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I'm planning to use this DC-DC converter output as a dual supply for an opamp circuit. The manual of the converter is here.

In my case Vin will be 12V and Vout will be +/-12V. And in that case they recommend 2.2µF/25V for Cin and 1µF/25V for Cout. enter image description here

But right below they have EMC compliance circuit where cap values are different and they also use some more caps like CY and C2. enter image description here

My questions are:

1-) Should I ignore the EMC compliance circuit and use 2.2µF/25V for Cin and 1µF/25V for Cout?

2-) What type of capacitors must be used for Cin and Cout?

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  • \$\begingroup\$ 2. What does the datasheet say about them? \$\endgroup\$
    – winny
    Commented Feb 16, 2022 at 18:03

2 Answers 2

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  1. It depends if you need to pass official EMC testing, for example if you want to manufacture and sell the device. Also it depends on how much ripple your power supply tolerates or how much ripple your load tolerates, and how much common mode noise you can live with.

  2. Ceramic would be likely best for a switch mode power supply.

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  • \$\begingroup\$ The datasheet says "Choosing suitable filter capacitor values is very important for a smooth operation of the modules, particularly to avoid start-up problems caused by capacitor values that are too high." Would that indicate that low ESR capacitors might need series resistors, and so ordinary electrolytic caps would be usable (without resistors)? \$\endgroup\$ Commented Feb 16, 2022 at 18:11
  • \$\begingroup\$ @AndrewMorton If the datasheet would mean that, maybe it would mention it. Now it does not mention and does not show a symbol for an electrolytic. The problem is usually not the ESR of the capacitor, it's just the capacitance, since a big capacitance takes long to charge and the power supply might give up starting because of the short circuit protection. It might take too much current for too many cycles or it takes too many cycles to get output voltage to acceptable level. \$\endgroup\$
    – Justme
    Commented Feb 16, 2022 at 18:17
  • \$\begingroup\$ @Justme What is the function of CY? \$\endgroup\$
    – GNZ
    Commented Feb 16, 2022 at 18:25
  • \$\begingroup\$ Also Im wondering what type of ceramic X7R or? \$\endgroup\$
    – GNZ
    Commented Feb 16, 2022 at 18:25
  • \$\begingroup\$ CY is a common mode suppression capacitor. It means that without it, the high frequency of driving the transformer coil primary can couple to transformer coil secondary and your output is isolated but floating on top of a high frequency square wave. CY shorts out the hight frequency square wave that might couple between transformer coils. Also, X7R should be fine, except for the Cy which may need to be a special high voltage capacitor with a specific safety rating if you intend to utilize the 2000 volts isolation the power supply can provide. \$\endgroup\$
    – Justme
    Commented Feb 16, 2022 at 18:31
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dual supply for an opamp circuit.
1-) Should I ignore the EMC compliance circuit

You should never ignore the RF effects of switching power supplies. Input cable emissions and interference with Op Amp input paths are essential considerations for your compatibility and SNR analog Design Specs. ( stated assumptions & goals)

2-) What type of capacitors match for Cin and Cout?

A simple impedance attenuation analysis will tell you that for these low currents if the difference between family ESR and other cost-performance tradeoffs matter in your selection criteria again depending on your Design Specs for vibration, EMI and temp range as the characteristics are diverse.

Ceramic types are less expensive than metallized film, but unstable over time and temperature and less mechanically stable.

Ceramic failure mode also tends toward short circuits, whereas metallized paper and film types tend towards the open circuit.

If you have no idea of EMI issues on the 12 V input path, use the best low ESR alum e-caps or X7R ceramic if you have no vibration issues. These are only 1W supplies, so it may possible to cost reduce that with Y5V/25V and ferrite beads.

Cy

This plays a critical role in reducing the CM switching noise due to unbalanced leakage feedthru capacitance while having DC isolation. The switching frequency is used to compute the coupling impedance from 0V output to 0V input relative to the differential load of 12V^2/1W = Z load

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