A few weeks ago, I decided to purchase a DC-DC Converter Step-Up module for another project I am working on. I need to step the 3 volts from a LiPoly battery up to about 7.5 volts at 100 ma.
The usual Chinese sources had various kinds but I found the MT3608-based 2 amp Adjustable Step-Up Converter Module to be the right price at less than USD$1 (including shipping). These modules are based on the AEROSEMI MT3608 “2A, High Efficiency, 1.2MHz Current Mode Step-Up Converter”. The datasheet can be located in the usual places but this link will provide the AEROSEMI datasheet (PDF). The OEM claims ~94% efficiency. User reviews were not too bad either. I always order at least two of any Chinese manufactured modules because you never know if one may be bad but since these were so inexpensive, I ordered four (4).
Once the modules arrived, I quickly set out to test the units. All but one (1) passed the test. I contacted the seller and they ended up responding with the usual “please test it again and find the right drivers then let us know“. Okay, drivers?
Since I needed a fixed voltage anyhow, I decided to pull the 100KΩ pot from the PCB and replace it with a fixed resistor. I could see that there is a 2.2KΩ fixed resistor that had to be part of the feedback control loop but I wasn’t sure how the pot was being implemented because all three legs went to different locations on the PCB and two of them weren’t tied together (see schematic). I didn’t find a schematic online, so I traced the circuit, which was pretty close to the OEM’s reference design. Here’s a PDF of the schematic. Without removing them to test, I could not tell what value the input and output capacitors were. I can assume that they are at least 1 uF but the schematic shows them as 10 uF.
In my haste, I desoldered and pulled the pot off the failed unit but then realized that it was a 1KΩ pot not a 100KΩ pot! Well that was likely the problem and after quickly adding a fixed value resistor in place of the pot, the failed unit came to life. Great, problem solved!! I can’t use that module with a 1KΩ pot and I didn’t have any of the 100KΩ pots in stock, so this previously “failed” module became my fixed voltage module.
After calculating the values for a 7.5 volt output, I soldered two (2) fixed value resistors to the PCB. I had a problem getting the module to start up reliably, so I added a 22 uF electrolytic capacitor to the input and output, which cured the problem. I wanted to test efficiency so I created a spreadsheet to do so. Overall, 96% efficiency was achieved at 7.3 volts/130 ma output. I think this module will work for my use. The only issue that I see is the ~400 ma of input current, even at 96% efficiency.
I have an adjustable power supply on order. When it comes in, gets built and I get some extra time, I’ll check the noise and performance better.
I trust the schematic and the spreadsheet helps.
Peace and blessings,