Switchmode Power Supply Foldback Current Limiting Control
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Solution Overview
Problem
Conventional switchmode power supplies have a foldback current limiting starting point that is too high, typically at around 2 V, which is not suitable for applications requiring a lower voltage threshold, such as portable devices with reduced battery cells, as it does not effectively reduce power dissipation under fault conditions like short circuits.
Innovation Solution
A voltage regulation deactivating unit is introduced to increase the sensed auxiliary voltage, allowing the output voltage regulator to enter foldback current limiting mode at lower output voltages by adding resistance in parallel to the voltage sensing resistor, effectively reducing the starting point for foldback current limiting to around 0.6 V, and a supply voltage stabilizing unit ensures stable power supply to the integrated circuit across varying load conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the foldback current limiting starting point is set at around 2 V in conventional switchmode power supplies, then the circuit operates normally under standard conditions, but it fails to effectively reduce power dissipation under fault conditions like short circuits in applications requiring lower voltage thresholds
Solution Approach 1:
The patent changes the voltage threshold parameter for foldback current limiting by adding a parallel resistance (R22) to the voltage sensing resistor (R9). This modification alters the voltage division ratio in the feedback network, effectively lowering the foldback starting point from around 2 V to around 0.6 V, enabling the power supply to reduce power dissipation at lower output voltages suitable for portable devices with reduced battery cells
2Adaptability or versatility
If the auxiliary voltage is increased by adding parallel resistance to the voltage sensing resistor, then the foldback current limiting starts at lower output voltages, but the voltage sensing accuracy may be affected
Solution Approach 1:
The patent introduces a controllable switch (T2) as an intermediary element that selectively connects the parallel resistance (R22) to the voltage sensing resistor (R9). This switch is controlled by a control signal that activates only under fault conditions (when output voltage drops below a threshold), allowing the system to maintain accurate voltage sensing during normal operation while enabling foldback current limiting at lower voltages when needed
3Loss of energy
If the power supply is designed to operate efficiently at very low output power levels, then it meets energy saving requirements, but maintaining stable operation across varying load conditions becomes more challenging
Solution Approach 1:
The patent employs a feedback mechanism where the auxiliary voltage, derived from the output voltage through the modified sensing network, is fed back to the control circuit. This feedback allows the controller to dynamically adjust the primary-side switch duty cycle, maintaining stable operation across varying load conditions while enabling efficient low-power operation and foldback current limiting when required
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
The solution allows for a lower voltage starting point for foldback current limiting, reducing power dissipation and preventing overheating in fault conditions, and ensures stable operation across a wider range of input voltages and load conditions without modifying the existing integrated circuit hardware.
Implementation Method 1
a transformer (W3) having a primary-side main winding (W3-1) and a secondary-side main winding (W3-3)
Implementation Method 2
a primary-side switch (T100) which is operable to open a primary-side current that flows through the primary-side main winding (W3-1)
Implementation Method 3
a feedback unit which is operable to generate an auxiliary voltage that is a function of a secondary-side output voltage
Data Source
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AI summary
The present invention relates to a switchmode power supply and a method to control a primary-side switch (T100) in a switchmode power supply. A switchmode power supply comprises a transformer (W3) having a primary-side main winding (W3-1) and a secondary-side main winding (W3-3), a primary-side switch (T100) for interrupting a primary-side current that flows through the primary-side main winding (W3-1), a feedback unit (W3-2) to generate an auxiliary voltage as a function of a secondary-side output voltage (Vout), and a control circuit comprising an output voltage regulator unit (102) that is operable to reduce the primary-side current flow in response to a sensed value (Vaux) of the auxiliary voltage and to perform a foldback current limiting. The control circuit further comprises a voltage regulation deactivating unit (104) that is operable to increase the sensed value of the auxiliary voltage, when the output voltage reaches a defined lower threshold.