With the continuous miniaturization of electronic equipment, especially computers, the volume of power supply is required to be miniaturized. Therefore, the switching power supply has begun to replace the linear regulated power supply characterized by heavy power frequency transformer, and the power supply efficiency has been significantly improved. The reduction of the power supply volume means the deterioration of the heat dissipation capacity, so the power consumption of the power supply is required to be small, that is, the efficiency must be improved on the premise of the constant output power.

The power dissipation of the power supply with the same volume is basically the same. Therefore, in order to obtain greater output power, the efficiency must be improved. Meanwhile, high power supply efficiency can effectively reduce the stress of the power semiconductor device and is conducive to improving its reliability.

Progress of power semiconductor devices: the basis of high efficiency power conversion

The progress of power semiconductor devices, especially the progress of powermosfet, leads to a series of progress in power conversion: the extremely fast switching speed of powermosfet increases the switching frequency of switching power supply from 20kHz of bipolar transistor to more than 100kHz, effectively reducing the volume of passive energy storage elements (inductors and capacitors). The low-voltage powermosfet makes the low-voltage synchronous rectification a reality. The on voltage of the device is reduced from about 0.5V of the Schottky diode to 0.1V of the synchronous rectifier or even lower, which improves the efficiency of the low-voltage rectifier by at least 10%. The improvement of the on voltage drop and switching characteristics of the high-voltage powermosfet improves the primary efficiency of the switching power supply. The reduction of the power consumption of the power semiconductor device also reduces the volume of the heat sink and the whole machine.

There is an unwritten viewpoint in the power supply field: the efficiency of non regulated voltage is higher than that of regulated voltage, the efficiency of non isolated voltage is higher than that of isolated voltage, and the efficiency of narrow range input voltage is higher than that of wide range input voltage. VICOR's 48V input power module achieves 97% efficiency. AC input switching power supply needs power factor correction. Since power factor correction has the function of voltage stabilization, in applications with low requirements for output ripple (such as the output is connected with a battery or a super capacitor), the circuit topology of power factor correction and unregulated isolation converter can be adopted. In 1986, there were products abroad, with an efficiency of more than 93%.

Among the power modules with DC48V input voltage, the modules with efficiency above 93% almost invariably adopt the scheme of front stage voltage stabilization and rear stage non regulating isolation, and eliminate the output capacitance of the first stage and the output inductance of the second stage, simplifying the circuit structure.

Many domestic switching power supplies pay relatively little attention to the structural design. Sometimes, the temperature rise of each part of the power supply is uneven, some places are overheated, some places have almost no temperature rise, and even large losses are generated on the PCB. A good switching power supply should be the components that generate heat evenly distributed in the PCB

In addition, the temperature rise of the heating element is basically the same, and the PCB should have as little loss as possible, which is particularly important in the design of the module power supply and the adapter of the plastic shell.

While the efficiency is improved, the electromagnetic interference of the power supply is reduced

Among the various losses of switching power supply, the loss caused by electromagnetic interference will not be ignored when the power supply efficiency reaches a certain level. On the one hand, the electromagnetic interference itself consumes energy, especially the improvement of power efficiency often requires soft switching technology or zero voltage switching or zero current switching technology (whether it is specially set or inherent in the circuit itself). The application of these technologies slows down the change rate of voltage and current in the switching process or eliminates the switching process, and the electromagnetic interference becomes very small, There is no need to set a circuit for suppressing electromagnetic interference (this circuit is lossy) as in the conventional switching power supply circuit.

Switching power supply enters the era of high efficiency power conversion

After careful analysis, it seems that high-efficiency power conversion is very simple. Even some circuit topologies were introduced more than 20 years ago (for example, the two-stage conversion topology was introduced in an19 of applicationnote in the data manual of unitrode 82 / 83, and this power conversion topology was also used in tek2235 oscilloscope). However, due to the technical level at that time, In particular, people's recognition of the limitations (always think that the efficiency of the two-stage conversion is lower than that of the single-stage conversion. In fact, the two-stage conversion can realize the inherent zero voltage switch in fact, and the single-stage conversion requires special additional circuits and control methods) has not been recognized and applied. With the improvement of device performance and people's understanding, two-stage conversion has become one of the main methods of high-efficiency power conversion.

epilogue

Nowadays, for switching power supply design engineers and manufacturers, advanced power semiconductor devices can be easily obtained, and advanced circuit topologies and control methods have been applied. All they have to do is to find ways to improve their technical level and create better application opportunities and market shares.