source:power adapter Popular:adapter release time:2021-04-25 10:32:40 Article author:sznbone
Since the 1980s, in the field of power adapters, with the development of insulated gate field effect transistors (MOSFET tubes, or just FET tubes), bipolar high-power transistors have been gradually replaced by FET tubes. In future designs, more FET devices will be used.
However, there are still some occasions where bipolar transistors will continue to be used (such as linear rectifiers and low-power applications). Because of its relatively low price, it still shows advantages when applied to linear rectifiers. Since bipolar transistors still show advantages in a few occasions, and most power adapters in some applications initially use bipolar transistors, for a designer, they should be familiar with their main characteristics and understand the early days. The design method is very important to solve possible failures.
To design a power adapter using bipolar transistors, first of all, the voltage and current ratings of the device must be reasonably selected. The maximum voltage and current stress of the device depends on the selected topology, input voltage and its fluctuation range and output power. This chapter gives the calculation formulas and derivation process of these stresses when analyzing each topology.
For the overall reliability of power devices, the design method of the bipolar transistor base drive circuit and the selection of transistor voltage and current ratings are equally important. The general rules and typical design schemes of base drive circuit design will be discussed below.
Supplement: FET is a voltage-driven device. On the rising and falling edges of the drive pulse, current flows into and out of the gate respectively. When driving high-frequency and high-power devices, the transient drive current may be very large, and the gate voltage in the on-state may also be very high, some devices are as high as 8V. Therefore, driving high-power FETs will be more difficult than imagined. Moreover, the FET is more reliable when it is completely turned on and off. When it is used in linear applications, because the switch tube is only partially turned on and the power loss is not ideal, the FET is suitable for switching circuits.
Compared with a MOSFET, a bipolar transistor is a current-driven device, and the base current changes in proportion to the collector current. This ratio is defined as the gain of the transistor. The driving voltage required by the base is very small, about 0.6V, and the characteristics of the base are similar to those of a diode. Bipolar transistors are mostly used in linear applications and are also used in other circuits. In the future, the demand for bipolar transistors will not decrease.
The main goal of the ideal base drive circuit of the Brazilian standard power adapter bipolar transistor
The base input current should be large enough so that when the gain of the transistor is the smallest, the transistor with the smallest B value is used at the maximum input current and minimum input voltage, and this current is used to drive the transistor, the collector-emitter voltage The saturation value will be about 0.5~3.0V.
It must be considered in the design that the B value of the transistor has a 4 times variation range. The current-voltage curve (LV) in the power adapter manufacturer's device manual (picture) usually corresponds to the typical or commonly used B value. It can be assumed that the minimum B value is half of the typical value given in the curve, and the maximum B value is twice the typical value.
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