RoadTest: Richtek EVB_RT7275GQW Evaluation Board
Evaluation Type: Evaluation Boards
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: Other brands buck converters, like TI.
What were the biggest problems encountered?: Modifying the board for 5V.
A very good review is already done by Peter Oakes (http://www.element14.com/community/roadTestReviews/1884).
It is not my intention to duplicate his work. In this review I will mainly focus on two subjects:
First test were with the unmodified board. Here is a picture of the setup. The resistor is 4.7 ohm, a little bit to large value to test the 1.05 output voltage.
Input voltage comes from a programmable power supply:
First I checked the unloaded output voltage versus the input, from 18 - 3V in steps of 1V.
As you can see around 5V the output is unstable and noisy, dropping to 0 for lower input values.
There are quite a number of testpoints available on the evaluation board, see the documentation (http://www.richtek.com/assets/evb_file/UG7275GQW-00.pdf) for details.
On of the signals is PG (Power good output test point) PG voltage is 5.1V when soft-start is complete and the output voltage is in regulation. Below is a graph of the PG signal versus the input voltage (still without load). This PG signal can be used to switch off the power in case there is no proper input signal.
The evaluation board is designed for 1.05 volt output. Other output values can be set by replacing R23 and R28 according to the following formula:
Unfortunately this is not an easy task, since both resistors are 0603 type smd components. Furthermore according to page 14 of the RT7275 IC datasheet (RT7275/76 - Richtek Technology Corporation) a larger value for L1 is advised.
For an evaluation kit I would expect user selectable output power, with a jumper or solder bridge.
For my tests I removed r23 and replaced it with a potentiometer, as can be seen in the next pictures.
Next graph shows a power up with a load 0f 4.7 ohm at 5V output. Within 2.5 ms the full output signal is available.
Next graph shows the output voltage as function of the input voltage with 5W load (5v, 4.7ohm).
Again we see a very stable output as long as the input voltage is larger than 6 volt.
I quickly looked at the efficiency, Peter Oakes did this also, but in my case its at 5v output level.
The numbers I got here are very good.
This mainly concludes my review. I'm planning to use this evaluation kit for a solar USB charger. I will keep you informed about the progress, either here or somewhere else on element14.
Please let me know when you have any questions.
I am actually running a series of tutorials right now on building your own bench power supply if your interested
there are also several options right here on Newark / Farnel you can choose, the first thing you need to ask yourself is what are my power requirements, how many channels, what max voltage, Max Current, do I want it programmable, remote sensing etc etc
Oh, and what is my budget
clem57 this one was depreciated from our work lab, so I would say try to find one on a flea market or hamfest. You also can build one yourself, lots of building instructions are available in the magazines or on the web. As this is a linear one it is not very efficient compared to switch mode power supplies which are more common nowadays. Furthermore this one is very noisy due to the fan inside.
I like your short to the point approach, easy to read and follow along while still giving all the important information.