Isolated power supply for a digital isolator

Question

I am using a small microcontroller board to communicate with other devices through a MAX3225 and several analog components. To isolate the communication interface, I would like to use the ADuM1402. Additionally, an isolated +5V and GND is required for the isolated side of the ADuM, the MAX3225, and the other analog components. To achieve this, I am considering using a DC/DC-Converter, specifically the TEA 1-0505E (TEA 1E) or TBA 1-0511HI (TBA 1HI).

I have never worked with DC/DC-Converters before and I have a few questions:

1. Is using a DC/DC-Converter the best / a valid solution for my problem?
2. Which of the two converters (TEA 1E or TBA 1HI) is more appropriate for my situation?
3. Both converters have an "unregulated output." Do I need to take any additional steps to regulate this output? (If yes, I would appreciate further infos)
4. Are there any other components I need to use in conjunction with the DC/DC-Converter to make it work properly?

Answer 1

1. Is using a DC/DC-Converter the best or a valid solution for my problem?

• Without knowing more about your setup, I can't say if it's the best, but I can tell you it is VALID.

2. Which of the two converters (TEA 1E or TBA 1HI) is more appropriate for my situation?

• The TEA 1-0505E has 1,500V isolation (1.5kV), while TBA 1-0511HI has 3,000V (3kV) isolation between its input and output pins, so it depends on how much isolation do you need. Also, the TBA model seems to have somewhat more stable output voltage at various loads.

3. Both converters have an "unregulated output." Do I need to take any additional steps to regulate this output? (If yes, I would appreciate further infos)

• Their output voltage is basically a reflection of their input voltage, so as long as your input voltage is a stable, regulated 5V, their output should also be a fairly stable 5V. Both of the ICs you plan on using (ADuM1402 and MAX3225) should be fine with the voltage tolerances which are less than 10%.
  Correction: They require at least a 10% load to keep the output voltage more stable.

4. Are there any other components I need to use in conjunction with the DC/DC-Converter to make it work properly?

• It should contain everything it needs. The only component suggested is an input fuse, if necessary.
  Correction: They also suggest a low ESR, 22uF input capacitor.

Answer 2

1. An isolating DC/DC converter is a valid option. Whether it is the best solutiom depends on what your circuit has that we don't know and could be used to make isolated 5V.

2. It depends what your circuits need. We don't know your circuit consumption or tolerance to supply voltage.

3. It depends if your chips and circuits work with whatever unregulates voltage the regulators output. You need to check your range of output current you need and how much each regulator will vary the output and if that's OK for your circuit and chips.

4. That would be answered by manufacturer example schematics and appnotes. Usually modules like this don't work without external components, or at least don't pass any electromagnetic compliance tests, unless a bunch of filter capacitors and inductors are added, and an Y capacitor between input and output to limit common mode voltage swings.

Answer 3

A galvanically isolated DC-DC converter is the expected approach.

One "gotcha" with non-regulated DC-DC converters (in addition to the usual maximum output current, isolation and regulation specifications) is a minimum load requirement. Many have a requirement of something like 10% of full-load current in order to stay within the specified limits.

If you fail to provide sufficient loading, the output may rise considerably, which could conceivably damage the ICs being supplied. You can add a resistor (maybe use it to power an LED) if the minimum current drawn by the circuitry is insufficient. In the case of a 1W 5V DC-DC and 10% you would need to draw about 20mA minimum, which is a bit much for a modern single LED.