"Introduction
At present, in the field of high-end electronic equipment, high-grade DC / DC power modules mainly rely on foreign imports. Therefore, it is urgent to realize the localization of high-quality DC / DC power modules as soon as possible. Taking the design of a class H DC / DC power module as an example, this paper introduces the design idea of class H DC / DC power module in detail from the aspects of circuit and technology.
1 Main Indicators of module
The DC / DC power module is used for the whole machine. The required input voltage is 20 ~ 35V, the output voltage is 10 ~ 10.1v / 10 ~ 10.2v (- 55 ℃ ~ + 125cic), 5 ~ - 5.1V / 5 ~ - 5.2v (a 55 ℃ ~ + 125 ℃), the load regulation rate and voltage regulation rate are less than 1%, and the output ripple voltage is less than 40mv
2 circuit design
2.1 unbalanced dual power and voltage output design in DC / DC converter design
For two modules with unequal output power, there are two main design methods: one is to use transformer winding, and use coupling inductance and low-voltage voltage voltage stabilizing circuit for secondary voltage stabilizing method. Second, the secondary multi winding of the transformer is used to output two relatively independent voltages respectively. Although method 1 can improve the stability of the circuit and ensure the accuracy of the output voltage, it will increase the loss of the circuit, because the smaller the difference between the input and output voltage of the secondary voltage stabilizing circuit, the smaller the power consumption of the voltage stabilizing circuit. The two output powers of the project differ greatly (55W and 2.5 w respectively), and the variation range of the main power is also large. If method 2 is adopted, because the feedback can only control one circuit of voltage, the accuracy of one circuit of output voltage can only be guaranteed, and the other circuit of voltage can only be pre stabilized by transformer and filter inductance. The large change of main circuit output power will inevitably lead to the large change of secondary voltage of auxiliary circuit transformer, so the accuracy of output voltage can not be guaranteed. Therefore, this design uses two outputs to control and feedback independently, which can not only accurately control the output voltage, but also reduce the loss caused by secondary voltage stabilization.
2.2 design of transformer
When designing the transformer, the magnetic core material should be reasonably selected first. The main factors to be considered for the core material are its loss at the operating frequency and the applied flux density. After determining the working frequency of the module, the specific model of the material can be determined according to the manual provided by the manufacturer, and then find out the magnetic flux saturation density of the module under the worst environmental conditions, and then determine the maximum magnetic flux density, so as to ensure that the transformer will not work at the saturation point and improve the reliability of the module.
If the set working frequency is 500 kHz, it can be found from the manual that PC40, pc95 series of TDK company and n67 series of Siemens company can meet the requirements. After determining the specific magnetic core model, shape and size, you can find the magnetic flux saturation density BS of the model at 125 ℃, and then select the maximum magnetic flux density of 0.2bs according to the derating design. After determining Bmax, you can calculate the number of turns on the primary side of the transformer according to the following formula:
Where, KF is the waveform coefficient (4 in case of square wave) and is the switching frequency (Hz); AE is the effective area of the magnetic core (M2), Bmax is the magnetic flux density (T), VI is the input voltage (V), and NP is the number of turns on the primary side.
The number of secondary side turns of transformer can be calculated from the number of primary side turns:
Where NP is the number of turns on the secondary side and viin is the minimum input voltage on the primary side. The losses associated with the transformer mainly include hysteresis, eddy current and resistance losses. The hysteresis loss is related to the number of turns of the winding. It determines the work done by the magnetic force in each working cycle. This loss can be given by the following formula:
Where, KH watt is the hysteresis loss constant of the material, VC is the core volume, in cm3, FSW is the switching frequency, in Hz, and Bmax is the maximum working magnetic flux density, in t.
As can be seen from equation (3), the loss is directly proportional to the quadratic of the maximum switching frequency and working magnetic flux density. Therefore, in the design, on the premise of selecting the magnetic core with high permeability and low high frequency loss as the transformer magnetic core, Bmax should be set reasonably, and the hysteresis loss of the transformer should be reduced by reasonably designing the number of turns.
2.3 design of feedback compensation circuit
As we all know, negative feedback loop is the core part of switching power supply. It keeps the output voltage constant mainly by using error amplifier to reduce the error between output voltage and ideal reference voltage. Because there are load changes and input voltage changes in practical application, the error amplifier is required to respond quickly to these changes, and will not oscillate and cause the instability of the whole system.
The design of error amplifier compensation circuit shall follow the following principles: firstly, at all frequencies with gain greater than 0dB, the closed-loop phase shall not exceed - 360 °, and in the actual design, it is generally less than 300 °; Secondly, the crossing frequency of the closed-loop gain is as high as possible and the DC gain is as large as possible, which is conducive to improving the regulation accuracy and transient response of the system; Finally, the slope of the closed-loop gain curve should decrease with - 20dB / Dec.
Because different power supplies adopt different control methods, the corresponding compensation methods are also different. In this paper, the current mode control is adopted, and the compensation method is pole zero compensation. The circuit is shown in Figure 1. This compensation method has a pole at the DC, and the output regulation performance can be improved by increasing the open-loop gain of the error amplifier. A zero point is introduced at or below the lowest pole frequency of the output filter to compensate the phase lag caused by the filter poles.
Because the circuit with two outputs for independent control and feedback is complex and the assembly density is high, the interference between leads, between leads and components, and between leads and casing will inevitably increase; In addition, the circuit works in the switching state. Due to pulsating current and noise between unit circuits, it is easy to form coupling noise through common impedance such as internal resistance of power supply and lead wire.
In the design, firstly, a filter link can be added in the circuit to suppress the coupling noise. In addition, the common impedance can be reduced as much as possible in the conductor layout, the grounding point can be set reasonably, and the internal resistance of the power supply can be reduced to reduce the noise. At the same time, the signal line can be separated from the noise source and the lead flowing through the pulsating current to reduce the coupling noise.
Secondly, the generation of noise should be reduced as much as possible in the design. The short-circuit peak current caused by the reverse recovery time of switching diode is the main cause of module noise. In this design, Schottky diodes with fast reverse recovery time can be used, and reasonable selection can be made according to specific output conditions. Appropriate derating design can also better suppress noise. For the noise generated by high-frequency transformer, decoupling and shielding methods can be used in module design to reduce the interference. The output ripple peak value of the module is less than 40 mV.
3 product reliability design
Since the quality level of the product is required to be level H, the product shall not only pass the identification QCI inspection, but also pass the QML inspection. The test conditions shall be in accordance with gjb548a-96 and gjb2438a-2002. The main test conditions of QML are listed in Table 1.
It can be seen from table 1 that the test of this product is quite severe. Since the product is required to work reliably for a long time in the full temperature range (- 55 ℃ ~ + 125 ℃), reliability design should be considered in both circuit design and process structure.
3.1 reliability optimization design
First of all, in terms of design, the circuit structure should be simplified as much as possible, not only to realize the circuit performance, but also to simplify the variety and quantity of components, so as to reduce the reduction of reliability caused by the failure of components. Secondly, after the preliminary design is completed, the reliability comprehensive analysis software should be used for analysis and verification, and the deficiencies should be found for improvement and improvement. Finally, various parameters are reasonably designed to make the product work in the best state.
3.2 derating design
The electrical stress of components includes current stress and voltage stress. The derating design of components can effectively improve the reliability of products. Derating coefficient is generally 0.5 ~ 0.8 times. If the input voltage is 16 ~ 40V, the rated voltage of the adopted input capacitor is 50V and should have 2.5 times the withstand voltage capacity; The output voltage is 5V, and the reverse withstand voltage of rectifier diode is 45V; The output voltage is 10 V, and the reverse withstand voltage of the selected rectifier diode is 100 v.
3.3 thermal design
Because the DC / DC converter is a power module and the product has to work reliably at 125 ℃ for a long time, thermal design is very important. Thermal analysis software and infrared thermal imager can be used for analysis, design and improvement. The specific technical measures are as follows:
(1) In the input / output filter circuit, the sheet monolithic capacitor with high reliability can be used to replace the traditional tantalum capacitor, so as to avoid the failure of tantalum capacitor at high temperature;
(2) Try to improve the efficiency of the product. Low power consumption components can be selected. The reasonable design of transformer can effectively reduce the internal power consumption of the product and reduce the temperature rise of the product. In the miniaturization design of the product, the heat source shall be evenly distributed in the layout (such as VMOS tube, rectifier tube and other heat sources) to avoid heat concentration in a part of the product;
(3) The heat source shall be in full contact with the substrate and the substrate and the shell. The current carrying welding method can be adopted instead of bonding method, which can reduce the thermal resistance and avoid the heat accumulation of the product;
3.4 process reliability design
In order to meet the requirements of various stress tests of products, the process design can be carried out through the following aspects:
(1) Thick film multilayer wiring process can be adopted in the process, the number of wiring layers is more than three, and the line width and line spacing can be 200 μ m. Generally, the resistance in the circuit shall be film resistance, and laser repair technology shall be adopted to make the resistance meet the circuit design requirements.
(2) Because the product is a product with high shell temperature (125 ℃), the control of water vapor content is a difficulty. Through the transformation of relevant equipment and process research, as well as repeated experiments on vacuum baking process parameters, appropriate data can be obtained to effectively control the atmosphere during sealing and make the moisture content less than 5000 PM; The oxygen content is less than 2000 PM, so as to ensure the reliability of the product for long-term use.
(3) In the internal structure of DC / DC converter, there is a certain temperature stress between substrate and shell, substrate and components. In order to meet the h-level requirements of the product, it is necessary to analyze the stress concentration point and mechanical strength. Through the metallization design of the substrate surface and the control of the thickness of the surface film layer of the substrate, the above stress conditions can be satisfied between the substrate and the base. The problem of bonding strength between components and substrate can also be effectively solved through reasonable selection of relevant components and reasonable welding process for large components. In the internal structure of DC / DC converter, the components with the largest volume and the heaviest weight are transformer and inductor. Due to the brittleness and fragility of its magnetic material, there is also a large difference in the coefficient of thermal expansion between it and A12O3 substrate. Therefore, in the design, the internal structure can adopt the symmetrical distribution of substrate to bond the transformer and inductor directly to the base. In this way, cracking caused by temperature stress can be avoided; Secondly, it can reduce the substrate area, so as to avoid the formation of unbalanced gravity points on the substrate, resulting in the cracking of the substrate in mechanical impact; thirdly, it is also good for the heat dissipation of the transformer.
(4) By selecting appropriate conductive adhesive and epoxy adhesive, the process parameters, operation procedures and the matching characteristics between various adhesives and bonding elements can be studied and optimized, and the purpose of high reliable bonding can also be achieved; For example, for the transformer, selecting a suitable adhesive to bond the transformer will not only have a certain tensile strength, but also have a certain toughness. Its temperature characteristics match with the magnetic tank material, but also ensure that the magnetic tank will not crack in the temperature cycle.
(5) Optimize welding and bonding technology. The composition, strength characteristics and temperature coefficient of different types of solder paste and solder wire shall be compared and analyzed to determine the best welding material, so as to ensure the high reliability of welding elements; Right 25 μ M Au and 200 μ Through the test of MA1 wire process, it can be determined to cure and avoid the failure of gold aluminum wire bonding.
4 verification results
At present, the product has successfully passed the direct QML inspection of the Fifth Institute of the Ministry of information industry and has passed the appraisal. The main technical indexes and measured indexes of the products are listed in Table 2., Read the full text, technical section
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