Do you know some maintenance tips for medical equipment switching power supplies? With the rapid development of medical electronic technology, the types of medical equipment are increasing. Medical equipment is increasingly closely related to modern medical diagnosis and treatment. Any medical equipment cannot do without a safe and stable power supply, and most of them are switching power supplies. In the process of daily diagnosis and treatment, it is often encountered that the equipment cannot be used due to power failure. At this time, clinical medical engineers of medical service institutions are required to provide rapid and efficient services to clinical departments based on their own experience and professional knowledge. Due to the particularity of medical equipment, the interchangeability of equipment power supplies is poor, and some even lack technical drawings, which brings great inconvenience to maintenance work.
Medical equipment switching power supplies can generally be divided into two categories: AC/DC and DC/DC. The input of the primary power supply AC/DC converter is 50/60Hz, 220V AC, which must be rectified and filtered. Large-volume filter electrolytic capacitors are indispensable, and the AC input must be added with EMC filtering and safety standard devices. The secondary power supply DC/DC converter is used for power conversion. It is the core part of the switching power supply. In addition, there are circuits such as startup, overcurrent and overvoltage protection, and noise filtering. The output sampling circuit detects the output voltage change and compares it with the reference voltage. The error voltage is amplified and pulse width modulated (PWM) circuit, and then controls the duty cycle of the power device through the drive circuit, so as to achieve the purpose of adjusting the output voltage.
The damage of the switching power supply can be roughly divided into: ① damage to inductive, capacitive and resistive devices; ② damage to power semiconductor devices; ③ damage to PWM IC; ④ damage to optocoupler; ⑤ damage to other power devices such as crystal oscillator and fan.
According to the power supply workflow, it can be divided into: ① AC input failure; ② DC/DC converter failure; ③ drive circuit failure; ④ PWM circuit failure; ⑤ sampling circuit failure. There are many types of switching power supply failures, which cannot be listed one by one here. The following is a discussion of typical maintenance techniques in the above two categories combined with actual maintenance examples.
1 Input circuit failure
The input circuit of the switching power supply of medical equipment generally includes switches, fuses, AC anti-interference circuits and soft start circuits. Faults in switches, fuses and AC anti-interference circuits are easy to find. If a switch is damaged, it can be replaced directly. However, if a fuse is damaged, it is best to check whether the load is seriously short-circuited, and replace it with a fuse of the same ampere. Monitor the total input current when the power is on. Faults in AC anti-interference circuits are generally caused by the failure of capacitors due to long-term use. The soft start circuit is one of the protection circuits of the switching power supply. The input circuit of the switching power supply is mostly designed with a rectifier plus capacitor filter circuit. At the moment of closing the input circuit. Since the starting voltage on the capacitor is 0, a large instantaneous impact current will be formed.
For this reason, medical equipment switching power supplies generally have a soft start circuit to prevent impact current in the input circuit.
Common soft start circuits include thermistor impact current protection circuits, SCR-R circuits, circuits composed of relays and resistors, circuits using timing triggers and current limiting resistors, and circuits composed of zero-triggered optocoupler thyristors and bidirectional thyristors. The following takes thermistor impact current protection circuits as an example to briefly explain its working principle: Thermistors are divided into positive temperature coefficient thermistors (PTC) and negative temperature coefficient thermistors (NTC). The normal resistance of PTC is low. When there is an excessive abnormal current flowing through, the resistance value of PTC increases rapidly due to its own heating, and becomes larger, which plays a role in current limiting; the resistance value of NTC thermistor is large at the moment of power supply connection, which can achieve the effect of limiting the impact current.
When the circuit is in normal working state, the resistance heats up and its resistance value becomes smaller.
Due to the thermal inertia of the thermistor, it takes time for the NTC thermistor to restore the original resistance value. When the power is turned on quickly after power failure, it cannot play a current limiting role. Many infusion pumps and some low-power medical equipment power supplies use PTC thermistor current limiting or NTC thermistor impact current protection circuit design. Among them, PTC thermistors are easily damaged when encountering lightning or strong currents, and they are always in a low resistance state and burn out when powered on. NTC thermistors often have open circuit failures, resulting in a primary power supply DC without AC access.
2 Photocoupler failure
An optocoupler (OpTIcal Coupler) is also called a photocoupler, or optocoupler for short. It is a device that uses light as a medium to transmit electrical signals. Usually, an infrared light-emitting diode and a photosensitive semiconductor are packaged in the same tube shell. When an electrical signal is applied to the input end, the light-emitting diode emits light, and the photosensitive semiconductor receives the light to generate an electrical signal, which flows out from the output end, thereby realizing the "electric-light-electric" conversion. It is widely used in signal isolation, switching circuits, pulse amplification, solid-state relays (SSR) and other circuits. In addition, a linear optocoupler can be used to form an optocoupler feedback circuit, and the duty cycle can be changed by adjusting the current at the control end to achieve the purpose of precise voltage regulation.
Optocouplers can achieve electrical isolation, and have the advantages of strong anti-interference ability, long service life, and high transmission efficiency [3]. However, circuit failures caused by the degradation of optocoupler performance are still common in medical equipment switching power supplies.
Example 1: Many clinical medical engineers have come into contact with the power supply of the Philips BV25 X-ray machine. Among them, the failure to start up due to poor performance of the photocoupler has almost become a common problem of this power supply. The BV25 main power supply adopts a contactless soft start circuit design. When 220V is connected, a transformer provides a group of 28V and multiple groups of 7V power supplies. After rectification and voltage regulation, the 28V is rectified and stabilized to obtain a +15V voltage to provide power to the power control board, and 7V is supplied to each group of photocouplers. If H1 on the power board is green, it can be roughly judged that the 28V and 7V outputs are normal. Poor performance of thyristors V1-V3 and photocouplers (4N25) B1-B6 will cause startup failure. To determine whether V3 is damaged, it is necessary to remove it for measurement, otherwise it is easy to misjudge.
Example 2: OHMEDA 2000 baby incubator, the temperature continues to rise after reaching the set value, and reports "E013". Check the maintenance manual for the prompt "Header not switching off". After eliminating the thermal switch fault, the most likely cause is the poor performance of the optocoupler in the SSR. After replacing the device, the temperature box works normally.
Like other switching power supplies, power devices are indispensable for medical equipment switching power supplies. Among them, power diodes, thyristors (SCRs) and power field effect tubes are used more frequently. During the maintenance process, power devices are the focus of inspection. Damage to such devices will cause power-on protection or fuse burnout. When such devices are found to be damaged during maintenance, in addition to replacing devices with the same parameters, peripheral high-voltage capacitors and current limiting or current detection resistors must also be checked.
Example 1: The Alcon Universal II ultrasonic emulsification instrument has no display on the power-on panel, the "Standby" light flashes, and the switching power supply has a "squeaking" sound, which can roughly determine that the power supply has protection action. The power supply uses PWM ICs such as UC3842, UC3843 and UC3854. The current detection terminals of each IC indicate overcurrent, and the voltage of each power supply terminal jumps. After excluding the damage of PWM IC and peripheral circuits, the power devices were checked in detail. The switch tube (IRF460) of one power supply was broken down. After replacing the field effect tube, the peripheral circuit was checked again. It was found that the C26 high-voltage capacitor (1KV) connected to it was broken down. After replacing C26, the power was turned on and the main +24V output was normal. All the connections of the machine were restored. The voltage of each group was normal and the whole machine worked stably.
Example 2: The monitor of SHIMADZU OPESCOPE 50N X-ray machine had no display and the indicator light was flashing. The total power supply of the X-ray machine was 220V, while the monitor was powered on with 110V. Before sending it for repair, the operator added 220V to the monitor alone, but the indicator light did not light up. The monitor power supply adopts the STR 54041 switching power supply thick film module design. Its DS pole has been broken down, and D1722 has been broken down. After replacement, the output of each power supply is normal after connecting the dummy load. After the circuit connection is restored, the indicator light is on, there is a "click" sound in the machine, but there is still no display. After inspection, it is found that the row tube Q9 and the fuse resistor R71 are damaged. After replacement, the whole machine works normally.
3 PWM IC and peripheral circuit failure
The combination of power control chip and switch tube is widely used in medical equipment switching power supply, and one power supply may even be used in multiple places. The basic working principle of PWM switching voltage or current stabilization power supply is that when the input voltage, internal parameters and external load change, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal, adjusts the conduction pulse width of the main circuit switch device, and makes the switching power supply and the output voltage or current and other controlled signals stable.
The switching frequency of PWM is generally fixed, and the control sampling signal can form a single-loop, dual-loop or multi-loop feedback system to achieve the purpose of voltage stabilization, current stabilization and constant power.
At the same time, some additional functions such as overcurrent protection, anti-magnetic bias and current sharing can be realized.
When repairing the switching power supply, when the rectifier filter circuit and the switch tube are normal, it is usually necessary to check whether the PWM IC and the peripheral circuit are normal, which will achieve twice the result with half the effort. PWM IC basically has IC power supply, reference voltage, drive pulse, current detection and sampling adjustment circuit. The PWM IC power supply is generally obtained by reducing the voltage of the main power supply through a resistor, which is usually called the starting resistor. If the resistor is open or becomes larger, the power supply to the IC will be low, which will cause the power supply to fail to start.
When the power supply is normal, focus on checking whether the reference voltage and drive pulse are normal, then monitor whether the voltage at the current sensing end is normal, and then carefully check the sensing branch. The method to judge the fault of the PWM IC itself is generally to measure the impedance between the pins or input the nominal voltage to the power supply end to observe whether the reference voltage is accurate.
Example 1: The +5V and +12V power boards of the North American GS anesthesia machine have no output. The primary PWM IC chip of the machine's switching power supply is UC3845, and the fuse and main power devices are intact. It is planned to check the power supply, reference voltage and current detection pin voltage of the PWM IC chip first, and it is found that +300V is normal, and there is no voltage input at pin 7.
The reason is that the 100K startup resistor is open. After replacement, the PWM IC power supply is normal, the output pulse waveform of pin 6 is stable, and the +5V and +12V output voltages are correct.
Example 2: Stryker laparoscopic monitor, the power supply consists of switch tube BUK456, UC3824 and peripheral circuits. UC3842 is damaged due to a short circuit between pin 6 and pin 5, the DS pole of BUK456 is broken down, the current detection resistor is open, and the series resistor at the pulse output end is open. After replacing the above devices, it works normally. If only the damaged device of the peripheral circuit is replaced, and the UC3842 itself is not found to be damaged, the replaced device will be damaged again at the moment of power on. Therefore, the PWM IC itself should be excluded during maintenance.
4 Other power supply component failures
During maintenance, we often encounter some failures that are not caused by complete damage to electronic devices. For example, the capacitance becomes smaller, the circuit board has poor hidden contact, the power supply is unstable due to excessive dust or poor heat dissipation, and the power supply stops vibrating due to failure of some fan control circuits. Since such problems are somewhat difficult to detect through traditional detection methods, replacement is adopted to eliminate them based on experience and analysis.
When repairing the power supply of medical equipment, the dust must be handled first. It can be removed with a vacuum cleaner and a high-power air cooler. During the treatment process, it is necessary to reduce the static electricity of the human body and prevent the circuit board capacitor from discharging to the human body. For power supplies with a large number of fans, the fan speed must be checked, especially those with speed control or speed detection. If you are not sure, you can use the replacement method to solve it.
Example 1: Hitachi 7170A biochemical analyzer +5V switching power supply, after starting up and working normally for a few minutes, the power indicator light turns from green to off, the +5V output stops, the cooling fan has no obvious abnormality, the power components and PWM IC are normal, but after cleaning, the fan power supply is not connected and there is no output. After replacing the ordinary 2-wire CPU fan, there is still no output. After connecting the fan speed measurement line and connecting it to the circuit board, the power input is normal and can work continuously. Therefore, it can be concluded that the original fan has stopped vibrating due to the reduction of speed over a long period of time. In the past, this kind of problem also occurred in +24V. At that time, the new power module was replaced because the equipment was used in a hurry and it returned to normal.
Example 2: After the TOSHIBA 240A type B ultra-continuous working time is long, the power supply shell becomes hot and over-temperature protection often occurs. This type of failure is generally caused by excessive internal dust or low internal fan speed, which deteriorates the working environment of the entire power supply. After the power supply is removed and thoroughly dusted, the power supply temperature drops significantly after replacing the fans at the bottom and back of the power supply, and the machine works normally, and there has been no failure for 1 year.
5 Summary
There are many types of medical equipment, and high-power and high-current switching power supplies are widely used in medical equipment. Switching power supply failures account for more than 60% of medical equipment failures. Therefore, mastering the maintenance of switching power supplies is a basic skill for every clinical medical engineer, and it is also a difficult point. This article only discusses the maintenance technology of medical equipment switching power supply based on actual maintenance experience. I hope more experts in the same field can provide valuable suggestions and conduct further research on medical equipment switching power supply. The above are some maintenance tips for medical equipment switching power supply, I hope it can help you.
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