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Power Supply Issues, cont'd

A project log for GPS Driven Clock

This clock displays the time and day of the week directly from a GPS receiver.

BharbourBharbour 09/20/2021 at 15:450 Comments

In the previous log, I mentioned that the system was particular about the source of the 12VDC input. With certain wall warts and power bricks, the +12V would not turn on. In some cases, it would destroy the buck regulator chips on the power supply board.

The new power supply PCBs came back from OSHPark, and I built the first one up this weekend. After talking with a friend who knows a lot about power supply design, I decided to replace the inductors in the input power filter with simple wire bridges. Testing the new board with a variety of 12VDC  sources, the system will power up with all of the sources that it had problems with previously. It looks like the regulated wall wart power supplies do not like powering up into an inductive load. Most of the wall warts available now, are the regulated variety, so this is a useful piece of information.

I did a preliminary set of tests to see if removing the input filter inductors is going to cause more conducted emission problems from the power supplies. To perform these tests, a circuit called a Line Impedance Stabilization Network (LISN) is inserted between the power source and the thing being tested. A spectrum analyzer is connected to the LISN to observe the Radio Frequency energy coming out of the thing being tested.

I used two of the power supply boards, one with the input filter inductors populated and one with them bridged out. A 5 Ohm power resistor was used as a load.

At very light loads, the input filter inductors made a large difference. The signal levels below 1.5 MHz on the board with the filter inductors were about 50dB lower than the board without the filter inductors. I didn't test above 1.5MHz on this test setup.

The regulator board has two separate regulated outputs, 5V and 3.3V. Two separate regulators provide these outputs.

With either the 5V or 3.3V output loaded with the resistor, the filter inductors did not make a lot of difference between 1.5 and 11 MHz.

Between 11 and 50 MHz, the signal levels  out of the board without the filter inductors are about 12dB higher than the board with the filter inductors. The filter inductors are clearly doing something (beside blowing up regulator chips).

09/23/2021 As an added scrap of conformation on the inductors, I have another filter/regulator power supply board that uses a completely different switching regulator chip but a similar input filter. This board would not operate on wall warts either. I built up one of these boards this week with the filter inductors bridged out and this new board works fine with the wall warts that I tested with.

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