What is PWM "Dead Band"?

Pulse width modulation (PWM) is the abbreviation for pulse width modulation. Rectification and inverters are the most widely utilized in power electronics. A rectifier bridge and an inverter bridge are required for this. Three bridge arms are required for three-phase power. 

Taking two levels as an example, there are two power electronic devices, such as  IGBTs, on each bridge arm. A short circuit will result if the two  IGBTs are turned on at the same time.

As a result, by using a PWM wave with dead time, the upper and lower devices will not turn on at the same time. That is, if one device is switched on and then turned off, the other can be turned on after a period of inactivity.

Ⅰ.  What is a dead band?

H-bridges or 3-phase bridges made out of high-power tubes, IGBTs, and other components are commonly used in high-power motors, frequency converters, and other applications.

Each bridge's upper and lower half bridges can never be turned on at the same time, but when the high-speed PWM  drive signal reaches the control pole of the power element, it frequently has a delay effect for various reasons, causing one of the half-bridge components to not turn off when it should, causing the power components to burn out.

To avoid burning the power components, the dead band occurs when the upper half bridge is turned off and the lower half bridge is switched on after a period of time, or when the upper half bridge is turned on after a period of time after the lower half bridge is turned off. The dead band is the time when there is a delay. (That is, the upper and lower half bridge components are turned off.) Dead time control is not available in PWMs equipped with low-end single-chip microcomputers.

When the PWM is output, a protection period is established to prevent the upper and lower tubes of an H-bridge or half H-bridge from going on at the same time due to a switching speed problem. As a result, the upper and lower tubes will not have output at this time. Obviously. The waveform output will be stopped, but the dead time will only be a small percentage of the cycle.

The dead band will affect the output ripple, but it should not play a crucial role, because the PWM  wave itself has a tiny duty cycle and the vacant part is greater than the dead band.

Ⅱ.  PWM dead band in DSP

The upper and lower bridges of the same phase cannot be turned on at the same time during the rectification and inversion process, or the power supply will be short-circuited. In theory, the DSP's PWM won't switch on at the same moment, but the device's purpose is to prevent the PWM from being an immediate level jump. It always falls in a trapezoidal shape, through which the higher and lower bridges may pass. As a result, the higher and lower bridges are closed for a brief time and then selectively turned on to avoid direct connection of the upper and lower bridges. The dead band will cause the control performance to alter in real-time control. Difference.

The upper and lower bridge arms of the PWM's upper and lower bridge arms cannot be turned on at the same time. If both ends of the power supply are turned on at the same moment, a short circuit will occur. As a result, the two triggering signals must disconnect the triode when a certain amount of time has passed. The "dead band" is the name given to this location.

The average voltage output to the DC motor is determined by the duty cycle of the PWM; the  PWM does not regulate the current.

PWM stands for pulse width modulation, which is the process of adjusting the time ratio between square wave high and low levels. A 20% duty cycle waveform will have 20% high level time and 80% low level time, while a 20% duty cycle waveform will have 20% high level time and 80% low level time. There is 60% high level time and 40% low level time in the waveform with a 60% duty cycle. The higher the output pulse amplitude, that is, the higher the voltage, the higher the duty cycle and the longer the high level period.

If the duty cycle is 0%, the high time is also 0%, and no voltage is output.

When the duty cycle is set to 100%, the entire 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 continuously adjusted steplessly.

Ⅲ. PWM related concepts

1. Duty cycle

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

For example, if a PWM's frequency is 1000Hz, its clock cycle is 1ms, or 1000us. If the high level appears for 200us, then the low level must show for 800us, resulting in a duty cycle of 200:1000, or a duty ratio of 1:5.

2. Resolution

In other words, the minimum duty cycle can be met. For example, the theoretical resolution of 8-bit PWM is 1:255 (single slope), whereas the theoretical resolution of 16-bit PWM is 1:65535 (single slope) (single slope).

This is how often it happens. The resolution of 16-bit PWM.  for example is 1:65535. T/C must be counted from 0 to 65535 to reach this resolution. Counting from 0 to 80 begins at 0..... Then its minimum resolution is 1:80, but it is also quicker, implying a higher PWM output frequency.

3. Double slope / single slope

Consider a PWM that counts from 0 to 80 and then back to 0 to 80... There is only one slope here.

Consider a PWM that counts from 0 to 80, then back to 0... This is a double incline.

The counting time of the double slope is doubled, resulting in a half-slower output PWM frequency, but the resolution is 1: (80+80)=1:160, which is doubled.

Assuming that the PWM is a single slope, set the highest count to 80 and a comparison value of 10, then when T/C counts from 0 to 10 (the counter continues to count up until it reaches the set value of 80), the microcontroller will control whether a specific IO port outputs 1 or 0 or is reversed according to your settings. It is the most fundamental principle of PWM in this fashion.