Current-Mode Converter Ramp Timing for Low On-Time Stability
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Solution Overview
Problem
Switching voltage converters experience stability issues due to delays in the slope compensation ramp generation, leading to pulse skipping and jittery operation, especially during narrow PWM pulses, caused by limited bandwidth in the amplifier and propagation delays in the slope compensation circuit.
Innovation Solution
A slope compensation ramp generation circuit that initiates the slope compensation ramp signal before the start of each switching cycle, ensuring a sufficient slope is established by the time the clock's falling edge occurs, using a one-shot circuit, inverters, a latch, and passive components to generate a ramp signal with an initial large slope that progressively decreases, addressing the delays and bandwidth limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If slope compensation ramp generation is delayed due to amplifier bandwidth limitations and circuit propagation delays, then the converter can use simpler circuit components, but the converter experiences pulse skipping and jittery operation during narrow PWM pulses
Solution Approach 1:
The slope compensation ramp generation is initiated before the PWM pulse begins, using a one-shot circuit triggered by the PWM signal. This preliminary action ensures the ramp is ready for immediate comparison with the inductor current, eliminating delays during narrow pulses and preventing pulse skipping while maintaining circuit simplicity
2Device complexity
If the slope compensation ramp has a constant slope throughout the switching cycle, then the circuit design is simpler, but the loop response becomes unstable during low on-times
Solution Approach 1:
The slope compensation ramp transitions from a static constant slope to a dynamic decreasing slope, where the ramp slope is largest at the beginning of the switching cycle and progressively decreases. This dynamic adjustment provides sufficient slope for accurate current control during low on-times while maintaining reasonable circuit complexity through the use of a simple RC circuit
Data Source
AI summary
A voltage converter includes a power stage, a compensator circuit, and a modulator circuit. The power stage circuit has an input voltage terminal, an output voltage terminal, and a pulse width modulation (PWM) control input. The compensator circuit has a voltage input coupled to the output voltage terminal and has a compensation output. The modulator circuit has a compensation voltage input and a PWM control output. The compensation voltage input is coupled to the compensation output, and the PWM control output is coupled to the PWM control input. The modulator circuit includes a slope compensation ramp generator circuit that is configured to generate a slope compensation voltage ramp having a slope that decreases during at least a portion of a switching cycle.