DC-DC Converter
July 2020
For this project I wanted a general DC-DC converter that could step voltage down to a desired level with a potentiometer. This project had a portable power supply project in mind but I wanted a design versatile enough to be used in other projects with similar requirements.
The requirements for this project were as follows:
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Adjustable output
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Single rail
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Minimum output voltage must not be higher than 2V
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Minimum 1A maximum output current
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Large output span ≥ 30V
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Input > 30V
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This design will be a buck topology DC-DC converter based around the adjustable version of the LM2575 IC using the following reference design as a start point. This will satisfy all of the above requirements.
Output capacitor selection
To reduce ripple the datasheet recommends to pick a value several times larger than the above capacitance value to consolidate BoM the same capacitor as the suggested input capacitor will be used (100uF).
Inductor selection
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Because this project didn't have a specific use in mind I used an inductor that will work optimally in most scenarios. Using the below equation and chart from the manufacturer we can find out what inductor is suitable for a certain input voltage, output voltage and load current. I found at high load (more than 0.5A) the most common area on the chart was H680 and H1000. I selected the H680 because the vendor didn't have any suitable inductors for the H1000 region this does limit the design as at certain E*T values the supply should be close to or at full load to operate correctly.
One likely conversion is 24V to 12V, I used this example to see what load current it will become sub-optimal at. As you can see just under 0.6A where it enters the H1000 region of the chart.
Potentiometer and resistor selection
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R1 must be between 1k and 5k.
Knowing this we can determine that our potentiometers maximum value should be between:
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And
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50k is a common value potentiometer between the 2 which we can use to determine R1:
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1.8k is the closest e10 value.
Schottky diode selection
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voltage rating = 1.5*maximum output voltage
1.25*40 = 46.25V
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current rating = 1.2*maximum output current
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1.2*1 = 1.2A
PCB schematic and layout
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The schematic follows the reference design with screw terminals at the input and output. Ceramic capacitors were also placed across the output of the converter.
The layout was simple enough taking care to keep the loop area as small as possible and leaving space for the heatsink.
Once the PCB and parts arrived I assembled the boards and they work great up to 24V I am yet to try higher input voltages but given they have worked well even at high load (1A) I have no doubt that they will work as expected. As you can see in the photo the potentiometer is dual ganged, the vendor only had dual ganged potentiometers in the size and value I needed but this comes as a positive because I can use the unconnected pins if I wanted to use 2 boards to make a dual tracking supply.
