“For offline power supplies, a flyback topology is a reasonable solution. However, if the end application of the design does not require isolation, then the off-line inverted buck topology offers higher efficiency and lower BOM count compared to the design. This power supply design article will discuss the advantages of inverted bucks for low-power AC/DC conversion.
For offline power supplies, a flyback topology is a reasonable solution. However, if the end application of the design does not require isolation, then the off-line inverted buck topology offers higher efficiency and lower BOM count compared to the design. This power supply design article will discuss the advantages of inverted bucks for low-power AC/DC conversion.
One of the most common is an off-line power source, also known as an AC power source. As more and more products integrate typical home functions, the industry’s demand for low-power offline converters with output capabilities below 1W is growing. For these applications, the most important design aspects are efficiency, integration, and low cost.
When deciding on a topology, a flyback topology is usually the first choice for any low power offline converter. However, this may not be the best approach if isolation is not required. Assuming that the end device is a smart light switch that the user can control via a smartphone app, in this case the user is unlikely to be exposed to the exposed voltage during operation, so isolation is not required.
For offline power supplies, the flyback topology is a reasonable solution because it has a low bill of materials (BOM), only a few power stage components, and the transformer is designed to handle a wide input voltage range. But what if the end application of the design does not require isolation? If so, the designer may still want to use a flyback topology considering that the input is offline. Controllers with integrated field-effect transistors (FETs) and primary-side regulation can create small flyback solutions.
Figure 1 shows a typical schematic of a non-isolated flyback converter designed using the UCC28910 flyback switching power supply IC with primary side regulation. Although this scheme is feasible, the offline inverted buck topology has higher efficiency and less BOM compared to the flyback power supply. This power supply design article will discuss the advantages of inverted bucks for low-power AC/DC conversion.
Figure 1: This non-isolated flyback design using the UCC28910 flyback switching power supply IC converts AC to DC, but an offline inversion topology can do the job more efficiently.
Figure 2 depicts an inverted buck power stage. Like the flyback power supply, it contains two switching elements, a magnetic element (which is a power Inductor rather than a transformer), and two capacitors. As the name suggests, an inverted buck topology is similar to a buck converter. The switch produces a switching waveform between the input voltage and ground, which is then filtered through an inductor-capacitor network. The difference is that the output voltage is regulated to a potential lower than the input voltage. Even if the output “floats” below the input voltage, it can still power downstream Electronic circuits normally.
Figure 2: Simplified schematic of an inverted buck power stage
Setting the FET on the low side allows the flyback controller to drive it directly. The inverted buck topology shown in Figure 3 is designed using the UCC28910 flyback switching power supply IC. A 1:1 coupled inductor acts as a magnetic switching element. The primary winding acts as an inductor for the power stage. The secondary winding provides timing and output voltage regulation information to the controller and charges the controller’s local bias supply (VDD) capacitors.
Figure 3: Typical inverted buck topology design using the UCC28910 flyback switching power supply IC
One disadvantage of the flyback topology is the way the energy is transferred through the transformer. This topology stores energy in the air gap during the on-time of the FET and then transfers it to the secondary during the off-time of the FET. The actual transformer will have some leakage inductance on the primary side. When energy is transferred to the secondary side, the remaining energy is stored in the leakage inductance. This energy is not available and needs to be dissipated using a Zener diode or a resistor-capacitor network.
In a buck topology, leakage energy is delivered to the output through diode D7 during the off-time of the FET. This reduces component count and increases efficiency.
Another difference is the design and conduction losses of each magnetic element. Because the inverted buck topology has only one winding to transfer power, all power transfer current goes through it, which achieves good copper utilization. The flyback topology does not have such good copper utilization. When the FET is on, current flows through the primary winding, but not in the secondary winding. When the FET is off, current flows through the secondary winding, but not in the primary winding. Therefore, in a flyback design, the transformer stores more energy and needs to use more copper to provide the same output power.
Figure 4 compares the current waveforms of the primary and secondary windings of a buck converter inductor and a flyback transformer with the same input and output specifications. The waveform of the buck converter inductor is in the single blue box on the left, and the primary and secondary windings of the flyback converter are in the two red boxes on the right.
For various waveforms, conduction losses can be calculated as the square of the rms current multiplied by the winding resistance. Because the buck converter has only one winding, the total conduction losses in the magnetic field are losses in this one winding. However, the total conduction loss of a flyback converter is the sum of the primary and secondary winding losses. Additionally, the physical size of the magnetics in a flyback converter is larger than an inverted buck design at the same power level. The stored energy of both elements is equal to ½L×IPK2.
For the waveform shown in Figure 4, according to calculations, the energy stored in the inverted buck design is only 1/4 of that in the flyback design. Therefore, the size of an inverted buck design is much smaller compared to a flyback design of the same power.
Figure 4: Comparison of Current Waveforms in Buck and Flyback Topologies
Flyback topologies are not always the best solution for low power offline applications when isolation is not required. The inverted buck topology can provide higher efficiency and lower BOM cost due to the use of smaller transformers/inductors. For designers in power electronics, it is imperative to consider all possible topological solutions to determine the best match for a given specification.