Output Driver Gate Slew Shaping for Low EMI Switching Delay
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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 output signals.
Innovation Solution
A switching device with a control terminal that has a conduction threshold, where the control signal has a faster slew rate before crossing the threshold and a slower slew rate after, utilizing an energy storage device like a capacitor to manage the signal levels and reduce switching delay while minimizing emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the rise or fall of the control signal is slowed down to reduce radiated electromagnetic emissions, then radiated emissions are reduced, but switching delay increases
Solution Approach 1:
The control signal is pre-charged to the conduction threshold level before being applied to the switching device gate. This preliminary action ensures the signal reaches the threshold quickly (reducing switching delay) while the actual transition is controlled by a separate mechanism that limits the rate of change after threshold crossing (controlling radiated emissions).
Solution Approach 2:
The control signal generation is divided into two distinct phases: a fast charging phase that quickly brings the signal to the conduction threshold, and a controlled transition phase that limits the slew rate after threshold crossing. This segmentation allows each phase to be optimized independently for its specific function.
2Loss of time
If the control signal transitions quickly to reduce switching delay, then switching delay is reduced, but radiated electromagnetic emissions increase
Solution Approach 1:
The control circuit dynamically adjusts its behavior based on the signal level. Before the conduction threshold, the circuit allows fast charging. After the threshold is crossed, the circuit automatically transitions to a mode that limits the rate of change. This dynamic adaptation resolves the contradiction by changing the system's characteristics at the critical moment.
Solution Approach 2:
The slew rate parameter of the control signal is changed at the conduction threshold. Before threshold crossing, the effective slew rate is high (fast charging). After threshold crossing, the slew rate is reduced to a controlled value. This parameter change allows the system to achieve low switching delay while maintaining controlled radiated emissions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively reduces radiated electromagnetic emissions while minimizing switching delays, enhancing the accuracy and reliability of output signals in applications like magnetic field sensors.
Implementation Method 1
A capacitor may be selectively coupled to the control terminal of the switching device after being charged to a level associated with the conduction threshold
Data Source
Figure 1~2
Figure 3
Figure 3A
AI summary
A switching device (300) for driving a load (14) is provided. The switching device comprises a control terminal (304) and has a conduction threshold which, when crossed by a control signal (305, Vgate) coupled to the control terminal, causes the switching device to conduct. A control circuit (306, 308, 310, 312, 314, 316, 318, 320, 322) for generating the control signal is also provided. The control circuit is configured to generate a control signal (305) having a first slew rate (between TO and Tl') prior to the control signal crossing the conduction threshold (at Tl') and a second slew rate after the control signal has crossed the conduction threshold. The first slew rate being larger than the second slew rate.