Converter Circuit Dynamic On-Time Control
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Solution Overview
Problem
Constant on time DC-DC converters require a large, in-phase feedback ripple, making ceramic capacitors unsuitable, and necessitate the use of more expensive solid capacitors for stability and transient response.
Innovation Solution
A converter control circuit comprising an error amplifier, ramp signal generators, comparators, and a control signal generator to dynamically control switch ON and OFF times based on error and ramp signals, allowing for variable on and off times proportional to output and input voltages, thereby stabilizing the circuit and improving transient response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a constant on time converter circuit is used, then the transient response performance is excellent and the internal structure is simple, but the feedback ripple must be large and in-phase with output inductor current, making ceramic capacitors unsuitable
Solution Approach 1:
The patent applies dynamics by making the on-time variable rather than constant. The on-time is dynamically adjusted based on the ratio of output voltage to input voltage through the ramp signal generation mechanism. This dynamic approach allows the use of ceramic capacitors while maintaining stability and achieving excellent transient response performance.
Solution Approach 2:
The patent changes the parameter of on-time from a fixed constant value to a variable value that depends on the voltage ratio. By generating ramp signals with slopes proportional to output and input voltages, the on-time automatically adjusts according to operating conditions, resolving the contradiction between stability requirements and capacitor selection flexibility.
2Ease of manufacture
If ceramic capacitors are used as output capacitors, then the cost is reduced and size is minimized, but the feedback ripple cannot be large enough and in-phase with output inductor current
Solution Approach 1:
The dynamic adjustment of on-time based on voltage ratios enables ceramic capacitors to be used effectively. The ramp signal generators create time-varying control signals that compensate for the lower ripple characteristics of ceramic capacitors, maintaining adequate feedback ripple performance while achieving cost and size reductions.
Solution Approach 2:
By changing the on-time parameter from constant to variable, the system adapts to the different ripple characteristics of ceramic capacitors. The variable on-time compensates for the reduced ripple amplitude, ensuring the feedback ripple remains sufficient for proper converter operation while enabling the use of smaller, cheaper ceramic capacitors.
3Reliability
If variable on and off times proportional to output and input voltages are used, then the circuit stability is improved and transient response is enhanced, but the control circuit complexity increases
Solution Approach 1:
The patent segments the control function into separate ramp signal generators, each dedicated to generating ramp signals proportional to specific voltages. This segmentation allows independent optimization of each generator and simplifies the overall control architecture while achieving variable on-time proportional to the voltage ratio.
Solution Approach 2:
The ramp signal generators serve multiple functions: they provide the timing reference for on-time control, encode the voltage ratio information, and enable automatic adaptation to different operating conditions. This multi-functionality reduces the need for additional separate control circuits, minimizing the increase in overall circuit complexity.
Data Source
AI summary
A converter control circuit for converting an input voltage to an output voltage comprises: an error amplifier, coupled to an output voltage or a feedback output signal from the output voltage, and a reference signal, operable to generate an error signal accordingly; a ramp signal generator, generating a first ramp signal and a second ramp signal; a first comparator, coupled to the error signal and the first ramp signal, operable to generate a first comparing signal accordingly; a second comparator, coupled to the error signal and the second ramp signal, operable to generate a second comparing signal accordingly; and a control signal generator, coupled to the first comparing signal and the second comparing signal, operable to generate a control signal to turn switches in the converter circuit ON and OFF accordingly.