How to reduce output noise and lower input RMS currents using the i6A series of non-isolated DC-DC converters

In a system requiring multiple rail voltages, board mounted DC-DC converters are often used in conjunction with a bulk single output AC-DC power supply, rather than using a multiple output AC-DC supply. This can reduce wiring complexity, improve regulation and save cost, if not all the voltages are needed for every system configuration.

System circuitry can be noise sensitive, particularly when digital imaging techniques are being used in medical equipment or high-resolution microscopes. Multiple DC-DC converters operating at different switching frequencies, particularly if the frequencies vary with input voltage or output load, can dramatically interfere with image resolution. Even using fixed frequency converters, the tolerance on individual switching circuits can lead to sub-harmonic beat frequencies.

Significant filtering on the inputs and outputs of the converters is one solution, although it can be complex over a wide frequency bandwidth and very time consuming for the development engineer. A second solution is to use fixed switching frequency DC-DC converters and synchronise them to a master clock. With just one frequency to be concerned with, on-board and system filtering is made simpler and easier. Although somewhat uncommon, there are some DC-DC converters that have a synchronisation capability. The i6A series of 250W non-isolated DC-DC converters, for example, has this option, and multiple units can be configured to synchronise their operating frequencies.

In addition to output noise reduction, the i6A offers a 180-degree phase shift (delay) function, which reduces the RMS input current, ripple and noise, the amount of input capacitance needed and losses in the input filters. Figure 1 shows a comparison of the input currents of two converters with a 180-degree phase shift and without a phase shift.

Figure 1: Phase shifting input current comparison

By offsetting converter B’s input current by 180 degrees, the peak and RMS input current is reduced significantly.
The i6A converters can be configured to synchronise the switching frequencies and enable phase shifting in four ways:

1. Master / Slave, with no phase shift
One module is designated as the “master” and the other module or modules will operate at the master’s switching frequency. All the modules draw input current at the same time.

2. Master / Slave, with 180 degree phase shift
One i6A module is designated as the “master” and other module(s) will operate at the master’s switching frequency, but 180 degrees out of phase. Any module with a jumper between pins 4 and 33 will have the phase shift function activated.

3. Using an external clock with no phase shift
All the i6A modules will operate at the same frequency as the external clock (no master / slave). With no phase shifting, all the modules draw input current at the same time.

4. Using an external clock with a 180 degrees phase shift
All the i6A modules operate at the same frequency as the external clock (no master / slave), and any module with a jumper between pins 4 and 33 will have the phase shift function activated.

The i6A non-isolated DC-DC series from TDK-Lambda of step down converters (the input voltage has to be higher than the output) in the industry standard 1/16th brick footprint. All models feature wide range input voltages, as high as 9 to 53V and have wide range output adjustment from 3.3 – 15V to 3.3 – 40V. Operating efficiencies can be as high as 98%.

For more information about the i6A series, please visit the i6A webpage.

Jun29

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