Oscillator circuits with a CMOS Schmitt trigger-inverter are commonly used in applications that relate to relay driver circuits. It is possible to devise a fail-dangerous occurrence from the circuit, in which an open fault happens at the input circuit between the capacitor and input inverter IC. The causes of this self-oscillation event contribute to the failure of other parts of the circuit. This paper presents countermeasures for self-oscillation of the capacitor open-circuit self-oscillation in a CMOS Schmitt trigger-invertor oscillator circuit for a fail-safe relay drive. The proposed circuit replaces a normal 2-pin capacitor with a special 4-pin designed capacitor which connects a parallel resistor between the input CMOS inverter and the ground source. This paper carried out experimentation using Failure Modes and Effects Analysis (FMEA).The results showed that the output logic was high when the circuit had an open fault. Thus, the new designed circuit had no fail-dangerous occurrences.
Słowa kluczowe: CMOS inverter, Self-oscillation, Open-fault, Fail-safe
W artykule opisano środki zaracze zapobiegające samooscylacjom w obwodzie pojemnościowym CMOS przerzutnika Schmitta w obwodzie przekształtnika. Metoda poleg ana zastąpieniu dwukońcówkowej pojemności obwodem z czterema końcówkami z rezystorem miedzy wejściem przekształtnika a masą.
Keywords: przekształtnik CMOS, samooscylacje, przerzutnik Schmitta
In the field of power electronics, especially motor drive systems, the motor current cut-off in terms of both fail-safe and cost-effectiveness is important because cut-off relays have additionally been provided and a driver circuit should be embedded in the motor drive control systems. Meanwhile, CPU-based safety-related control circuits are being utilised in the industry. However, such CPU-based safety-related control applications require certain subsidiary safety measures, e.g. diagnostic functions and redundant hardware structure, since their circuits are not simple. Moreover, CMOS elementary logic gates , which are widely used and easily available, are not normally applicable to safety-related controls because of the possibility of self-oscillation in the case of input open-faults. A key requirement of the safety relay drive circuit is no faildangerous occurrences when a fault occurs in the operation. The problem with such a circuit is self-oscillation (e.g. the input IC open-fault). The proposed relay drive circuit will be designed in the form of a fail-safe circuit  that does not use integrated circuits, a CPU, or complex systems. The paper [4-5] presents how to solve such problems by employing a special 4-pin designed capacitor covered by a metal shield between the input CMOS inverter and the ground source, which resolves the parasitic capacitance issue within the CMOS inverter. However, the solution could create a new problem where the capacitor or metal shield has an open-fault occurring spontaneously. Fault diagnosis of the analogue circuit is an area of great importance in the design, manufacturing and utilisation processes for electronic devices. For diagnostic methods, there are two main causes of such situations. The first is the difficulty in diagnosing analogue circuits due to the non-linear characteristics and tolerances of the system's elements. The second is new challenges such as limited access [...]
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