Power adapter adapter multi-output hybrid adjustment system

　　The use of integrated magnetic components and various simulation techniques has demonstrated the possibility of a wide range of combinations. In many cases, these combined converters are difficult to distinguish clearly. To understand the advantages and disadvantages of each combination, for example, in the Cuk converter, the input and output ripple voltage can be suppressed to zero by combining the output and input chokes. Therefore, the combination of converters can provide some very useful features.

　　A power adapter designer who has a full understanding of various combinations can choose the most suitable combination for a particular application. This is indeed a very powerful design tool.

　　A complete introduction to many technologies is beyond the scope of this book, but the following chapters introduce a particularly useful structure. If more information is needed, interested engineers should study the many excellent papers and books in the references.

　　Buck converter, in series with DC-DC converter

　　The following section only considers a more widely used and easy-to-understand buck converter structure. The regulator is connected in series with a square wave self-excited voltage feedback DC-DC converter. The figure shows a block diagram of the converter using MOSFETs. This structure is particularly useful for multi-output offline switch mode power adapters.

　　In other words, in the example of Figure 2.18. 1, the MOSFET, L1 and D1 in the input buck converter reduce the input high voltage (300V) to a more easily operated, pre-regulated 200V DC voltage and supply push-pull DC MOSFET2, MOSFET3 and T1 of the transformer. The main output of the DC converter is connected to the buck converter to form a closed-loop control with a constant output, so the other auxiliary outputs are approximately unchanged.

　　In push-pull operation, the field effect tube of the DC transformer has to withstand at least twice the overvoltage of the pre-regulated power adapter voltage. In a buck converter, the converter field is greatly reduced by reducing the voltage and removing input changes. The overvoltage that the effect tube bears and improves the reliability.

　　The 5V output voltage forms a closed control loop through the amplifier A1 and the optocoupler OC1, which has a very good regulating effect. Therefore, the buck converter maintains the DC power adapter added to the DC transformer at a level that makes the 5V output voltage constant. Since the input of the converter part is almost completely isolated from the AC power adapter, the overvoltage suffered by the converter's switching elements is much smaller, which will reduce the output ripple and improve reliability.

　　Furthermore, due to the closed-loop control, the voltage per turn of the DC transformer will remain constant (for the first order), while the other outputs of the same transformer will be semi-stable.

　　For the instantaneous input voltage, the larger input capacitor C1 plays a natural filtering role, and the buck converter and filters L1 and C2 have good noise immunity. The small low-pass filter at the output further removes the output switching and rectifier recovery noise. Since the DC transformer works under the condition of full duty cycle (square wave), the rectified output is almost DC, and the output circuit only needs a small high-frequency filter. This is particularly economical when a large amount of output is required.

　　In some applications, the converter's operating frequency will be synchronized with the buck converter to prevent low-frequency intermodulation components that will generate additional output ripple.

　　The use of power FETs in the buck converter can make it work at high frequencies without excessive switching losses. That is, in each half cycle of the converter, the buck converter can provide several power pulses to reduce the inter-modulation ripple.

　　If bipolar switching elements are used in a buck converter, the buck drive combination will experience low-frequency instability at light loads. This is due to the direct positive feedback of the DC transformer modulating the storage time of the bipolar transistor. Because it is outside the normal control loop, this effect is difficult to compensate. The negligible storage time of the field effect switch in the buck converter eliminates this problem.

　　It should be noted that the DC transformer is voltage feedback, and the capacitor C2 is large enough to maintain the voltage almost unchanged throughout the cycle. This provides a low-impedance ripple-free pre-adjusted DC input for the DC transformer, and can perform secondary-side duty cycle control when additional secondary-side adjustment is required. The influence of cross regulation is also reduced. Without additional secondary side adjustment, the auxiliary output adjustment can reach ±5% when the 5V output is closed-loop, or the auxiliary output adjustment can reach ±2% when the high-voltage, low-current output is closed-loop.

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