I. What is deadband?

The deadband that occurs in a circuit is the phenomenon in which the output voltage remains unchanged when the input voltage is within a certain range. For example, in a pulse width modulation (PWM) circuit, when the amplitude of the input signal exceeds a certain threshold, the switching tube opens and the amplitude of the output signal increases. However, when the input signal amplitude drops to a certain range, the amplitude of the output signal remains unchanged, resulting in a dead zone.

PWM (Pulse Width Modulation) deadband is a concept that is critical in PWM control systems, especially in applications involving power electronics and motor control. The following is a detailed explanation of PWM deadband, including its definition, role, principles, implementation methods, and considerations in practical applications.

Definition of PWM Deadband

PWM deadband, also known as “dead time” or “dead interval”, refers to the time interval between the closing of the previous channel and the opening of the next channel due to the response time and delay of the electronic components (e.g., transistors, field effect tubes, and other switching components) during the switching process of the PWM signal. The time interval that exists between the switching of the previous channel off and the switching of the next channel on. During this time, the PWM signal output will be temporarily interrupted and both channels will be off at the same time, thus avoiding two adjacent PWM signals (usually complementary) being high or low at the same time at the same moment.

The significance of the PWM (Pulse Width Modulation) deadband is to prevent a short circuit between the rising and falling edges of the PWM signal. In many electronic systems, especially those that use MOSFETs as switching elements, deadtime is used to ensure that when one MOSFET turns off, the other does not turn on immediately, thus avoiding a pass-through caused by both MOSFETs turning on at the same time.

Straight-through causes power to flow directly from the input to the output, bypassing the load, which not only wastes energy, but can also damage the MOSFETs or other circuit components, and may even cause overheating problems. In addition, a straight-through affects the performance of the system because it alters the intended operating state of the circuit.

Dead time is an intentionally signal-free period of time during the high and low level transitions of a PWM signal. During this time, the control circuit does not send any switching signals to the MOSFETs, ensuring that the previous MOSFET is completely turned off before the next MOSFET begins to conduct. This effectively avoids the straight-through problem and ensures the safe and stable operation of the circuit.

When designing PWM control circuits, it is very important to set the dead time reasonably, which needs to be determined according to the specific requirements of the circuit and the characteristics of the MOSFET. A dead time that is too long will reduce the efficiency of the system, while one that is too short may not be effective in preventing straight-through. Usually, the setting of the dead time needs to be optimized by experiment or simulation to achieve the best working result.

II. The role of PWM deadband

PWM deadband plays a vital role in power electronic equipment and motor control systems, mainly in the following aspects:

Protection of circuit components :

In PWM control, if there is no dead time, when the two complementary PWM signals in the switching, may be due to the response delay of the switching elements and lead to a short period of simultaneous conduction, thus generating a great current shock. This current shock may not only damage the switching elements, but may also cause serious damage to the entire circuit system.

The introduction of dead time prevents this to a certain extent and protects the circuit components from damage.

Preventing short-circuit faults :

In special cases, such as H-bridge circuits in motor control, a short-circuit fault will result if two neighboring switching elements are energized at the same time. Such a fault not only damages the circuit components, but can also lead to more serious safety issues.

The dead time setting ensures that only one switching element is on at any one time, thus preventing short-circuit failures.

Improved system stability :

The introduction of dead time also reduces the noise and ripple in the system by minimizing the sudden changes in current and voltage that occur when switching elements are switched too quickly.

This helps to improve the stability and reliability of the system, especially in applications that require high precision.

III. The principle of PWM deadband

The principle of PWM deadband is based on the response time and delay characteristics of the switching elements. When the PWM signal switches from one state to another, the switching element needs a certain amount of time to respond and switch its state. This time is the response time or delay time of the switching element.

In PWM control, to ensure that two complementary PWM signals are not in a high or low state at the same time, a dead time needs to be introduced during their switching. This time interval is long enough to ensure that the previous switching element is completely turned off before turning on the next switching element.

Causes of Deadband:

The generation of dead zones is related to the characteristics of the electronic components. Generally, a switching type circuit consists of a diode and a MOSFET tube.The input of the MOSFET tube has a gate, and when the gate is equal to the threshold voltage, the MOSFET tube will turn on. However, the gate passivation effect causes the gate to switch slower, which results in the output voltage remaining constant over a certain range.

Effect of Deadband:

The presence of dead zones may lead to circuit instability or performance degradation, for example, in PWM circuits, dead zones can lead to distortion of the output signal waveform and frequency variations, thus affecting the accuracy and reliability of the circuit. In addition, in areas such as motor control, dead zones can lead to overshooting phenomena, stall phenomena, etc.

The solution to the deadband:

1. Add a compensation circuit: by adding a compensation circuit at the input or output, the compensation circuit can adjust the output of the circuit to eliminate the deadband phenomenon.

2. Select suitable devices: Selecting devices with high-speed switching performance can effectively reduce the occurrence of deadband phenomenon.

3. Adjusting the operating state: by adjusting the threshold voltage and operating state of the switching tube, the occurrence of deadband phenomenon can be reduced or eliminated.

IV. Conclusion

In circuit design and application, deadband is a common phenomenon that adversely affects circuit performance. Therefore, it is necessary to understand the causes and effects of deadband and adopt corresponding solutions to ensure the reliability and stability of the circuit.