Design of a missile-borne secondary power supply

The power supply is the power source of all electronic equipment and is the basic component to ensure the normal operation of electronic equipment. According to relevant statistics, power supply failure accounts for about 40% to 50% of the failure rate of electronic equipment. For this reason, some basic requirements must be put forward for the power supply, including practical performance requirements and electrical performance requirements. This is especially true for the secondary power supply on board. It must be carefully considered. In addition to meeting the power supply capacity, its grounding method, efficiency, and the choice between switching power supply and linear power supply must also be considered.

1 Basic requirements for secondary power supply
1.1 High reliability
The mean time between failures (MTBF) is an important indicator for measuring the reliability of power supply. In the general standard, it is stipulated that the reliability index is greater than or equal to 3,000 h as the minimum requirement.
1.2 High safety
The switching power supply designed and manufactured should meet the safety index requirements specified in the relevant standards or specifications, such as heat dissipation requirements, electrical strength requirements, and protection against electric shock, so as to prevent power failures from endangering personal and equipment safety under extreme conditions or harsh environmental conditions.
1.3 Good maintainability
When the power supply fails, the fault phenomenon and location should be diagnosed in time, and the fault can be effectively solved or the faulty module can be replaced.

2 Secondary power supply design ideas
Due to its space and system requirements, the secondary power supply design needs to be miniaturized, have good electromagnetic compatibility, and high DC-DC efficiency to meet the power requirements of each component. The test and debugging of the linear integrated voltage regulator is relatively simple. If the two are combined, it will provide convenience for the subsequent stage design of the product. After comprehensively considering the linear voltage regulator, switching voltage regulator or composite design, and analyzing the advantages, disadvantages and feasibility of various solutions, this secondary power supply will be designed by combining the linear integrated voltage regulator with the DC-DC, that is, the composite design. This design has a relatively high efficiency and can meet the power requirements of each component. For the power supply circuit with relatively high ripple requirements, a linear voltage regulator is used.

3 Specific design analysis of the secondary power supply
3.1 Power supply grounding
design There is another key point and difficulty in designing the power supply, which is grounding. Grounding is very simple in the literal sense, but it may be the most difficult technology to master for those who have experienced the frustration of electromagnetic interference. In fact, grounding is the most difficult technology in electromagnetic compatibility design. Facing a system, no one can come up with an absolutely correct grounding solution, which will leave some problems. The reason for this situation is that there is no systematic theory or model for grounding. When considering grounding, people can only rely on past experience or experience from books. But grounding is a very complex issue. A good solution in other occasions may not be the best here. Grounding design relies heavily on the designer's intuition, that is, his understanding and experience of the concept of "grounding". There are many grounding methods, and the specific method to be used depends on the structure and function of the system.
3.1.1 Single-point grounding
Single-point grounding includes single-point grounding of unit circuits, between circuits, and between devices. Figure 1 shows a schematic diagram of single-point grounding. Its advantage is that it can suppress conducted interference. When single-point grounding is used, since each circuit and device is connected to one grounding point, the closed loop of interference current in the signal ground system is eliminated. The interference voltage on the equipment ground will not enter the signal circuit through the grounding circuit. Such grounding uses a long wire, and the impedance of the grounding wire itself is considerable, which is not good for high-frequency signal grounding. When the wiring length reaches 1/4 of the signal wavelength or an odd multiple thereof, the ground wire impedance becomes very high, and it is not a grounding wire but more like a radiating antenna.

3.1.2 Multi-point grounding
In a multi-point grounding system, each circuit and device has multiple points of parallel grounding. Because it can be grounded nearby and the grounding wire is short, the high-frequency standing wave effect can be reduced. However, this grounding method has multiple ground loops. The 50 Hz mains in the public ground is easily coupled to the signal loop through the public ground loop. Engineering practice shows that if the return lines of the power supply and signal can be separated, the return lines of strong signals and weak signals can be separated, and sensitive signals such as weak signals and pyrotechnic signals use separate return lines, the interference caused by the loop will be greatly reduced. Figure 2 shows a schematic diagram of multi-point grounding.
3.1.3 Hybrid grounding
Hybrid grounding includes both the characteristics of single-point grounding and the characteristics of multi-point grounding. For example, the power supply in the system requires single-point grounding, while the RF signal requires multi-point grounding. In this case, the hybrid grounding shown in Figure 3 can be used. For DC, the capacitor is open and the circuit is single-point grounded. For RF, the capacitor is conductive and the circuit is multi-point grounded. Figure 3 shows a schematic diagram of hybrid grounding.

In practical applications, when the signal frequency is lower than 1 MHz, single-point grounding is used; when it is higher than 10 MHz, multi-point grounding is used; when the frequency is between 1 and 10 MHz, if the length of the grounding wire is greater than 1/20 wavelength, single-point grounding is used; otherwise, multi-point grounding should be used. The secondary power supply of the missile is a low-frequency circuit, so single-point grounding is selected, and when designing the circuit board, attention should be paid to making the ground wire as wide as possible and running in a straight line to ensure clean grounding.
3.2 Power switching design
Because the product includes "preheating" and "preparation" when working, and only includes "preheating" when working normally, the power switching part must also be designed, as shown in Figure 4.
When the power supply is in the preheating state, the transient current of the 27 V power supply reaches 5.6 A; in the preparation state, 27 V preheating and 28.5 V preparation are powered at the same time, and the current reaches 5.25 A; after leaving the carrier, the power supply is a single 28.5 V preparation power supply, and the current reaches 5.25 A. According to the voltage and current characteristics, the selected diode should meet the requirements of large rated current, high reverse working voltage, meet the use requirements, and its package should be easy to install, and it is installed on the shell of the placement chamber to facilitate the heat dissipation of the diode.
3.3 Linear voltage regulator power supply circuit design
There is a 12 V power supply in this power supply, which is mainly used to power several microwave components. The power supply ripple level is required to be high. In order to meet the requirements and make full use of the advantages of linear power supply, a linear voltage regulator circuit is specially selected for design. Specific values ​​are not given here, only examples are given to illustrate the effect of selecting a suitable capacitor on eliminating ripples, as shown in Figure 5.

A current limiting protection resistor is connected to the power input to reduce the voltage drop of the integrated voltage regulator, reduce the power consumption of the integrated voltage regulator itself, improve the efficiency of the module, and protect the module from instantaneous short-circuit current. The capacitor C connected to the input/output terminal plays a role in filtering and improving the transient effect of the load, thereby reducing the output ripple. All integrated voltage regulators in the circuit use a fixed positive voltage output series. After superimposing an AC component on the input voltage of 30 V, observe the output. The output ripple size here is the key point, so it is necessary to select a suitable capacitor for filtering. Whether or not to use a suitable capacitor has a great difference in output ripple, as shown in Figure 6 and Figure 7 respectively.

3.4 Switching power supply circuit design
This power supply also uses a DC-DC module circuit. Here we analyze its advantages and disadvantages and propose solutions. No specific values ​​are given here, only principle analysis is done. As shown in Figure 8.
This circuit is a relatively typical switching power supply circuit. Its biggest advantage is high efficiency. The module used in the circuit can achieve a utilization rate of 90%. The biggest disadvantage of the switching power supply is that the output ripple is large. In addition to the input rectification pulsation component, it is mainly the switching frequency fundamental ripple, which is sawtooth-shaped. At the same time, the peak switching noise generated by the power switch tube in the on-off transition state overlaps on the sawtooth wave. When the output ripple is observed with an oscilloscope, when the scanning frequency is low, only the low-frequency component of the rectification pulsation may be observed, and the switching frequency fundamental ripple is modulated by the low frequency. Observe the fundamental ripple, the scanning frequency should match the switching frequency, as shown in Figure 9.

The solution to this problem is to add filter capacitors to the output port, and when wiring the PCB board, the output copper wire should be as wide as possible, and the line spacing should not be too large. The output parallel capacitor should be as close to the module power supply as possible to reduce interference.

4 Key technologies and solutions The
main problem is about filtering. Whether it is a linear power supply or a switching power supply, excessive output ripple is an undesirable problem. In addition to adding filter capacitors in principle to solve this problem, it is also necessary to use actual engineering experience, such as making the PCB board wiring as wide as possible, the spacing as small as possible, and the output capacitor as close to the module as possible, which will be of great help in reducing ripple energy. There are also problems encountered in actual engineering. The output current of some circuits in the secondary power supply of the seeker is large. It is necessary to select a suitable integrated voltage regulator, and at the same time consider its efficiency and heat dissipation. At present, there are many types of linear integrated voltage regulator products, which are small in size, good in stability, high in precision, low in noise, strong in ripple suppression, and good in electromagnetic compatibility. However, its efficiency is low and it is a power device with large heat dissipation. Drawing on previous experience, the linear power supply is placed on the surface of the placement cabin, the DC-DC module is located inside the electronic cabin to power the processor, the integrated voltage regulator has fewer peripheral circuit components, and the integrated voltage regulator and external devices are placed separately and connected with wires.

5 Conclusion
The key components in this design are all mature products with reliable quality; at the same time, grounding, filtering, space, heat dissipation and electromagnetic compatibility design are fully considered. This makes the secondary power supply design simple, with a small number of components and a small size, and ultimately can achieve small output ripple, good voltage regulation characteristics, and easy design, assembly and debugging, meeting subsequent engineering requirements.