Power supply selection: An essential guide

Introduction

Power supplies are vital to functioning electronic systems, as they deliver the energy needed for efficient running of gadgets. Whether you're an engineer, a hobbyist, or a technical enthusiast, choosing the correct power supply is critical for achieving peak performance. This guide will walk you through the essential issues and factors in selecting a power supply, allowing you to make informed decisions based on your technical needs.

Figure 1: Block diagram of a power supply(Source)

Classification based on topologies for selecting power supplies

Topology refers to the arrangement and design of components in a power supply. It significantly influences voltage accuracy, load response, performance, and reliability while also meeting specific needs such as size, control, and noise reduction. There are different kinds of accessible topologies, and understanding the strengths and limits of each topology allows you to make an informed decision customized to your specific application.

• Isolated Fly-back converter- The topology uses a coupled inductor for energy storage and isolation. The coupled inductor is a significant magnetic component that enables energy transfer to the secondary and load during the switching element's off-time. The simplicity and low component count of the Isolated Fly-back Converter make it a cost-effective method of AC to DC power conversion. DC-DC converters mainly use it for low-power applications (<100w), such="" as="" mobile="" chargers,="" CRT="" televisions,="" and="" monitors.<=""/>
• Forward converter - This topology uses a transformer and an output inductor for energy transfer during the switching element on time, with forward converters applied in AC power supplies and DC-DC converters. In contrast, energy in the output stage is stored via the inductor and capacitor, not in the transformer.
• Buck converters- The buck converter excels in applications that require a regulated output voltage lower than the input, ensuring efficiency and reliability. This step-down DC-DC converter uses an inductor and capacitor for energy storage, maintaining stability. This is especially useful in battery-powered or automotive systems hampered by fluctuating voltages.
• Boost converters - The boost converter excels as a DC-DC power supply, elevating any input voltage to a fixed, regulated output. The Boost topology stores energy in an inductor before releasing it through a diode and capacitor, making it perfect for solar systems, battery boosters, or telecom equipment.

To learn more about switching topologies, please check with Farnell - Voltage Topologies – Part 1 of 2 and Voltage Topologies – Part 2 of 2

Input considerations

Electromechanical generators are favored for AC electricity production, characterized by polarity reversal. AC is typically derived from local power networks as a single or three-phase source before being converted to DC by electronic equipment.

• AC power sources - Domestic and light industrial settings generally use single-phase power. It is typically derived from one phase of a three-phase AC system configured to supply three single-phase sources or, alternatively, from a phase-to-phase connection for a higher nominal voltage from the same source system. Three-phase AC power systems employ either a three or four-conductor arrangement, with phases 120 degrees apart, offering consistent power without pulsations. Transmitting three times the power of a single-phase system, three-phase power is prevalent in commercial and industrial settings demanding higher power delivery. There are two primary three-phase connections: the delta and the star (or wye) connection. These efficient and flexible configurations cater to diverse operational requirements and power demands.

Figure 2: AC distribution system (Source)

• Single-phase variations and Three-phase efficiency – Understanding regional voltage differences and three-phase systems is essential for power supply selection. For example, Europe typically runs on 220-240VAC single phase, and the Americans favor 100-127AC. Modern SMPSs offer a wide range of voltages, accommodating global standards. However, most industrial applications prefer three-phase power for its efficiency and smooth delivery, with 220-240VAC derived from a 400VAC phase system in Europe and parts of Asia and 100-127VAC from a 200VAC system in the Americas and Japan.
• Battery chemistries- The battery chemistry is of four types: VRLA, NiCad, NiMH, and Lithium (including Lithium Ion & Lithium Polymer).
  • Valve Regulated Lead Acid (VRLA) batteries find extensive use in industrial control, UPS, security systems, and telecommunications for standby power during mains failure.
  • Lithium batteries, commonly used in portables, offer higher power density and lighter weight than VRLA or Nickel batteries.
  • Nickel Cadmium (NiCad), an older portable technology, boasts high power density and discharge rates (20-30C) but suffers from a memory effect, requiring a complex charging regime for recovery. Nickel Metal Hydride (NiMH), a newer NiCad evolution, avoids the memory effect in non-cycled systems.

Output considerations

• Output regulations: A regulated power supply converts unregulated AC to a constant DC, ensuring that the output current remains constant even when the input voltage changes. The initial set accuracy determines the power supply output point, usually under predefined parameters such as nominal input and 50% load. This is critical for external power supply with output cables, as they can cause voltage drops with a tolerance of less than 1% of the nominal voltage. The main parameters and techniques considered for DC output regulations are line and load regulations, cross regulations, temperature co-efficient, remote sensing, etc.

  Line regulation- Line regulation measures the power supply output change with variation in input voltage over a specified range, typically the full specified input voltage range. It's expressed as a percentage.

  

  VOUT (Nominal) is the output voltage at the nominal line input voltage.

  VOUT (Max) is the maximum output voltage measured over the specified input range.

  VOUT (Min) is the minimum output voltage measured over the selected input range.

  Load regulation- Load regulation gauges a power supply's ability to maintain output within specified limits over a set load range, expressed as a percentage. The load range depends on the product type and design outlined in the datasheet.

  

  where VOUT (Nominal) is the nominal output voltage.

  VOUT (Max) is the output voltage at maximum output current.

  VOUT (Min) is the output voltage at minimum output current.

  Remote sense- Remote sense adjusts the output at the load instead of the supply terminals, compensating for voltage drops. With this feature, power supplies and DC-DC converters have two sense connections—one for positive and another for return—allowing connection at the load, even at a distance from the supply.

Figure 3: Remote sense(Source)

Transient load response - Transient response measures the speed and effectiveness of a power supply or DC-DC converter in adapting to sudden changes in current demand.

Figure 4: Response of a power converter to abrupt changes in load current(Source)

• Peak load applications: A few power supplies specify their peak load capability, which indicates their ability to manage short-term loads that surpass the nominal continuous power rating. In such instances, the average power required is frequently far less than the peak demand. High-peak current applications, like those in factory automation, medical devices, robotics, power tools, machining, packaging, testing, dispensing systems, and printers, involve electromechanical elements such as print heads, pumps, motors, and drives. Their average power demands are significantly lower than the peak power requirements.

Figure 5: Ripple & Noise for Regulated, Switched Power supply(Source)

Figure 6: Ripple & Noise for Regulated, Linear Power supply(Source)

• Noise and Ripple - Noise and ripple are unwanted fluctuations in DC power supplies. These are mainly categorized into four types:

  a. A low-frequency rumble (120Hz) occurs twice the AC line frequency with clockwork precision.

  b. High-pitched squeals (kHz-MHz) arise from the switching frequency and exhibit erratic behavior.

  c. Random bursts (anytime): A periodic noise contributes to unpredictability without any discernible pattern.

  d. Pulse party crashers (MHz): A switching noise introduces high-frequency spikes.

Minor voltage variations at the power supply's operating frequency cause imperfect conversion from the AC manifest, which in turn causes ripples. It depends on the load and can affect sensitive instruments. Noise, on top of ripple, comes from various sources like switching and external electronics. It's unpredictable, high-frequency, and can negatively impact sensitive circuits. Both are measured in peak-to-peak voltage and specified in the power supply's datasheet. Understanding them is crucial for choosing the right supply for your application, especially for sensitive tasks.

• Output protection - Output protection in power supplies and DC-DC converters prevents the power solution and end equipment from damage, safeguards against overload, and protects the end equipment from over-voltage and excessive fault current. Circuits protect the power supply and load from excessive current by limiting the current or power in case of overload or short circuit at the output. The following techniques are used for protection against overloads:

  Trip & Restart or ‘Hiccup’ mode - This protection mode's power supply goes for a preset shutdown when it detects an overload and then attempts to restart. If overload persists, it repeats the cycle until removed. The duration of the Off-time and voltage thresholds vary depending on the impedance of the overload.

  Constant power limit - This is used in multi-output power supplies to monitor and limit primary power, enabling power trading across outputs without exceeding the overall power rating.

  Constant Current Limit: Output current is maintained at a predetermined level beyond the maximum load, accommodating high inrush loads like capacitors, lamps, and motors during power-up. The removal of overload causes the supply to return to constant voltage mode.

  Foldback Current Limit: Voltage and current decrease simultaneously with load impedance reduction during overload, preventing excessive dissipation in the series pass element. This feature is widely used in linear power supplies with crowbar over-voltage protection to limit fault current.

• Series & Parallel Operation: When an application uses several power supplies, it's essential to evaluate the system's earth leakage current to meet safety regulations.

  Power supply and DC-DC converters can be connected in series, but caution is advised to avoid interference between them. The overall output voltage must not be greater than the operational output-to-earth breakdown voltage of either one of the power supplies.

Figure 7: Block diagram of series operation(Source)

Connecting two or more power supplies in parallel is a standard solution for power increase. Make a star formation with the load in the center to ensure almost equal lead lengths. Avoid looping one power supply to the next to prevent connector overloads and poor sharing.

Figure 8: Block diagram of parallel operations(Source)

Comparative study on the standard features and benefits side-by-side

• Linear Power Supply: These have a consistent low-hum output and are ideal for noise-sensitive applications. However, they are unsuitable for high-power applications due to lower efficiency and increased thermal dissipation. Applications of linear power supplies include medical devices, low-noise amplifiers, sensors, controllers, lab equipment, and telecommunication devices such as sampler circuits, multiplexers, demultiplexers, and A/D and D/A converters.
• Switching Mode Power Supply (SMPS): These highly efficient power supplies use high voltage supplies and switches for output voltage regulation. The SMPSs are significantly lighter and more efficient than their linear counterparts. Despite potential noise, it is ideal for compact designs in applications like industrial equipment, LED drivers, and battery chargers.

Figure 9: Structure and appearance of linear power supplies(Source)

Figure 10: Structure and appearance of switching mode power supplies(Source)

Figure 11: Adjustable power supply(Source)

Decision Matrix

Choose a power supply based on your priorities

How DC-DC converters are more effective in comparison to voltage regulators. (Source-element14 community)

Risk mitigation and safeguarding strategies

The correct power supply selection involves considerations beyond voltage and amperage. For technical applications, safety and protection are also significant concerns. The crucial concerns for safeguarding the circuits and ensuring reliable operation are:

• Overvoltage and Overcurrent: These are the most common threats to the equipment. Overvoltage occurs when the supply delivers a higher voltage than the circuit can handle, potentially damaging components or causing malfunctions. Similarly, overcurrent happens when the circuit draws more current than the supply can provide, leading to overheating and potential fire hazards.

  Protection measures:

  Overvoltage protection (OVP) - Most power supplies have built-in OVP circuits that automatically shut down the output when the voltage exceeds a preset limit. Some offer adjustable thresholds for fine-tuning protection.

Figure 12: Overvoltage Protection circuit diagram(Source)

Overcurrent Protection (OCP) - Like OVP, OCP circuits trip and cut off the current if it surpasses a defined threshold. Adjustable OCP allows tailoring protection to specific circuit needs.

Figure 13: Overcurrent Protection circuit diagram(Source)

• Short circuit protection: Short circuits occur when unintended connections allow current to flow through unintended paths, potentially causing excessive heat and component failure.

  Protection measures-

  Foldback Current Limiting- This feature automatically reduces the output current when a short circuit is detected, minimizing damage and allowing for safe recovery.

Figure 14: Foldback Current Limiting circuit diagram(Source)

• Electronic Fuses: used for instantly cut off the current during a short circuit and offer fast reset capabilities.

• Grounding and Isolation: Proper grounding prevents electrical shocks and equipment damage. Isolation minimizes noise and interference.

  Protection Measures- For safe current drainage, connect the power supply's ground to a proper earth ground. Power supplies with good isolation ratings should be chosen to increase circuit stability and minimize noise coupling.

  Grounding:

• Thermal considerations: Power supplies generate heat during operation. Exceeding safe operating temperatures diminishes lifespan and compromises performance. Protection Measures-
  1. Adequate Ventilation: Ensure proper airflow around the power supply to prevent overheating.
  2. Temperature monitoring: Some power supplies offer internal temperature sensors and alarms to alert potential thermal issues.

• Environmental Factors: Dust, moisture, and extreme temperatures can affect its performance and lifespan.

  Protection measures-
  1. Enclosure Rating: Choose a power supply with an appropriate IP rating (Ingress Protection) to withstand dust and moisture exposure.
  2. Operating Temperature Range: Ensure the power supply's operating temperature range matches the environment.

The following devices constitute the primary components that protect electronic equipment from damage.

Standard and compliances

It is essential to adhere to established standards for optimal power supply performance in various contexts.

• Safety measures certifications: Industry standards such as UL, CE, and FCC certifications validate a power supply's adherence to safety and electromagnetic interference (EMI) regulations. Having these certifications for power supplies guarantees a secure operating environment.
• Compliance related to energy efficiency: Power supplies must be energy efficient. They must comply with standards like 80 PLUS for computer power. Choosing an energy-efficient solution supports global sustainability initiatives and leads to long-term cost savings.
• RoHS compliance: Power supplies must adhere to the RoHS directive and prioritize environmentally responsible choices. Hazardous material should be used sparingly for a cleaner, greener planet.
• Temperature tolerance: Power supplies should account for thermal limits by adhering to standards like EN 50155 for railways or MIL-STD-810 for the military. The power supply must be capable of handling varying temperatures, ensuring reliable performance across diverse environments.
• Ripple and noise standards: Power supplies must comply with industry benchmarks such as MIL-STD-1275 for military applications or EN 61000-6-3 for residential environments.
• REACH and WEEE regulations: REACH regulates the exchange of supply chain information through a supplier portal in the European Union. Simultaneously, the Waste Electrical and Electronics Equipment Regulations (WEEE Regulations) seek to increase electronic equipment reuse, recycling, and recovery. This directive establishes criteria for collection, treatment, recycling, and recovery, putting financial obligation on producers.

At a glance - what element14 offering you in different varieties of power supplies

Conclusion

Professionals must acquire skills in various power supply types and understand the subtleties of different power supply types. For technical audiences, it is critical to consider crucial aspects and stay up to date on evolving technology. This comprehensive guide is a valuable resource that provides technical professionals, engineers, and enthusiasts with the knowledge to make informed decisions and ensure success in diverse technological applications.