Advantages of Inverted Buck for Low Power AC/DC Conversion

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Advantages of Inverted Buck for Low Power AC/DC Conversion [Copy link]

　For offline power supplies, the flyback topology is a reasonable solution. However, if the end application of the design does not require isolation, the offline inverted buck topology has higher efficiency and a smaller BOM count. This power design article will explore the advantages of inverted buck for low-power AC/DC conversion.

　　One of the most common power supplies is the offline power supply, also known as the AC power supply. As more and more products integrate typical household functions, the industry's demand for low-power offline converters with output capabilities below 1W is increasing. For these applications, the most important design aspects are efficiency, integration, and low cost.

　　When deciding on a topology, the flyback topology is usually the first choice for any low power offline converter. However, if isolation is not required, this may not be the best approach. Let’s say the end device is a smart light switch that the user can control via a smartphone app. In this case, there is no possibility that the user will come into contact with exposed voltages during operation, so isolation is not required.

　　For offline power supplies, the flyback topology is a reasonable solution because of its low bill of materials (BOM) count, only a few power stage components, and the transformer is designed to handle a wide input voltage range. But what if the design's end application does not require isolation? If so, the designer may still want to use the flyback topology, given that the input is offline. Controllers with integrated field effect transistors (FETs) and primary-side regulation can create a small flyback solution.

　　Figure 1 shows a typical schematic of a non-isolated flyback converter, designed using the UCC28910 flyback switcher IC with primary-side regulation. Although this solution is feasible, the offline inverted buck topology has higher efficiency and a smaller BOM count compared to the flyback power supply. This power design article will explore the advantages of inverted buck for low-power AC/DC conversion.

Advantages of Inverted Buck for Low-Power AC/DC Conversion
　　Figure 1: This nonisolated flyback design using the UCC28910 flyback switcher IC converts AC to DC, but an offline inverted topology can do the job more efficiently.

　　Figure 2 shows the inverted buck power stage. Like the flyback supply, it contains two switching elements, a magnetic element (a power inductor instead of a transformer), and two capacitors. As the name implies, the inverted buck topology is similar to a buck converter. The switches produce a switching waveform between the input voltage and ground, which is then filtered by the inductor and 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 electronics normally.

Advantages of Inverted Buck for Low Power AC/DC Conversion

Figure 2: Simplified schematic of an inverted buck power stage

　　By placing the FET on the low side, the flyback controller can drive it directly. The inverted buck topology shown in Figure 3 is designed using the UCC28910 flyback switcher IC. The 1:1 coupled inductor acts as the magnetic switching element. The primary winding acts as the 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) capacitor.

Advantages of Inverted Buck for Low Power AC/DC Conversion

Figure 3: Typical inverted buck topology design using the UCC28910
flyback switcher IC

　　One disadvantage of the flyback topology is the way 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. A real transformer will have some leakage inductance on the primary side. When the energy is transferred to the secondary side, the remaining energy is stored in the leakage inductance. This energy is not usable and needs to be dissipated using a Zener diode or a resistor-capacitor network.

　　In the buck topology, the leakage energy is transferred to the output through diode D7 during the FET's off time. This reduces the number of components and improves efficiency.

　　Another difference is the design and conduction losses of each magnetic component. Because the inverted buck topology has only one winding to transfer power, all the power transfer current goes through it, which allows for 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 the secondary winding. When the FET is off, current flows through the secondary winding, but not the primary winding. Therefore, in the flyback design, the transformer has more energy to store and requires more copper to provide the same output power.

　　Figure 4 compares the current waveforms of the buck converter inductor and the primary and secondary windings of the flyback transformer with the same input and output specifications. The buck converter inductor waveform is in a single blue box on the left, and the flyback converter primary and secondary windings are in two red boxes on the right.

　　For each waveform, the conduction loss can be calculated as the square of the rms current multiplied by the winding resistance. Since the buck converter has only one winding, the total conduction loss in the field is the loss of this one winding. However, the total conduction loss of the flyback converter is the sum of the losses of the primary and secondary windings. In addition, the magnetic components in the flyback converter are physically larger than those of the inverted buck design at the same power level. The energy storage of both components is equal to L×IPK2.

　　For the waveform shown in Figure 4, according to calculations, the energy required to be stored in the inverted buck design is only 1/4 of that in the flyback design. Therefore, compared with the flyback design of the same power, the size of the inverted buck design is much smaller.

Advantages of Inverted Buck for Low Power AC/DC Conversion

Figure 4: Comparison of current waveforms in buck and
flyback topologies

　　When isolation is not required, the flyback topology is not always the best solution for low power offline applications. The inverted buck topology can provide higher efficiency and lower BOM cost due to the use of smaller transformers/inductors. For designers in the field of power electronics, it is necessary to consider all possible topology solutions for a given specification to determine the best match.