Research on Capacitive Load of Small DC/DC Switching Power Supply

In the application of DC/DC switching power supply, the external capacitor at the output load can play the role of filtering and suppressing interference. In some devices with large capacitive load dynamic jump, the power output terminal is required to have a fast response, which requires the switching power supply to have a strong ability to carry capacitive loads and have good stability. In the design process of the switching power supply, in order to fully understand and realize the special requirements of the customer's load use, it is necessary to analyze the two different state requirements of the capacitive load capacity of the switching power supply.

1 Capacitive load requirements

In the basic characteristics of switching power supplies, there are generally no strict specifications for capacitive load capacity. Generally, power supplies can carry quite capacitive loads, but considering the overcurrent protection capability of the power supply, especially the output short-circuit protection, the capacitive load capacity cannot be too large, otherwise the protection capability will deteriorate. For the capacitive load carried by modules with multiple outputs, the distribution principle is that the total energy storage of the capacitor cannot exceed 0.25J, including the sum of the energy storage of the capacitor of the main circuit and the energy storage of the auxiliary circuit (1/2∑CU2). Based on this calculation, the selection of the maximum capacitive load in the design is shown in Table 1 (the maximum capacitive load of modules with different circuits and output powers will be different. When actually selecting and using them, it is necessary to refer to the relevant given technical parameter manual) In actual design and application, the output capacitive load capacity of the switching power supply is debugged and tested by the following two requirements:

1) The power supply works stably, only the load changes from no load to full load, and the output voltage is stable

When the module works normally, the DC/DC switching power supply can be equivalent to a voltage source. Its output equivalent circuit diagram and simplified equivalent circuit diagram are shown in Figure 1 and Figure 2 respectively. Among them, U is the output voltage, RS is the equivalent internal resistance, RL is the output load resistance, C is the output capacitance,

According to Figure 2,

It can be assumed that the capacitor voltage UC is zero before the switch S is closed. From equation (1), we can get

From the above calculation, it can be seen that the capacitor voltage VC rises exponentially. In order to establish the output voltage faster and more stably, the power supply output internal resistance should be small. Generally, performance improvement and enhancement can be achieved through a high-gain, fast-response output voltage regulation feedback loop.

However, due to the energy storage of the output inductor, voltage feedback and front-end peak current control, the capacitor voltage is not a completely exponential waveform starting from zero, as shown in Figure 3.

During the stabilization of the output voltage, on the one hand, the energy storage of the output filter inductor is gradually supplemented, and on the other hand, the primary side of the feedback loop control circuit quickly outputs more power. The value of the output inductor is generally determined by the current ripple coefficient λ and the no-load characteristics of the power supply. In order to avoid excessive undershoot of the output voltage due to capacitive jump, which makes the control circuit reach its limit, the value of the inductor should be greater than the value calculated from the theoretical value of λ, but at the same time, the voltage overshoot amplitude when the output is unloaded should also be considered, so the output inductor cannot be too large. Large inductors are generally difficult to make and have high costs, so the actual value of the inductor can be obtained by experimental methods. Figure 4 shows the jump waveform of the output inductor current when the capacitive load suddenly changes when the output inductor Lout has different values. The maximum capacitive load capacity.

The experiment shows that the module with large output inductance carries a large capacitive load, the inductor energy storage helps to stabilize the output voltage, the current limiting protection circuit works for a short time, but the response time will be correspondingly longer.

2) The module starts with output capacitor, and the output voltage is stable

When the module starts with a large capacitor, the capacitor needs to be charged quickly to maintain a stable output voltage, and a large current will be generated at the moment of startup. If the large current lasts too long during the startup process, the protection function of the module control chip will reach its limit, and a poor startup phenomenon will occur, that is, the output voltage cannot be established normally; in addition, the size of the capacitive load directly affects the rise time of the output voltage, and application failures will occur in an environment with strict output voltage rise time requirements. Generally, the output voltage of a self-fed power supply is proportional to the supply voltage. Before the output reaches the normal voltage, the chip VCC cannot meet the power supply requirements. Therefore, the power supply method of the startup circuit and the energy storage of the VCC capacitor are also important factors in determining the capacitive load capacity.

2 Methods to improve the capacitive load capacity of switching power supply

If the auxiliary power supply is an independent power supply, the output voltage can be slowly built up by soft start. For self-feeding power supply, it is difficult to start if the soft start time is long. At this time, the VCC capacitor capacity should be increased. A large VCC capacity will affect the module that relies solely on the internal current loop of the chip for short-circuit protection. Therefore, it is necessary to find a balance between capacitive load and overcurrent (and short-circuit protection).

Appropriately reducing the value of the current limiting resistor is equivalent to increasing the output power of the power supply, which is used to increase the energy provided to the output load during the startup process. Adjusting the feedback loop parameters to speed up the desaturation speed of integrated components such as op amps and optocouplers can solve the serious overshoot problem when the capacitive load jumps, that is, improve the stability of the power supply.

3 Typical cases of capacitive load design

When designing the Forward DC/DC converter, the designers adjusted the current limiting resistor, the starting capacitor and other related parameters based on the two basic requirements of capacitive load capacity, and referred to the theoretical calculated values ​​for debugging and testing. The capacitive load capacity of the Golden Sun series model (VRB_LD_15W) DC/DC module has been greatly enhanced to meet the wide range of applications of customers in various special environments.

4 Application and summary of capacitive load capacity

The required size of capacitive load is closely related to the application environment. The capacitive load capacity is a balanced choice based on short-circuit performance and overload protection capability. In the design of modules with special specifications, the above parameters must be optimized and debugged to achieve the best design to meet specific application requirements.