Advanced Designs for High-Efficiency Low-Power DC/DC Converters

Aug 4, 2023

Circuit diagram with diodes, inductors and capacitors

DC/DC converter designs have a long history of advancements, beginning with the invention of switching converter technology. Compact size and cost-effectiveness are critical to applications requiring compact DC/DC converters, and RECOM is a leader in selecting the right designs for the optimal combination of cost and size. Our current designs include high-power density and high-efficiency converters that are small and have a low power dissipation. This article covers the history and recent advancements in low-power DC/DC converter technology.

Switching Topologies for Compact Low-Power DC/DC Converters
Conclusion

Converting one DC voltage to another in an electronic circuit has a history that spans over fifty years of increasing sophistication — modern designs boast incredibly high-power densities along with corresponding advances in efficiency to minimize power dissipation. RECOM’s latest designs incorporate innovations from high-power power supplies to benefit low-wattage converters in the smallest packages.

The first DC/DC converters were all low-noise linear designs that were simple to use but had two major drawbacks. First, the output voltage must always be lower than the input voltage. Additionally, a linear regulator is very inefficient and dissipates a significant portion of its input power as heat. Second, depending on the voltage difference between input and output, a linear regulator’s efficiency can be 60% or lower.

The invention of the switching DC/DC converter solved both problems but introduced a more complex design methodology. In contrast to linear designs, a switching converter exploits the energy-storing properties of inductive and capacitive components to transfer power in discrete packets. These pulses of power are stored either in an inductor's magnetic field or in a capacitor's electric field.

Block diagram of a power supply system — Fig. 1: Simplified block diagram of a switching regulator

The switching controller controls the transfer of power so that only the power required by the load is transferred in each switching cycle, which makes this topology very efficient. Optimal designs can achieve an efficiency of 97% or greater. Figure 1 shows the simplified block diagram of a switching regulator.

The ‘Switch’ function in Figure 1 is performed by power transistors that alternate between their highly efficient ‘on’ and ‘off’ states in a controlled sequence. This contrasts with the continuous operation of a linear regulator. A switching converter can produce an output that is either higher or lower than the input (step up or step down) or invert the voltage from input to output.

The output can be either regulated or unregulated. The output voltage of an unregulated converter changes significantly with variation in the load current or input voltage. In a regulated converter design, a feedback control loop (dotted line) provides feedback on the output voltage back to the switching block; which adjusts the switching operation to compensate for output voltage deviations from the desired value regardless of whether they are caused by changes in the input voltage (for example, a supply battery slowly being drained) or by changes in the load.

The simplest switching topologies share a common ground current path between input and output and are, therefore, non-isolated, with the inductive element being an inductor. An isolated converter, on the other hand, provides galvanic isolation between input and output because it transfers power via an electromagnetic field using the mutually-coupled windings of a transformer. As the output is electrically isolated from the input, the input voltage polarity relative to the output is irrelevant. In a linear regulator, the ground-return current flows directly between input to output; thus, isolation is not an option, and only three pins are required: V_in, Common Ground, and V_out.

Switching Topologies for Compact Low-Power DC/DC Converters

It is a common tradeoff in power supply design that better performance often comes with higher cost, increased complexity, and larger footprint. Since users of compact converters place a premium on both size and cost-effectiveness, how does RECOM meet these requirements in its low-power isolated converter products?

The push-pull topology is widely used for isolated converters. It is a cost-effective method of generating higher, lower, or inverted voltages since the transformer turns ratio can be set to determine the output voltage relationship. The topology is simple, reasonably efficient, and has relatively low electromagnetic emissions.

Circuit diagram of an oscillator with transformer and diodes — Fig. 2: Push-pull DC/DC converter with unregulated output

Figure 2 shows the block diagram of an isolated push-pull converter with an unregulated output. To save space, the oscillator and drive transistors can be combined in a dedicated push-pull transformer driver IC. For a regulated output, the simplest approach adds a linear regulator on the secondary side in series with the +V_out line, as shown in Figure 3. This approach achieves the desired goal and is suitable for the lowest-wattage DC/DC designs.

Circuit diagram of an oscillator and voltage regulator — Fig. 3: Push-pull DC/DC converter with regulated output

An example is the RECOM RYK series, where the linear regulator offers short-circuit protection as well as a regulated, low-noise output. This type of design can achieve an efficiency of around 65–75%. For converters above 1W or 2W, maximizing efficiency becomes a higher priority, which requires further design refinement. This is why, instead of secondary-side regulation, primary-side regulation is often used. In place of the linear regulator, the output voltage is monitored on the secondary side and compared to the desired voltage to generate an error voltage that is then fed back to the primary-side oscillator controller.

This adjusts the switching frequency to drive the error toward zero. Since this is an isolated design, the error signal must also be isolated. Figure 4 shows this approach used in RECOM’s regulated converters rated at 3W and higher, allowing for an efficiency of around 85%.

Schematic diagram of an electrical circuit with labeled components — Fig. 4: Secondary-side error signal provides feedback to the primary-side controller

A more sophisticated approach is needed for DC/DC converters with even higher power outputs. Not only does the linear regulator waste power in the manner discussed above, the two secondary-side diodes are also sources of loss. A power diode has a forward voltage drop of typically 0.5V, which translates to a power loss of 0.5W at 1A. The solution is to replace these components with a synchronous rectifier. This typically consists of two FETs and a controller, a technique known as synchronous rectification.

Figure 5 contrasts the two approaches. The FETs function as rectifiers by switching on during the forward part of the cycle and turning off during the reverse part of the cycle. The combination of fast switching and very low on-resistance RDS(ON) of about 10mΩ makes FETs ideal rectifiers.

Electrical circuit before and after simplification — Fig. 5: passive rectification (left) vs. synchronous rectification (right)

The disadvantage is that they must be actively driven, requiring additional timing and drive circuits to sense the internal voltages and correctly switch the two FETs synchronously with the output waveform. Diodes are passive devices that need no extra circuitry to function, but the increased efficiency from synchronous rectification more than offsets the added cost and complexity for higher-current converters.

This technique is used in RECOM’s RP20 series of 20W DC/DC converters, which can achieve an efficiency of up to 89%. This design also includes the isolated error signal described earlier. As shown in Figure 6, the RP20 can maintain high efficiency (85–89%) across a wide load range.

Schematic representation of an isolated DC/DC converter — Fig. 6: the RP20 incorporates numerous design techniques to boost efficiency

Conclusion

Advanced design techniques from larger power supplies are now being applied to low-wattage DC/DC converters, yielding higher efficiency and greater power density. As customer priorities change as power levels increase, appropriate modifications are necessary. RECOM is a leader in selecting the optimal design for the best combination of cost, size, and performance.

Applications

	Series
1	RYK Focus	Low cost 1:1 Input voltage range	Efficiency up to 81% 4kVDC/1 second isolation	Datasheet	Order now
2	RP20-A	2:1 input voltage range 1.6kVDC isolation	UL certified Efficiency up to 91%	Datasheet	Order now
3	RP20-AW	4:1 wide input voltage range 1.6kVDC isolation	UL certified Efficiency up to 90%	Datasheet	Order now
4	RP20-F	2:1 input voltage range 1.6kVDC isolation	UL certified Efficiency up to 89%	Datasheet	Order now
5	RP20-FR	Wide 4:1 input voltage range 2.25kVDC isolation	Efficiency up to 89% Six-sided continuous shield	Datasheet	Order now
6	RP20-FW	Wide 4:1 input voltage range 1.6kVDC isolation	UL certified Efficiency up to 89%	Datasheet	Order now