What is PWM "dead zone"?

qwqwqw2088

What is PWM "dead zone"? [Copy link]

PWM is pulse width modulation. In power electronics, the most commonly used are rectification and inversion. This requires the use of rectifier bridges and inverter bridges. For three-phase electricity, three bridge arms are required. Taking two-level as an example, each bridge arm has two power electronic devices, such as IGBTs. These two IGBTs cannot be turned on at the same time, otherwise a short circuit will occur.

Therefore, designing a PWM wave with a dead zone can prevent the upper and lower devices from being turned on at the same time. That is, when one device is turned on and then turned off, after a period of dead zone, the other device can be turned on.

1. What is a dead zone?

Usually, the ends of high-power motors, inverters, etc. are H-bridges or 3-phase bridges composed of high-power tubes, IGBTs and other components. The upper and lower half bridges of each bridge must not be turned on at the same time, but when the high-speed PWM drive signal reaches the control pole of the power component, it often produces a delay effect due to various reasons, causing a half-bridge component to not turn off when it should be turned off, causing the power component to burn out.

Dead zone means that after the upper half bridge is turned off, the lower half bridge is turned on after a delay, or after the lower half bridge is turned off, the upper half bridge is turned on after a delay, so as to avoid burning of power components. This delay time is the dead zone. (That is, the components of the upper and lower half bridges are all turned off) Dead zone time control is not available in the PWM equipped with ordinary low-end microcontrollers.

Dead time is a protection period set during PWM output to prevent the upper and lower tubes of the H-bridge or half-H-bridge from being turned on at the same time due to switching speed issues. Therefore, during this time, neither the upper nor lower tubes will have output, which will of course interrupt the waveform output. The dead time generally only accounts for a few percent of the cycle. However, when the duty cycle of the PWM wave itself is small, the empty part is larger than the dead time, so the dead time will affect the output ripple, but it should not play a decisive role.

2. PWM Dead Zone in DSP

During the rectification and inversion process, the upper and lower bridges of the same phase cannot be turned on at the same time, otherwise the power supply will be short-circuited. Theoretically, the PWM generated by the DSP will not be turned on at the same time, but due to the device, the PWM cannot be an instantaneous level jump, and it always decreases in a trapezoidal shape, which may cause the upper and lower bridges to be turned on directly. For this reason, an extremely short time is set, the upper and lower bridges are closed, and then selectively opened, to avoid the upper and lower bridges being turned on directly. In actual control, the dead zone will lead to poor control performance.

The transistors of the upper and lower bridge arms of PWM cannot be turned on at the same time. If they are turned on at the same time, the two ends of the power supply will be short-circuited. Therefore, the two trigger signals must disconnect the transistors for a period of time. This area is called the "dead zone".

The duty cycle of PWM determines the average voltage output to the DC motor. PWM does not regulate current. PWM means pulse width modulation, that is, adjusting the time ratio of the high level and low level of the square wave. A 20% duty cycle waveform will have 20% high level time and 80% low level time, while a 60% duty cycle waveform will have 60% high level time and 40% low level time. The larger the duty cycle and the longer the high level time, the higher the output pulse amplitude, that is, the higher the voltage. If the duty cycle is 0%, then the high level time is 0, and there is no voltage output. If the duty cycle is 100%, then 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 and steplessly.

3. PWM related concepts

1. Duty cycle

It is the ratio of the time the high level is maintained in the output PWM to the clock cycle time of the PWM.

For example, if the frequency of a PWM is 1000Hz, then its clock period is 1ms, which is 1000us. If the high level appears for 200us, then the low level must be 800us, so the duty cycle is 200:1000, which means that the duty cycle of PWM is 1:5.

2. Resolution

That is, what is the minimum duty cycle that can be achieved. For example, for an 8-bit PWM, the theoretical resolution is 1:255 (single slope), and for a 16-bit PWM, the theoretical resolution is 1:65535 (single slope).

The frequency is like this. For example, the resolution of 16-bit PWM reaches 1:65535. To achieve this resolution, T/C must count from 0 to 65535. If the count is from 0 to 80 and then from 0 to 80..., then its minimum resolution is 1:80. However, it is also faster, which means that the output frequency of PWM is high.

3. Dual slope / Single slope

Suppose a PWM counts from 0 to 80, and then counts from 0 to 80 again....... This is a single slope

Suppose a PWM counts from 0 to 80, and then from 80 to 0....... This is a dual slope

It can be seen that the counting time of the dual slope is doubled, so the output PWM frequency is half as slow, but the resolution is 1:(80+80)=1:160, which is doubled.

Assume that PWM is single slope, set the highest count to 80, and set a comparison value to 10. Then when T/C counts from 0 to 10 (the counter continues to count until it reaches the set value of 80), the microcontroller will control a certain IO port to output 1, 0, or invert the port according to your settings. This is the most basic principle of PWM.