“One of the most common power sources is an offline power source, also known as an AC power source. As the number of products designed to integrate typical household functions increases, so does the need for low-power offline converters that require an output capability of less than 1 watt. For these applications, the most important design aspects are efficiency, integration, and low cost.
One of the most common power sources is an offline power source, also known as an AC power source. As the number of products designed to integrate typical household functions increases, so does the need for low-power offline converters that require an output capability of less than 1 watt. For these applications, the most important design aspects are efficiency, integration, and low cost.
When deciding on a topology, flyback is usually the first choice for any low-power offline converter. However, if isolation is not required, this may not be the best approach. Suppose the end device is a smart light switch, which the user can control through an app on a smartphone. In this case, the user is not exposed to the exposed voltage during operation, so isolation is not required.
For off-line power supplies, the flyback topology is a reasonable solution because it has a 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 end application of the design does not require isolation? If so, the designer may still want to use flyback considering the input is offline. Controllers with integrated field effect transistors (FETs) and primary side regulation result in small flyback solutions.
Figure 1 shows an example schematic of a non-isolated flyback using the UCC28910 flyback switch with primary-side regulation. While this is a viable option, an off-line inverted buck topology will have higher efficiency compared to a flyback with a lower BOM count. In this power management design tip, I’ll discuss an inverted buck for low-power AC/DC conversion.
Figure 1 This non-isolated flyback design using the UCC28910 flyback switch converts AC to DC, but an offline inversion topology can do the job more efficiently.
Figure 2 shows an inverted buck power stage. Like a flyback, it has two switching elements, a magnetic (single supply Inductor instead of 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 out by the 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 electronics normally.
Figure 2 Simplified schematic of an inverted buck power stage.
Putting the FET on the low side means it can be driven directly from the flyback controller. Figure 3 shows an inverted buck using the UCC28910 flyback switch. A one-to-one coupled inductor acts as a magnetic switching element. The primary winding acts as a power stage inductor. 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 An example of an inverted buck design using the UCC28910 flyback switch.
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 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 the 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 the number of components and increases efficiency.
Another difference is the design and conduction losses of each magnetic element. Because an inverted buck has only one winding to transfer power, all the power transfer current goes through it, which provides good copper utilization. Flyback does not have such good copper utilization. When the FET is turned on, current flows through the primary winding instead of the secondary winding. When the FET is turned off, current flows through the secondary winding instead of the primary winding. Therefore, more energy is stored in the transformer and more copper is utilized in the flyback design to provide the same output power.
Figure 4 compares the current waveforms of the primary and secondary windings of a buck inductor and flyback transformer with the same input and output specifications. The buck inductor waveform is in the single blue box on the left, and the primary and secondary windings of the flyback are in the two red boxes on the right.
For each waveform, conduction losses are calculated as the rms current squared multiplied by the winding resistance. Because the buck has only one winding, the total conduction loss in the magnetic field is the loss of one winding. However, the total conduction loss of the flyback is the sum of the primary and secondary winding losses. Additionally, the physical size of the magnetic field in the flyback will be larger than an inverted buck design at similar power levels. The energy storage of any component is equal to ? L × IPK2.
For the waveforms shown in Figure 4, I calculated that the inverted buck would only need to store a quarter of the power that the flyback would need to store, so the footprint of the inverted buck design would be compared to an equivalent power flyback design much smaller.
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. Inverting the buck gives higher efficiency and lower BOM cost because you can use a potentially smaller transformer/inductor. It is important for power electronics designers to consider all possible topological solutions to determine the one that best fits a given specification.