DC-DC converters are used to improve the safety and reliability of power supply.

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DC-DC converters are used to improve the safety and reliability of power supply. [Copy link]

The design of the special power supply for DC motor servo drive uses DC-DC converter as the basis to realize the power conversion from 220V mains to +60V/20A. The power factor correction technology is used in the power supply design to improve the active power; in particular, the power supply is designed with a microcomputer control interface to work synchronously with the servo system, and realizes the power-on timing control to ensure that the +60V voltage lags behind the low-voltage output. Multiple filtering measures and twisted pair output mode are adopted to effectively reduce the output voltage ripple and improve the power supply output quality; it has perfect self-protection function and monitoring and detection function, which improves the safety and reliability of the power supply.

　　Power Design

　　Module series connection

　　System Structure

　　After rectification and filtering, the 220V AC voltage is rectified and filtered to obtain a DC voltage of about 320V, which is added to the input of the power module. The output voltage of a single DC-DC module is generally +48V. To obtain a DC output voltage of +60V, the module series connection method must be used. The design uses two PH600S280-28DC-DC modules (adjusted to +30V output) to obtain a +60V output voltage through series connection. The fast recovery diodes D1 and D2 are protection devices in the series connection method. The reverse withstand voltage of D1 and D2 is required to be greater than twice the rated output voltage of the power supply, and the current is greater than twice the rated output current of the power supply. The forward conduction voltage drop should be as small as possible. Since two power modules are connected in series to form a power supply system, it is difficult to complete the design within a limited package. Some module series connection schemes use two packages to complete the design, that is, design two 30V independent power supplies, and then connect them in series externally to form a +60V power supply system. The design rationally configures the space, installs a DC-DC module in each of the upper and lower covers of the power supply, uses a metal shell as a heat dissipation method, and adopts compact design and installation technology to encapsulate the entire power supply system in a smaller space, greatly reducing the size and weight of the entire power supply. The cross-sectional area is only 6×9 inches2, realizing a small-volume, high-power integrated power supply system design.

　　Power Factor Correction Measures

　　The bridge rectifier and large capacitor filter circuit of the switching power supply make the overall load appear capacitive, causing a difference in the current and voltage phase of the 220V AC input, resulting in a low power factor, a decrease in active power, and the generation of high-order harmonics that pollute the power grid. Therefore, power factor correction (PFC) measures must be taken. Based on factors such as cost control, circuit volume, and ease of application, we use passive power factor correction measures. The passive PFC has a simple structure. According to the overall load characteristics of the power supply, a power inductor with appropriate parameters is connected in series before the large filter capacitor. Here, a 10mH/8A toroidal core inductor is used. The overall load characteristics of the power supply are forced to be balanced to ensure that the power factor is not less than 0.8. Passive PFC uses passive components such as inductors, which are reliable and low-cost, and do not require modifications to the original electrical design. It is currently a commonly used PFC method.

　　Design features and key technologies

　　Microcomputer control and detection interface

　　The microcomputer control function can ensure that the +60V/20A power supply starts output only when the computer sends an enable signal and the servo system is working. The power supply has no output at other times. This way of working synchronously with the servo system has a series of advantages such as power saving, low heat generation, and flexible control. In a series of power supplies in a certain equipment power supply system, the +60V/20A power supply has a power consumption of , but a heat generation of , and a temperature rise of , which fully proves the superiority of using a computer control interface in power supply design.

　　As shown

　　Power-on sequence control

　　There is a power-on timing problem in the DC motor control system. Generally, the driving voltage powers on quickly, but it takes a certain amount of time to establish the control level after the control circuit voltage is powered on. In this way, if the power-on timing control is not performed, at the moment the system is powered on, the high voltage powers on faster than the low voltage, and the control level is relatively delayed, resulting in the loss of control of the servo system at the moment of power-on, and the motor has a short period of uncontrolled rotation, especially in the bipolar control mode. The traditional solution is to manually control the power-on timing by setting up high and low voltage switches, or to design a power-on timing control circuit in the control system, which will inevitably increase the complexity of the circuit, resulting in increased circuit cost and reduced reliability. The solution to this problem on the motor drive power supply is simple and effective. The working principle is: the CNT end is the module enable control end, which can control the working state of the module and serve as the control switch of the output voltage. Optocouplers are usually used to control the state of the CNT end. Just add an optocoupler to solve the power-on timing problem. As shown in Figure 2, the optocoupler input is controlled by the operating voltage +5V of the motor control circuit, so that the +60V power supply output must lag behind the low voltage +5V, realizing the power-on timing control function and fundamentally solving the above-mentioned problem.

　　Power supply protection function and electromagnetic compatibility measures

　　The module has overcurrent, overvoltage and overheat protection functions, and the rated output voltage can be adjusted within the range of ±10% using an external potentiometer. In the design of the power supply system, we use TVS surge absorbers at key locations such as the high-voltage input end after 220V rectification and the +60V output end to protect against voltage transients and surge impacts, protecting the power supply system in a bypass absorption manner, while reducing electromagnetic interference and improving the reliability and life of the power supply system.

　　The +60V output voltage ripple we measured in the experiment was 800mV~1000mV, which was obviously too large. By using polyester capacitors for filtering at the adjustment and output ends of the power supply system and twisted pair wiring inside the power supply, the output ripple of the +60V power supply system was finally controlled within 200mV~400mV, meeting the requirements.