Editing Dead-time distortion
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− | Dead time distortion (DTD) is a type of cross-over distortion caused by dead-time state of PWM power stages. The problem is present in all | + | Dead time distortion (DTD) is a type of cross-over distortion caused by dead-time state of PWM power stages. The problem is present in all {{param|PWM}} output drives. The longer dead time used, the more significant distortion becomes. |
==About dead-time== | ==About dead-time== | ||
Dead-time is a blanking time period (upper & lower transistors in off-state simultaneously) of half-bridge power stage. Both transistors are switched off for short period of time to prevent both transistors conducting simultaneously thus causing a short circuit from supply to ground. | Dead-time is a blanking time period (upper & lower transistors in off-state simultaneously) of half-bridge power stage. Both transistors are switched off for short period of time to prevent both transistors conducting simultaneously thus causing a short circuit from supply to ground. | ||
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===Dead time distortion=== | ===Dead time distortion=== | ||
− | These graphs show actual distortion on sinusoidal motor currents when | + | These graphs show actual distortion on sinusoidal motor currents when {{param|PWM}} frequency was 17.5 kHz and dead-time length 2 µs. No countermeasures were taken to reduce distortion. |
Notice the current dips in otherwise sinusoidal signal. There are 6 dips per sine cycle because in this experiment the test was done on a 3 phase motor which has 3 sine currents with 60 degree phase angle differences. Only one phase is displayed but distortion of two remaining phases will induce dips also in the neighbor phases. | Notice the current dips in otherwise sinusoidal signal. There are 6 dips per sine cycle because in this experiment the test was done on a 3 phase motor which has 3 sine currents with 60 degree phase angle differences. Only one phase is displayed but distortion of two remaining phases will induce dips also in the neighbor phases. | ||
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===Distortion elimination=== | ===Distortion elimination=== | ||
The bottom image shows the exactly same experiment with the one above but this time [[Argon (servo drive)]] dead time correction was turned on. Notice how dips almost completely disappear and PWM duty cycle setpoint shows more sine-like waveform. | The bottom image shows the exactly same experiment with the one above but this time [[Argon (servo drive)]] dead time correction was turned on. Notice how dips almost completely disappear and PWM duty cycle setpoint shows more sine-like waveform. | ||
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