martin,

    thanks for your write-up, but the circuit-schematic is wrong. The 100k-resistor goes straigt to pin 3. We actually modified this circuit to run as a CC/CV step-up-boster with just 3 additional parts. The original circuit is somewhat crude regarding the compensation-capacitor and the 100k.

    If you want to join us, i may send your our ltspice-project.

    http://www.360customs.de/2014/05/103050100w-led-applikation-treiber/

    google-translation is available on site.

    Regards,

    Christian

    Reply
      1. doctormord

        Hi Martin/Chris,
        thanks for the reply. I measured different loads connected to this converter, and can say, that more than 60W isn’t possible without heavy cooling. The best way to increase performance is to change the winding on the inductor to ~twice the winding-count with high-frequency-braid (wire).
        Regards,
        Christian

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