“The switching power supply will cause the output voltage to decrease when the load is short-circuited, and the output voltage will also increase when the load is open-circuit or no-load. In the maintenance, the dummy load replacement method is generally used to distinguish whether the power supply part is faulty or the load circuit is faulty. Regarding the selection of dummy loads, generally choose 40W or 60W bulbs as dummy loads (bulbs above 100W can be used as dummy loads for large-screen color TVs). The advantage is intuitive and convenient.
The switching power supply will cause the output voltage to decrease when the load is short-circuited, and the output voltage will also increase when the load is open-circuit or no-load. In the maintenance, the dummy load replacement method is generally used to distinguish whether the power supply part is faulty or the load circuit is faulty. Regarding the selection of dummy loads, generally choose 40W or 60W bulbs as dummy loads (bulbs above 100W can be used as dummy loads for large-screen color TVs). The advantage is intuitive and convenient.
The switching power supply will cause the output voltage to decrease when the load is short-circuited, and the output voltage will also increase when the load is open-circuit or no-load.
In the maintenance, the dummy load replacement method is generally used to distinguish whether the power supply part is faulty or the load circuit is faulty. Regarding the selection of dummy loads, generally choose 40W or 60W bulbs as dummy loads (large-screen color TVs can choose bulbs above 100W as dummy loads). and the output voltage level.
But the shortcomings are also obvious. For example, the thermal resistance of a 60W bulb is 500Ω, while the cold resistance is only about 50Ω. It can be seen from the table below: Assuming that the main voltage output of the power supply is 100V, when a 60W light bulb is used as a dummy load, the current of the power supply during operation is 200mA, but the main load current at startup reaches 2A, which is 10 times the normal operating current. , Therefore, using the light bulb as a dummy load will easily make the power supply difficult to start. Because the greater the power of the light bulb, the smaller the cold resistance, therefore, the high-power light bulb has a larger starting current and the power supply is more difficult to start.
When calculating the starting current and working current of the power supply, you can use the formula I=U/R to calculate: the load current when the power supply starts is 100V/50Ω=2A, and the load current when the power supply is working is 100V/500Ω=0.2A, but it should be noted that Yes: The above are theoretical calculations, and there may be discrepancies in practice. In order to reduce the start-up current, a 50W electric soldering iron can be used as a dummy load (900Ω in both cold and hot states) or a 50W/300Ω resistor, which is more accurate than using a 60W bulb.
Some power supplies can be directly connected to dummy loads, while others cannot. Specific problems need to be analyzed in detail. The following three types of situations are explained in detail.
DY class is a switching power supply of other excited type. For a separately excited power supply without line pulse synchronization (such as Changhong N2918 color TV), the line load can be disconnected and the dummy load can be directly connected. For the separately excited switching power supply with line pulse frequency locking and indirect sampling (such as Panda 2928 color TV), when the dummy load is directly connected (especially when the light bulb with high power such as 150W is connected), the output voltage may drop more or There is no output, because this type of power supply, although the addition of the line pulse only plays the role of synchronization and frequency locking, and does not participate in the oscillation, but the line synchronization pulse can advance the conduction time of the switch tube, and the power supply has the strongest load capacity at this time. , If the line load is disconnected, the line synchronization pulse will lose its effect, and the power supply’s ability to carry the load will inevitably decrease. The power supply voltage regulation sensitivity of the indirect sampling is low, and the output voltage will also be reduced. However, if the voltage regulator circuit of this type of power supply adopts direct sampling (the sampling voltage is taken from the secondary of the switching transformer), due to the high voltage regulation sensitivity, it can be disconnected from the line load and directly connected to the dummy load or even no-load for maintenance.
The second type of switching power supply for line pulse synchronization can disconnect the line load and directly connect the dummy load. This kind of switching power supply is purely a self-excited switching power supply. The purpose of introducing a forward traveling reverse pulse at the base of the switching tube is to synchronize the self-excited oscillation of the switching tube with the horizontal pulse, so as to interfere the pulse radiation of the switching power supply with the oblique bars of the screen. Restricted to line scan reverse travel, so no interference is visible on the screen. The line pulse added to the base of the switch tube only makes the switch tube turn on in advance during the cut-off period, and basically does not constitute an auxiliary excitation function, so it is called a switching power supply with line pulse synchronization. The way to judge whether it belongs to this type of power supply is that when the reverse pulse is disconnected, the switching power supply only makes a sound (because the oscillation frequency becomes lower), and the output voltage does not drop. Therefore, this power supply can disconnect the line scan circuit and repair it with a dummy load method.
The third type is the switching power supply of line pulse auxiliary excitation. The reverse pulse of the switching power supply not only completes the synchronization of the self-excited oscillation frequency of the switching power supply, but also constitutes an indispensable part of the switching tube feedback network. The working process of this switching power supply is as follows: after the switch is turned on, the switch tube generates self-excited oscillation. Under the rated load, its feedback network can only make the output terminal generate a voltage that is 40% lower than the normal output. This voltage enables the line scan to start, and the line pulse The feedback to the switch tube to assist the excitation, in order to achieve the rated voltage output. This has two purposes: First, it has the function of buck protection. Once the line scan circuit fails, whether it is open circuit or short circuit, the output voltage of the switching power supply will be reduced to 60% of the original value, reducing the damage range. Second, both the power supply and the line scan have a very short soft-start process, which reduces the failure rate of the power supply and the line scan. For this type of power supply, if the feedback line pulse circuit is removed, the output voltage of the power supply will drop by 40% to 60%, and even the output voltage will be very low. Obviously, this kind of power supply cannot directly disconnect the line scan and use the dummy load method to repair, because even if the power supply circuit is normal at this time, it is impossible to output the rated voltage. The method to distinguish the failure of the power supply and the line scan circuit is to use an external power supply to supply the line scan circuit alone. If the line scan circuit works normally, it means that the switching power supply is bad.
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