At some point, we’ve all suffered anxiety from a dead phone battery when we’re out and about. Though its severity may affect some worse than others, the truth is for most people, it’s become our primary method of contact. Battery drain will always depend on operation, but no matter how lightweight the OS is or minimal the usage, everyone needs to charge again, eventually.
Faced with this reality, there is intense motivation in several industries to improve performance at the circuit level. Low-voltage power supplies are ripe for this level of analysis due to their widespread adoption in consumer portable electronics. Engineers and manufacturers that can squeeze out even the slightest gain in optimization can position their products or technology as industry leaders in this key metric.
What Defines Low-Voltage, Anyway?
Values for low-voltage can differ wildly depending on the accepted definition; standards run anywhere from zero to 1,500 V at DC. From the board perspective, low-voltage values are likely closer to the minimum value than the maximum. One of the driving forces in circuits, both as a recent and long-term trend, is the tendency for packages to shrink and voltage requirements to drop on a per-chip basis. Miniaturization has enabled entirely new architectures for a bevy of end-use applications,
One compelling factor for low-voltage is the safety issue, especially for products primarily intended for users who may be more vulnerable for various reasons (age, health, etc.). Insulation is provided in power supplies to isolate the input and output sides, preventing shorts that could still be quite hazardous even at low-voltage settings. For these cases, the International Electrotechnical Commission (IEC) designates two low-voltage settings with further stipulations:
- Safety Extra low-voltage (SELV): Rated voltage is low enough that even in direct contact during operation or if a fault occurs, there is no danger to the user. Voltage maximums are set at 60 V DC/~42.4 V AC but may be lower for specific applications.
- Protective Extra low-voltage (PELV): PELV is identical to SELV, except grounding of the circuits is a requirement (as opposed to acceptable).
Investigating Low-Voltage Power Supplies and Building a Case
Low-voltage power supplies face an uphill battle compared to higher voltages in terms of efficiency. One of the primary advantages of high-voltage power is the low loss relative to low-voltage power at the same wattage. For the same power rating, a decline in voltage results in proportional current growth. As the current grows, there is greater loss due to Ohmic heating, reducing total efficiency. On the other hand, an increase in voltage requires a commensurate decrease in current; this is the driving justification for the large step-up transformers that carry electrical power at tens of thousands of volts across lengthy transmission lines. Also worth noting is that a large enough decrease in current may reduce a wire’s diameter (in other words, an increase in the gauge number).
One way that low-voltage power supplies increase efficiency is through the topology of the power system. Unregulated or linearly regulated power supplies are highly inefficient because they are constantly engaged and outputting voltage by emitting excess energy in the form of heat. The greater the difference between input and output voltage, the more heat the regulator emits and the greater the system’s inefficiencies. Furthermore, regulators can only operate from a higher input voltage to a lower targeted output voltage due to their method of operation.
A More Efficient Model
An alternative to the power transistor as a resistor model, low-voltage switched mode power supplies (SMPS) utilize a switch to turn on and off the circuit. Although topology can vary, the basic structure is that the source first charges a storage element (either a capacitor or inductor). The second step has the switch change position to engage the second half of the circuit, where the charged element adds its voltage or current to the load (usually in addition to the source).
Comparatively, an SMPS may reach efficiency values of 90~95%, whereas linear regulators are closer to 50%, an 80~90% improvement in energy consumption and corresponding heat dissipation. Balancing out the overall effectiveness of the circuit, SMPS requires some additional circuitry to prepare the input and output waveforms for better performance in-circuit and to reduce any noise associated with high-frequency inputs. Additionally, there is a tradeoff between the overall capability of the charging element and the time it takes to charge/discharge.
Common and Preferred SMPS Topologies
The three most common topologies associated with SMPS are the boost, buck, and buck-boost networks. While all three have specific use cases, advantages, and disadvantages, they only differ in the relative arrangement of the same core circuit elements: a switch, an inductor, and a diode on the front end. However, an alternate SMPS, charge pumps, offers arguably the most efficient power supply with numerous additional advantages for low power: they can scale voltages up or down by fractional or integer values and invert inputs from a single power rail. Their high charge/discharge cycle reduces the cost/size of capacitors. It enables quick charging of portable electronics, while their on-device usage reduces waste heat generation, ensuring long product service lives.
Your Contract Manufacturer Can Help Power Your Designs
Multiple arrangements of low-voltage power supplies are available with endless possible uses and motivating factors for a particular topology in a circuit. While charge pumps offer a compelling case as the most efficient low-voltage power supply with many real-world use cases, engineers may settle on other highly efficient SMPS topologies to perform their design’s crucial power delivery function.
If your electronics have issues regarding power performance or heat mitigation, VSE can help. At VSE, we’re a team of engineers who build electronics for our customers and coupled with our professional and experienced manufacturing partners, we deliver boards that meet design intent while exceeding expectations.
