CMOS Solid State Relay With Transformer Oscillator Isolation
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Solution Overview
Problem
Existing solid state relays face challenges with slow switching times, integration difficulties on integrated circuit chips, cross-talk issues in multi-channel designs, and high trigger current requirements for triac switches, making them unsuitable for high-bandwidth and low-voltage, high-current applications.
Innovation Solution
Incorporating the primary winding of an isolation transformer into the tank circuit of an oscillator, rectifying the oscillator's output to drive a relay switch circuit, and implementing the entire system, including a micro-transformer, oscillator, rectifier, and switch circuit, on a CMOS integrated circuit chip.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an opto-isolator is used for isolation, then isolation between control circuit and relay is achieved, but switching speed becomes slow and integration with solid state relay on IC chip becomes difficult
Solution Approach 1:
The patent replaces the opto-isolator (optical system) with a transformer isolator (electromagnetic system). The transformer couples the control circuit to the relay through magnetic fields, providing electrical isolation while enabling faster switching speeds and compatibility with IC chip fabrication processes.
Solution Approach 2:
The patent changes the isolation mechanism from optical to electromagnetic, fundamentally altering the physical parameter domain. This transformation enables the system to achieve both isolation and fast switching performance simultaneously, as electromagnetic coupling responds more rapidly than optical detection and conversion.
2Reliability
If a transformer isolator is used with a triac switch, then isolation is achieved, but the triac requires high trigger current (30-50 mA) which requires a larger transformer not readily implementable in IC chip process
Solution Approach 1:
The patent replaces the triac switch with a MOSFET switch, fundamentally changing the switching device type. MOSFETs have much lower gate trigger currents compared to triac trigger currents, enabling the use of smaller transformers that can be integrated into IC chip processes while maintaining isolation functionality.
Solution Approach 2:
The patent substitutes the triac (thyristor family device) with a MOSFET (field-effect transistor), replacing a device requiring high current triggering with one that operates with minimal gate current. This substitution enables miniaturization of the transformer and overall system integration on IC chips.
3Ease of operation
If a triac is used as a discrete element, then switching function is achieved, but it is difficult to fabricate in an integrated circuit chip process
Solution Approach 1:
The patent changes the device type from triac to MOSFET, which are standard components in CMOS fabrication processes. MOSFETs can be easily fabricated using standard IC chip processes, enabling full integration of the relay circuit on a single chip while maintaining the switching function.
4Reliability
If a transformer isolator is used, then isolation is achieved, but switching speed becomes slow due to size and higher inductance
Solution Approach 1:
The patent replaces the triac with a MOSFET, which has much lower input impedance and can be driven by smaller currents. This enables the use of smaller transformers with lower inductance values, which switch faster and reduce the time constant of the isolation stage, thereby improving overall switching speed.
5Ease of operation
If a triac is used with an inductive load, then switching function is achieved, but a snubber circuit must be used to address turn off delay caused by induced current
Solution Approach 1:
The patent replaces the triac with a MOSFET, which can be turned off by removing the gate voltage regardless of the load conditions. MOSFETs do not suffer from the same commutation problems as triacs when driving inductive loads, eliminating the need for snubber circuits and reducing turn-off delay.
6Ease of operation
If a triac is used, then switching function is achieved, but the relatively high forward resistance makes it less than ideal for low voltage, high current loads
Solution Approach 1:
The patent replaces the triac with a MOSFET, which has a much lower on-resistance when fully enhanced. MOSFETs are ideal for low voltage, high current applications because their resistance drops significantly when the gate voltage exceeds the threshold, minimizing power loss and improving efficiency.
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 approach enables a fast, high-bandwidth solid state relay with quick on/off capabilities, suitable for multi-channel applications and integration on a single CMOS chip, eliminating cross-talk and high trigger current issues.
Implementation Method 1
Another type of isolator is a transformer isolator that passes the control signal from the primary to the secondary
Implementation Method 2
A rectifier responsive to the a.c. signal from the oscillator circuit provides a d.c. drive signal
Data Source
AI summary
A solid state relay includes: an oscillator circuit responsive to a control signal for generating an a.c. signal; an isolation transformer having a primary winding which forms a part of the tank circuit of the oscillator circuit and a secondary winding; a rectifier responsive to the a.c. signal from the oscillator circuit for providing a d.c. drive signal; and a switch circuit responsive to the drive signal to open and close the relay.