All Isolated DC / DC converters have switching devices that generate electrical noise. When the power switching device is in an inductive load, such as a transformer, it is necessary to avoid the resonance of the parasitic inductance or capacitor of the power switching device. The DC / DC converter operated at a few hundred Hz moderates can produce noise up to 20 MHz.
This high frequency interference is fully infiltrated throughout the converter, shutting between the traces and the traces, jumping between the input to the output through the coupling capacitance within the device. Further, the flow of the pulse energy of the output is output, and the output port exhibits output ripple and noise, and the noise and ripple are reflected by the transformer to the input, forming the ripple current of the primary side. This pulse current produces low-frequency voltages in any inductive element, such as input and output wire, length, via or device pin of the PCB trace. Although it can reduce the input and output voltage ripple by increasing the capacitor by the primary and secondary side of the power supply, the common modulo noise is difficult to filter out because they appear in the two ports of the input or output, so the filter cannot be " Noticed. (figure 1).
Figure 1: Currency noise of the DC / DC converter (exceptions from DC / DC Knowledge Manual)
To design a very low ripple and noise power supply requires three separate filters, each filter handles different interference:
1: Output ripple filter. This filter reduces pulse output ripple caused by transformer transmission power. The output capacitor needs to absorb the pulse current generated by each switch cycle. In the gap time of the power transfer period, the output capacitor must provide energy to the load. The voltage on the output capacitor has risen and decreases each switch cycle, and thus a graphic feature of the sawtooth wave shape (Fig. 2).
Figure 2: Typical output ripple and noise waveform of DC / DC converter
The high-frequency switch noise over the saw tooth wave is added to the saw tooth wave each time the power transistor is turned on (VCE fast drop) or when the VCE is rapidly rises. Therefore, the peaks and troughs of high frequency noise and switch cycles are synchronized.
2: Input ripple filter. The power transistor is rapidly increased each time the power transistor is turned on, and the current will fall rapidly when it is closed. The input capacitor cannot filter out the ripple current overlay the common mode noise, but the noise of the power input is usually lower than the noise of the output, because the primary side of the power supply is a low impedance source, absorbs a lot of high frequency noise. A typical input reflection AC ripple current is shown in Figure 3.
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Figure 3: Reflection line current
3. The transformer is a high resistance source between the input and output, so the switching noise can easily couple the interlayer capacitance between the transformer windings to be coupled to the transformer. To reduce the noise, a capacitor can be installed on the isolation belt to provide a low impedance path between the output and input.
Our task is to design an isolation power source that output ripple and noise less than 5 mVP-P. This smoothed power supply is required in a very small signal and high sensitivity amplifier circuit or high resolution signal processing application, such as a 24-bit model converter.
We use the R1ZX-0505DC / DC converter, which is output via the board linear regulator, which provides a low noise output of only 30 mVP-P.
The first step is to add a 2NF capacitor between -vout and + VIN. The return path of the 2NF capacitor provides a lower impedance relative to the 100PF coupling capacitance of the transformer. The device can greatly reduce the output noise, but the impact on the input or output ripple is not large.
The second step is to increase capacitance in the input and output loop. Two 10μFMLCCs are connected in parallel to reduce ESR and install both ends of the input and output. The result is that the input and output ripples are greatly reduced, but the common mode noise in the output is still very obvious. The output waveform is shown in Figure 4.
What we need to do now is to filter out these high-frequency switch peaks, but they are not easy to filter out because they are common mode interference, thus increasing more capacitors or LC filters will not have any effect.
Figure 4: Input and output ripple waveform plus suppression capacitance
We tried different common mode inductors with a common-mode box until the best solution was found. The input requires a PI filter with a choke and a pair of capacitors for 50UH. Even if the CMRR of the linear regulator is high, it still needs to strictly control the input ripple. A similar common mode PI filter is placed at the output, but finally found a 10 μH choke is sufficient (Fig. 5 and Figure 6).
Figure 5: Complete filter design
Unfiltered output (only C3 only)
Filter output (with common mode choke)
Figure 6: Output waveform before and after filtering (the same ratio)
Conclusion: The output ripple and noise when full load is measured using a common mode filter and about 2 mVp -p. Changing the parameter values for any device make data worse, so this is the smallest value. The complete filtering circuit may look more complicated, but if you want to filter out all the different interference sources to get excellent low noise power (-68dB) must be designed. The device used by the filter circuit is small, so it only needs to take a small board space.
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