Switching power supply pursues high efficiency power

The continuous miniaturization of electronic equipment, especially computers, requires the size of power supplies to be miniaturized accordingly. Therefore, switching power supplies have begun to replace linear voltage-regulated power supplies characterized by bulky power frequency transformers, and the power efficiency has been significantly improved. The reduction in the size of the power supply means the deterioration of the heat dissipation capacity, so the power consumption of the power supply is required to be reduced, that is, under the premise of unchanged output power, the efficiency must be improved.

High-efficiency power conversion: the goal of switching power supply design

The power dissipation of power supplies of the same volume is basically the same. Therefore, in order to obtain greater output power, the efficiency must be improved. At the same time, high power supply efficiency can effectively reduce the stress of power semiconductor devices, which is beneficial to improving their reliability.

The losses of switching power supplies are mainly: passive component losses and active component losses

Switching losses have always puzzled switching power supply designers. Since power semiconductor devices have current and voltage on them simultaneously during the switching process, switching losses are inevitable. If the switching tube and output rectifier diode in the switching power supply can achieve zero voltage switching or zero current switching, its efficiency can be significantly improved.

The switching loss caused by the switching process accounts for about 5% to 10% of the total input power. Significantly reducing or eliminating this loss can increase the efficiency of the switching power supply by 5% to 10%. The most effective method is soft switching technology or zero voltage switching or zero current switching technology.

Among the many soft switching schemes, the more practical one is the high-power full-bridge converter, which usually adopts the phase-shifted zero-voltage switching control method. This control method requires the addition of a freewheeling inductor on the primary side to ensure that the switch tube is turned on under zero voltage state. Due to the large effective current flowing through, this additional inductor will generate heat (although much smaller than the RC buffer circuit), so it is not used in low-voltage power conversion.

The passive lossless snubber circuit is characterized by not destroying the conventional PWM control mode and is simple to design/debug. Nevertheless, the passive lossless snubber circuit and quasi-resonant/zero voltage switching operation mode also have some disadvantages, such as only being able to achieve soft switching at turn-off and not being suitable for large load range changes in flyback converters. Active clamping in soft switching is an effective method to improve the efficiency of single-tube forward/flyback converters. The initial patent restrictions have now expired and can be widely used.

Advances in power semiconductor devices: the foundation for high-efficiency power conversion

The progress of power semiconductor devices, especially PowerMOSFET, has led to a series of progress in power conversion: the extremely fast switching speed of PowerMOSFET has increased the switching frequency of the switching power supply from 20kHz of bipolar transistors to more than 100kHz, effectively reducing the size of passive energy storage components (inductors, capacitors). Low-voltage PowerMOSFET makes low-voltage synchronous rectification a reality. The on-voltage of the device is reduced from about 0.5V of Schottky diodes to 0.1V or even lower of synchronous rectifiers, which increases the efficiency of low-voltage rectifiers by at least 10%. The improvement of the on-voltage drop and switching characteristics of high-voltage PowerMOSFET has improved the primary efficiency of the switching power supply. The reduction in power consumption of power semiconductor devices also reduces the size of the heat sink and the entire machine.

There is an unwritten view in the power supply industry: unregulated voltage is more efficient than regulated voltage, non-isolated voltage is more efficient than isolated voltage, and narrow-range input voltage is more efficient than wide-range input. Vicor's 48V input power module has an efficiency of 97%. AC input switching power supplies require power factor correction. Since power factor correction already has a voltage stabilization function, in applications where output ripple requirements are not high (such as output connected to a battery or supercapacitor), power factor correction plus unregulated isolated converter circuit topology can be used. Foreign products have been available since 1986, with an efficiency of more than 93%.

In the power supply modules with DC48V input voltage, the modules with efficiency above 93% almost all adopt the solution of front-stage voltage regulation and back-stage non-regulation and isolation, and eliminate the output capacitor of the first stage and the output inductor of the second stage, simplifying the circuit structure.

Many domestic switching power supplies pay relatively little attention to structural design, and sometimes the temperature rise of various parts in the power supply is uneven, some parts are overheated, and some parts have almost no temperature rise, and even large losses are generated on the PCB. A good switching power supply should have heat-generating components evenly distributed on the PCB, and the temperature rise of the heating components is basically the same. The PCB should have as little loss as possible, which is especially important in the design of module power supplies and plastic shell adapters.

While improving efficiency: electromagnetic interference of the power supply is reduced

Among the various losses of switching power supplies, the losses caused by electromagnetic interference cannot be ignored after the power supply efficiency reaches a certain level. On the one hand, electromagnetic interference itself consumes energy, especially the improvement of power supply 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 rate of change 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 up a special circuit to suppress electromagnetic interference like in conventional switching power supply circuits (this circuit has losses).

Switching power supply enters the era of high-efficiency power conversion

A careful analysis shows that high-efficiency power conversion seems to be very simple, and some circuit topologies were even introduced more than 20 years ago (such as the two-stage conversion topology, which was introduced as early as AN19 of the Application Note of the UNITRODE82/83 data sheet, and the TEK2235 oscilloscope also uses this power conversion topology), but it was not recognized and applied due to the technical level at the time, especially the limitations of people's understanding (it was always believed that the efficiency of two-stage conversion was lower than that of single-stage, but in fact two-stage conversion can achieve the inherent zero voltage switch in fact, and single-stage conversion requires special additional circuits and control methods). The performance of devices and the improvement of people's understanding have made two-stage conversion one of the main ways of high-efficiency power conversion.

Conclusion

Today, for switching power supply design engineers and manufacturers, advanced power semiconductor devices are easily available, and advanced circuit topologies and control methods have begun to be applied. All that remains for them is to find ways to improve their technical level and create better application opportunities and market share.