Getting the Most out of a DC/DC Converter for High Efficiency

As modules, through-hole single in-line pins (SIP) versions have been popular, and the earliest models delivered around a Watt of unregulated power output in a compact SIP7 format, sufficient for many of the applications mentioned. Over the years, power density has steadily increased, with unregulated 3W now available in the even smaller SIP4 format (Figure 1).


Fully regulated parts soon appeared, initially using self-oscillating, variable frequency flyback circuits for minimal component count, but the latest versions are typically fixed-frequency IC-based for optimum converter efficiency over a wider load range, providing high power density, 4:1 input range, and features such as output trim and ON/OFF control. 2W in a DIP24 package was the initial benchmark, but soon SIP7 and SIP8 2W, 3W, and 6W versions appeared, and now the 12W benchmark has been reached (Figure 1).

As power levels have increased, the modules are increasingly used for powering entire sub-systems and even complete products, rather than just as auxiliary power supplies. As the main power conversion stage, agency-rated isolation becomes more important, and since SIP7 or SIP8 packages allow meaningful creepage and clearance between input and output pins, the trend has been to increase power from these packages rather than shrinking module size while keeping the same power.

As vendors introduced more SIP8 DC/DC modules with improved power density, some made optimistic claims to appear competitive with industry leaders. The headline power may only be available with heavy derating – for example, a nominal 9W part may deliver full power only if the case temperature is held impractically low, if the load duty cycle is limited, or if significant forced air cooling is applied, while the useful power may be much less. For instance, the 6W part from RECOM delivers more power at +75°C ambient with convection cooling than the nominal 9W part from a competitor.

Such discrepancies, where a nominal lower-power part delivers more useful power than a nominal higher-power part in real-life situations, indicate that the existing design topology has reached its limits. It may be possible to further increase power density with more expensive components, but a radical redesign is necessary to reach the next power level, such as replacing the inefficient bobbin transformer construction with a fully planar design. This is the approach used in the new RS12-Z series from RECOM.

Planar transformers use PCB tracks for the windings, which is technically challenging and requires multilayer PCBs, often with blind and buried vias. Achieving guaranteed isolation (3kVDC in the RS12-Z) demands careful PCB stack-up design. Additionally, for every input-output voltage combination, a different PCB layout is required with adjusted turns ratios, making manufacturing more complex.

However, planar transformers reduce labor in assembly and provide very repeatable performance compared with traditional wire-wound transformers. Along with improved efficiency, advanced thermal management enables the full 12W output at 75°C ambient across the 4:1 input range. In this product, heat generated in the converter is efficiently transferred to the metal case with low thermal resistance, and tinned case tabs further conduct heat into the host PCB.

A benefit of advanced IC-based converter circuits is the additional functionality available, solving important problems. For example, as a main sub-system or product power source, the converter should operate reliably when the input supply drops, typically from a discharging battery. Regulated switching converters draw constant power from their input for a constant load, so if the input voltage drops, current increases. If the converter continues operating below the minimum rated input voltage, the increased current can be damaging; therefore, an under-voltage lockout function is necessary and is built into many control ICs. This function also ensures the converter output switches off cleanly at a set minimum input voltage.

Another feature in circuits such as the RS12-Z is a control pin or ON/OFF function. This can place the converter into a sleep mode for minimum power consumption and extended battery life when used as an input supply. It can also delay or sequence the converter output with other power rails. Control can be purely primary-side or derived from isolated outputs via optocouplers. Figure 2 shows an example arrangement where DC/DC 2 is enabled after DC/DC 1 is powered with a fixed delay set by R1/C1, while DC/DC 3 is enabled only when DC/DC 2 output is within tolerance.

A trim feature is useful to adjust a DC/DC output to compensate for external voltage drops. A typical application is with parallel converters for redundancy, where output series diodes prevent failure of one DC/DC from affecting the other (Figure 3). In this case, with Schottky diodes, each output may be trimmed up by about 0.3V via R1 and R2 so the gated supply reaches the correct nominal voltage, 3.3V in this example. Converters are rated for maximum power output, so if the output is trimmed up, rated current should be reduced.


A trim pin can also be controlled by an external voltage for additional functions. One example is a production ATE system power rail cycling between allowed tolerances to achieve a margining function, testing resilience to power supply variations. Figure 4 shows a circuit comprising a sine-wave oscillator IC1 coupled to the trim pin of a RECOM RS12-Z DC/DC converter with a set DC offset matching the trim pin nominal voltage. VR1 controls amplitude variation, and IC2 sums this with a negative fixed offset to produce a positive offset on the trim pin with a superimposed sinusoid.


With additional external circuitry, the trim pin can implement remote sensing or control power sharing if current is sensed externally to the converters.

When used as the main power source for low-power equipment, a DC/DC converter will itself be powered from an external supply and may need to meet specific EMC specifications, typically EN 55032, Class A or Class B. To achieve this, particularly noisy or variable-frequency converters may require additional filter components that can occupy more board space than the converter itself.

Typical filter circuits (Figure 5) enable the RECOM RS12-Z converters to meet these limits using a combination of small electrolytic and ceramic capacitors and low-value inductors.


Modular DC/DC converters have evolved significantly since their introduction. Power density has increased dramatically, as has functionality, allowing them to serve as precision power sources for modern electronic systems. The RECOM RS12-Z series is an excellent example, with an innovative design suitable for nominal 12, 24, or 48V DC inputs, delivering a full 12W at 75°C.