Power Supply Current Limiting Circuit Segmentation
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Solution Overview
Problem
Conventional power supplies with a power limit of less than 100 VA are inadequate for handling short-duration high peak power demands, as they either require frequent fuse replacements or restrict high momentary current demands, and existing solutions do not meet revised UL 1950 reaction time requirements effectively.
Innovation Solution
A power supply with two independent current limiting circuits, one using a printed wiring board trace as a sensing element, allows for programmable current limiting based on time constant and level, enabling high pulse current delivery while maintaining average power within regulatory limits, and includes a latch-off or automatic restart feature to manage overloads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fuse is used to limit power output to meet UL 1950 requirements, then power safety is improved, but reaction time to overload becomes too long and fuses require frequent replacement
Solution Approach 1:
The power supply system is divided into multiple independent current limiting circuits, each with different trip levels and time delays. The first circuit handles short circuit protection with immediate response, while the second circuit handles overload conditions with delayed response, allowing differentiated protection strategies for different fault types without requiring fuse replacement
Solution Approach 2:
The circuit transitions from static fuse-based protection to dynamic electronic control with adjustable time delays and current thresholds. The programmable time constant allows the system to adapt its response based on the duration and severity of the overload condition, providing faster reaction times while maintaining safety
2Reliability
If electronic current limiting is used to maintain continuous power below 100 VA, then compliance with Limited Power Source requirements is achieved, but the ability to supply high momentary current demands is lost
Solution Approach 1:
The power supply employs periodic action through time-delayed current limiting circuits that allow high current pulses for predetermined durations (up to 5 seconds) before activating the limiting function. This enables the system to deliver peak power during short bursts while maintaining average power compliance over longer time spans
Solution Approach 2:
The system changes the parameter of current limiting from a fixed continuous threshold to a time-dependent variable threshold. By programming different time constants and current limit levels in different circuits, the power supply can dynamically adjust its output capacity based on the duration of the demand, allowing peak powers of 400-500 VA for short periods while maintaining 100 VA average power compliance
3Reliability
If the reaction time is reduced to meet revised UL 1950 five-second requirement, then safety response is improved, but the ability to handle legitimate high-power pulses is reduced
Solution Approach 1:
The protection system is segmented into multiple circuits with different response characteristics. The first current limiting circuit provides immediate protection for short circuit conditions, while the second programmable circuit provides delayed response for overload conditions, allowing the system to meet five-second safety requirements while still accommodating legitimate high-power pulses that occur within the time window
Solution Approach 2:
The system uses feedback through programmable time constants and current sensing to distinguish between hazardous overloads and legitimate high-power demands. By monitoring the duration and magnitude of current draws against predetermined thresholds, the circuit can selectively respond to different conditions, providing fast protection when needed while allowing authorized peak power delivery
Data Source
AI summary
A power supply (200) is provided having a plurality of independent current limiting circuits. A first current limiting circuit (6) provides protection against a short circuit or other extraneous load conditions, while a second current limiting circuit, using a trace on a PCB as a sensing element, is programmable on the basis of a time constant and a current level.