Compensation of dead time effect of hottest PWM in

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Compensation of dead time effect of PWM inverter

1 introduction

in PWM three-phase inverter, because the switch tube has a certain on and off time, in order to prevent the straight through phenomenon of two switching devices on the same bridge arm, a few microseconds of dead time must be set in the control signal. Although the dead time is very short and the output voltage error caused is small, due to the high switching frequency, the superposition value of the error caused by the dead time will cause the waveform distortion of the motor load current and cause large pulsation of the electromagnetic torque, which will degrade the dynamic and static performance and reduce the practical application effect of the switching device

this paper starts with analyzing the mechanism of dead time effect, and seeks the compensation method of dead time effect

2 the generation of dead time effect

is discussed by using a bridge arm in the inverter (as shown in Figure 1). It includes upper and lower switching devices V1 and V2, freewheeling diodes D1 and D2. The control signal connecting the two power devices comes from the PWM generator to generate two basic drive signals ub1 and ub2. The output voltage is connected to the motor load, and the direction of current I flowing to the load is set as positive

when the upper and lower power tubes are switched, there are two cases: V1 from on to off and V2 from off to on or V2 from on to off and V1 from off to on. The dead time must be injected to make the upper and lower switch tubes not conductive. At this time, the output current will be continued by D1 or D2, depending on the direction of current I, and the output voltage will be delayed due to the dead time, as shown in Figure 2

it can be seen from Figure 2 that the error wave will be caused between the output ideal wave and the actual wave. If the storage time and pulse rise and fall time of the switching device are ignored, the error wave can be regarded as a rectangular wave

3 compensation for dead time effect

3.1 compensation method for adjusting the reference waveform

assuming that the switching frequency is much greater than the fundamental frequency, the output current is a sine wave, and the voltage error caused by each dead time is approximately equal, the influence of dead time on the fundamental voltage can be expressed by the average voltage error of positive and negative half cycles of the current

the error wave area of each dead zone is: Δ E=tdud (1)

where: TD -- dead time( μ s)

ud - DC power supply voltage (V)

then the average error value in each fundamental wave period is: Δ U= (sign (I)) utd/t (2)

where: m - the number of switches in each cycle. Those who pay the conference fee before March 31 will enjoy a discount of 1500 yuan/person

t - fundamental cycle( μ s)

it can be seen that the voltage loss is independent of the current amplitude and related to the current direction. The waveform of the influence of the average error voltage on the inverter is shown in Figure 3. Where ur is the ideal fundamental wave. If the load is inductive, the angle of current lag ur is φ′。 Average error voltage Δ U is a rectangular wave, which has an inverse relationship with current I. after decomposition, the fundamental wave is Δ u1。 Then the actual fundamental voltage U1 is the ideal fundamental wave ur and the error fundamental wave Δ Superposition of U1

in sine modulated PWM inverter, the realization of controlling the pulse width waveform is obtained by comparing the reference wave with the modulated wave. Therefore, the compensation of dead time effect can be realized by adjusting the reference wave according to the direction of load current

according to the above analysis, the dead time compensation circuit can be formed by the elastic buckling of the structural strain, as shown in Figure 4

device A1 detects the direction of load current I, and the output of A2 is a rectangular wave, which is added to the reference wave to generate a reference wave after the water is first evaporated and adjusted. When I 0, the reference wave becomes corrected; When I 0, the reference wave becomes more negative. According to such an appropriate adjustment, the error caused by the dead time can be eliminated, and the output fundamental voltage will be the same as the original reference wave. In Figure 4, R2 is the gain adjustment, so that the square wave amplitude and the error average amplitude value Δ U is equal. R2 is proportional to dead time and switching frequency. R3 is offset adjustment, which is set considering the positive and negative voltage imbalance caused by the unequal time delay of each power switch

this circuit is suitable for sine modulated PWM inverter. The pulse width must be obtained by directly comparing the reference wave with the modulated wave. This compensation method requires a current sensor to feed back the direction of the current. Its characteristic is that the hardware circuit is simple and easy to implement

3.2 dead time effect compensation based on pulse adjustment

according to the dead time effect analysis in Figure 2, the software programming method can also be used to compensate the dead time effect by changing the switching time. It is only necessary to detect the polarity of the load current without detecting the phase of the current. The specific method is as follows:

when I 0, figure 2 (a) is the ideal wave, and Figure 2 (b) shows the error between the actual wave caused by the dead time and the ideal wave without dead time. To eliminate this error, the pulse time can be changed by software, as shown in Figure 2 (c). Add another positive pulse before the dead time generator generates an asymmetric pulse, The "material certification and the technical service ability of the material supplier are also the requirements for the utilization of engineering materials. After the synthesis of positive pulse and dead time, the actual wave and ideal wave generated are the same in width and position, as shown in Figure 2 (d); when I 0, figure 2 (E) , compared with the ideal wave, the actual wave generated with dead time increases a positive pulse. If a negative pulse is added before the dead time generator generates an asymmetric dead time pulse, the synthesized actual wave is consistent with the ideal wave in width and position

this method can be realized by using 80C196MC special chip for motor control, which contains a PWM waveform generator. The pulse time of the waveform generator is adjusted before the dead time counter to compensate for the dead time effect. This method is independent of carrier frequency and only related to the polarity of load current. Taking the U-phase switch tube signal generator as an example, the programming is based on the polarity of the load current and a calculated state variable CNT representing down/up. Down indicates that the switch tube is open and up indicates that the switch tube is closed. This determines whether to add or subtract pulses during correction

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