What You Should Know About Paralleling DC-DC Converters?

Posted by mark taylor on June 4th, 2019

There are a lot of situations which demand high load current that is greater than a single DC-DC converter can provide. In such cases, two or more DC-DC converters are connected in parallel to meet the requirement. Using two DC-DC converters in parallel, the current load is effectively doubled.

You must be wondering as to why not use a really big DC-DC? Well, a more prominent converter can be less efficient than their smaller counterparts, for a start. Also, bigger passives will take up space and designing such power systems is not terribly straightforward, either – thus, paralleling is the more comfortable & quicker option. Also, using two smaller DC to DC converter in place of one big will spread the heat dissipated across a larger area on the PCB, making them reliable. Let’s get into the details.

Load current must be shared equally

Two DC to DC converters connected parallel automatically shares the load evenly. Even if they appear to be identical, the output voltages will still be slightly different due to component tolerances. The one having higher output voltage will typically provide the entire load current, operating at its full extent while its partner will relatively be doing little work. This provides the required amount of power, but it’s hardly optimal.

The DC to DC converter doing the more significant share of the work will be creating a thermal hot spot, which can be problematic, and it’ll have a shorter expected lifetime under these conditions. Forcing DC/DC converters to share the load equally mitigates the hot spots and optimizes lifetime for all the modules. Additionally, for parallel DC to DC converter operating in redundant configurations, if in case one module fails, the others must increase its output to compensate.

Synchronizing switching frequencies reduces ripple

There are always some discrepancies between switching frequencies in the used converters, even when you are operating identical parts from the same input bus. Even a slight mismatch in switching frequencies might result in unwanted beat frequencies, which means the switching frequencies can interfere with each other. The resulting frequencies create ripple currents at the converters’ input. These ripple currents lead to a loss in the interconnection between the converters and can put additional stress on input bypass capacitors. In extreme situations, ripple currents falsely trigger over-current conditions.

Hence, DC to DC converter has the ability to be synchronized can be selected to avoid this problem.