Two high-efficiency power supply designs and topology analysis

Power supplies play a vital role in reducing power consumption, so in the face of regulatory standards and higher consumer demands, it is urgent to review their design methods. Although traditional topologies can be improved to achieve higher performance requirements, it is obvious that products that follow the old design methods will have low cost-performance. In this article, we will propose two design methods that can meet higher performance requirements and control target costs, and compare them with traditional topologies.

Traditional topology

There are several factors to consider when choosing a topology for a particular application, including whether the input voltage range is universal or specific to a particular region, whether the output voltage is single or multiple (current size is also an important condition), and performance targets, especially performance under different loads. Traditionally, when mass-producing power supplies, cost, the design engineer's familiarity with the topology, and whether the components are easy to purchase are considered. Other factors include whether the design is easy to implement.

Whether the current and design methods are well known in the power supply industry chain, etc.

The more popular traditional design methods are mainly single-switch forward, dual-switch forward and half-bridge structures, which provide solid solutions to meet current needs. However, as mentioned above, emerging standards require power supplies to achieve higher efficiency than before. In the past, typical desktop power supplies could achieve a maximum efficiency of 60% to 70%, but now the power supply is required to achieve a minimum efficiency of 80% at 20%, 50% and 100% of the rated load. At the same time, there has been a recent trend to achieve 70% or more efficiency at loads below 20%, and standby power consumption can continue to decrease. We will explore the advantages and disadvantages of three traditional topologies and introduce two new topologies.

1 Single switch forward

The topology in Figure 1 is quite popular, mainly due to the low component count and simple design requirements, but the high efficiency requirements for different load conditions have brought new challenges to this topology. At near or full load, the efficiency of this topology is limited by the 50% duty cycle. At lighter loads, switch losses are the main cause of poor efficiency. Many newer designs use a power factor correction (PFC) front end to reduce harmonic currents. At a PFC output voltage of 400 V, the single-switch forward method is forced to use a switch greater than 900 V, which increases the cost of the FET.

Figure 1 Single switch forward topology

2 Double switch forward

Figure 2 is another fairly common topology that is an upgraded version that solves the switch voltage limitation problem. This is still a hard switching circuit with high switching losses. The problem it brings is that a gate drive transformer or chip drive circuit is required to drive the high voltage side MOSFET.

Figure 2 Dual switch forward topology

The half-bridge transformer in Figure 3 is another option for high power requirements. In contrast to the single-switch or dual-switch forward transformer, the half-bridge transformer can operate in both quadrants and reduce the current in the primary FET. The transformer structure and output rectification are more complex than the single forward topology, and there is also the problem of high switching losses.

Figure 3 Half-bridge topology circuit structure

Emerging Topologies

In order to meet the requirements of higher performance, the industry has developed several new topologies. These new circuit topologies are not necessarily newly invented, but have recently been applied in large commercial quantities. Among them, the two most popular topologies are active clamp forward and dual inductor plus capacitor (LLC).

1 Active Clamp Forward

The active clamp forward topology in Figure 4 is a long-standing soft switching structure. Although this structure is similar to the traditional forward topology, it has been considered difficult to implement in the past and is therefore mainly used in special fields such as telecommunications. However, with the introduction of new ICs, the implementation of this structure has become very simple.

Figure 4 Active clamp forward topology using ON Semiconductor NCP1562

In this topology, the transformer is reset by the capacitor in series with the auxiliary switch during the entire off time of the main switch, which eliminates the dead time in the single-switch forward structure. Its main advantages include low switching losses, operation at duty cycles above 50%, and reduced current stress on the primary switch. At the same time, this structure also provides self-driven synchronous rectification, eliminating the need for dedicated gate drive circuits. Coupled with the increasingly lower prices of low-voltage MOSFETs, the use of MOSFETs and synchronous rectification has become a viable solution for achieving low output voltage and high current rectification.

The use of active clamp devices and control of active clamp FETs may appear to increase circuit complexity, but this is offset by savings in snubber circuits, reset circuits, and lower overall switching requirements. This structure can also operate over a wide input voltage range, making it suitable for a variety of applications, including video game consoles.

The main disadvantage of this topology is that it is not used in high volume applications, such as computers, and is therefore unfamiliar to desktop designers. However, with the introduction of products from companies such as ON Semiconductor, this topology has become easier to implement. Using this topology in larger volume applications can also reduce component costs. Another disadvantage of this topology is that it requires higher voltage rated switches than a two-switch forward or half-bridge transformer. <-- 2007-12-5 23:37:38--> 2 LLC Resonant Half-Bridge

The LLC topology in Figure 5 is particularly suitable for applications requiring high output voltages, such as LCD and plasma TVs.

Figure 5 LLC resonant half-bridge topology

Like the active clamp topology, this is a soft switching topology that achieves ultra-high efficiency due to ultra-low switching losses. Other advantages include no output inductor required, which reduces the overall cost of implementation. Finally, due to the half-bridge configuration, the stress on the primary components can be reduced.

On the other hand, this structure also has some disadvantages, the most important of which is the increase in complex magnetic design, high ripple current on the output capacitor and variable frequency. At the same time, this structure is also more difficult to design for a wide input voltage range.

Although we cannot use a single topology as a solution for all applications, we can decide which circuit structure to use based on the specific situation. Here, we use a 12V, 20A output transformer design to compare the differences between the various structures mentioned above, focusing on major design issues such as primary switches, rectifiers, magnetics, storage capacitors, etc. Although there are other differences, they are beyond the scope of this article. The differences between the various topologies are summarized below.

● Primary switch: In the input voltage range of 300-400Vdc, the primary peak current of the active clamp transformer is the lowest, and the single-switch and dual-switch forward topologies have the same

Similar RMS current, but with larger conduction losses due to the MOSFET voltage rating.

● The DC secondary rectifier voltage stress is lowest for the resonant half-bridge transformer, followed by the active clamp, and then the single-switch and dual-switch forward transformers. The stress on the conventional circuit topology is higher due to the switching surge.

● The hold time requirement can be achieved by increasing the capacitor value or the transformer input range.

● In terms of magnetics, the resonant half-bridge provides significant simplification by removing the output inductor, but it is quite challenging in transformer design. Compared with the traditional forward transformer, the output inductance of the active clamp transformer can be reduced by about 13% at the same frequency.

● Since the resonant half-bridge transformer has no output inductor, the output capacitor current ripple is the highest.

● The switching frequency of the active clamp forward transformer can be pushed higher (200-300kHz), while the hard switching topology is below 150kHz. The resonant half-bridge is a variable frequency transformer. When fully loaded with low power supply voltage, its minimum frequency is usually set at 60-70kHz; when working with high power supply voltage and light load, the maximum frequency can reach hundreds of kHz.