70s of the 20th century and the early 20kHzPWM type switching power supply applications in the world caused the so-called "20kHz power technology revolution. Inverter power conversion method can be divided into the power frequency conversion and high frequency conversion. Power frequency conversion using discrete devices or manifold 50Hz square wave signal, and then to push the power switch, the signal-frequency step-up transformer 220V AC. This inverter power structure is simple, reliable, but due to the defects of the circuit structure itself, does not fit in with inductive loads, such as refrigerators, fans, pumps, fluorescent. In addition, this power inverter frequency transformer, thus bulky, heavy, high prices. Mainly used in large-scale solar photovoltaic power plants.
this to transform thinking at that time are used in power inverter system as soon as power devices are expensive, and a big loss, the study of the high-frequency high-power inverter has remained stagnant. 1980s, with the improvement of the quality of the power MOSFET technology matures and magnetic materials, high frequency conversion power inverter to market.
high frequency conversion power inverter by high-frequency DC / DC conversion technology, the first low-voltage DC into high frequency low voltage AC and high voltage DC rectifier and then a pulse transformer step. DC / DC converter using PWM technology, which can be obtained in this stable DC voltage, the voltage can directly drive the load of AC energy-saving lamps, incandescent lamps, TV. Class sine transform or sine transform to the high-voltage DC 220V, 50Hz Class sine wave AC or 220V, 50Hz sine wave AC. This inverter using high-frequency transform (now more 20kHz ~~ 200kHz), and therefore sizes small, light weight, and then the use of secondary width modulated and a secondary voltage regulator techniques, and thus the output voltage is very stable, and load capacity high performance and low cost, is the product of choice for renewable energy generation systems. AC photovoltaic system in the developed countries, small and medium-sized general use, but in the country, due to technical reasons and confusion in the market, some of the power inverter manufacturers has been to promote industrial frequency to transform power inverter, and some in order to reduce costs even silicon steel sheet, inverter power with low silicon flooding the market, making the AC photovoltaic system costs increased, will hinder the promotion of the exchange of PV systems for the development of the industry is very unfavorable.
domestic small and medium-sized power inverter power high frequency conversion problems analysis
At present, the problem of high frequency transform small and medium-sized power inverter power reliability is not high. Our many years of research, production and use Description: The main factors affect the high-frequency transform small power inverter power life electrolytic capacitors, optocouplers,Grid tie inverter, and magnetic materials.
Practice has proved that: the pursuit of longer life expectancy from the design aspects, rather than relying on the consumer. Measures to reduce the junction temperature of the device to reduce the device's electrical stress, lower operating current and high-quality magnetic materials can greatly improve its reliability. Domestic reason why the reliability of the high frequency conversion power inverter doubt, an important reason is that in order to reduce costs, some manufacturers still use the first generation of magnetic material developed in the 1970s, as TDK's H35, FDK H45 due to lower the saturation magnetic flux density of this magnetic material and the Curie point, and thus a long time when power is larger easily fault. We use the third-generation of magnetic material developed in the late 1980s, such as TDK's H7C4 FDK H63B and H45C, Siemens N47 and N67, not only to improve the conversion efficiency (3), but also improves the power inverter reliable sex. In fact, the TV and computer use in switching power supplies also proved the reliability of the high-frequency transform the way. The prolonged use of the user also prove the production of high-frequency transform small and medium-sized power inverter power with high reliability and efficiency, fully comparable with MASTERVOLT company products.
To improve the efficiency of the power inverter, it must reduce its loss. Inverter power loss usually can be divided into two categories: conduction losses and switching losses. The conduction loss is due to conduction device has a certain resistance Rds, so when the current flows will generate a certain power, the power loss Pc is calculated by the following formula: Pc = I2 × Rds. Device turn-on and turn-off process, the device is not only flows through the larger current, but also withstand high voltage, the device will also have a greater loss, this loss is called switching losses. Switching losses can be divided into the opening loss, turn-off losses and capacitor discharge loss.
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opening loss:
Pon = (1/2) × Ip × Vp × ts × f;
turn-off losses:
Poff = 1/2 × Ip × Vp × ts × f;
capacitor discharge loss:
Pcd = (1/2) × Cds × Vc2 × f;
total switching loss:
, Grid tie inverter ; Pcf = Ip × Vp × ts × f + (1/2) × ; Cds × Vc2 × f.
: Ip maximum current flowing through the device switching process;
Vp device switching withstand voltage maximum;
ts for the opening of the off-time;
f is the operating frequency;
Cds power MOSFET drain-source parasitic capacitance.
modern power theory: To reduce these losses, we must implement zero voltage or zero current transition of power switches, the resonant-type transformation structure.
photovoltaic system with small and medium-sized power the Development Outlook Inverter
resonant switching power supply with the development, the idea of resonant converter is also used in the inverter power system constitutes a resonant high-efficiency power inverter. The inverter is a zero voltage or zero current switching technology is used in the DC / DC converter, and thus the switching loss largely can be eliminated, even when the switching frequency of more than more than 1MHz, the power efficiency will not be significantly reduced. Experimental results show: reduced by 30% to 40% in the case of the same working frequency, the loss of the resonance type transform comparable non-resonance type transform,power inverter. Currently, the operating frequency of the resonant power of up to 500kHz to 1MHz.
Also worth noting is that the PV system with small and medium-sized power inverter power source is moving modular direction, that is, using different combinations of modules, could constitute a different voltage waveform conversion system .
, power inverter ; There is no doubt that the PV system with a small power inverter power will using high-frequency transform circuit structure. Some technical details, also different from the the power inverter other occasions, such as, in addition to the pursuit of high reliability, high efficiency, should also characteristics of the photovoltaic industry, controls, inverters effectively combined, ie PV inverter power supply should be designed with over-voltage, under voltage, short circuit, overheating, polarity reversed protection. Doing so not only reduces the cost of the system, and improve the reliability of the system.
With the continues specification of PV systems, high-frequency transform small and medium-sized power inverter power will be gradually recognized by the market, and its use will promote the benign development of the photovoltaic industry.
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