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Most of the switching power supplies currently in use, whether self-excited or excited, are circuits that are controlled by a PWM system. In this type of switching power supply, the switching transistor is always on/off periodically, and the PWM system only changes the pulse width of each cycle. PWM system control is continuous control. The non-periodic switching power supply is different, and the pulse control process is nonlinear and continuously changing, and there are only two states: when the output voltage of the switching power supply exceeds the rated value, the pulse controller outputs a low level, and the switching tube is turned off; when the switching power supply output voltage Below the rated value, the pulse controller outputs a high level and the switch is turned on. When the load current decreases, the discharge time of the filter capacitor is prolonged, the output voltage does not decrease at a constant speed, and the switch tube is in an off state until the output voltage drops below the rated value, and the switch tube is turned on again. The cut-off time of the switch depends on the magnitude of the load current. The on/off of the switching transistor is controlled by the level switch sampling from the output voltage, so this non-periodic switching power supply is extremely suitable for supplying power to intermittent or variable loads.
The initial non-periodic switching power supply adopts its excitation circuit structure, and the operational amplifier constitutes a voltage comparator, which turns the output sampling voltage into a control level and controls the output pulse of the excitation oscillator. When the output voltage maintains the rated voltage, the comparator outputs a high level, and the oscillator turns off the output pulse to turn off the switch. When the output voltage decreases, the comparator outputs a low level, and the oscillator outputs a pulse to turn the switch on. After the non-periodic switching power supply enters the household appliance, in order to simplify the circuit, most of the self-excited oscillation mode is adopted, and the voltage regulator tube is directly used as the level switch. Because its control process is the time ratio of the oscillating state and the suppressed state (or blocking state), it is called an oscillating suppression converter (RINGING CHOKECONVERTER, RCC type switching regulator for short). The obvious difference in the circuit is that the PWM switching power supply consists of a separate sampling error amplifier and a DC amplifier to form a pulse width modulation system; the RCC type power supply only consists of a voltage regulator to form a level switch to control the on/off of the switching tube.
The flyback self-excited converter is the RCC (Ringing Choke Converter) circuit that we usually refer to. The working mode of the transformer (storage inductor) is in a critical continuous state, which can easily realize current-mode control. It is a single-pole system in structure. It is easy to get a fast and stable response and is widely used in switching power supplies below 50W. Since the critical continuous mode is maintained and the primary current rise of the transformer is affected by the input voltage, the switching operating frequency is affected by the input voltage and the output current, and the duty cycle is also affected by the input voltage. The operating frequency is highest at the highest input voltage and no load. It is also because the operating frequency fluctuates greatly, and the design of the filter circuit is correspondingly difficult.
Compared to its shortcomings, the advantages of RCC current are also outstanding. The first is that the circuit structure is simple, only a small number of separate originals are required to obtain the voltage output performance that can be realized by a dedicated chip, and an efficient and reliable work can be obtained through good design. Second, many drive-related difficulties (drive waveforms, transformer saturation, etc.) are well solved in self-excited converters. Moreover, since it always works in the full energy transfer mode, the secondary rectifier diode is conducting current to zero, the reverse recovery current and loss are small, and the resulting ringing is much smaller than the incomplete energy transfer mode, so the output The high frequency noise is also much smaller. In addition, the primary side supervisor is always zero current, so the efficiency is high. Early RCC converters were only suitable for switching power supplies with low power below 100W. In recent years, with the deepening of research, the improved RCC circuit solves many difficult problems such as cross conduction and transformer saturation, which is cheap, efficient and reliable.
Performance is highly appreciated by people. Its working form is a full energy transfer type, which is easy to use with current. It is a single-pole system in structure and is easy to get a fast and stable response. In order to reduce the switching losses of conventional RCC converters, improve efficiency, and increase the range of input voltage, the improved RCC circuit incorporates constant current excitation and delay conduction circuits. Due to the addition of constant current excitation and delayed conduction circuit, the oscillation analysis is somewhat different from that of the traditional RCC converter. Although its circuit is more complicated, its performance is greatly improved, and it can work normally in the range of DC127V-DC396V, providing 250W. The above power, its cost performance has been greatly improved.
Based on the above characteristics, RCC circuits are widely used in low-cost and high-performance power supply equipment, such as low-voltage small-power modules, home appliances, and mobile phone chargers.
The accessories are part of the design circuit, detailed circuit analysis and more information can be found in the PDF document!
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