Output Driver Slew-Rate Control for Low-EMI Fast Switching
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Solution Overview
Problem
Existing output driver circuits face a trade-off between reducing radiated electromagnetic emissions and minimizing switching delay, as slowing the rise or fall of control signals to reduce emissions can result in undesirable delays and inaccuracies in data transmission.
Innovation Solution
A switching device with a control terminal and a control circuit that generates a control signal with distinct slew rates before and after crossing the conduction threshold, allowing for faster initial slew rate changes to reduce switching delay while maintaining a slower slew rate to minimize radiated emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the rise or fall of the control signal is slowed to reduce radiated electromagnetic emissions, then radiated emissions are reduced, but switching delay increases
Solution Approach 1:
The control signal transitions from a static slew rate to a dynamic, multi-stage slew rate profile. The circuit dynamically adjusts the slew rate in two stages: a first slew rate for the initial portion of the transition and a second slew rate for the remaining portion. This dynamic adjustment allows optimization of both radiated emissions and switching delay by having different slew rates at different phases of the switching event.
Solution Approach 2:
The invention changes the slew rate parameter during the control signal transition. By implementing a first slew rate that differs from a second slew rate, the system optimizes the balance between switching speed and radiated emissions. The control circuit monitors the control signal voltage and adjusts the slew rate parameter based on whether the signal is above or below a reference voltage, effectively using parameter changes to resolve the contradiction.
2Object-generated harmful factors
If a single slow slew rate is used for the entire control signal transition, then radiated emissions are reduced, but switching accuracy and speed are degraded
Solution Approach 1:
The control signal transition is segmented into two distinct phases: a first portion and a second portion. Each phase has its own dedicated slew rate (first slew rate and second slew rate respectively). This segmentation allows the first portion to be optimized for one objective (e.g., faster switching for accuracy) while the second portion is optimized for another objective (e.g., slower transition for reduced emissions), thereby resolving the contradiction between switching accuracy and radiated emissions.
Solution Approach 2:
The control circuit dynamically switches between two different slew rates based on the instantaneous voltage level of the control signal relative to a reference voltage. When the control signal is below the reference voltage, one slew rate is applied; when above, a different slew rate is applied. This dynamic switching enables precise control over the transition characteristics, maintaining switching accuracy while managing radiated emissions.
Data Source
AI summary
A switching device for driving a load is provided. The switching device comprises a control terminal and has a conduction threshold which, when crossed by a control signal coupled to the control terminal, causes the switching device to conduct. A control circuit for generating the control signal is also provided. The control circuit is configured to generate a control signal having a first slew rate prior to the control signal crossing the conduction threshold and a second slew rate after the control signal has crossed the conduction threshold. The first slew rate may be faster than the second slew rate.