5. Electromagnetic compatibility study of high frequency switching power supply
The electromagnetic compatibility problem of high frequency switching power supplies is special. Usually, it
involves di/dt and dv/dt generated by the switching process, causing strong conducted electromagnetic interference and
harmonic interference. In some cases, it also causes strong electromagnetic field radiation. It not only seriously pollutes
the surrounding electromagnetic environment, but also causes electromagnetic interference to nearby electrical equipment,
and may also endanger the safety of nearby operators. At the same time, the control circuit inside the switching power supply
must also withstand the electromagnetic interference of the main circuit and industrial applications. Due to the above
specificities and specific difficulties in measurement, research work on electromagnetic compatibility of switching power
supplies is still in its infancy. Obviously, in the field of electromagnetic compatibility, there are many cutting-edge topics in
cross-science that need to be studied. Such as: typical field and system near field, conducted interference and radiation
interference modeling; printed circuit board and switching power supply EMC optimization design software; low intermediate
frequency, super audio and high frequency strong magnetic field on human health; high power switching power supply EMC
measurement Methodological research, etc.
6. Switching power supply design, testing technology
Modeling, simulation, and CAD are new, convenient, and cost-effective design tools. In order to simulate a switching
power supply, simulation modeling is first required. The simulation model should include power electronics, converter circuits,
digital and analog control circuits, as well as magnetic components and magnetic field distribution models, circuit distribution
parameter models, etc., as well as the thermal model, reliability model and EMC modeling of the switch. The various models
vary widely, so the direction of modeling should be: digital-analog hybrid modeling; hybrid hierarchical modeling; and the
formation of a unified multi-level model of various models (similar to a circuit model, with a block diagram, etc. ); automatically
generate models to enable simulation software with automatic modeling to save user time. Based on this, a model library can
be built.The switching power supply CAD, including main circuit and control circuit design, device selection, parameter
optimization, magnetic design, thermal design, EMI design and printed circuit board design, reliability estimation, computer-
aided synthesis and optimization design. The simulation-based expert system for the switching power supply CAD can
optimize the designed system performance, reduce the design and manufacturing costs, and can do manufacturability
analysis. It is one of the development directions of 21st century simulation and CAD technology. Expert systems for
designing DC-DC switching converters and MATSPICE software for simulation have been developed abroad.
In addition, the development, research and application of thermal testing, EMI testing, and reliability testing of
switching power supplies should be vigorously developed.
7. Low-voltage, high-current switching power supply development
(1) Requirements for low-voltage, high-current switching converters
The speed and efficiency of data processing systems are increasing. The logic voltage of the new generation
microprocessor is as low as 1.1~1.8V, and the current is 50~100A. The power supply is low voltage, high current output
DC-DC converter module. Also known as the voltage regulator module (VRM). The requirements of the new generation
of microprocessors for VRM are: low output voltage, large output current, high current rate of change, fast response, etc.
1 In order to reduce the electric field strength and power consumption of the IC, the microprocessor supply
voltage must be reduced, so the output voltage of the VRM should be reduced from the conventional 3V to less than
2V or even 1V.
2 When running, the power input current is >100A. Due to parasitic L and C parameters, the voltage disturbance
is large, and L should be minimized.
3 The microprocessor starts and stops frequently, continuously starts from the sleep state, works, and then goes to
sleep state. Therefore, the VRM current is required to be abruptly changed from 0 to 50A, and then dropped to 0, and the
current change rate is 5 A/ns.
4 The design should control the disturbance voltage ≤10%, and allow the output voltage to vary by ±2%.
(2) Using waveform interleaving technology
The parasitic impedance of the line, the ESR of the capacitor, and the ESL have a large effect on the voltage
regulation of the VRM during load changes. High frequency, high power density and fast new VRM must be developed.
A variety of topologies have emerged, such as: synchronous rectification Buck converter (using a power MOS transistor
instead of a switching diode); to prevent output voltage disturbances due to high frequency parasitic parameters when the
current is greatly changed, there are literatures that use multiple input channels or The multiphase DC-DC converter,
called the Interleaving technique, is used to ensure that the VRM output ripple is small, the output transient response is
improved, and the output filter inductance and capacitance are reduced.
(3) Voltage ripple and impulse voltage problems
1 voltage ripple and ESR. For a load with a voltage below 1V and a current of 100A or more, the load resistance
is 10mΩ or less, which is lower than the internal equivalent series resistance of the filter capacitor, and a voltage ripple
problem occurs. Now, suppose that this power supply can be realized by a buck-boost or boost converter, but the ripple
current flowing through the capacitor is above 100A, and the efficiency is less than 50%. In this regard, the buck converter
includes a series filter inductor to suppress ripple current. However, the load resistance is equivalent to ESR, and the ripple
current flows through the capacitor and the load, respectively, and its operation mode is different from the current filter circuit.
In order to explore the ripple voltage operation mode, the equivalent circuit is first simulated. In the simulation, according to
the value of Crc, there are four operating modes of ripple voltage. There are four modes of operation for the relationship
between the voltage ripple value and rc/R. The larger the C, the smaller the ripple rate. In order to further reduce the
low-voltage and high-current output voltage ripple, that is, to reduce the ESR value of the filter capacitor, certain methods
and strategies must be adopted.
2 Shock voltage caused by load mutation. For the load of digital circuits, in order to respond quickly to various
modes of conversion, it is very important that the output voltage corresponds to the transient response characteristics of
the load change. At this time, if the rate of change of the current is large and the impact generation time is shorter than the
switching period Ts, it is difficult to expect an output voltage stabilization effect by feedback. At present, there is no way for
technology, and it is in the stage of simulation research.
(4) Exploring the possibility of omitting the filter capacitor
If the output voltage fluctuates due to sudden load changes and the fluctuation duration exceeds the switching period,
the feedback can be adjusted to some extent. The LC filter circuit plays a decisive role in this voltage adjustment effect. In
order to achieve the purpose of voltage regulation, it is necessary to increase the switching frequency, reduce the L and C
values, and extend the cutoff frequency as far as possible to the high end. Some people consider using two asymmetric
inverters (with transformers) to output two-phase square waves. The output voltage of each inverter is connected to a common
load through half-wave rectification, and the cutoff frequency is extended to the high end.
The switching frequency is determined by the switching time of the MOSFET. In order to improve the switching
efficiency and exceed its limit value, a multi-phase switching method can be used in practice to increase the switching
frequency equivalently. However, there are limits to the number of phases. In addition, the reason for the change is only
on the load side, and it is very effective to keep the cutoff frequency as low as possible. To achieve this, the use of electrical
double-layer capacitor filters may be the future direction. Of course, it is necessary to consider how to simultaneously
reduce the equivalent series resistance and equivalent series inductance of the double layer capacitor.
(5) Portable equipment and fuel cells
For portable appliances such as laptops, cell phones, and digital cameras, power is the most problematic part.
The power of portable devices has always been the world of traditional batteries, and the traditional batteries can not fully
meet the requirements of users in terms of light weight and long-term use. For this reason, fuel cells composed of solid
polymer materials have recently attracted attention. The fuel cell uses methanol as a fuel and platinum as a catalyst.
The structure is an electrolyte membrane sandwiched between electrodes, and the energy density can be 10 times that of
a lithium battery. The operating temperature below 100 °C includes power generation at room temperature, and the
single-cell voltage is about 1~2V. Hydrogen is the most desirable fuel, but from a practical point of view, it is convenient to
use a combination of methanol and platinum catalyst. However, it has problems with the followability of the load change,
so in order to protect the electrode, it needs to be used in combination with the capacitor.
The fuel cell has the advantage of being easy to maintain and can be used for a long time. When the power is
insufficient, only the fuel can be replenished, and it is not necessary to charge for a long time.Above, the low-voltage,
high-current switching power supply is the center, and the future development direction of the switching power supply is
discussed. According to Moore's Law, IC integration will increase by a factor of two every 18 months, so it is difficult to
determine how much the voltage will be reduced. If this trend continues unrestricted, it can be expected that the requirements
for power supply will be higher and higher. To meet these requirements, the first is to develop new semiconductors and
capacitors. In addition, building a micro-structure model of components from a circuit perspective may also become a key
point to solve the problem. Therefore, the need to break the boundaries of disciplines for collaborative research at various
levels will become more and more important in the future.
8. Low voltage, high current DC-DC converter module
According to IEEE Spectrum, the large-scale integrated circuits used in data processors in 2005 will have a
transistor density of 100 million/cm2 and a clock frequency of 1 GHz.In order to meet the needs of the next generation of fast
microprocessors, portable communication devices, servers, etc., it is necessary to develop VRMs with high current (50~100A),
low output voltage (less than 1V), and high current rate of change (5A/ns). . Research new topologies, apply high-performance
components, research new structures and packaging technologies, and make equivalent microprocessors and integrated
packages with VRM. Figure 2 shows an idea of a microprocessor integrated with a VRM.