Dead-Time Generating Circuit for Brushless DC Motor Control
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Solution Overview
Problem
Existing dead-time generating circuits for three-phase brushless DC motor drive circuits face challenges in maintaining consistent dead times due to variations in threshold voltages and pulse widths, leading to potential damage to switching elements and reduced controllability, especially during changes in rotation direction or brake control.
Innovation Solution
A dead-time generating circuit that includes a constant current circuit, a current generating circuit, a control circuit, and a charge/discharge circuit, where the charge/discharge circuit compares capacitor voltage with a threshold voltage and generates signals to control charging and discharging based on a delay time, ensuring consistent dead times at both rise and fall timings of the control signal, even with varying pulse widths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a dead time is provided to prevent simultaneous conduction of upper-stage and lower-stage elements, then element damage is prevented, but motor efficiency and torque decrease
Solution Approach 1:
The patent applies dynamics by making the dead time variable rather than fixed. The dead time generation circuit adjusts the dead time duration dynamically based on the PWM duty ratio, using shorter dead times at low duty ratios and longer dead times at high duty ratios, thereby optimizing both protection and efficiency across different operating conditions
Solution Approach 2:
The patent changes the parameter of dead time duration based on the PWM duty ratio. By using the duty ratio signal to control the dead time generation circuit, the system varies the dead time parameter adaptively, preventing element damage while minimizing efficiency losses during normal operation
2Reliability
If a dead time is provided to prevent simultaneous conduction of switching elements, then element damage is prevented, but acceleration time increases
Solution Approach 1:
The system dynamically adjusts dead time based on operating conditions, using shorter dead times during normal acceleration when duty ratios are moderate, and extending dead time only when necessary for protection, thereby minimizing acceleration time while maintaining reliability
Solution Approach 2:
The dead time parameter is changed adaptively according to the PWM duty ratio and operational state, reducing dead time during phases requiring rapid response and increasing it only when protection is needed, thus optimizing acceleration performance
3Reliability
If different dead times are used for transitions from first state to second state and vice versa, then element protection is ensured, but controllability suffers
Solution Approach 1:
The patent intentionally introduces asymmetry in the dead time generation by using different circuit paths for rising and falling edges of the PWM signal. This asymmetric design allows different dead time durations for forward and reverse transitions, ensuring element protection while maintaining consistent and predictable control behavior through deliberate asymmetric timing
Solution Approach 2:
The system dynamically selects different dead time values based on the transition direction (rising or falling edge of PWM signal), using the asymmetric circuit configuration to provide appropriate protection for each transition type while maintaining overall system controllability
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution ensures consistent dead times, preventing damage to switching elements and improving controllability by maintaining equal dead times at both rise and fall timings, even with short pulse widths, thus enhancing the reliability of motor control operations.
Implementation Method 1
a constant current circuit 102 configured to generate a constant current whose magnitude is determined by an external resistor
Implementation Method 2
a charge/discharge circuit 104 configured to control charging or discharging of a capacitor using the capacitor-charge current
Implementation Method 3
the charge/discharge circuit is also configured to compare a voltage of the capacitor with a threshold voltage, and to generate a comparator signal when the voltage of the capacitor exceeds the threshold voltage
Data Source
AI summary
A dead-time generating circuit includes a constant current circuit; a current generating circuit generating a capacitor-charge current; and a control circuit receiving a dead time control signal and a comparator signal. The control circuit generates a dead time generating signal based on the dead time control signal and the comparator signal, and a charge/discharge signal based on the dead time generating signal. Charging or discharging of a capacitor is controlled by the capacitor-charge current in accordance with the charge/discharge signal. A voltage of the capacitor is compared with a threshold voltage in order to generate a comparator signal when the voltage of the capacitor exceeds the threshold voltage. The control circuit generates the charge/discharge signal for a duration starting from a time when the delay time has elapsed from the rise or fall timing of the dead time control signal until the control circuit receives the comparator signal.