PWM Signal Generator with Level-Inversion Inhibition
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Solution Overview
Problem
Existing PWM signal generators using hysteresis comparators face instability in duty ratio due to manufacturing variations and vehicle power-supply noise, leading to inconsistent headlamp intensity during daytime driving.
Innovation Solution
A PWM signal generator with an oscillator circuit, comparator circuit, and level-inversion inhibiting circuit that forcibly maintains the inverted state of the output signal to prevent level-inversion, ensuring a stable duty ratio by changing the comparison levels irrespective of the signal levels, thereby reducing the impact of noise and manufacturing variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a hysteresis comparator is used to generate PWM signal, then chattering due to power-supply noise can be prevented, but manufacturing variation of circuit constants causes duty ratio instability
Solution Approach 1:
The invention separates the chattering prevention function (hysteresis comparator) from the duty ratio determination function (level-inversion inhibiting circuit). The hysteresis comparator generates a preliminary output signal with noise immunity, while the level-inversion inhibiting circuit independently controls the comparison levels to ensure stable duty ratio, preventing the coupling of these two functions that caused the original contradiction.
Solution Approach 2:
The level-inversion inhibiting circuit acts as an intermediary between the hysteresis comparator and the final PWM output. It receives the output signal from the hysteresis comparator and actively controls the comparison levels at the comparator input, mediating the effect of manufacturing variations before they reach the duty ratio determination, thus stabilizing the output while preserving noise immunity.
2Object-affected harmful factors
If feedback resistor and power supply resistor are used in hysteresis comparator, then noise immunity is improved, but resistance value variation causes threshold voltage variation
Solution Approach 1:
The level-inversion inhibiting circuit implements active feedback control by monitoring the output signal from the hysteresis comparator and dynamically adjusting the comparison levels at the comparator input. This feedback mechanism compensates for threshold voltage variations caused by resistor manufacturing tolerances, maintaining stable duty ratio while preserving the noise immunity provided by the hysteresis structure.
Solution Approach 2:
The invention dynamically changes the comparison level parameter based on the output signal state. When the output signal level-inverts, the level-inversion inhibiting circuit immediately adjusts the comparison levels to prevent premature re-inversion, effectively changing the operating parameters of the hysteresis comparator to maintain stable duty ratio despite resistor value variations.
3Manufacturing precision
If external resistors are used to mitigate variation, then manufacturing variation is reduced, but threshold voltage still varies due to internal resistors
Solution Approach 1:
The level-inversion inhibiting circuit serves as an intermediary that compensates for the residual threshold voltage variations that external resistors cannot eliminate. It actively controls the comparison levels and timing of level-inversion, mediating the effect of internal resistor variations to ensure consistent duty ratio across products, thereby achieving both manufacturing variation mitigation and duty ratio consistency.
Data Source
AI summary
After an output signal S4 is level-inverted, first and second shorting FETs 55, 56 as a level-inversion inhibiting circuit inhibit level-inversion so that the signal is maintained to the inverted state. Thereafter the inhibition of level-inversion is released, when the signal is subsequently level-inverted at a proper time according to a desired duty ratio of a PWM signal S1. Thus chattering can be prevented and thereby a PWM signal S1 of a stable duty ratio can be generated, even if the level of a reference signal S3 fluctuates due to a noise or the like during vehicle acceleration, for example.