Power Switch Circuit Back-to-Back Transistor Soft Start Control
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Solution Overview
Problem
In power switch circuits using transistors in back-to-back series connection, differences in threshold voltages can lead to unsmoothed equivalent resistance variation, causing inrush current and soft start failure during startup.
Innovation Solution
A control circuit is used to determine the type of series connection between the transistors and generate control voltages to ensure that one transistor is fully turned on before the other, preventing overlapping switching intervals and thus avoiding inrush current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If two transistors are connected in back-to-back series to prevent parasitic diode conduction, then the power switch can be completely turned off, but the transistors may switch at different times due to threshold voltage differences, causing unsmoothed equivalent resistance variation and inrush current during soft start
Solution Approach 1:
The control circuit performs preliminary action by completely turning on one transistor before turning on the other during soft start. This sequential activation ensures that the equivalent resistance changes smoothly without abrupt transitions, preventing inrush current while maintaining the back-to-back connection's ability to block parasitic diodes.
Solution Approach 2:
The control circuit dynamically adjusts the switching sequence of the two transistors based on their threshold voltage characteristics. By adapting the turn-on timing to individual transistor properties, the system maintains smooth equivalent resistance variation during soft start while preserving reliable off-state operation through the back-to-back configuration.
2Reliability
If the control circuit controls one transistor completely before controlling the other, then the switching intervals do not overlap and soft start failure is prevented, but the control complexity increases
Solution Approach 1:
The control circuit uses feedback from voltage detection at the connection node to determine when one transistor is completely turned on. This feedback mechanism automatically regulates the switching sequence without requiring complex external control logic, reducing control circuit complexity while ensuring reliable soft start operation.
Solution Approach 2:
The control circuit leverages the inherent voltage characteristics at the transistor connection node to self-regulate the switching sequence. By using the node voltage as a natural reference point, the circuit determines optimal turn-on timing without additional sensing components or complex control algorithms, minimizing added complexity while improving soft start reliability.
Data Source
AI summary
A power switch circuit including first and second transistors, and a control circuit is provided. A first end of the first transistor serves as an input terminal of the power switch circuit. A second end of the first transistor is coupled to a node. A control end of the first transistor receives a first control voltage. A first end of the second transistor serves as an output terminal of the power switch circuit. A second end of the second transistor is coupled to the node. A control end of the second transistor receives a second control voltage. The control circuit detects a voltage of the node to determine a type of series connection between the first and second transistors, and generates the first and second control voltages to control a turned-on state of another of the first and second transistors after turning on one of the first and second transistors.