Testing a DC-DC Converter Module

DC.DCPNG

I needed a 5V rail for a project that would be sourcing current for a USB battery charger from a 12V wall adapter. At these kinds of loads (<= 1A) a linear voltage regulator like the LM7805 is impractical, as it has to drop a lot of voltage, and dissipate a lot of power. That’s why I opted to use a buck converter instead.

I needed it quick so instead of going though the trouble of picking a switching controller, a MOSFET, and inductor, a diode and a capacitor, and going through all of the calculations, I decided to try out a little DC-DC converter modules from Digikey. That being said, there is a lot to learn from building a DC-DC converter circuit from scratch. Going through all the calculations and choosing the right controller, FET, inductor, etc. is a great way to learn how to find a part and how to address the trade-offs that engineering is all about. But I was in a rush, so I ordered some of these modules. I was browsing Adafruit and saw the little LM78xx pin compatible buck converters which instantly reminded me of the mailbag video Dave put up on the eevblog where he tested a similar product. Not wanting to pay the 15 bucks for the Adafruit modules I headed over to Digikey and found the V7805-1000R buck converter module from CUI Inc. I ordered a couple along with the recommended input and output capacitors and waited for them to arrive so I could test their efficiency and see how close I got the their claimed 97% efficiency.

97% efficiency is awesome. Switch mode converters are almost perfect from an efficiency point of view. The efficiency in this case is the ratio of the input power to the output power. At 100% efficiency, the input and output power would be equal, and the device itself wouldn’t have to dissipate any power.

Lets take the case of the LM7805 (which isn’t really fair as linear voltage regulators are a totally different animal, which have their own advantages over switching converters). In my application the input voltage is 12V and the output voltage is 5V at a 1A load. This means the regulator has to drop 7V at 1A, which is 7W. It’ll probably get a little toasty, and will probably need a heat sink. The efficiency, is the output power (5W) divided by the input power (12W) times 100. The efficiency would be about 42% in this case.

The datasheet for the V7805-100R shows the peak efficiency occurs at the minimum input voltage of 6.5V at a load of around 200mA.

Datasheet_Efficiency

My test will be at an input voltage of 12V, so the efficiency numbers probably won’t reach the 97% shown in the datasheet.

My test circuit will consist of a DC power supply set to 12V, a multimeter used as an ammeter on the input, another meter on the output used as an ammeter, and a third meter to measure the voltage at the input and output. The meters I used are as follows: 1. Fluke 77 IV, 2. a BK Precision Test Bench 390a, and 3. a Craftsmen 82141. The voltages were measured at the terminals of the device, and the DC supply was adjusted to compensate for the multimeter burden voltage.

Test_setup.jpg
Left – multimeters used as ammeters, Right – DC-DC converter module circuit

The table below shows the data gathered.

Vin (V) Vout (V) Iin (mA) Iout (mA) Pin (mW) Pout (mW) Efficiency (%)
12.045 5.06 30 50 361.35 253 70.01522
12.1 5.07 50 100 605 507 83.80165
12.04 5.06 70 151 842.8 764.06 90.65733
12.08 5.09 100 201 1208 1023.09 84.69288
12.07 5.12 120 250 1448.4 1280 88.37338
12.03 5.11 150 314 1804.5 1604.54 88.91881
12.01 5.01 160 354 1921.6 1773.54 92.29496
12.02 5 190 405 2283.8 2025 88.66801
12 5 210 450 2520 2250 89.28571
12 5 230 504 2760 2520 91.30435
11.99 5 250 546 2997.5 2730 91.0759
11.99 5 280 603 3357.2 3015 89.80698
12.02 4.99 330 721 3966.6 3597.79 90.70211
12 4.99 400 863 4800 4306.37 89.71604
11.98 4.99 450 981 5391 4895.19 90.80301
12.01 4.99 510 1111 6125.1 5543.89 90.51101

The graph of efficiency versus load is shown below, the peak efficiency is around 92%. Not bad for a 12V input.

Capture

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4 thoughts on “Testing a DC-DC Converter Module

  1. Pingback: Bench Testing a Switch Mode Drop In Replacement For the LM7805 | Hackaday

  2. Pingback: Bench Testing a Switch Mode Drop In Replacement For the LM7805 | Technology News

  3. Pingback: Bench Testing a Switch Mode Drop In Replacement For the LM7805 | Ad Pub

  4. Pingback: Building a Better Voltage Regulator | RETROVOLTAGE

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