PWM is a pulse width modulation, and most commonly used in power electronics is rectification and inverting. This requires the use of rectifier bridges and inverters. For three-phase electricity, three bridge arms are needed. Taking two levels as an example, there are two power electronics on each bridge, such as IGBT. These two IGBTs cannot be turned on simultaneously, otherwise there will be short circuits.
Therefore, the PWM wave designed to be dead can prevent up and down two devices from being turned on simultaneously. That is to say, when a device is turned on, it will pass through a dead zone, and the other can be turned on.
What is the dead area?
Typically, high-power motors, inverters, etc., both ends of high power tubes, IGBT and other components. The upper half bridge and the lower half bridge of each bridge are absolutely not to be turned on simultaneously, but the high-speed PWM drive signal often produces a delayed effect due to various reasons due to various reasons. A semi-bridge component is not turned off when it should be turned off, resulting in burning power components.
After the upper half bridge is turned off, it is delayed for a while to open the lower half of the bridge or turn off the second half bridge, and then the upper half bridge is delayed for a while, thereby avoiding the power element burns. This delay time is the dead zone. (It is, the components of the lower half bridge are all turned off) dead time control is not in the PWM equipped in the usual low-end microcontroller.
When the dead time is the PWM output, in order to make the upper and lower tabs of the H-bridge or half H bridge will not be set by the switch speed problem, it will not be output at this time, the upper and lower tubes will not be output, of course The waveform output is interrupted, and the dead time is generally only a few percent. However, the PWM wave itself duty cycle hours, the empty part is larger than the dead zone, so the death zone will affect the output of the output, but it should not play a decisive role.
PWM dead zone in DSP
During the rectified inverter, the upper and lower bridges of the same phase cannot be turned on simultaneously. Otherwise, the power supply will be short, theoretically, the PWM generated by the DSP will not simultaneously, but the reason for the device cannot be instantaneous level. Always trapezoidal decline, this will cause the upper and lower bridges to pass through, for this, set a very short time, the upper and lower bridges are closed, and the selective opening is opened, avoiding the up and down bridge, the actual control, the dead zone will result in control performance. Difference.
The triode of the upper and lower bridge arm of the PWM cannot be turned on simultaneously. If it is turned on, it will be shorted at both ends of the power supply. Therefore, the two trigger signals should be disconnected in the triode for a period of time. This area is called "dead zone".
The duty cycle of the PWM decides the average voltage output to the DC motor, and the PWM is not adjusting current. PWM means pulse width adjustment, that is, adjustment square wave high and low levels of time ratio, a 20% duty cycle waveform, there will be 20% high level time and 80% low electricity time, and one The waveform of 60% duty cycle has a high level time and 40% low level time. The larger the duty cycle, the longer the high level time, the higher the output pulse, the higher the voltage. . If the duty cycle is 0%, then the high level time is 0, there is no voltage output. If the duty cycle is 100%, the full voltage is output. Therefore, by adjusting the duty cycle, the purpose of adjusting the output voltage can be achieved, and the output voltage can be adjusted continuously.
PWM related concept
Duty cycle
It is the ratio of the time that the time maintained in the PWM is maintained with the time of the clock cycle of the PWM. For example, a PWM frequency is 1000 Hz, then its clock cycle is 1ms, which is 1000US. If the high level is 200 su, then the low level is definitely 800US, then the duty cycle is 200: 1000, also That is to say, the duty cycle of PWM is 1: 5.
Resolution
That is, the duty cycle can minimize how much, such as 8-bit PWM, theoretical resolution is 1: 255 (single slope), 16-bit PWM theory is 1: 65535 (single slope). The frequency is like this, such as 16-bit PWM, its resolution reaches 1: 65535, to achieve this resolution, T / C must be counted from 0 to 65535 to achieve, if the count is counted from 0 to 80 after 80 0 starts to count 80 ......., then its resolution is 1:80, but it is fast, that is, the PWM output frequency is high.
Double slope / single slope
Suppose a PWM counts from 0 to 80, then count from 0 to 80 ....... This is a single slope
Suppose a PWM counts from 0 to 80, then counts from 80 to 0 ....... this is a double slope
It can be seen that the count time of the double slope is doubled, so the PWM frequency output is slower, but the resolution is 1: (80 + 80) = 1: 160, it is doubled. Suppose PWM is a single slope, setting up the highest count is 80, and we set a comparison value 10, then T / C counts from 0 to 10 (at this time the counter is still up to the top, until count to the set value 80 ), The microcontroller will control an IO port based on your settings 1 or output 0 or the port is reverse, so that the most basic principle of PWM. , Read the full article, original title: What is the PWM "dead zone"?
Article Source: [Micro Signal: ENGICOOLARABIC, WeChat Public Number: Shenzhen Innovation Design Research Institute] Welcome to add attention! Please indicate the source of the article.
Our other product:
