Optimized design of DC-DC power supply system

    Abstract: DC-DC power system efficiency is a very critical issue in compact electronic equipment. The interaction between various parts of the DC-DC power supply system and the main factors affecting the efficiency of the DC-DC power supply system are analyzed in detail, and it is pointed out that: combined with the input characteristics of the DC-DC converter, reasonably configure the power supply parameters and select the DC-DC operation points can effectively improve DC-DC power system efficiency.

    Keywords: DC-DC converter system efficiency DC-DC operating point power supply internal resistance

With the miniaturization of electronic devices, power supply for compact electronic devices is a very important issue. At present, DC-DC converters are commonly used in compact electronic equipment that requires power-saving power supply. The purpose of applying DC-DC converters is, on the one hand, to perform voltage conversion and provide appropriate operating voltages for some devices, but more importantly, to ensure high system efficiency and small size while converting voltage. Under normal circumstances, an excellent DC-DC converter has a conversion efficiency of over 95%. Higher system efficiency can not only extend the power life cycle, but also further reduce the size of the equipment.

After analysis, it is not difficult to find that the system efficiency of the DC-DC power supply is limited by the energy-consuming components of the power system itself, such as the internal resistance of the power supply, filter impedance, connecting wires and contact resistance; on the other hand, it is limited by the DC-DC conversion The working status of the converter is also closely related to the power supply parameters. Reasonable configuration of these design parameters can improve system efficiency. The energy consumption of the internal resistance of the power supply will reduce the efficiency of the power supply itself, and also affect the input voltage of the DC-DC converter, thus also affecting the conversion efficiency of the DC-DC converter. In extreme cases, the DC-DC converter will enter an abnormal state. In severe cases, the system will completely stop working. Even if it can work normally, the system efficiency will be seriously lost. Therefore, rationally selecting the power supply voltage, reducing the internal resistance of the power supply, and correctly selecting the operating point of the DC-DC converter in the design can effectively improve the system efficiency of the DC-DC power supply. The key to the optimal design of a DC-DC power supply system is to correctly analyze the interaction between various parts of the electronic equipment (especially between the power supply and the DC-DC converter) and find out the main factors that affect the efficiency of the power supply system.

1 Power distribution in general electronic equipment

Generally, electronic systems containing DC-DC power supplies can be divided into three parts: power supply, voltage regulator (DC-DC converter) and load, as shown in Figure 1. The actual power supply part can be a power supply group or a regulated or unregulated DC power supply. During analysis, it can be equivalent to an ideal voltage source Vs and a power supply internal resistance Rs. Among them, Rs includes power supply output impedance, series filter resistance, wire resistance, contact resistance, etc. These resistances are energy-consuming components and will seriously affect the power supply efficiency. Power supply efficiency (Es) is defined as the ratio of the power drawn by the voltage regulator to the total power delivered by the power supply:

Es=Pi/Ps=[(IiVi)/(IiVs)] ·100% (1)

The voltage regulator consists of a control IC and related peripheral components. Some of the characteristic parameters of the control IC can be found in the data sheet provided by the manufacturer. The conversion efficiency (Ed) of a voltage regulator is defined as the ratio of the power supplied to the load section by the DC-DC converter to its input power:

Ed=Po/Pi=[(IoVo)/(IiVi) ]·100% (2)

According to the manufacturer's description, the DC-DC conversion efficiency Ed is a function of the input voltage Vi, the output voltage Vo and the load current I0. However, under normal circumstances, the conversion efficiency Ed is insensitive to changes in load current I0. When the change in load current Io exceeds two orders of magnitude, the change in efficiency will not exceed a few percentage points. Therefore, under non-extreme conditions, DC-DC conversion The conversion efficiency of the converter can usually be approximated as a constant. However, in extreme cases, the conversion efficiency will be seriously lost, which can be seen from the input characteristics of the DC-DC converter (shown as ① in Figure 2). For now, consider the DC-DC converter as a two-port black box.

The load part is similar to the power part and can also be equivalent to the effective load Rl and the energy-consuming component RP connected to it. The efficiency of the load section (Ei) is defined as the ratio of the actual power drawn by the effective load to the output power delivered by the converter:

El=Pl/Po=(IoVl/IoVo)·100% (3)

The efficiency of the entire system should be the product of the efficiencies of the three parts Es, Ed, and Ei. Since these three parts influence each other, the key to system optimization design lies in correctly analyzing the interaction between the three parts and rationally configuring the relevant parameters between them to maximize the efficiency of the entire system.

2 Key factors affecting system efficiency

Assume the system efficiency is Ea, then Ea=EsEdEl

From formulas [1], [2], [3] we can get:

Ea=[(EdViVi)/(VsVo)]·100% (4)

In the actual system, Vo and Vl are determined by the load circuit requirements, and the power supply part should provide appropriate and stable voltage to the load. Due to the special design inside the DC-DC converter, Ed only changes within a small range under normal conditions. As long as the operating point is appropriately selected, Ed is approximately a constant. Only Vi and Vs are optional, so the key to ensuring higher system efficiency lies in the values ​​of Vi and Vs. Vi and Vs depend on the interaction between the power supply and the voltage regulator. According to the circuit principle, there is this relationship between them:

Vs=IiRs+Vi (5)

Obviously, in order to reduce the loss of the internal resistance of the power supply, when Vs is selected, Ii should be made as small as possible, while Vi should be as large as possible. This is also consistent with the input characteristics of the DC-DC converter (as shown in Figure 1 ① shown) are consistent. It can be seen from the input characteristics of the DC-DC converter that a larger Vi within the effective operating range means that the regulator will draw less current, so the loss in the internal resistance of the power supply will be reduced, while the output will be almost unchanged. This improves the efficiency of the power supply section. So how do you make the regulator draw less current from the power supply while keeping the output almost constant? This requires a reasonable selection of the operating point of the DC-DC converter.

3 Selection of working point of DC-DC converter

① in Figure 2 is the input characteristic curve of a general DC-DC converter. It can be seen that its input has a certain dynamic range, and its input characteristic curve can be clearly divided into three intervals. When Vi

In Figure 2, ② is the resistive load characteristic of the power supply, which is determined by equation (5). The intersection of ① and ② is the operating point Q of the DC-DC converter. In order to make ① and ② have an intersection and fall within the effective working area of ​​the DC-DC converter, Vs and Rs must be selected reasonably. Among them, Rs determines the slope of the characteristic equation, and Vs determines the intersection point of the characteristic agenda and the horizontal axis. Therefore, changing Vs and Rs can move the operating point Q. Combined with the above analysis, the working point efficiency of Vi should be as high as possible.

In addition, it can be seen from the figure that when Vs is selected, the operating point is determined by the internal resistance Rs of the power supply. To prevent the operating point from ever entering the non-effective operating area, the load line slope (-1/Rsmax) should have a limit, that is, the internal resistance Rs of the power supply should have an upper limit Rsmax. The figure shows:

Rsmax=(Vs-Vmin)/Iimax (6)

Among them: Iimax=Po/EdVmin (7)

From formulas [6] and [7], we get:

Rsmax=[(Vs-Vmin) ×Ed×Vmin]/Po (8)

That is to say, the total resistance R between the power supply part and the DC-DC converter should always be smaller than Rsmax. Otherwise, the operating point of the DC-DC converter will enter an abnormal operating area, seriously losing system efficiency, or even causing the DC-DC converter to stop working completely. This is particularly important in actual design. The DC-DC power supply has very high requirements on the resistance Rs between the power supply part and the DC-DC converter. For example, to convert a 5V power supply into a 3.3V output and provide a load current of 2A, if the DC-DC converter MAX797 chip (Vmin=4.5V) is selected and a conversion efficiency of 90% is guaranteed, Rs should not be greater than 0.307 Ω ; If a conversion efficiency of 95% is required, Rs should not be greater than 0.162 Ω . It can be seen that the DC-DC power supply has very high requirements on the resistance Rs between the power supply part and the DC-DC converter. Rs is also a key factor affecting system efficiency.

Based on the above analysis, the efficiency of DC-DC power supply system in compact electronic equipment is a very important issue. The key to optimal design of DC-DC power supply system is to correctly analyze the interaction between the power supply and the voltage regulator, and reasonably configure the parameters of the power supply and the operating point of the regulator, which can effectively improve the efficiency of the entire system.