Self-Oscillating PWM Phase Shift Using a Forward-Path Delay
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Solution Overview
Problem
Adjusting the hysteresis of comparators and delay elements in self-oscillating pulse-width modulators with sufficient precision is difficult, making it challenging to achieve the required 180° phase shift for oscillation.
Innovation Solution
Implementing a phase shift in the forward branch using a delay element, such as a latch or current-starved inverter, which allows for the use of a comparator without hysteresis and enables precise adjustment of the time delay, thereby achieving the necessary 180° phase shift in the control loop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If hysteresis of the comparator is adjusted to achieve 180° phase shift, then oscillation can be generated, but the adjustment is difficult to perform with sufficient precision
Solution Approach 1:
The patent introduces a delay element as an intermediary component between the comparator output and the feedback path. This delay element provides a controllable time delay that translates to a controllable phase shift, replacing the need to precisely adjust comparator hysteresis. The delay element acts as a mediator that converts the difficult hysteresis adjustment problem into a more manageable delay time adjustment problem.
Solution Approach 2:
The patent changes the adjustment parameter from comparator hysteresis voltage to delay element time delay. By using a delay element with adjustable delay time (such as a variable delay line or programmable delay), the phase shift can be precisely controlled by changing the delay parameter rather than adjusting hysteresis, which is difficult to measure and control accurately.
2Measurement precision
If delay element is used in the feedback loop, then phase shift can be controlled, but the delay must be precisely adjusted to achieve 180° phase shift
Solution Approach 1:
The patent employs feedback mechanisms where the delayed output signal is fed back to the comparator input. This feedback loop allows the system to self-adjust and stabilize at the desired 180° phase shift condition. The feedback ensures that any deviations from the target phase shift are corrected, reducing the need for extremely precise manual adjustment during manufacturing.
Solution Approach 2:
The system is designed to automatically achieve and maintain the correct phase shift through its own operation. The delay element combined with the feedback loop creates a self-regulating system that naturally settles into oscillation at the frequency where the total phase shift is 180°, eliminating the need for external precision adjustment tools or procedures.
3Reliability
If comparator with hysteresis is used, then oscillation can be maintained, but the hysteresis adjustment is relatively difficult
Solution Approach 1:
The delay element serves as an intermediary that provides the necessary phase shift without requiring hysteresis adjustment. This intermediary component decouples the oscillation maintenance function from the hysteresis parameter, allowing the comparator to operate with fixed or minimal hysteresis while the delay element handles the phase shift requirement.
Solution Approach 2:
Instead of using hysteresis to create the phase shift needed for oscillation, the patent inverts the approach by using a delay element to provide the phase shift and allowing the comparator to operate with minimal or no hysteresis. This inversion of the traditional approach simplifies the adjustment process while maintaining reliable oscillation.
Data Source
AI summary
A self-oscillating pulse-width modulator comprises an input, an output and a control loop. The control loop comprises a forward branch and a feedback loop. The forward branch is connected between the input and the output and comprises a comparator and a delay element. The feedback loop loops back to the input a feedback signal which depends on an output signal present at the output. The phase shift of the open control loop is 180° at the frequency of an oscillation to be generated by the modulator. At least a part of the phase shift is realized in the forward branch by means of the at least one delay element.