Power supply design
The build vs. buy decision
A reliable power supply is an essential part of any electronics-based design. And provisioning power for a product involves several choices, the first of which is whether to buy a ready-built power supply or to design a discrete unit.
Creating a power supply from scratch might appear an attractive option if your product or application has particular needs. However, embarking on a custom design takes time, something that is a precious commodity when time-to-market pressures are high. Also, unless you have an experienced power supply design engineer on your team, there will need to be a rapid growth of knowledge to learn current power systems design practice.
Having a good understanding of pertinent regulations and legislation is also essential, since power supplies and their attached products fall under safety and energy efficiency specifications. If your product is for healthcare or medical applications, it will also have to pass more stringent safety regulations. For the above reasons, most engineering teams opt to select a power supply module or unit they can quickly incorporate into the design.
Power supply types and key selection criteria
Electronics-based products and applications generally operate on low voltage DC supplies. For products designed to work from mains power, the power supply must convert from AC mains to a DC output voltage – using an AC-DC power supply. Within some applications, more than one DC voltage might exist, so further power conversion is required – using a DC-DC power converter. You might also use a DC-DC converter if your power source is a battery, for example in a vehicle or for a handheld portable device.
Figure 1: A trace of DC output showing noise, ripple
and transients over changing load conditions |
Whether you need an AC-DC or a DC-DC power supply, there are some essential criteria. Firstly, for your application to operate reliably, the DC output voltage needs to be well regulated within permitted tolerances across all load conditions - termed line regulation. Also, the output voltage must be free of harmful transient voltage spikes, noise and ripples that potentially interfere with the operation of the end product.
Electrical noise may come from the power supply itself, or be induced from an external source. Ripple voltages are small voltage variations of the output voltage that might also be present as a result of the power conversion process. Transient spikes occur when there is a rapidly changing load condition - dV/dt - and can create instantaneous voltages that are significantly higher than the output voltage.
The power conversion circuitry also serves to provide isolation between the input voltage and the output voltage, a factor particularly crucial for mains-powered equipment. Power supplies are typically architected around topologies such as linear and switched mode. For reasons explained later, the switched mode topology, which includes isolated fly-back conversion, and non-isolated buck and boost, is the most popular.
Unfortunately, any power conversion process will involve some energy losses, impacting the overall energy conversion efficiency. For example, a typical AC-DC power supply may have an efficiency rating of 85 - 95 % when operating at full load. The energy lost during conversion, usually by inductors and switching transistors, results in waste heat that needs to be dissipated away from the power supply.
The energy efficiency, and the amount of energy consumed by an AC-DC power supply when it has no load or is in standby, is subject to international regulation and will be covered later in this article.
AC-DC converter topologies
The majority of all AC-DC power supplies use a form of the switched-mode topology. The linear topology is rarely used today apart from some specific applications. One notable downside is its use of a bulky mains transformer to isolate and reduce the mains voltage to a usable lower level. That said, linear power supplies may find use in high-quality audio applications where their low noise characteristics make them a viable alternative.
Switched-mode topologies benefit from using a semiconductor switching circuit that operates in a frequency range, typically 20 kHz to 150 kHz, where the physical size of the transformer is significantly reduced compared to a linear topology. Switched-mode power supplies are more energy-efficient thanks to the switching methods used. However, ripple, noise, and the generation of conducted and radiated EMI from the switching process needs careful attention.
Figure 2: Functional block diagram of a typical isolated fly-back converter AC-DC power supply
The isolated fly-back converter (see Figure 3, above) - is an example of a topology used for AC-DC power supplies delivering up to 120 watts. The use of a compact transformer keeps costs to a minimum and provides the essential input to output isolation for product safety regulations. The AC input is filtered and rectified before the switching stage. The power stage typically uses MOSFETs, driven hard-on and hard-off by a driver signal, the frequency and duty-cycle of which varies according to load conditions. Regulation of the DC output takes place by feeding back a control voltage to the driver circuit. An opto-isolator performs an electrical isolation between the primary control circuit and the DC output. Another frequently used topology for AC-DC supplies is the forward converter, which is also used in DC-DC converters.
As previously mentioned, the switching losses have a direct impact on the power supply's energy efficiency. Ensuring the switching occurs when no current is flowing, known as zero voltage switching (ZVS), has a significant impact on overall efficiency. The half-bridge converter topology is a popular choice for this. Higher efficiencies result in less waste heat, leading to smaller power supplies with a higher power density.
Recent innovations in semiconductor process technology include using silicon carbide (SiC) and gallium nitride (GaN) switching transistors as a replacement for MOSFETs. These new devices reduce switching losses resulting in higher energy efficiency.
Depending on the application, AC-DC power supplies are typically available as either Class I or Class II. A Class I power supply uses the protective earth as a safety function while Class II supplies do not. Low power wall plug supplies are typically Class II.
DC-DC converter topologies
Discrete three-pin or 3SIP linear DC regulators find application in some designs, but there are now drop-in switching regulators available that offer far better efficiencies. This results in lower operational temperatures without the need for heatsinks.
Popular DC-DC converter topologies include the non-isolated types of buck, boost, and buck/boost, and isolated types of fly-back, forward, half-bridge, and full-bridge. As with AC-DC power conversion, the use of a transformer between input and output provides galvanic isolation, a requirement for many safety requirements. Non-isolated types will have a direct electrical path between input and output. They all use switching semiconductors, a drive circuit, and regulated versions incorporate a feedback loop.
A buck converter provides an output voltage that is lower than the input, and a boost converter steps up the input voltage to provide a higher output voltage. A buck/boost converter can either step up or step down the DC input voltage.
Figure 3: A simplified diagram of basic concept of a buck/boost converter |
Figure 4 (right) illustrates a simple buck/boost converter circuit. When the switching MOSFET is on, current flows into the inductor, storing energy in its magnetic field. When the MOSFET switches off, energy from the inductor’s collapsing magnet field flows to the capacitor and the load. The capacitor keeps the output load supplied with power when the MOSFET turns on. A blocking diode prevents current flow from the capacitor to the inductor. Switching frequency and the value of the inductor and capacitor determine the output voltage.
The isolated fly-back converter is another popular topology for DC-DC converters. Regulation in an isolated converter tends to rely on using an opto-isolator to transfer a feedback signal to the drive circuits.
A new approach that some DC-DC converter manufacturers are taking dispenses with the opto-isolator and instead senses the output voltage via the transformer's primary winding during the driver off period. This approach reduces the component count further in addition to increasing converter reliability.
Power supply standards
International standards exist that stipulate several key power supply attributes. These include power efficiency, no-load power consumption, safety parameters such as isolation voltages, and electromagnetic interference (EMI).
The primary energy efficiency standards are the US Department of Energy Level VI and the European Union Eco Design 2019/1782 (formerly termed EU CoC Tier 2). These standards cover limits such as no-load power consumption, and average efficiencies when the power supply is in standby.
The international safety standard IEC-62368 covers most consumer and commercial equipment, replacing two previous standards IEC 60950 (ICT) and IEC 60065 (AV). Safety standards for power supplies relate to not only the power supply itself but the complete product, and there may be regional variations or adaptations of the standard's specification. Critical safety criteria include isolation techniques and leakage currents.
Any equipment used for medical applications must conform to the IEC 60601-1 specification that emphasises the safety of a connected patient and the equipment operator. Another important directive - IEC 60335 – covers household equipment and battery chargers, where user safety is also key.
Ensuring a power supply does not create excessive levels of electromagnetic interference is covered by the EMI standards CISPR32 and FCC 20870.
Conclusion
Opting to buy a suitable AC-DC or DC-DC power supply is a prudent step that will save significant design time. You’ll avoid having to up skill your engineering team by adding a power designer, and the worries associated with submitting your design in time and in accordance with the relevant safety compliances. A broad range of standard products is available, which can be customised for specific applications. Power supply manufacturers have design teams and tools available to assist in selection, and Avnet Abacus’ power specalists are also on hand to help. Get in touch in your local language to discuss your requirements.
