1 The basic working principle of medical equipment switching power supply
Most medical devices are inseparable from safe, stable power supplies, and a large part of them are switching power supplies. Due to the special nature of the medical equipment itself, the interchangeability of the switching power supply of the equipment is poor, which adds certain difficulty to the maintenance work. Therefore, it is especially important to understand and master the basic working principle of switching power supplies for medical devices. The switching power supply is generally composed of a main circuit and a control circuit. The main circuit is a circuit that transmits the electric energy in the AC power grid to the load, and the control circuit is a circuit that controls the working state of the main circuit according to the input/output conditions. . At present, the control loops in most medical equipment switching power supplies are integrated circuits, also known as integrated controllers. Since the integrated controller is applied to the switching power supply of the medical device, the number of the switching power supply control components is greatly reduced, and the structure and working procedure of the switching power supply are also simplified, further improving the safety and reliability of the power supply itself. The medical device switching power supply includes the following components: switching transformer, output rectification, and so on. The specific working principle of the switching power supply is as follows: the AC power input is converted into a DC power supply after being rectified and filtered, and the switching power is controlled by the PWM signal, and the converted DC power source is directly applied to the primary of the switching transformer, and then the secondary of the switching transformer is The high-frequency voltage is induced, and then rectified and filtered to supply the load, and the output part is fed back to the control circuit by a certain circuit to control the duty ratio of the PWM, thereby achieving the purpose of stable output. Figure 1 shows the working principle of the switching power supply for medical equipment.
It can be clearly seen from Fig. 1 that the power conversion circuit composed of the switching transformer and the switching element is the core component of the whole switching power supply, wherein the switching transformer needs to withstand the high frequency working state, so generally a special alloy magnetic is used. The core is fabricated, and the switching elements are divided into two categories, one is an automatic excitation switching element, and the other is a passive switching element. Switching power supplies are controlled in three ways: pulse width modulation, pulse frequency modulation, and hybrid modulation. The most common pulse width modulation is shown in Figure 1. The width and frequency of the pulse can be controlled using the corresponding modulation components to provide a rectangular pulse train for the entire circuit. The types of medical equipment are relatively large, and the circuit patterns of medical equipment switching power supplies are also different, but the working principle of these switching power supplies is similar, and the main structure is also very similar, generally can divide the circuit of the switching power supply into The three parts are the rectifier circuit, the line filter, and the inrush current suppression circuit. Among these three parts, the rectifier circuit has the most obvious effect. It can convert the AC power in the input circuit into DC power for the device. The filter mainly plays the role of attenuating high-frequency interference. The main function of the surge current suppression circuit is Suppress the inrush current in the line.
Figure 1 Schematic diagram of the working principle of the switching power supply of medical equipment
2 Analysis of common faults in medical equipment switching power supply
2.1 Fault classification
According to the damage of the components of the switching power supply, the faults can be classified into the following types: inductive, capacitive, and faults caused by damage of the resistive device; faults caused by damage of the power semiconductor components; faults caused by damage of the optocoupler; crystal oscillator, A fault caused by damage to the power hardware device such as a fan.
According to the working process of the switching power supply, the fault can be divided into the following types: PWM circuit fault; DC converter fault; AC input fault; drive circuit fault. The above faults are just the most common types of faults in medical equipment switching power supplies.
2.2 Causes of failure
The cause of the failure of the switching power supply of medical equipment can be summarized as follows: due to the DC voltage generated by the input circuit, the diode is damaged, and the power tube is damaged in serious cases; when the leakage current of the high-voltage filter capacitor in the input circuit exceeds the limit, The load capacity of the switching power supply itself is reduced, and in severe cases, the resistance of the power supply starting current limit is burned; if the high voltage filter capacitor fails or the rectifier diode of the secondary winding of the high frequency transformer is overheated, the ripple voltage is increased. Therefore, it is easy to cause abnormality of the whole machine; the damage of the power switch tube causes the oscillator to stop working, resulting in no DC current output; due to the damage of the optocoupler or the voltage adjustment part, the self-stabilization function of the switching power supply is lost, thereby making the voltage output Unstable, this will cause the whole machine to work abnormally; because the load at the output or the no-load current is too small, or the output of one of the outputs is faulty, the power supply will enter the protection state immediately after starting, resulting in no DC output of each channel. .
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