DC/DC Converter Current Limit Protection via Dynamic Off-Time
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Solution Overview
Problem
Conventional DC/DC converters face issues with excessive inductor current during short circuits or initial charging, leading to potential damage due to insufficient protection against overcurrent conditions, with existing frequency foldback methods being inadequate in preventing switch current runaway and unnecessarily constraining output current capability.
Innovation Solution
A regulator circuit with a signal responsive switch and a latch circuit that adjusts the switch's off-time based on sensed current levels, using a diode to measure current during the off-state and delaying recoupling until the current ramps down, allowing for smaller external components and higher power densities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If frequency foldback provisions are used to increase off-time to prevent excessive inductor current, then switch current protection is improved, but output current capability is unnecessarily constrained during normal operation
Solution Approach 1:
The patent implements dynamic off-time adjustment based on real-time current sensing. The off-time is extended only when excessive current is detected through current sensing during the on-state, and only by the minimum amount necessary to allow current ramp-down. During normal operation, the off-time remains at its nominal value, preserving output current capability while providing protection only when needed.
2Reliability
If a fixed minimum off-time is used to allow current ramp-down, then switch current protection is improved, but component sizes must be larger to handle the time constraint
Solution Approach 1:
The patent makes the off-time dynamic rather than fixed. The off-time is adjusted based on actual current conditions - extended only when excessive current is detected and only by the minimum necessary amount. This dynamic approach allows the use of smaller inductors and capacitors that would otherwise be required to handle fixed minimum off-time constraints, thereby reducing overall component sizes.
3Reliability
If the switch off-time is extended to prevent current runaway, then switch protection is improved, but power density is reduced due to larger component requirements
Solution Approach 1:
The patent implements dynamic off-time adjustment that extends the off-time only when and only as much as needed to prevent excessive current. This minimizes the impact on switching frequency and power density. Combined with reduced component sizes enabled by the same technique, the overall power density is maintained or improved while still providing adequate protection.
Solution Approach 2:
The current sensing mechanism allows the system to self-regulate the off-time based on actual current conditions without external intervention. The system automatically extends off-time only when excessive current is detected, eliminating the need for conservative fixed timing that would reduce power density.
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 effectively prevents excessive switch current and reduces component sizes, ensuring safer operation and higher power densities by dynamically adjusting the switch's off-time in response to current conditions.
Implementation Method 1
The third input of the regulator circuit is coupled to a circuit element operable to conduct unidirectional current, such as a diode. The circuit element is connected in series with the ground input power supply terminal. Current through the diode and ground terminal is measured directly during the off-state of the switch
Implementation Method 2
When the switch is turned off, energy stored in the inductor is transferred to the capacitor 14
Data Source
AI summary
A converter coupled to a DC voltage input and connectable to a load, includes a signal responsive switch coupled between a first circuit point and a second circuit point. Current flow is directed by the switch, when in a closed state, to the second circuit point to bypass the load. A regulator circuit has an output coupled to a control input of the switch. The regulator circuit has a first input for receiving a sensed load parameter, a second input for receiving a sensed current level signal at the second circuit point when the switch is in its closed state, and a third input for receiving a sensed current level signal at the second circuit point, measured directly, when the switch is in an open state. A fixed minimum time is set for the switch to be in the open state. The third input inhibits re-closing of the switch, providing current limit protection for the switch.