What Is a Chopper Amplifier?

A chopper circuit (also known as a DC chopper circuit) refers to the use of electrical power to "chop off" a portion of a sine wave for certain needs. (For example, when the voltage is 50V, the next 50 ~ 0V is partially cut off, and the output voltage is 0.) Later, it was borrowed from the DC-DC switching power supply, mainly during the voltage regulation of the switching power supply. The original linear power supply was "cut" by the line into pulses. Change direct current to another fixed or adjustable voltage direct current. Also known as DC-DC converter (DC / DC Converter).

A DC chopper circuit is a power electronic converter that converts a constant voltage DC current into a voltage adjustable DC power. It is also known as a DC chopper or

DC / DC converter. The basic idea of using a chopper to achieve DC conversion is to cut a constant DC voltage or current into a series of pulsed voltages or currents through fast on and off control of power electronic switching devices. Under certain filtering conditions, under load The average voltage or current that can be obtained can be less than or greater than the power supply. If the operating frequency of the switching device is turned on or off, or the proportion of the switching device is turned on or off, the pulse width of this pulse sequence can be changed to achieve the adjustment of the average output voltage and current

Currently, choppers are widely used in electric traction. For example, stepless speed regulation of DC motors such as subways, electric locomotives, trolleybuses and battery trucks. Compared with the traditional method of regulating the resistance in series in the circuit, it not only has better starting,

As early as 1940, the Germans used the idea of mechanical switches to adjust the DC voltage to control the speed of DC motors. In 1960, Americans used transistor choppers to control the excitation system of diesel generators. The device is used to control the battery car. In the early days, it was mainly used in DC traction speed control systems for urban trams, subways, and electric vehicles. With the continuous development of self-shutdown power electronic switching devices and PulseWidth Modulation (PWM) technology, DC choppers have significant advantages such as high efficiency, small size, light weight, and low cost. They are widely used in switching power supplies. New technology fields such as source power factor correction and superconducting energy storage. Generally speaking, DC chopper circuits have two different application areas: One type of load requires that the output voltage can be adjusted and controlled within a certain range, that is, the circuit is required to output a variable DC voltage, such as a DC motor load, in order to change its speed , Requires a variable DC voltage power supply; the other type of load requires that the output voltage of the circuit can be maintained constant regardless of the power supply voltage change or load change, that is, output a constant DC voltage, such as switching power supplies. These two different requirements can be fulfilled by a certain type of control system based on the principle of feedback control. ^[2]

6 basic chopper circuits: buck chopper circuit, boost chopper circuit, buck-boost chopper circuit,

There are many types of DC chopper circuits. According to their circuit structure and function classification, there are four basic types: buck chopper circuit, boost chopper circuit, and buck-boost. ) Chopper circuit and Cuk chopper circuit. The first two are the most basic circuits, and the last two are the combination of the first two basic circuits. Sepic chopper circuit and Zeta chopper circuit derived from basic chopper circuit are also more typical circuits. The combination of the basic chopper circuit can also constitute a composite chopper circuit and a multi-phase multiple chopper circuit.

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Today's soft switching technology has made a qualitative leap in DC / DC. A variety of ECI soft switching DC / DC converters designed and manufactured by American VICOR Corporation have a maximum output power of 300W, 600W, 800W, etc., and the corresponding power density is (6 , 2, 10, 17) W / cm ^ 3, the efficiency is (80-90)%. Japan's NemicLambda company recently introduced a high-frequency switching power supply module RM series using soft switching technology. Its switching frequency is (200-300) kHz, the power density has reached 27W / cm ^ 3, and a synchronous rectifier (MOS-FET instead) is used. Schottky diode), which improves the overall efficiency of the circuit to 90%. ^[1]

(PFC) Chopper circuit active power factor correction (PFC)

In principle, the active power factor correction can use any type of DC chopper circuit topology, such as Buck, Boost, Sepic, and Cuk. The following uses Boost circuit as an example to introduce the working principle of active power factor correction (PFC) using peak current control method. The main circuit is composed of a single-phase bridge rectifier and a Boos chopper circuit. The PWM control circuit in the dashed box includes: voltage error amplifier, current error amplifier, multiplier, PWM generator and driver, etc. The load can be resistance or resistance. Or a switching power supply. ^[5]

The working principle of PFC is as follows: After a given reference voltage Uref is compared with the output voltage Uo transformed by the detection circuit, it is input to the voltage error amplifier; the detection value of the rectified voltage ud and the output signal of the voltage error amplifier are added to the input of the multiplier At the end, the output of the multiplier is used as a reference signal for current feedback control. After comparing with the input current detection value, a PWM signal is generated, which is amplified and isolated to provide a gate drive signal for the IGBT to control the on and off of the switching device T, so that The waveform of the input current (ie, the inductor current) iL is basically the same as the waveform of the rectified voltage ud, thereby improving the power factor at the input.

DC voltage and speed regulation of chopper circuit

In most small and medium-capacity DC speed control systems, it is common to adjust the armature voltage of a DC motor to achieve speed regulation. There are two commonly used speed regulation schemes: one is to use a controllable rectifier circuit (such as a thyristor rectifier circuit) to obtain an adjustable DC voltage for the motor; the other is to use a non-controllable rectifier circuit to obtain a constant DC voltage and then use DC Chopper mode is used to adjust the voltage.