Offline power is one of the most common power sources, also known as AC power. As the number of products designed to integrate typical household functions increases, so does the demand for low-power offline converters that require less than 1 watt of output capacity. For these applications, the most important design aspects are efficiency, integration, and low cost.

How does an inverted buck provide a topological choice for a non-isolated flybackWritten by JOHN DOROSA

Offline power is one of the most common power sources, also known as AC power. As the number of products designed to integrate typical household functions increases, so does the demand for low-power offline converters that require less than 1 watt of output capacity. For these applications, the most important design aspects are efficiency, integration, and low cost.

When deciding on the 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 method. Assuming that the terminal device is a smart light switch, the user can control it through a smart phone application. In this case, the user will not 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) count, only a few power-level components, and the transformer is designed to handle a wide input voltage range. However, what if the end application of the design does not need to be isolated? If this is the case, considering that the input is offline, the designer may still want to use flyback. Controllers with integrated field effect transistors (FET) and primary side regulation will produce small flyback solutions.

Figure 1 shows an example schematic diagram of a non-isolated flyback using a UCC28910 flyback switch with primary side regulation. Although this is a viable option, the offline inverted buck topology will have higher efficiency than a flyback with a lower BOM count. In this power management design tip, I will discuss inverted buck for low-power AC/DC conversion.

How does an inverted buck provide a topological choice for a non-isolated flyback

Fig. 1 This kind of non-isolated flyback design using UCC28910 flyback switch can convert AC to DC, but the offline inversion topology can accomplish this work more effectively.

Figure 2 shows the power stage of the inverted buck. Like flyback, it has two switching elements, a magnetic element (a single-supply Inductor instead of a transformer) and two capacitors. As the name suggests, the inverted buck topology is similar to a buck converter. The switch generates a switching waveform between the input voltage and ground, which is filtered out by the inductance capacitor network. The difference is that the output voltage is adjusted to a potential lower than the input voltage. Even if the output “floats” below the input voltage, it can still normally supply power to downstream Electronic devices.

How does an inverted buck provide a topological choice for a non-isolated flyback

Figure 2 A simplified schematic diagram of the inverted buck power stage.

Placing the field effect Transistor on the low side means that it can be driven directly from the flyback controller. Figure 3 shows an inverted buck using UCC28910 flyback switch. One-to-one coupled inductors are used as magnetic switching elements. The primary winding acts as a power-level inductor. The secondary winding provides timing and output voltage regulation information to the controller, and charges the controller’s local bias supply (VDD) capacitor.

How does an inverted buck provide a topological choice for a non-isolated flyback

Figure 3 An example of an inverted buck design using UCC28910 flyback switch.

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 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 Zener diodes or resistor-capacitor networks.

In the step-down topology, the leakage energy is transferred to the output terminal through the diode D7 during the off period of the FET. This can reduce the number of components and increase efficiency.

Another difference is the design and conduction loss of each magnetic component. Because an inverted buck has only one winding to transmit power, all power transmission current passes through it, which provides good copper utilization. Flyback does not have such a good copper utilization. When the FET is turned on, the current flows through the primary winding instead of the secondary winding. When the FET is disconnected, the current flows through the secondary winding instead of the primary winding. Therefore, more energy is stored in the transformer, and more copper is used in the flyback design to provide the same output power.

Figure 4 compares the current waveforms of the primary and secondary windings of a step-down inductor and a 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, the conduction loss is calculated as the square root mean square current multiplied by the winding resistance. Because there is only one winding for buck, 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 losses in the primary winding and the secondary winding. In addition, the physical size of the magnetic field in the flyback will be larger than the inverted buck design at similar power levels. The energy storage of any component is equal to ½ L × IPK2.

For the waveform shown in Figure 4, I calculated that the inverted buck only needs to store a quarter of the stored power of the flyback. Therefore, compared with the flyback design of the same power, the footprint of the inverted buck design is Much smaller.

How does an inverted buck provide a topological choice for a non-isolated flyback

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. Inverted buck can provide higher efficiency and lower BOM cost, because you can use a possibly smaller transformer/inductor. For designers of power electronic devices, it is important to consider all possible topological solutions to determine the most suitable topology for a given specification.

John Dorosa is an applications engineer for Texas Instruments.

The Links:   STK621-061 LM190E08-TLJ7