H-bridge level inverter PWM control design plan
Abstract: This article mainly explores the implementation of high-power high-voltage inverter H-bridge-level inverter, mainly studies on control policies implemented in the system, and implementation methods and performance for several control strategies. Analysis and comparison.
Keywords: h bridge, PWM;
1 Introduction
The H-bridge-level multi-level transformer uses a plurality of power units in series to implement high voltage output, which outputs more multi-electrode-powered phase PWM control, to achieve lower output voltage harmonics, smaller DV / DT and common mode voltage and smaller torque pulsation. In order to achieve high pressure, simply add the number of units, the technical difficulty of this type of implementation is small. Each power unit is a separate DC power source, which is independent of each other, and the control of one unit does not affect other units. The implementation of the H-bridge-level inverter and single bridge inverter is in the control mode of the PWM, and this paper discusses the PWM control method of the H-bridge level inverter.
2 h bridge-level inverter structure
Each power unit is an independent DC power supply, which is shown in Figure 1 below:
Figure 1 block diagram of the power unit structure
According to the description of the power unit according to the above figure, the power unit of such implementation can generate three levels, namely + UDC, 0, -UDC. When S1 and S4 are turned on, S2 and S3 are closed, the load obtains a voltage + UDC; when S2 and S3 are turned on, S1 and S4 are turned off, the load obtains voltage -UDC; when S1 and S3 (or S2 and S4) are turned on. When S2 and S4 (or S1 and S3) are closed, the load obtains voltage 0 (Note: In the process of control, strictly avoid the same bridge arm two power devices simultaneously, that is, the two control signal requirements of the same bridge arm is opposite). It can therefore be seen that different PWM waveforms can be generated when using different PWM control strategies.
3 carrier movement control theory
In general, N levels of inverter modulation require N-1 triangular carrier. In the carrier phase modulation method, all triangles have the same frequency and amplitude, but the phase of any two adjacent carriers must have a certain phase shift, its value is
(1)
The modulation signal is usually three-phase sinusoidal signals that are adjustable in the amplitude and frequency. By modulating the wave and carrier comparison, the drive signal of the required switching device can be generated [1].
4 PWM control strategy
The inverter is usually output in a sine wave. For single-phase bridge, its output can generally be divided into single-polarized modulation and bipolar modulation (limited to space, concrete implementation, see reference). Based on the H-bridge-based inverter, it is also possible to output a waveform similar to the single-phase bridge output, and its PWM control strategy should be adjusted slightly. The waveforms output from the two modes of single polarity modulation and bipolar modulation have different performance. Since the single polarity modulation can output three levels, the bipolar modulation can only output two levels, so the bipolar The DV / DT of sex is large, and the motor is insulated. In the product design process, unipolar modulation waveforms are usually used as the final output waveform. This paper uses the SPWM generated by the H-bridge-level inverter and the SPWM generated by carrier phase transformation as the control signal of each power unit, and the output of the single polarity SPWM waveform is realized. Break several PWM control strategies:
1) Single bridge arm chopping: The so-called single bridge arm chip method is S1 and S2 as a half cycle control signal, and S1 is turned on when the positive half cycle is turned on; the S2 is closed when the negative half cycle is closed, S2 is turned on. The control signal of S3 is a SPWM signal.
Fig. 2 S3 control signal waveform diagram 3 S1 control signal waveform diagram 4 power unit output waveform
The control signal of S4 is opposite to the control signal of S3. With such control, the waveform as shown in Fig. 4 can be output, although the output waveform is similar to the waveform of the single-phase bridge, the DV / DT is smaller, but this method causes the power of the two bridge arms. balanced.
2) Bipolar modulation: bipolar modulation of the H-type inverter is the same as the bipolar modulation of the single-phase bridge, the method of generating the control signal is the same, the difference is that one is a single bridge arm, one is a double bridge arm . In order to solve such a problem, the control signal input to S1 and S4, S2 and S3 and S1 and S1 and S4 are reversed. This control method can only occur in combination of two switches, namely S1 and S4 simultaneously turn it off, and S2 and S3 are simultaneously closed; S2 and S3 are simultaneously turned on. A waveform similar to a single-phase bridge bipolar modulation can be output. Although the DV / DT of the output waveform is large, it will generate high harmonics, which increases the impact of the system, but the power of the power unit two bridge arms is equal, and the control method is simple and easy to implement. Due to the high voltage inverter system, there is a distance between the output unit of the control signal and the power unit, which is connected to the fiber, which uses this method to reduce the use of the fiber, reduce the cost of the product, and also decrease The difficulty of on-site wiring.
3) Unipolar modulation: Although the single bridge arm chopping can achieve a similar output waveform similar to single-phase bridge unipolar modulation, this control method has inherent defects, here is introduced another control Way. As shown in FIG. 1, S1 is controlled by the control signal control shown in Fig. 5, and S3 is controlled by the control signal control, S2 and S4, respectively, the reverse signal of S1 and S2 control signals, respectively. The control signal shown in FIGS. 5 and 6 is a symmetrical SPWM signal of 180 degrees. Since the fundamental phase difference is 180 degrees, the duty cycle of the two-channel control signal corresponds to 1, ie, complementary. The output waveform will appear four combinations: S1 conduction, S3 is closed, output + UDC; S1 conduction, S3 is turned on, output 0; S1 is closed, S3 closed output 0; S1 is closed, S3 is turned on, output -udc. See the left side of the dashed line in the figure, the first three switches will appear, and the threatened three switches in the right side of the dashed line can output the PWM wave as shown in FIG. 7 satisfying the sinusoidal variation.
Figure 5 Left bridge arm control signal Figure 6 Right bridge arm control signal Figure 7 H bridge single polarity modulation output waveform
The PWM control implemented in this manner realizes the conversion of the single polarity SPWM to the bipolar SPWM to realize the power balance of the left and right bridge walls, and the inverter output voltage harmonics obtained in this manner is very low. The output does not require a filter, called a perfect-harmonic inverter. In the inverter control, DSP control is usually used. Since the DSP can only output two levels, unipolar SPWM can not be directly implemented, and the aid of an additional device is required. This method uses the combined logic relationship of the power unit (logic relationship as a table. 1), instead of the function of the external device, saving the device, reducing the development cost and development difficulty, simple control, easy to implement.
Table 1 logical relationship
5 Conclusion
In high-power high-voltage inverter technology, PWM control technology is one of its core technologies. A good PWM control strategy is a guarantee of product performance. In this paper, this paper mainly targets PWM control method for H-bridge-level high-voltage high voltage inverters. It has been discussed, giving three implementations, and analyzed and compare the methods and performance of their implementation. Technology area
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