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Switching Power Supply Principles and trends

Section frequency switching power supply circuit

High frequency switching power supply consists of the following components:

1, the main circuit From the AC power

input, DC output of the entire process, including:

1, input filter: its role is to network the existing clutter filtering, but also hindered the clutter generated by the unit back to the public grid.

2, rectification and filtering: The Power AC power directly to DC rectifier is smoother, for the next level of transformation.

3, inverter: will be rectified DC into high frequency alternating current, which is the core of high-frequency switching power supply, the higher the frequency, size, weight and the smaller the ratio of output power.

4, the output rectifier and filter: the load required to provide reliable DC power supply.

2, control circuit The one hand, sampling from the output, compared with the standard-setting, and then to control the inverter, to change their frequency or pulse width, to output stability, on the other hand, according to data provided by the test circuit, The protection circuit identification on the machine control circuit to provide various protective measures.

3, detection circuit Addition to providing protection circuit is run in a variety of parameters, but also provide a variety of data display instruments.

4, auxiliary power Provide all the different requirements of a single circuit power supply.

II switch control regulator principle Switch K to a certain time interval repeatedly connect and disconnect, the switch K connected, the input power source E through the switch K and the filter circuit provided to the load RL, the switch is in the period, power E to provide energy to the load; when the disconnect switch K, the input power cut off energy E will be provided. Can be seen, input power to provide energy to the load is intermittent, for the load to get the energy to provide continuous, switching power supply must have a storage device, the switch is part of the energy will be stored in the switch disconnected to the load release. In the figure, the inductor L, capacitor C2 and diode D circuit composed of, they have this feature. Inductor L to store energy when the switch off, stored in the inductor L in the energy released through the diode D to the load, the load by a continuous and stable energy, due to diode D so that the load current continuously, so as freewheeling diode. AB average in the voltage between the EAB can be expressed as:

EAB = TON / T * E Type of TON for the switch each time to connect, T to switch off the work of the cycle (that is, the time switch to turn on and off-time TOFF TON of and).

By the above equation, change the switch to turn on the proportion of time and duty cycle, AB average value between the voltage also will change, therefore, with the load and input supply voltage automatically adjusts the TON and the ratio of T the output voltage V0 can be maintained. TON, and connect time to change the duty cycle that is changing the pulse duty cycle ratio, which is known as "time ratio control" (TimeRatio Control, abbreviated as TRC).

By TRC control theory, there are three ways:

1, pulse width modulation (Pulse Width Modulation, abbreviated as PWM)

Constant switching cycle, by changing the pulse width to change the way the duty cycle.

2, pulse frequency modulation (Pulse Frequency Modulation, abbreviated as PFM)

Conduction pulse width of the constant switching frequency by changing the way to change the duty cycle.

3, mixed modulation Conduction pulse width and switching frequency are not fixed and can change each other's way, it is the mixture of these two ways.

III development and trends in switching power supply

1955 U.S. Royer (GH.Roger) invented the transistor self-oscillating push-pull single-transformer DC-DC converter, frequency conversion control circuit to achieve the beginning of 1957, the United States Charles race (JenSen) invented the self-excited push-pull pair of transformers, 1964, U.S. scientists have proposed to abolish the frequency transformer switching power supply series idea, which is the power to decline in volume and weight were a fundamental way. To 1969 because of pressure to improve high-power silicon transistors, diode reverse recovery time shortened components to improve, and finally made a 25 kHz switching power supply.

Present, switching power supply to small, light weight and high efficiency characteristics are widely used in a variety of computer-driven devices, communications equipment, etc. Almost all the electronic equipment is the rapid development of today's electronic information industry essential a Power Source. Currently on the market the use of switching power bipolar transistors made of 100kHz, with 500kHz made of MOS-FET power, Although practical, but its frequency needs to be improved. To improve the switching frequency, it is necessary to reduce switching losses, and to reduce switching losses, it needs high-speed switching components. However, the switching speed of the increase will be distributed by the circuit inductance and capacitance, or diode stored charge of the generated surge or noise. This will not only affect nearby electronic equipment, will greatly reduce the reliability of power supply itself. Which, with the switch to prevent Kai - closed by the voltage surge occurs, can use RC or LC buffer, while the diode stored charge due to current surge can be used such as magnetic core made of amorphous magnetic buffer . However, for more than 1MHz frequency, resonant circuit to be used to make the switch voltage or current through the switch was a sine wave, this can reduce the switching losses, but also to control the occurrence of surge. This switch is called resonant switch. Current switching power supply of this very active, because this approach does not require substantial increase in switching speed can be reduced theoretically to zero switching losses and noise are small, is expected to be of a high-frequency switching power supply main ways. At present, many of the world countries are committed to the converter a few trillion Hz practical research.

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