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Sunday, April 28, 2013

Power Supply Refactoring - closing the hood

After many hours of work I finally can say it's done. Even though I don't consider it the perfect work, I can definitively consider it a major improvement in respect to the previous version of the PSU:



And in respect to the inner workings:



The tests carried so far were quite satisfying. Some optimizations had to be performed, and others are still on the pipe to be performed. For instance, one of the first issues I found was the imbalance of thermal dissipation between the 4 power transistors:

This was fixed by swapping the hottest transistor (which happened to be on the rightmost edge) and putting it in second place (counting from the left). This resulted in 3 of the 4 transistors sharing a relatively homogenous temperature. Only the first (the one closest to the fan) tended to be cooler (by 10ºC from the tests done so far). In the future I may try to swap it by a spare transistor, and see if there are improvements.

As I don't currently have an electronic load or any similar testing device, I had to play with using a manually built load consisting of a piece of nichrome wire wound resistor which we can see here glowing:

Another aspect requiring testing was the ripple. As such, one of the obvious tests was to compare it with the performance of my commercial power supply and see the differences. I didn't expect miracles. Even though the Korad KA3005P can be seen as a low cost PSU, it has a very good construction and well engineered design. They really went to town in minizing noise at the output. The comparison revealed good results in spite of my minimalist design. In average the Korad measured about 14 mVpp, while my PSU measured roughly 33 mVpp:

The test setup:

The result from the oscilloscope:

In yellow is the signal from the Korad, and in blue is the signal from my PSU. In both cases the input is AC coupled, 1x attenuation in the probe, and amplitude set to 10mV per division. No analog or digital filtering was being applied. For the triggering, the AC line trigger was selected in an attempt to sychronize with whatever ammount of stray 50 Hz could exist in the signal. As we can see in the image, there is no obvious dominance of the AC line frequency, just a sort of broadband white noise. When the fan turns on there is a slight disturbance of the signal to be checked.

However I cannot consider these tests absolutely conclusive, as even after shutting down both PSUs, the same noise pattern persisted with similar amplitude. It is not an oscilloscope probe issue because after swapping the probes, the results were still consistent.In a preliminary discussion of the results this would cause me to lean upon differences in (external) line and RF immunity rather than noise intrinsic to the power supplies. I have several switching power supplies operating close to the two PSUs under test, so this could be an issue.

Going back to the construction details, one unforeseen issue was found regarding the two led meters bought from China (panel ammeter and voltmeter): in spite of both being powered by a floating voltage in respect to the measured circuit, there was a low value resistance between the power ground and the signal ground. As such this would prevent the nr. 2 secondary tap from the transformer, from being used to also power these meters. The reason is the relation between the power ground and control electronics ground, which affects the output voltage.

As such, and given the fact that there were no more secondary windings available in the transformer, an extra transformer had to be added. I searched the scrap parts bin, and found a small ac wall transformer that seemed adequate. Opened it up with the dremel tool, and attached it to the chassis:

The AC output (the twisted red and brown wires to the left of the transformer) goes to the main control board where a rectifier bridge, filter capacitors and linear regulator produce the steady 5 Volts used by the meters.

A view of the control board, which includes elements such as the LM723 linear regulator (the core of the power supply voltage and current regulation), a PIC12F683 for controlling the fan based on the temperature, 
and bridge rectifiers, filters and regulators for the different voltages required by the circuit:

Several heatsinks were required, given the large voltage differential between the input and output (16 Volts in the case of the fan power supply). In the case of the fan, only around 100 mA of power are required, but considering this differential, we end up with a total of 1.6 Watts of dissipation in the regulator. Without a heatsink it risks getting damaged.

All in all the result is good and we now have a working power supply with good performance. In the form of improvements/tasks to be accomplished is the following list of items:
  • properly measure the ripple/reduce the injected noise (even though as is it is very good, compared to many stabilized PSUs, as for example the ones found in computers);
  • try to swap the cold transistor with a spare one,  and check if the dissipation is evenly distributed by the transistors;
  • improve the fan control algorithm, by adding some hysteresis for smooth operation (while still ensuring fact action);
  • fix slight voltage drift over time (possibly a potentiometer issue);
  • Add overtemperature indicator LED to the front panel (for not having to burn the fingers on the heatsink);
  • Check slight disturbance (few millivolts) of the output voltage when fan turns on.
In conclusion, the single benefit of this kind of project is the joy associated to the design and step-by-step construction of the PSU, and of course the knowledge that is obtained and/or profounded in the process. Also worth mentioning is the freedom to implement things that could be nice to have in commercial units. Apart from this, considering the cost of comparable commercial lab grade power supplies, and the time spent, it is clear that buying the off the shelf unit is better if the single objective is to have the instrument in the bench ready for use.