Research on Reliability Tolerance Design Technology of LED Street Light Constant Current Drive Power Supply

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

3.2 LED constant current drive power reliability index

The LEDs used to drive the power supply load have a nominal forward drive current of 700 mA. The relationship between LED luminous flux and forward current is as follows: when the forward current is less than 700mA, the luminous flux of this LED increases with the increase of current; after the forward driving current reaches 700mA, the current continues to increase, and the luminous flux will not be obvious. The change is saturated, but the heat loss of the LED will increase dramatically. The reliability index of the driving power supply is given below according to the requirements of the LED.

(1) Output current fluctuation range: (700 ± 15) mA.

(2) Operating temperature: −30～60°C.

(3) Pass rate requirement: 0.95.

3.3 LED street light constant current drive power supply modeling

The accuracy of the EDA-based reliability tolerance design depends on the accuracy of the circuit simulation model. This paper uses Pspice to model and simulate the circuit.

(1) Transformer model. The transformer is a key component of a single-ended flyback drive. The existence of parasitic parameters in the transformer directly affects the operational reliability of the entire circuit. Considering that the equivalent circuit model of the transformer can accurately simulate the influence of the parasitic parameters of the transformer on the driving power supply, this method is used to model the transformer.

Let the magnetic resistance of the transformer core be Rm, the primary and secondary voltages are U1 and U2, respectively, the primary and secondary currents are i1 and i2, respectively, the primary and secondary turns ratio is n1/n2, and the excitation current is imp, so according to the formula ( 3) ~ (6) can obtain a transformer model considering the magnetizing inductance. The model of the further transformer considering parasitic inductance, winding loss and core loss is shown in Figure 3. In the figure, Rac1 and Rac2 are the parasitic resistances of the primary and secondary windings, Ll1 and Ll2 are the primary and secondary parasitic inductances respectively, Rf is the equivalent resistance of the transformer core loss, and Lmp is the primary excitation inductance.

Figure 3 Transformer equivalent circuit model

(2) Component temperature model. The parameters of electronic components (resistors, capacitors) vary with temperature. Changes in the parameters of the components may cause drift in the output signal of the circuit and may even affect the normal operation of the circuit. For example, the simplified temperature effect model of a resistor in Pspice is given by equation (7).

Where Res represents the resistance of the resistor at a certain temperature; Tc1, Tc2 and T0 represent the linear temperature coefficient, the nonlinear temperature coefficient and the normal temperature value (30 ° C), respectively.

Temperature effects of resistors and capacitors can be more accurately described using a linear temperature model. The regulator TL431 uses linear and squared temperature coefficients. Table 1 lists the temperature coefficients of the components used in each circuit.

(3) Single-ended flyback. The Pspice model of the LED street lamp constant current drive power supply is shown in Figure 4. By comparing the simulation and test waveforms of the drain current and the drain and source voltages of the switch, it can be seen that the established model of the LED constant current drive power can simulate the working state of the actual circuit.

Table 1 Temperature coefficient of each component

4 LED street lamp constant current drive power supply reliability tolerance design

The process of designing the reliability tolerance of the LED street lamp constant current driving power supply by the above reliability tolerance design method is as follows:

(1) According to the circuit schematic diagram of the LED street lamp constant current driving power supply, the design factor for determining the sensitivity analysis in the circuit is 19, and each factor takes two levels, and the value changes to 2%. The L20 (219) orthogonal table was used for 20 tests. The absolute value of the relative sensitivity of each component calculated from the expression of relative sensitivity is shown in Fig. 5. The key components that affect the output are: three-terminal regulator U12 (TL431) and its related voltage divider resistors R11 and R14, and the sampling resistor Rs. The relative sensitivities of these four key components are: SR(U12)=+7.17mA, SR(Rs)=−6.10m, SR(R11)=+3.09mA, SR(R14)=−2.54mA.

(2) In the sensitivity analysis of the previous step, the four key elements obtained have three resistors and a voltage regulator that provides a reference voltage. Generally, the common resistance is 6%, 2.5%, 1%, 0.5%, 0.25%, and 0.1%. The regulator U12 has three levels of accuracy: 0.5%, 1%, and 2%. According to the determined factors and the number of levels, the uniform design table U7 (76) is selected, and according to the use table, four columns of 1, 2, 3, and 6 are selected for the test. The selection of uniform test factors and their levels are shown in Table 2. The accuracy of other non-critical resistor components is set to 5%, and the accuracy of non-critical capacitors is set to 20%.

Figure 4 LED street lamp constant current driving power supply simulation model

Figure 5 Relative sensitivity of constant current drive circuit components

Table 2 Selection of uniform test factors and their levels

Take a uniform design design of a set of tolerances, U12 accuracy ± 2%, Rs accuracy ± 2%, R14 accuracy ± 1%, R11 accuracy ± 0.1%, substituted into the circuit model for Monte-Carlo analysis, the results are shown in Figure 6. Shown. It can be seen from the statistical histogram that the output current of the LED driving circuit is approximately normal distribution, and the average value μ is 700 mA, and the standard deviation σ is 14.441 mA.

Figure 7 shows the probability density distribution law and constraints of the output current under the tolerance distribution scheme. The probability density of the output current is normally distributed, that is, Iout~N(μ, σ2). Let the constraints be x1, x2, then calculate the reliability of the output current, that is, calculate the probability that Iout is between x1 and x2, and the calculation method is as shown in equation (8).

Figure 6 Constant current drive circuit output current statistical histogram

Figure 7 Constraint and probability density distribution of output current

The yield of the output current under the tolerance distribution scheme is 0.706 95. The calculation results of the pass rate of other tests are shown in Table 3.

Table 3 Output current pass rate

(3) If the pass rate of the output current is α, the functional relationship between the pass rate and the influencing factors through regression analysis is

Regression analysis significant statistical statistic F0.95=0.044, less than the significance level of 0.05, it can be considered that the linear relationship between α and U12, Rs, R14, R11 is significant.

(4) Substituting all the tolerance allocation schemes into the above relationship, a total of 73 tolerance distribution schemes with a pass rate of α≥0.95 are obtained. Since the sampling resistor is a power resistor, its price increases greatly with the change of accuracy. The above tolerance allocation schemes are rearranged to make the accuracy of the sampling resistor as large as possible. The tolerance distribution scheme U12 was selected to be 0.5%, Rs was 1%, and R14 and R11 were both 0.1%.

(5) Adding the obtained tolerance allocation scheme to the PSpice software for Monte-Carlo analysis, the output current center value μ(Iout)=700.988 mA, and the standard deviation σ(Iout)=4.747 mA. After the calculation according to the constraint conditions, the yield rate α(Iout)=0.9961 of the output current under the distribution scheme is obtained, and the result of the tolerance calculation is larger than the required pass rate index of 0.95. Figure 8 shows the simulation results of the LED constant current drive power supply output current and the experimental results when the ambient temperature changes from −30°C to 90°C. The trend of simulation and measured output current is basically the same. It can be seen that the temperature model of the established components more accurately reflects the actual situation.

Figure 8 Comparison of output current simulation results with experimental results

The goal of tolerance design is that in the worst case, the output characteristics are still within the allowable tolerances. Through temperature simulation and testing, it is found that the output current rise value ΔIout ≤ 3mA at the lowest limit temperature −30°C; at the highest limit temperature 60°C, the output current drop value |ΔIout| ≤3mA. Considering the temperature effect, the allowable output current range of the LED driver power supply is narrowed from 685 to 715 mA to 688 to 712 mA. After calculation, considering the temperature effect, the reliability of the LED constant current driving power supply is α (Iout)=0.9876, which still meets the requirements.

5 Conclusion

In this paper, mathematical methods such as orthogonal test, uniform test and regression analysis are applied to the reliability tolerance design process of electronic circuits. Using this method to design the reliability tolerance of the LED street lamp constant current driving power supply, the following conclusions can be drawn:

(1) The key components that affect the output current of the LED constant current driving power supply are the voltage regulator U12 and the related voltage dividing resistors R11 and R14 and the sampling resistor Rs in the voltage generating circuit.

(2) In order to ensure the qualification rate of 0.95, the tolerance distribution scheme of each key component is as follows (according to the accuracy grade): U12 is 0.5%, Rs is 1%, R14 and R11 are 0.1%, and other non-critical resistors. The accuracy of the components is set to 5%, and the accuracy of non-critical capacitors is set to 20%.

(3) In order to ensure the yield of 0.95 in the wide temperature range of −30°C～60°C, the linear temperature coefficient of R14 and R11 can be taken as −200ppm/°C, and the linear temperature coefficient of power resistor Rs It can take 100ppm/°C, and the linear and square temperature coefficients of TL431 can be 14ppm/°C and −1ppm/°C2 respectively.

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