1 Introduction
Since the parameter values ​​of a large number of components in an electronic circuit are deviated from the nominal value due to processing dispersion, external environment and degradation effects during actual operation, the output characteristics of the electronic circuit are shifted, and some component parameters in the circuit A small shift in value can cause dramatic changes in output characteristics [1-2]. The presence of parasitic parameters in electronic circuits, fluctuations in input characteristics, and external electromagnetic interference can also affect the stability of electronic systems. The reliability tolerance design of the electronic circuit can effectively solve the above problems and improve the reliability of the electronic system. The Led Street Lamp constant current driving power source studied in this paper is a dedicated power source for driving series high-power LED street lamps, and its reliability directly affects the reliability of the entire LED lighting system. The forward current, operating voltage, and the environment in which the LED drive power is applied affect the luminous flux and lifetime of the LED. If the deviation between the output characteristics of the LED driver and the design requirements is within the allowable range, the LED illuminator will still work normally, otherwise the illuminator will malfunction. EDA-based reliability tolerance design for LED driver power enables parallel analysis and design of circuit performance and reliability, and finally results in key components and tolerance designs that affect circuit performance.
Reliability design for electronic circuits has attracted the attention of many scholars. In recent years, some scholars have used mathematical tests such as uniform test and orthogonal test to optimize the parameters of the switching power supply control circuit and electronic ballast. This can greatly improve the reliability of electronic circuits under the premise of ensuring performance indicators [3-4]. However, these methods focus on the system design and parameter design of the electronic circuit, and there is no tolerance analysis and design for the circuit. Reliability tolerance analysis methods include Monte-Carlo analysis, worst case analysis, interval analysis, and affine analysis. Since these methods need to establish a mathematical model of the circuit, although some simple circuits can be subjected to tolerance analysis, But when it comes to larger-scale electronic circuits, there is nothing that can be done [1-3]. Using EDA's powerful modeling and calculation capabilities to design reliability tolerances for electronic circuits can better solve this problem. And this method has been applied to the tolerance design of hybrid contactors, controllers and filters for railway locomotives [5-7]. However, these methods only consider the processing dispersibility of components, and there is still a problem of large workload.
Firstly, based on the previous work, this paper puts forward the reliability tolerance design considering temperature effect, and fully uses the mathematical methods of orthogonal test, uniform test and regression analysis to further improve the reliability tolerance design method based on EDA. By designing the reliability tolerance of the LED constant current driving power supply, on the one hand, it is beneficial to control the consistency of the output characteristics of the circuit; on the other hand, under the premise of ensuring the reliability index of the circuit, the production cost is reduced.
2 Reliability tolerance design method
The circuit tolerance design process includes sensitivity analysis, tolerance analysis, and tolerance distribution. Through the sensitivity analysis, the key components that affect the output characteristics can be found. Through the tolerance analysis, the influence of the deviation of each design variable on the output characteristics can be obtained, and the tolerance distribution scheme can be tested by using the tolerance distribution. The allowable deviation of the output characteristics is assigned to each relevant design variable to provide a basis for the design of each variable. Finally, the temperature effect simulation is used to analyze the reliability of the electronic system operating over a wide temperature range.
2.1 Sensitivity analysis based on orthogonal test
Circuit sensitivity refers to the sensitivity of the output characteristics of the circuit to the parameters of each circuit component. Relative sensitivity is usually used to judge the degree of influence of factors on the target characteristics, which is defined as the ratio of the relative change in the output characteristics of the circuit to the relative change in the component parameters [8]. Let f = f(x1, x2,..., xn), where f is the output characteristic of the circuit and xi is the input characteristic of the circuit. If x10, x20, ..., xn0 are the parameter center values ​​of n components, then the relative sensitivity 
Mathematical expression for
When there are many internal components in the system, the workload of sensitivity analysis will be very large, so the test design method must be used to arrange the test reasonably. Orthogonal test design is a method for multi-factor testing. It selects some representative points from the comprehensive test to test. These points are characterized by “uniform†and “tidy†and high efficiency. And a wide range of applications. In the orthogonal test design, the range analysis method is generally used for sensitivity analysis.
2.2 Tolerance analysis based on uniform test
The most common method of tolerance analysis is Monte Carlo analysis. The principle is a statistical analysis method for analyzing the deviation of circuit performance parameters by the sampling value of the circuit component parameters when the parameters of the circuit components obey a certain distribution. The analysis results of this method are the closest to the actual situation, but the disadvantage is that a large number of experiments are required. There are many problems in the process of tolerance analysis, and there are many problems at each level. This problem can be solved by a uniform test design method. The advantage of the uniform test design over the comprehensive test and the orthogonal test design is that the number of tests is greatly reduced, the test cycle is shortened, and at the same time, representative. In this paper, a uniform test method is used to select representative sets of solutions for tolerance analysis in the tolerance allocation scheme space.
2.3 Tolerance scheme selection based on regression analysis
Set n observation data of p sensitive components by uniform test 
Its multiple linear regression model can be expressed as
In this paper, the relationship between the pass rate and each related design variable is obtained by solving the regression coefficient matrix. After finding the linear regression equation, it is also necessary to test the regression equation for significance to check whether there is a significant linear relationship between the relevant variables. With this relationship, a reliable tolerance design can be obtained.
2.4 Reliability Tolerance Design Process
The flow chart of the new reliability tolerance design method proposed by EDA simulation technology is shown in Figure 1. It can be seen from the flow chart that the design method of reliability tolerance of electronic circuit mainly consists of the following two processes: the first process is 1-3 in the flow chart, which mainly determines the nature and reliability requirements of the circuit, and uses appropriate EDA software. simulation. The second process is 4-9 in the flow chart. Using the EDA model of the electronic circuit, the mathematical analysis methods such as Monte-Carlo analysis, orthogonal test, uniform test and regression analysis are used for tolerance analysis and distribution, and finally the tolerance design is obtained. result. If there is no tolerance distribution scheme that meets the requirements, you need to perform 10 to re-design the parameters of the circuit.
Figure 1 Flow chart of reliability tolerance design of electronic circuit
3 LED Street Lamp constant current driving power supply reliability index and modeling
3.1 LED constant current drive power supply works
As a load of a driving power source, LEDs often require dozens or even hundreds of combinations to form a light-emitting component. The type of LED driver and how the LED load is connected is directly related to its reliability and lifetime. Applying a single-ended flyback switching power supply to drive multiple white LEDs in series is a valuable LED driver. Its principle block diagram is shown in Figure 2. The circuit achieves a constant output current by means of current negative feedback.
Figure 2 LED constant current drive power supply block diagram U - front pole rectified output voltage Rc, Cc, VDc - RCD circuit