With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating. Currently, switching power supplies, characterized by their small size, lightweight design, and high efficiency, are widely used in almost all electronic devices and are an indispensable power supply method for the rapid development of today's electronic information industry.
What causes a switching power supply to have a low output voltage? The following are some possible reasons for a low output voltage in a switching power supply:
1) The 220V AC voltage input circuit and rectifier filter circuit cannot supply the operating voltage to the switching transistor, which exceeds the control range of the pulse width modulation circuit.
2) Overcurrent in the load circuit increases the load on the switching power supply, resulting in a decrease in output voltage.
3) When the power-on/power-off interface circuit is in standby mode, the switching power supply operates at a low-frequency vibration level, and its output voltage is the same as that in standby mode. This type of fault only occurs in models without a backup power supply, where the CPU's operating voltage in standby mode is supplied by the switching power supply.
4) The power on/off interface circuit fails to operate in a state between power on and standby, causing the power supply to operate at a frequency between standby and power on, resulting in the power supply output voltage being higher than the standby value and lower than the power on value.
5) The protection circuit malfunctions and operates in the conducting state, causing the power supply to enter a weak and narrow pulse power supply, resulting in a decrease in the output voltage of the switching power supply.
6) Damage to the diodes, filter capacitors, and current-limiting resistors in the rectifier output circuit causes the output voltage to decrease.
7) There is a problem with the pulse width modulation circuit. It cannot respond tangentially to changes in the output voltage of the switching power supply, and the adjustment direction of the base voltage of the power switch transistor is incorrect, resulting in a low output voltage of the switching power supply.
8) An increase in the positive feedback resistor in the positive feedback circuit leads to a deterioration in the function of the discharge diode and a lack of positive feedback, resulting in a longer oscillation period. This reduces the oscillation frequency, which in turn causes a low output voltage of the switching power supply.
9) Self-excited switching power supplies operate at lower frequencies due to lack of horizontal inversion, resulting in low output voltage.
12V switching power supply output voltage is too low
Besides the voltage regulation control circuit, the output voltage will also be very low. The following are other reasons that can also cause a low output voltage:
1) A short circuit fault exists in the switching power supply load (especially a short circuit or poor performance in a DC/DC converter). In this case, all loads in the switching power supply circuit should be disconnected to distinguish whether there is a defect in the switching power supply circuit or the load circuit. If the voltage output of the disconnected load circuit is normal, it means that the load is too heavy. Or if it is still abnormal, it indicates that the switching power supply is faulty.
2) Alternative methods can be used to assess faults in the rectifier diodes and filter capacitors on the output voltage side. A faulty switching transformer can not only cause a drop in output voltage but also result in insufficient excitation of the switching transistor, thus damaging the transistor.
3) The 300V filter capacitor is faulty, resulting in an excessively low output voltage.
Power supplies employ a voltage feedback mechanism. When the voltage is low, the switching chip increases the output duty cycle or frequency to increase the output voltage. When the voltage is high, the switching chip decreases the output duty cycle or frequency to stabilize the output voltage. Switching power supplies are familiar to most users, but they can sometimes encounter minor problems during use. It's crucial to detect these faults promptly and perform repairs. While switching power supply circuits vary in complexity, power output, and output voltage, following the information summarized below should provide a more efficient and effective solution.
The repair of switching power supplies involves the following six steps:
Switching power supply repair steps: 1. When repairing a switching power supply, first use a multimeter to check whether each power component is short-circuited, such as the power rectifier bridge, switching transistors, and high-frequency high-power rectifier diodes; check whether the high-power resistors that suppress surge current are burnt out. Then check whether the resistance of each output voltage port is abnormal. If any of the above components are damaged, they need to be replaced.
Step 2 of switching power supply repair: If the power supply still does not work properly after the first step is completed, then the power factor module (PFC) and pulse width modulation component (PWM) need to be tested. Consult relevant materials to familiarize yourself with the function of each pin of the PFC and PWM modules and the necessary conditions for the modules to work properly.
Step 3 of switching power supply repair: Next, for power supplies with PFC circuits, measure whether the voltage across the filter capacitor is approximately 380VDC. If the voltage is around 380VDC, the PFC module is working normally. Then, check the operating status of the PWM component, measuring its power input terminal VC, reference voltage output terminal VR, and start/control terminal voltages to ensure they are normal. Power the switching power supply using a 220VAC/220VAC isolation transformer. Use an oscilloscope to observe whether the waveform at the CT terminal of the PWM module relative to ground is a linear sawtooth or triangular wave. For example, the CT terminal of the TL494 shows a sawtooth wave, while the CT terminal of the FA5310 shows a triangular wave. Check whether the waveform at the output terminal V0 is an ordered narrow pulse signal.
Step 4 in switching power supply repair: In practical experience, many switching power supplies use UC38×× series 8-pin PWM components. Most power supplies fail to work because the power supply startup resistor is damaged or the chip performance deteriorates. When R is open-circuited, there is no VC, and the PWM component cannot work. A resistor with the same power rating as the original needs to be replaced. When the PWM component's startup current increases, the value of R can be reduced until the PWM component works normally. When repairing a GEDR power supply, the PWM module was a UC3843. No other abnormalities were found. After connecting a 220K resistor in parallel with R (220K), the PWM component worked, and the output voltages were normal. Sometimes, due to external circuit faults, the 5V voltage at the VR terminal is 0V, and the PWM component also fails to work. When repairing a Kodak 8900 camera power supply, this situation was encountered. Disconnecting the external circuit connected to the VR terminal changed VR from 0V to 5V, and the PWM component worked normally, with all output voltages normal.
Step 5 in switching power supply repair: When there is no voltage of approximately 380VDC on the filter capacitor, it indicates that the PFC circuit is not working properly. The key testing pins of the PFC module are the power input pin VC, the start pin Vstart/control, the CT and RT pins, and the V0 pin. When repairing a Fuji 3000 camera, a test revealed no 380VDC voltage on the filter capacitor on the board. The waveforms of VC, Vstart/control, CT and RT, and the V0 waveform were all normal. Measurements at the gate (G) of the MOSFET showed no V0 waveform. Since the FA5331 (PFC) is a surface-mount component, after prolonged use, a poor solder joint developed between the V0 terminal and the board, preventing the V0 signal from reaching the G of the MOSFET. After resoldering the V0 terminal to the board, a multimeter was used to measure a 380VDC voltage on the filter capacitor. If the PFC also fails to work when the Vstart/control pin is low, the relevant circuitry connected to its terminals should be checked.
In summary, to fully understand the basic structure of switching power supplies and the characteristics of PFC and PWM modules, as well as their basic operating conditions, and by following the steps and methods described above and performing hands-on repairs of switching power supplies, you can quickly troubleshoot switching power supply faults.
Switching power supply repair falls under the category of chip-level repair. As a crucial starting component in industrial equipment, the power supply plays a vital role in ensuring the normal operation of such equipment. Below, we'll outline the steps involved in switching power supply repair.
1. Check the power supply: To check the power supply, you should not only use a multimeter to check the voltage magnitude, but also use an oscilloscope to check the voltage waveform.
2. Check the crystal oscillator: To check if the crystal oscillator is oscillating, you can use an oscilloscope to check the waveform at the crystal oscillator pins.
3. Check the reset signal: Check whether the reset signal is normal and whether the reset pulse is correctly sent to the reset pin of the CPU chip.
4. Check the bus: Any open or short circuit on the data bus, address bus, or control bus can cause a fault. You can determine this by testing the resistance to ground of the parallel buses and comparing whether there is a fault on a certain line, or by observing the waveforms of each bus.
5. Check the interface chip: Interface chips are the most common type of component to fail. They can be replaced or tested with specialized instruments to determine if they are damaged.
6. Replace components: Through circuit testing and component inspection, troubleshoot the identified faults, including circuit repair, component replacement, and modification.
7. Power Supply Test: After troubleshooting, an offline load test must be performed before powering on the machine. Only after passing the test can the power supply be used for load testing.
