With the high development of medical electronic technology, there are more and more types of medical equipment, and the relationship between medical equipment and modern medical diagnosis and treatment is becoming increasingly close. Any medical equipment cannot do without a safe and stable power supply, and most of them are switch mode power supplies. In the daily diagnosis and treatment process, it is often encountered that equipment cannot be used due to power failure. In this case, clinical medical engineers from medical service institutions need to combine their own experience and professional knowledge to provide rapid and efficient services to clinical departments. Due to the particularity of medical equipment, the interchangeability of equipment power supply 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 primary power supply AC/DC converter input is 50/60Hz, 220V AC power, which must be rectified and filtered. Large filtering electrolytic capacitors are indispensable, and the AC input must be equipped with EMC filtering and use safety standard devices. The DC/DC converter of the secondary power supply is used for power conversion, which is the core part of the switching power supply. In addition, there are circuits such as startup, overcurrent and overvoltage protection, noise filtering, etc. The output sampling circuit detects changes in the output voltage and compares it with the reference voltage. The error voltage is amplified and pulse width modulated (PWM) by the circuit, and then controlled by the driving circuit to adjust the duty cycle of the power device, thereby achieving the goal of adjusting the output voltage.
The damage of switching power supplies can be roughly divided into: ① damage to inductive, capacitive, and resistive components; ② Damage to power semiconductor devices; ③ PWM IC damaged; ④ Damaged optocoupler; ⑤ Other power components such as crystal oscillators and fans are damaged.

According to the power supply workflow, it can be divided into: ① AC input fault; ② DC/DC converter failure; ③ Drive circuit failure; ④ PWM circuit malfunction; ⑤ Sampling circuit malfunction. There are many types of faults in switch mode power supplies, and we cannot list them in detail here. Below, we will discuss typical maintenance techniques in the above two categories based on actual maintenance examples.

1.Input circuit malfunction

The input circuit of switch power supply for medical equipment generally includes switches, fuses, AC anti-interference circuits, and soft start circuits. It is easy to detect faults in switches, fuses, and AC anti-interference circuits. If the switch is damaged, it can be replaced directly. However, if the fuse is damaged, it is best to check whether the load is severely short circuited and replace it with a fuse of the same ampere number to monitor the total input current when powered on. AC anti-interference circuit failures are generally more common due to the prolonged use of capacitors. The soft start circuit is one of the protection circuits for switch mode power supplies. The input circuit of the switch mode power supply is mostly designed with a rectifier and capacitor filter circuit, which is used when the input circuit is closed. Due to the starting voltage on the capacitor being 0, a significant instantaneous surge current will be generated.

For this reason, switch power supplies for medical equipment are generally equipped with a soft start circuit to prevent surge current in the input circuit.

Common soft start circuits include thermistor anti surge current circuit, SCR-R circuit, circuit composed of relay and resistor, circuit using timing trigger and current limiting resistor, and circuit composed of optocoupler thyristor and bidirectional thyristor triggered by zero crossing. Taking the example of anti surge current and circuit of thermistors, the working principle is briefly explained: thermistors are divided into positive temperature coefficient thermistors (PTC) and negative temperature coefficient thermistors (NTC). The normal resistance value of PTC is low. When there is excessive abnormal current flowing through, the resistance value of PTC rapidly increases due to its own heating, which increases the resistance and serves as a current limiting function; NTC thermistors have a high resistance value at the moment of power supply connection, which limits the impact current.

When the circuit is in normal working condition, the resistance heats up, causing its resistance value to decrease.

The NTC thermistor anti surge current circuit requires time to restore the original resistance value due to the thermal inertia of the thermistor. When the power is cut off and quickly connected again, it does not have a current limiting effect. Many infusion pumps and some low-power medical equipment power supplies use PTC thermistor current limiting or NTC thermistor anti surge current circuit design. Among them, PTC thermistors are prone to damage when exposed to lightning or strong currents, always in a low resistance state, and when powered on, they burn out the fuse. NTC thermistors often have open circuit faults, resulting in a DC power supply without AC connection.

2. Optocoupler malfunction

Optocouplers, also known as optocouplers, are commonly referred to as optocouplers. It is a device that uses light as a medium to transmit electrical signals. Usually, infrared light-emitting diodes and photosensitive semiconductors are packaged in the same tube shell. When an electrical signal is added 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, thus achieving "electrical optical electrical" conversion. It is widely used in signal isolation, switching circuits, pulse amplification, solid-state relays (SSRs) and other circuits. In addition, a linear optocoupler can be used to form an optocoupler feedback circuit, which can achieve precise voltage stabilization by adjusting the control terminal current to change the duty cycle.

Optocouplers can achieve electrical isolation, as well as strong anti-interference ability, long service life, and high transmission efficiency. However, when encountering circuit failures due to the performance degradation of optocouplers, it is still quite common in switch mode power supplies for medical equipment.

Example 1: Many clinical medical engineers have been exposed to the power supply of the Philips BV25 X-ray machine. Among them, the inability to turn on the power supply due to poor performance of the optocoupler has almost become a common problem. The BV25 main power supply adopts a contactless soft start circuit design. When 220V is connected, one transformer provides a set of 28V and multiple sets of 7V power supplies. After rectification and stabilization of 28V,+15V voltage is obtained to provide power to the power control board, and 7V is supplied to each group of optocouplers. If H1 on the power board is green, it can be roughly judged that the 28V and 7V outputs are normal. Poor performance of thyristor V1-V3 and optocoupler (4N25) B1-B6 can lead to startup failure. To determine whether V3 is damaged, it is necessary to remove and measure it, otherwise it is easy to misjudge.

Example 2: OHMEDA 2000 baby incubator, if the temperature continues to rise after reaching the set value, it will report "E013". The maintenance manual prompts "Header not switching off". After troubleshooting the thermal switch fault, it is most likely due to poor performance of the optocoupler inside the SSR. After replacing the device, the temperature box worked normally.

Like other switch power supplies, power devices are essential for medical equipment switch power supplies. The commonly used ones include power diodes, thyristor (SCR), and power field-effect transistors. During the maintenance process, power devices are the key inspection objects. Damage to such devices can lead to startup protection or blown fuses. When damage is found to such devices during maintenance, in addition to replacing devices with the same parameters, it is also necessary to check the external high-voltage capacitors and current limiting or current detection resistors.

Example 1: There is no display on the startup panel of the Alcon Universal II ultrasonic emulsifier, the "Standby" light is flashing, and there is a "squeaking" sound when switching on and off the power supply. It can be roughly judged that the power supply has a protective action. This power supply uses PWM ICs such as UC3842, UC3843, and UC3854. The current detection terminals of each IC indicate overcurrent, and the voltage at each power supply terminal jumps. After excluding damage to the PWM IC and peripheral circuits, focus on checking the power

3. PWM IC and peripheral circuit faults

The combination of power control chips and switching tubes is widely used in switch mode power supplies for medical equipment, and one power supply may even be used in multiple places. The basic working principle of PWM switch stabilized or current stabilized power supply is to control the circuit through closed-loop feedback of the difference between the controlled signal and the reference signal in the case of changes in input voltage, internal parameters, and external loads, adjust the conduction pulse width of the main circuit switch device, and make the switch power supply and output voltage or current controlled signals stable.

The switching frequency of PWM is generally fixed, and the control sampling signal can form a single loop, double loop, or multi loop feedback system to achieve stable voltage, current, and constant power.

At the same time, it can achieve some additional functions such as overcurrent protection, anti bias magnetization, and current sharing.

When repairing switch mode power supplies, when the rectifier filter circuit and switch tube are normal, it is usually necessary to check whether the PWM IC and peripheral circuits are normal, which can achieve twice the result with half the effort. PWM ICs generally have IC power supply, reference voltage, driving pulse, current detection, and sampling adjustment circuits. PWM IC power supply is generally obtained by reducing the voltage of the main power supply through a resistor, commonly referred to as the starting resistor. If the resistor is open or increases, providing low power to the IC will cause the power supply to not start.

When the power supply is normal, focus on checking whether the reference voltage and driving pulse are normal, then monitor whether the voltage at the current sensing end is normal, and then carefully inspect the sensing branch. The method to determine the fault of PWM IC itself is generally to measure the impedance between pins or input the nominal voltage to the power supply terminal, and 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 components are intact. The plan is to first check the voltage of the PWM IC chip's power supply, reference voltage, and current detection terminal pins. It was found that+300V is normal, and there is no voltage input on pin 7.

The reason is that the 100K starting resistor is open circuit. After replacement, the PWM IC power supply is normal, the 6-pin output pulse waveform is stable, and the+5V and+12V output voltages are correct.

Example 2: The Stryker cavity mirror monitor consists of a power supply consisting of switch tubes BUK456, UC3824, and peripheral circuits. UC3842 was damaged due to a short circuit between pin 6 and pin 5. The DS pole of BUK456 was broken down, and the current detection resistor was open. The pulse output terminal was connected in series with a resistor that was open. After replacing the above device, it worked normally. If only the damaged peripheral circuit components are replaced and UC3842 itself is not found to be damaged, the replaced components will be damaged again at the moment of startup. Therefore, in maintenance, it is necessary to eliminate the fault of the PWM IC itself.

4.Other power supply components malfunction

During maintenance, there are often malfunctions that are not caused by complete damage to electronic components. Such as reduced capacitor capacity, poor concealed contact of circuit board parts, unstable power supply due to excessive dust or poor heat dissipation, and power outage caused by some fan control circuit faults. Due to the difficulty of using traditional detection methods for such problems, replacement methods are adopted based on experience and analysis to eliminate them.

When repairing the power supply of medical equipment, the first step is to deal with dust, which can be removed by using a vacuum cleaner and a high-power air cooler. During the treatment process, it is necessary to reduce static electricity on the human body and prevent circuit board capacitors from discharging on the human body. For power supplies with a large number of fans, it is necessary to check the speed of the fans, especially those with speed control or speed detection. When uncertain, replacement methods can be adopted to solve the problem.

Example 1: The Hitachi 7170A biochemical analyzer has a 5V switch power supply. After starting up and working normally for a few minutes, the power indicator light turns green and the+5V output stops. The cooling fan has no obvious abnormalities, and the power components and PWM IC are normal. However, after cleaning, the fan power supply is not connected and there is no output. After replacing with a regular 2-wire CPU fan, there is still no output. After connecting the speed measurement cable of the fan to the circuit board, the power input is normal and can work continuously. Therefore, it can be concluded that the power supply stops vibrating due to the decrease in speed of the original fan over a long period of time. Previously, there were similar issues with+24V, but due to urgent equipment usage, a new power module was replaced and returned to normal.

Example 2: After continuous operation for a long time, the power supply casing of TOSHIBA 240A B-ultrasound becomes hot and often experiences overheating protection. This type of malfunction is generally caused by excessive internal dust or a decrease in the speed of the internal fan, resulting in a deterioration of the overall working environment of the power supply. After removing the power supply and thoroughly removing dust, replacing the fan at the bottom and back of the power supply, the temperature of the power supply significantly decreased, and the machine worked normally without any malfunctions for one year.

5.Summary

There are various 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 over 60% of medical equipment failures. Therefore, mastering the maintenance of switch mode power supplies is a basic skill and also a challenge for every clinical medical engineer. This article only explores the maintenance technology of medical equipment switch power supplies based on practical maintenance experience, hoping that more peer experts can provide valuable opinions and further research on medical equipment switch power supplies.