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Improving the reliability of industrial AC/DC power supplies by choosing topologies

The key to improving power supply reliability is to reduce thermal, voltage, and current stress on power components, which is primarily a function of input voltage and required power. However, you can choose a topology that helps alleviate these stresses.

The key to improving power supply reliability is to reduce thermal, voltage, and current stress on power components, which is primarily a function of input voltage and required power. However, you can choose a topology that helps alleviate these stresses.

Likewise, while thermal stress is a function of power rating, power supply efficiency also plays an important role. Therefore, in the pursuit of reliability, it is extremely important to explore topologies and circuit components that provide high efficiency.

In our 94.5% efficiency, 500 W industrial AC/DC reference design, the front-end power factor correction (PFC) stage is an interleaved transition mode boost topology, although the single-stage continuous conduction mode (CCM) boost topology is also A viable option.The topology selection is mainly based on the consideration of device pressure; the staggered topology, due to the two-stage parallel operation, combines the power components (boost inductors, switching metal oxide semiconductor field effect transistors)[MOSFET]and rectifier diodes) are reduced by a factor of two. Figure 1 shows a simplified diagram of the two topologies.

Figure 1: Interleaved and single-stage boost PFC

Transition-mode PFC has advantages in reducing switching stress due to significantly reduced turn-on stress. When the input voltage is less than half the output voltage, the voltage switching in filter mode is zero; even if the input voltage is higher, the voltage switching level is significantly reduced. Under all conditions, the MOSFET and rectifier have zero current switching (ZCS). ZCS operation results in almost elimination of reverse recovery in the rectifier diode, which also helps reduce stress and reduce electromagnetic interference (EMI). While reducing EMI does not provide direct reliability benefits, a reduction in the number of EMI filter components and the potential for noise pickup from sensitive circuit segments can indirectly help improve overall power supply reliability.

When thermal stress is considered, the interleaved transition-mode boost topology is again more favorable than the CCM topology. In the staggered transition mode topology, the components operate at lower temperatures; more components share nearly the same power loss compared to the CCM topology. Operating under reduced temperature conditions has a considerable impact on power supply reliability, especially in systems without forced ventilation.

In addition, the interleaving operation greatly reduces the ripple current in the input and output capacitors. This is an important consideration, especially with aluminum electrolytic output capacitors, which are one of the weakest links in determining overall power supply reliability. In PFC applications, the ripple current is the most important factor in determining the life of the output capacitors (voltage ratings are limited to 450V/500V due to size, cost and availability). It should be seen that the reduction in ripple current is not only a derating of the specification, but more notably the reduction in temperature due to the reduction in power dissipation.

For the DC/DC stage, the Inductor-inductor-capacitor (LLC) topology is preferred because it has reduced switching stress, although it does increase current stress. Operating at full load just above the resonant frequency minimizes the increase in current stress while avoiding reverse recovery of the output synchronous MOSFET body diode due to ZCS turn-off.

The design achieves close to 95% efficiency without adding too much complexity. The efficiency of the PFC grade is higher than 98% at 230 V and 96.5% at 115 V. The efficiency of the LLC grade is higher than 96.5%. Topology and component selection are factors that affect this performance.

Another important point to consider is the efficiency of the circuit over its operating range: it may not always operate at or near full load during its lifetime. Therefore, it is very important to achieve good efficiency over a wide operating area. This is where controller selection for PFC and LLC power stages becomes critical.

The two controllers used in this design (UCC28064A for PFC and UCC256301 for LLC) have control techniques that offer efficiency advantages over a wide operating range, as shown in Figure 2. In addition, the UCC24612, the synchronous rectifier controller and driver used in this design, reduces output rectifier losses by achieving near-ideal diode emulation and indirectly reduces primary-side losses. The contribution of these controller devices to improving overall reliability is not insignificant.

Figure 2: PFC and LLC Stage Efficiency

In industrial power applications, you must choose a topology that reduces stress on the components. Interleaved transition-mode boost and LLC topologies are better choices than other topologies because of reduced component stress. Topology selection should consider distributing power losses to more components, and improving efficiency is important because thermal stress is directly related to it.