PWM means pulse width modulation, and the PWM output waveform is a series of pulses with varying duty cycles.
The concept of modulation is mentioned here, that is, a square wave and a modulated signal are included in the PWM wave. The modulated signal can be obtained through demodulation (usually a low-pass filter or integrator).
For example, the PWM signal output by the inverter is a pulse width modulated wave with a square wave as the carrier wave and a sine wave as the modulation signal.
The PWM voltage signal is applied to the motor. Since the motor is an inductive load, the modulation wave (sine wave) that flows through the motor windings is mainly.
From the point of view of signal composition, PWM includes the fundamental wave (modulated sine wave) and harmonics. Because the carrier is a square wave, the harmonic content is very rich, and it has high frequency harmonics.
DC converter classificationNon-isolated DC converters can be divided into three types: single-tube, double-tube and four-tube according to the number of active power devices used. There are six types of single-tube DC converters, namely Buck converter, Boost converter, Buck/Bomt converter, Cuk converter, Zeta converter and Sepic converter器ç‰ã€‚ Among the six single-tube converters, buck and boost converters are the most basic, and the other four are derived from them. The double-tube DC converter has a double-tube buck-boost (Buck/tk) 0st) converter. Full-bddge converter (Full-bddge converter) is a commonly used four-tube DC converter.
Isolated DC converters can also be classified according to the number of active power devices used. There are two types of single tube: Forward and Flyback. There are four types of double transistors: Double transistor forward converter, Double transistor flyback converter, Push-pull convener and Half-bridge converter. The four-tube DC converter is a Full-bridge converter.
An isolated converter can achieve electrical isolation between input and output. Usually, a transformer is used to achieve isolation. The transformer itself has a function of voltage transformation, which is beneficial to expand the application range of the converter. The application of the transformer is also convenient to realize multiple outputs of different voltages or multiple same voltages.
A converter without isolation can be combined with a converter with isolation to obtain characteristics that a single converter does not possess.
According to energy transfer, there are two types of DC converters: unidirectional and bidirectional. The charger with two-way function charges the battery when the power supply is normal. Once the power supply is interrupted, it can return the battery power to the grid to provide emergency power to the grid for a short time. The converter for DC motor control is also bidirectional. The electric energy is transferred from the power supply to the motor when the motor is working, and the electric energy of the motor is fed back to the power supply when braking.
DC converters can also be divided into self-excited and other controlled types. The converter that realizes the self-sustained periodic switching of the switch tube by means of the positive feedback signal of the converter is called a self-excited converter, and the Royer converter is a typical push-pull self-excited converter. The control signal of the switching device in the other-controlled DC converter is generated by a special control circuit.
When the working state of the converter is constant, the loss of switching on or off once is also constant. Therefore, the higher the switching frequency, the greater the switching loss. At the same time, the oscillation of the distributed inductance and parasitic capacitance of the circuit will be aroused during the switching process, which will bring additional loss, so the switching frequency of the hard-switching DC converter cannot be too high. During the turn-on or turn-off process of the soft-switching DC transformer, the voltage applied to it is zero, that is, zero-voltage-switch-ing (ZvS), or the current through the device is zero , That is, zero-current switching (Zero-current.switching, ZCS).
Related links: Briefly describe the role of pwm converters
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