Analysis Of Typical Failure Mode Of FX0041 Abnormal Output And Research On Improvement Measures

FX0041 is a classic power operational amplifier of the author's company.It is widely used in voltage-controlled current source circuits to drive inductive loads such as servos or motors.Due to the characteristics of the FX0041 device itself and the complexity of the application scenario,the typical failure mode of the FX0041 output abnormality is self-oscillation.Regarding the possible oscillation phenomenon of the negative feedback system,a large number of literatures use the two-port or return ratio method to analyze the stability of the feedback circuit.This method of measuring loop gain will break the loop.Breaking the loop will cause two problems: one is that the DC operating points on both sides are usually different,and the other is that the small signal AC impedance seen from both sides is different from the closed loop situation.Although the Rosenstark's open-circuit/shortcircuit method can measure the loop gain relatively accurately,it is usually an inaccurate and error-prone method to calculate the loop gain by breaking the loop.However,most engineers still use a simple two-port or return ratio method to analyze the stability of the feedback circuit.Due to these problems,this thesis will implement the closed-loop Middlebrook's double-injection method based on Pspice.This method injects an ideal AC voltage and current source into the circuit without changing the DC operating point and small-signal AC impedance.The loop gain of the lower measurement circuit overcomes the shortcomings of the loop gain measurement under the open loop.Although the closedloop analysis method can measure the loop gain more accurately,it is not convenient to analyze the cause of instability of the operational amplifier circuit.Therefore,this article combines the respective advantages of closed-loop and open-loop methods to analyze the stability of the FX0041 voltage-controlled current source.Based on the Rosenstark's open-circuit/short-circuit method and Middlebrook's double-injection method,the FX0041 voltage-controlled current source circuit is determined to be suitable for breaking the loop node.Since the two-port method needs to consider the feedback structure,and the return ratio method does not need to consider the feedback structure,it is more general.By establishing the FX0041 transistor-level micromodel and extracting the key parasitic parameters of the printed circuit board,the return ratio simulation method is used to analyze the results: the low-frequency pole formed by the inverting input stray capacitance and load inductance attenuates the phase margin,which leads to FX0041 outputs oscillation.Then,the frequency compensation is realized by the way of feedback-lead,which solves the stability problem of the circuit.The return ratio method,Rosenstark's open-circuit/short-circuit method and Middlebrook's double-injection method all assume that the signal is transmitted in one direction,but in fact the signal is transmitted in both directions.Only reasonable approximate analysis can get accurate results.In order to verify the accuracy of the simulation analysis results,the FX0041 voltage-controlled current source evaluation board was designed and manufactured,and the accuracy of the stability analysis and the effectiveness of frequency compensation were verified by the transient overshoot percentage measurement.Therefore,it provides a set of complete and feasible solutions for engineers to design voltage-controlled current source circuits and measure loop gain,which has high practical value.