What should I pay attention to when designing and developing a switching power supply DC-DC control chip?

Chip design is crucial, and chip design is also a national key development project. Therefore, we should have a certain understanding of chip design. The power supply is the heart of all electronic equipment, and its quality directly affects the reliability of electronic equipment. The switching power supply is more so, more and more people pay attention to it. At present, the development of computer equipment and various high-efficiency portable electronic products tends to be miniaturized, and their power consumption is relatively large. It is required that the supporting battery power system be smaller, lighter, and more efficient. High-efficiency DC must be used. / DC switching power supply.

At present, the main development direction of power electronics and circuits is modularization and integration. Special chips with various control functions have developed rapidly in recent years. Integration and modularization make the power supply products small in size and high in reliability, which brings great convenience to applications. On the other hand, in the switching power supply DC-DC converter, since the input voltage or the load on the output terminal may fluctuate, the average DC output voltage should be kept within the required amplitude deviation range, and complicated control techniques are required. Therefore, the research of various PWM control structures has become a research hotspot. Under such a premise, designing and developing a switching power supply DC-DC control chip is very valuable in terms of economy and scientific research.

DC-DC converters use the switching of one or more switching devices to transform a certain level of DC input voltage into another level of DC output voltage. At a given DC input voltage, one of the control methods for controlling the average output voltage by adjusting the on-time of the circuit switching device is to use a fixed frequency for switching, and adjust the length of the conduction interval to control the average output voltage. This method is also called pulse width modulation [PWM] method. PWM can be divided into two categories in terms of control methods, namely voltage mode control (voltage mode control) and current mode control (current mode control). The basic principle of the voltage control method is to compare the output signal of the error amplifier with a fixed sawtooth wave to generate a PWM signal for control. From the perspective of control theory, the voltage control method is a single-loop control system. The voltage-controlled converter is a second-order system, which has two state variables: the voltage of the output filter capacitor and the current of the output filter inductor. The second-order system is a conditionally stable system. Only after careful design and calculation of the control circuit, the closed-loop system can work stably under certain conditions.

Current-mode control refers to comparing the output signal of the error amplifier with the peak current of the inductor sampled. Therefore, the duty ratio of the output pulse is controlled so that the peak current of the output inductor changes as the error voltage changes. The current control type is a first-order system, and the first-order system is an unconditional stable system. Based on the traditional PWM voltage control, the current negative feedback link is added to make it a double-loop control system, so that the inductor current is not an independent variable, so that the second-order model of the switching converter becomes a first-order System, signal. Compared with a single closed-loop voltage control mode, current mode control is a double closed-loop control system. The outer loop is formed by the output voltage feedback circuit, and the inner loop is formed by the transformer sampling the output inductor current. In double-loop control, the current inner loop is controlled by the voltage outer loop, that is, the inner loop current rises in each switching cycle until the error voltage threshold set by the voltage outer loop is reached. The current inner loop performs pulse-by-pulse comparison quickly and instantaneously, and monitors the dynamic change of the output inductor current. The voltage outer loop is only responsible for controlling the output voltage. Therefore, the current control mode has a much larger bandwidth than the voltage control mode.

The current-mode control mode has many advantages: the linear adjustment rate (voltage adjustment rate) is very good; the entire feedback circuit becomes a first-order circuit. Since the feedback signal circuit is reduced by one order compared to the voltage-type, the control of the error amplifier The loop compensation network is simplified, the stability is improved and the frequency response is improved, with a larger gain bandwidth product; with instantaneous peak current limiting function; simplifying the design of the feedback control compensation network, load current limiting, flux balance and other circuits , Reducing the number and cost of components, which is of great significance for improving the power density of the switching power supply, achieving miniaturization and modularization.

Of course, there are also disadvantages. For example, when the duty cycle is greater than 50%, the system may be unstable and may generate sub-harmonic oscillations. In addition, there are limitations in the choice of circuit topology. In the boost type and buck-boost type In the circuit, there is an error between the peak current and the average current because the energy storage inductor is not at the output. Sensitive to noise, poor noise resistance, etc. For such shortcomings, there has been a solution, ramp compensation is a necessary method.

If you want to know more, our website has product specifications for DC-DC control chip, you can go to ALLICDATA ELECTRONICS LIMITED to get more information