Methods for realizing high power density secondary modules

　　The DC/DC secondary power supply is the most rapidly developing high-frequency switching power supply. It mainly powers various ICs on communication boards, and its trend is low voltage, high current, low thickness, high power density and high efficiency, etc. The products are divided into two categories: low-power non-standard products and medium-to-high-power standard products. The latter is the current development direction, such as full brick, half brick, 1/4 brick and 1/8 brick. The power density of standard products is getting higher and higher, the output voltage is getting lower and lower, the output current is getting higher and higher, and the product process is getting more and more difficult.

　　A company producing DC/DC secondary power modules must have a fast product response capability to meet the rapidly growing needs of communication customers. In the past few years, many power supply companies generally chose active clamp forward circuits as the main circuit and aluminum substrates as the main process when developing DC/DC standard brick products. The reason is that there are too many literatures on

　　The active clamp forward circuit is positively introduced in these documents. The advantages of the active clamp forward converter are described as follows:

　　(1) Due to the auxiliary switch SA, the transformer's excitation energy can be fed back and utilized;

　　(2) Full demagnetization of the transformer and operation in quadrants I and III can reduce the size of the transformer;

　　(3) No switching voltage spikes, eliminating the absorption circuit;

　　(4) It can work in a duty cycle range greater than 0.5, thereby increasing the transformer ratio (Np: Ns), reducing the conduction loss of the primary switch and transformer; reducing the reverse voltage of the secondary diode and thus reducing its conduction loss; reducing the size of the filter inductor L, etc.;

　　(5) When the output voltage is low and the current is high, the MOSFET used for the secondary side rectification can be easily and synchronously driven. It is not easy to make this circuit into a good product because its dynamic relationship is complex and the dynamic index is difficult to optimize. Especially when the dynamic is large, it is very easy to damage the auxiliary tube. A similar patent is the complementary drive half-bridge circuit, which is also a US patent. It also has the same dynamic problem and is very easy to damage the tube when lightly loaded.

　　Principles and advantages of existing high power density secondary module product solutions

　　The standard DC/DC secondary power supply (half brick, 1/4 brick) module adopts a two-stage circuit scheme in which Buck and DC/DC transformers are cascaded, as shown in Figure (1). At first glance, this scheme has more components than the single-stage scheme (such as the active clamp forward circuit), which is not very reasonable. However, through analysis, it can be found that: the total number of power devices has not increased (because in low-voltage and high-current output applications, the output synchronous rectification MOSFET must be connected in parallel to achieve the efficiency index even in the single-stage scheme); the total area of ​​the magnetic core and capacitor components has not increased (because smaller magnetic cores and capacitors can be used). However, since the two-stage scheme is easy to optimize and design, the electrical performance of the module has been greatly improved. The specific analysis is shown in the introduction below.

　　The first stage of this scheme is a synchronous rectifier Buck converter, which adjusts the unstable input voltage to the appropriate input voltage of the DC/DC transformer at the subsequent stage, and controls the PWM duty cycle of the Buck converter through feedback output and error amplification, thereby stabilizing the output. The circuit and working waveform of the DC/DC transformer are shown in Figure 2:

　　The two primary side main pipes S1 and S2 are controlled by square wave, each with a duty cycle of 50% and complementary. There is the following steady-state relationship:

　　Vo=V1/N, where N=Np/Ns

　　And because V1=dVin, the total input-output relationship of the two stages is: Vo=dVin/N. This relationship is the same as that of the forward circuit.

　　As can be seen from the figure, the driving voltage of the secondary synchronous rectification is 2Vo, which is a constant that is independent of the load and input voltage. After a careful analysis of this solution, it can be seen that it has the following advantages:

　　(1) Since the maximum withstand voltage of the input MOSFET is Vinmax and the maximum voltage of the output MOSFET is 2Vomax, all power devices can be packaged in SO-8;

　　(2) Since the DC/DC transformer is implemented by two iron cores, the conduction loss of the secondary PCB connection can be greatly reduced when large current is output;

　　(3) Since the inductor is placed on the primary side, its design is easier and the conduction loss when placed on the secondary side is also reduced;

　　(4) The secondary MOSFET is easy to self-drive and its driving voltage is fixed;

　　(5) The design of the maximum operating duty cycle can be determined by the transformation ratio of the DC/DC transformer, which is very flexible;

　　(6) The circuit has excellent dynamic performance and the loop is easy to design and compensate;

　　(7) It can facilitate the layout of multi-layer PCBs, making it easier to increase the power density of the module, etc.

　　In summary, this solution has many advantages that single-stage circuits do not have in applications with wide input voltage range, high power density, low thickness, low voltage and high current output (such as half-brick and 1/4 brick modules), and can achieve higher efficiency, better dynamics, higher power density and higher reliability.

　　Innovations in existing high power density secondary module product solutions

　　This solution is actually a very simple two-stage circuit cascade. Its innovation is to challenge the framework of traditional single-stage circuits and consider the realization of products from different angles. In fact, two-stage PFC and single-stage PFC are very similar examples. In AC/DC research, everyone has gradually realized that single-stage PFC may be a misunderstanding, and two-stage is the best choice for AC/DC product development. In DC/DC research, especially in those occasions with wide input range, low voltage and high current output, I think the same problem exists, but before this solution was proposed, no one thought about it in this direction. So this idea is a more practical idea and a real innovation.

　　The development of a power supply product is actually a process of optimizing and compromising a multivariable system, so innovation is a more comprehensive way to solve the problem. If it can be achieved with the simplest circuit, then it is the best innovation. This is the innovation of this solution. Their solution looks so simple that many people will ignore it, but after it becomes a product, it has the best cost performance.

　　in conclusion

　　This article starts with introducing new methods for implementing existing high power density secondary modules , and compares ideas, innovations, and product optimization. It is hoped that power developers will not stick to traditional views and ideas, but consider issues from multiple angles.