A switching power supply is a type of power supply that uses modern power electronics technology to control the on and off time ratio of switching transistors to maintain a stable output voltage. Switching power supplies are generally composed of a pulse width modulation (PWM) control IC and MOSFETs. With the development and innovation of power electronics technology, switching power supply technology is also constantly evolving.
1. Reasons for high output voltage of switching power supply
(1) Models with voltage doubler rectification in the mains are incorrectly operating in voltage doubler rectification mode when the mains power is normal (only applicable to some new remote control color TVs).
(2) Problems with pulse width modulation circuits.
(3) The capacitance of the oscillation capacitor decreases.
(4) The main load (horizontal scanning circuit) is not working, causing the load on the switching power supply to become lighter, resulting in an increase in the output voltage. (This only applies to power supplies with indirect sampling in the voltage regulation loop, i.e., the voltage regulation sampling is not directly taken from the B+ output).
2. Troubleshooting methods and repair procedures
(1) Method to determine if the rectifier filter circuit is operating in voltage doubler rectification mode: Measure the collector voltage of the switching transistor. If it is more than 1.4 times higher than the AC supply voltage, it can be determined that the high output voltage of the switching power supply is caused by the high collector voltage of the switching transistor. The voltage doubler rectifier circuit should be checked. In areas where the mains voltage is relatively normal, the voltage doubler rectifier filter circuit can be removed to reduce the power supply failure rate.
(2) Use the substitution method to determine whether the oscillation timing capacitor is faulty.
3. Methods for diagnosing pulse width modulation circuit faults:
(1) Adjusting AC voltage method:
Adjust the AC input voltage using an AC voltage regulator and monitor the B+ output voltage, keeping it slightly above the normal value. (This is because if the sampling is normal, the negative feedback voltage regulation loop will be activated.) Then measure the base, emitter, and collector voltages of each stage of the transistors in the pulse width adjustment circuit, and the voltage drop between pins ① and ② of the optocoupler. Check if these values conform to the voltage regulation principle or show the same trend. If the measured value is opposite to the value expected by the voltage regulation principle, it indicates that there is a fault in that stage, which cannot correctly transmit the voltage regulation information, causing the voltage regulation to fail. The relevant components should be checked one by one.
(2) Segmentation method (applicable to direct sampling power sources)
Using the optocoupler in the voltage regulation loop as a dividing line, the circuit is segmented to determine the fault range. Short-circuit terminals ③ and ④ of the optocoupler and observe the change in B+.
A significant drop or complete halt in B+ output indicates that the hot plate section is functioning correctly. The fault lies in the B+ sampling circuit and the optocoupler.
• If the change is not obvious or there is no change, it indicates a fault in the hot plate section. A detailed inspection of the pulse width adjustment circuit in this section is necessary. Pay particular attention to the circuitry that forms the operating voltage of the pulse adjustment circuit, such as the filter capacitors and rectifier diodes; a replacement method should be used. Also, check and replace each adjusting transistor and related components, and check for open circuits in the copper foil.
1. Reasons for low output voltage of switching power supply
(1) The 220V AC voltage input circuit and the rectifier filter circuit provide insufficient operating voltage to the switching transistor, which exceeds the control range of the pulse width modulation circuit.
(2) Overcurrent in the load circuit causes an increase in the load on the switching power supply, resulting in a drop in output voltage.
(3) The power-on/power-off interface circuit is in standby mode, causing the switching power supply to operate in a low-frequency oscillation state, and its output voltage is the same as in standby mode. This type of fault should only occur in models without a backup power supply, where the CPU's operating voltage in standby mode is provided by the switching power supply.
(4) The power on/off interface circuit is in a state between power on and standby due to some reason, which causes the switching power supply to operate at a frequency between standby and power on, resulting in the output voltage of the switching power supply being higher than the standby value and lower than the power on value.
(5) The protection circuit is in the conducting state due to a fault, causing the power supply to enter a weak and narrow pulse power supply, which causes the output voltage of the switching power supply to drop.
(6) Damage to the diodes, filter capacitors, and current-limiting resistors in the rectifier output circuit can cause the output voltage to drop.
(7) The pulse width modulation circuit is faulty and cannot respond to changes in the output voltage of the switching power supply in a tangent manner. It also adjusts the base voltage of the power switch transistor in the wrong direction and magnitude, resulting in a low output voltage of the switching power supply.
(8) When the positive feedback resistor in the positive feedback circuit increases, the performance of the discharge diode deteriorates, and the positive feedback is insufficient, resulting in a longer oscillation period. This leads to a decrease in the oscillation frequency, which in turn causes a low output voltage of the switching power supply.
(9) Self-excited switching power supplies operate below low frequency due to lack of horizontal inversion, resulting in low output voltage.
2. Troubleshooting methods and steps
(1) Determine faults by measuring the collector voltage of the output transistor.
(2) Measure the voltage at each output terminal of the switching power supply to determine the fault.
(3) Some have a large output voltage drop ratio, while others have a small output voltage drop ratio.
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