Design Method of High Efficiency and Ultra-Wide Input Voltage Range DC-DC Converter

　　One of the parameters of an isolated DC/DC converter is the input voltage range over which the converter can operate normally. For those industrial standard brick products used in the 48V input telecommunications market, the input voltage range is usually 36V~75V, or a ratio of 2:1 between the highest and lowest input voltages. But there are many applications that expect the converter to handle a wider input voltage range. For example, in some system applications, the distributed input voltage has large transients and surges, and the duration is very long, which requires filters to filter out.

　　As an example, Table 1 shows the steady-state and transient ranges of distributed voltages in different railway system standards. Military vehicle design specifications also require a similar wide input voltage range to accommodate changes in distributed voltages. Another reason for using a wide input voltage range DC/DC converter is to create a "universal" product that can be used in different DC systems. For battery systems with nominal values ​​of 12V, 24V and 48V, three different input voltage versions are generally required. Instead, a converter that can operate from 9V to 75V provides a single solution. This single solution can save production costs and reduce inventory.

　　While it is desirable to have a wide input converter, there is a major problem: Traditionally, the wider the input voltage range over which the module operates, the worse the converter performance. Generally speaking, for a given size, such as a quarter brick, the efficiency and power that the converter can handle decreases as the input voltage range becomes wider. This is a natural result, because while designing for the highest input voltage, you must also handle the very large input currents that are introduced when the input voltage is the lowest. For a converter with a 2:1 input range, the product of the maximum input voltage and the maximum input current is twice the power that needs to be handled, which is acceptable as a reasonable compromise. However, when a converter is designed to handle an 8:1 input range, the product of the maximum input voltage and the maximum input current is 8 times the power that needs to be handled, which is an extreme result. This is very serious for the power circuits associated with the converter's isolation transformer .

　　Due to the above limitations, there are not many commercial DC/DC converters that can handle a wide input voltage range, but the few "ultra-wide" 4:1 input converters typically handle less than 1/2 the power in a given physical size, which is compared to the power handled by a converter with only a 2:1 input voltage input range in the same size. In addition, the conversion efficiency of wide input converters is generally 10%-25% lower than that of 2:1 input converters.

　　One way to reduce this loss in wide input range converters is to separate the converter's regulation function from its isolation function, as shown in Figure 1. In this figure, the first stage of the converter is a non-isolated step-down converter, while performing the voltage regulation function by varying the duty cycle. The second stage of the converter provides electrical isolation without any voltage regulation, and can generally also perform further voltage reduction based on the transformer ratio. This is how SynQor, a leader in high-efficiency DC/DC converters, designs all of its products.

　　The advantage of this two-stage design is that only the first stage sees a wide range of input voltages. While the losses from the wide input voltage must be borne by this first stage, they are not severe because the first stage does not require an isolation transformer. For the isolation stage with a transformer, there is no need to face a wide input voltage range. In this two-stage design, the input voltage is always the same as the intermediate bus voltage in the two-stage solution. This allows the isolation stage to be optimized for a single operating condition and makes it very easy to implement a synchronous rectification based design for the isolation stage, which can greatly reduce power consumption. The efficiency improvement in the isolation stage goes a long way to offset any additional losses incurred in the regulation stage.

　　Figure 2 shows SynQor's new IQ64 series 8:1 ultra-wide input half-brick DC/DC converter. Table 2 shows the different input voltage ranges of SynQor's new series. As can be seen from the table, in addition to the normal 2:1 input range, there are also products with 4:1 input range and even 8:1 input range. The maximum power level and typical efficiency for 3.3V output voltage are also shown in this table. Although there is some reduction in power level and efficiency as the input voltage range becomes wider, it is not very obvious. This is the result of the two-stage topology solution for power circuit design.

　　In addition to being able to meet the needs of different input voltage ranges, SynQor's InQor series of industrial-grade DC/DC converters are fully sealed and very rugged, capable of being used in harsh environments, which are often associated with systems with such challenging technical requirements.