Low-Side Contactor Coil Drive With Normally-On Solid-State Switching
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Solution Overview
Problem
Conventional low-side contactor coil drives in aerospace electrical systems require support for transient and steady-state currents, which traditional systems address using relays, but these are costly and unreliable, and low-power solid-state devices like JFETs are not practical due to high current requirements.
Innovation Solution
A low-side contactor coil drive circuit utilizing a combination of solid-state switches, including NPN BJT, PNP BJT, and N-channel JFET, with a normally-on JFET that controls the state of other switches, and an optocoupler for control, eliminating relays and minimizing leakage current through resistor configurations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If relays are used for low-side contactor coil drives, then the circuit can handle high currents and maintain normally-on state, but the system suffers from high cost, reliability problems, and large physical area
Solution Approach 1:
The patent replaces the mechanical relay system with a solid-state circuit consisting of transistors (Q1, Q2, Q3), resistors (R1-R4), and an optocoupler. This substitution eliminates mechanical moving parts, improving reliability while reducing physical size and complexity. The solid-state switches can handle the required high currents without the wear and failure modes inherent in mechanical relays.
Solution Approach 2:
The optocoupler serves as an intermediary device that provides electrical isolation between the control circuit and the power circuit. This allows the solid-state switches to be controlled without direct electrical connection, enhancing reliability by preventing ground loops and electrical interference while maintaining the normally-on functionality.
2Device complexity
If low-power solid state devices like JFETs are used, then device complexity and physical area are reduced, but they cannot support the required transient currents of 1-10 A
Solution Approach 1:
The patent segments the current handling function across multiple solid-state devices. Transistor Q1 handles the main high current path, while transistors Q2 and Q3 manage control and protection functions. This segmentation allows each device to operate within its optimal current range while collectively handling the full 1-10 A transient current requirement.
Solution Approach 2:
The patent combines multiple solid-state switches (BJT and JFET) in a complementary configuration to achieve both high current handling capability and low power consumption. The merging of different transistor types allows the circuit to leverage the high current capability of BJTs while using JFETs for efficient control, resolving the contradiction between power handling and device complexity.
3Device complexity
If solid state switches are used to eliminate relays, then cost and physical area are reduced, but leakage current may cause unintended switching
Solution Approach 1:
The patent converts the potentially harmful leakage current into a beneficial feature by designing the circuit with normally-off transistors Q2 and Q3 that actively suppress any leakage through Q1. The leakage current that would normally cause problems is instead used to verify the normally-on state, while the control transistors prevent unintended switching by maintaining proper bias conditions.
Solution Approach 2:
The circuit incorporates feedback through the interconnection of transistors Q2 and Q3, which monitor and respond to leakage currents in real-time. This feedback mechanism ensures that any leakage current is immediately counteracted, preventing unintended switching while maintaining the desired normally-on operational state.
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 provides a reliable, cost-effective, all-solid-state circuit capable of handling high currents with low voltage drop and reduced leakage, ensuring the circuit remains on when the controller is unpowered and turns off as commanded, addressing the limitations of traditional relay-based systems.
Implementation Method 1
an N-channel JFET. The third switch drain can be connected to the second switch base
Implementation Method 2
The solid state switch controller can include an optocoupler, for example
Data Source
AI summary
A low-side contactor coil drive circuit can include an input line and a first solid state switch having a first switch base, a first switch collector, and a first switch emitter. The first switch collector can be connected to the input line and the first switch emitter is connected to ground. The circuit can include a second solid state switch having a second switch base, a second switch collector, and a second switch emitter. The second switch emitter can be connected to the input line in parallel with the first switch collector. The second switch collector can be connected to the first switch base. The circuit can include a third solid state switch having a third switch gate, a third switch source, and a third switch drain. The third switch drain can be connected to the second switch base.