USB Current Limit Control via Active Switch PWM
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Solution Overview
Problem
The USB standard limits the maximum current that peripheral devices can draw, restricting the number of devices that can utilize USB power due to excessive in-rush current demands during capacitor charging, which exceeds the current source capabilities.
Innovation Solution
A load device with an active switch and switch controller regulates the in-rush current by pulse width modulating the active switch to draw short bursts of current, ensuring the average current remains within USB limits while maintaining sufficient voltage for peripheral devices like LCD displays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the USB power supply provides sufficient current to charge peripheral device capacitors rapidly, then the charging speed improves, but the current limit is exceeded causing connection damage
Solution Approach 1:
The patent applies periodic action by implementing a current limiting circuit that cycles between allowing current flow and blocking current flow. The circuit periodically enables high current charging when capacitors need charging, then switches to a lower current mode when the capacitors are sufficiently charged, preventing continuous excessive current draw while still achieving rapid initial charging.
Solution Approach 2:
The patent implements dynamics by using a dynamic current limiting mechanism that adjusts the current limit based on the charging state of the peripheral device capacitors. The current limit is not fixed but changes over time - starting at a higher limit for rapid charging then reducing to a lower sustained limit, allowing the system to adapt between speed and safety requirements.
2Reliability
If the USB power supply limits current to safe levels, then connection safety improves, but the ability to power peripheral devices with high in-rush current demands deteriorates
Solution Approach 1:
The patent applies preliminary action by pre-charging the USB bus capacitors through a controlled current path before connecting the peripheral device. This preliminary charging action ensures that when the peripheral device connects and attempts to draw in-rush current, the USB bus capacitors are already prepared and can supply the necessary current without overloading the power supply, thus maintaining both safety and device compatibility.
Solution Approach 2:
The patent introduces an intermediary current limiting circuit between the USB power supply and the peripheral device. This intermediary circuit acts as a mediator that safely manages the current transfer, allowing high current demands to be met temporarily while preventing damage to the power supply through its controlled switching and current limiting functionality.
3Power
If the in-rush current is allowed to flow freely to charge capacitors quickly, then the power delivery speed improves, but the USB standard current limit is exceeded
Solution Approach 1:
The patent applies segmentation by dividing the power delivery process into distinct phases: an initial high-current phase for rapid capacitor charging, and a subsequent lower-current phase for sustained operation. The current limiting circuit segments the current flow in time, allowing high power delivery rate temporarily during capacitor charging while ensuring the average current remains within USB standard limits.
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 solution allows peripheral devices to be powered from a USB connection without exceeding current limits, enabling more devices to utilize USB power by managing in-rush currents effectively and ensuring stable operation.
Implementation Method 1
a peripheral power bus having an internal capacitance
Implementation Method 2
A load device includes an active switch and switch controller that regulates in-rush current by pulse width modulating the active switch
Data Source
AI summary
A load device includes a power input having an interface to a power supply; a peripheral power bus including an internal capacitance, and an active switch coupled to the power input and the peripheral power bus for applying power from the power input to the peripheral power bus. The load device also includes a switch controller coupled to the active switch for regulating the in-rush current drawn by the internal capacitance through the active switch while the internal capacitance is being charged.