How to find the right power supply: Page 2 of 4

March 09, 2018 // By René Koch, Tobias Herrmann
In a networked world, connectivity is increasing at an unprecedented rate. This often leads to faults caused by concatenations and ground loops. Power supplies in particular are affected. Again, the increasing demands on efficiency lead to higher electromagnetic emissions. This means that the filter components take up more space. It is therefore important to find the best compromise for each application.

LEI's Chi-Bi LLC converters have been designed as single-stage AC-DC converters using a resonant LLC circuit with a high Q-factor, which enables highly efficient power conversion with low electromagnetic emissions. In addition, they are highly competitive products offering a relatively high power density.

Single-stage power supply versus universal power supply

Since the advent of the switch mode the output voltage of power supply units may optionally be regulated with a greater input voltage deviation, finally allowing a PSU with universal input voltage range (90 ~ 264 V) to be used worldwide. Even though this proves advantageous with regard to the product specification and reduces the variety of part numbers, universal product development always creates (unnecessary) hidden costs. It is obvious that mobile applications such as mobile phones, tablets, and laptops require a greater range, as users are expected to carry their devices with them when they travel. However, stationary applications such as network infrastructure, printers, TV sets, set-top or OTT boxes are not used in different locations and are therefore suitable for narrow-range power supplies.

Each design window (the range between minimum and maximum values) achieves optimal mid-range performance with proper balance, with both extremes showing reduced performance. This also applies to the input voltage range. When designing a universal power supply unit, the average operating range is approx. 180 VAC -  a voltage that is not used anywhere in the world, so that the power supply unit always operates at an operating point that is not optimal.

When designing a single-stage power supply, however, the minimum and maximum values deviate by about 10 % from the center of the design window, which is the voltage at which the power supply is actually used. This leads to a significant increase in performance compared to universal products.

Efficiency of a power supply vs input voltage.
Source: Finepower

Considering the performance of the components in terms of both price and electrical performance, differences can also be observed, especially with regard to the components of the primary side (AC side) of the power supply. For example, 400 V types are required as buffer capacitors for high-voltage operation (230 V), while a larger capacity is required for low-voltage operation (120 V) to meet requirements such as bridging times. Table 1 shows the result of the requirement profiles for input buffer capacitors, which illustrates that a universal power supply design requires a larger capacitor that is usually more expensive.

Table 1: Requirement profiles for the input buffer capacitors

The equivalent series resistance (ESR), the loss factor (tan∂) and the price are not listed here, but it is not difficult to estimate the outcome. This results in a potential performance increase for single-stage products in terms of efficiency, electromagnetic emissions, size and cost.

What are the advantages of LLC converters?

Nowadays, the vast majority of energy-efficient products (< 75 W) are built with flyback converters. A flyback converter is a simple converter that uses the transformer as a storage medium before the energy is supplied to the output. This two-stage converter type allows a very wide control range with a relatively simple control mechanism. The input energy is stored in the transformer during the switch-on time of the converter and is transferred to the output during the switch-off time. The isolation of the energy flow is the key to the wide operating range (control range) of flyback converters. The disadvantage of this technology, however, is that the transformer must be able to store all this energy and must therefore be of adequate size.

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