Repairing the Switching Power Supply of Color TV by Using the Dummy Load Method

The output voltage of the switching power supply will decrease when the load is short-circuited. Similarly, the output voltage will increase when the load is open or unloaded. In 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, 40W or 60W bulbs are generally selected as dummy loads (large-screen color TVs can use bulbs above 100W as dummy loads). The advantage is that it is intuitive and convenient. According to whether the bulb is lit and the brightness of the light, it can be known whether the power supply has voltage output and the level of the output voltage. But the disadvantages are also obvious. For example, the hot resistance of a 60W bulb is 500Ω, while the cold resistance is only about 50Ω. According to the table below, it can be seen that: assuming that the main voltage output of the power supply is 100V, when a 60W bulb is used as a dummy load, the current of the power supply when working is 200mA, but the main load current at startup reaches 2A, which is 10 times the normal working current
. Therefore, using a bulb as a dummy load can easily make the power supply difficult to start. Since the greater the power of the bulb, the smaller the cold resistance, the starting current of a high-power bulb is greater, and the power supply is more difficult to start. The starting current and working current of the power supply can be calculated according to I=U/R: the load current when the power supply starts is 100V/50Ω=2A, and the load current when the power supply works is 100V/500Ω=0.2A. Note: The above is a theoretical calculation, and there may be discrepancies in practice. In order to reduce the starting current, a 50W soldering iron can be used as a dummy load (the resistance value in both cold and hot states is 900Ω) or a 50W/300Ω resistor, which is more accurate than using a 60W light bulb. (2) Analysis of power supply connected to dummy load Some power supplies can be directly connected to dummy loads, while some cannot. The following is a detailed analysis. The first category: the switching power supply with line pulse synchronization can disconnect the line load and directly connect the dummy load. This type of switching power supply is a purely self-excited switching power supply. The purpose of introducing a forward line reverse pulse at the base of the switching tube is to synchronize the self-excited oscillation of the switching tube with the line pulse, and to limit the interference of the pulse radiation of the switching power supply to the diagonal stripes of the screen to the reverse line scanning, so that no interference can be seen 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. Therefore, it is called a switching power supply with line pulse synchronization. The method to determine whether it belongs to this type of power supply is that when the line 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 scanning circuit and use the dummy load method for maintenance. The second category: switching power supply with line pulse auxiliary excitation. The line reverse pulse of this 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 feedback network of the switching tube. The working process of this switching power supply is: after starting up, the switching tube generates self-excited oscillation. Under rated load, its feedback network can only make the output end generate a voltage lower than 40% of the normal output. This voltage starts the line scanning, and the feedback of the line pulse is used to assist the excitation of the switching tube to achieve the rated voltage output. This has two purposes: one is the voltage reduction protection function. Once the line scanning circuit fails, whether it is open or short-circuited, the output voltage of the switching power supply is reduced to 60% of the original value, so as to reduce the scope of damage. 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%, or even the output voltage will be very low. Obviously, this power supply cannot be directly disconnected from the line scan and repaired using the dummy load method, because even if the power supply circuit is normal at this time, it is impossible to output the rated voltage. The method to distinguish between power supply and line scan circuit failure is to use an external power supply to power the line scan circuit separately. If the line scan circuit works normally, it means that the switching power supply is bad. The third category: separately excited switching power supply. For separately excited power supplies 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 a separately excited switching power supply with line pulse frequency locking and indirect sampling (such as the Panda 2928 color TV), when directly connected to a dummy load (especially when connected to a high-power bulb such as 150W), the output voltage may drop significantly or there may be no output, because for 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 oscillation, the line synchronization pulse can advance the conduction time of the switch tube, and the power supply has the strongest load-carrying capacity at this time. If the line load is disconnected, the line synchronization pulse will lose its function, and the power supply's load-carrying capacity will inevitably decrease. In addition, the voltage stabilization sensitivity of the power supply with indirect sampling is low, and the output voltage will inevitably decrease. However, if the voltage stabilization 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 stabilization sensitivity, the line load can be disconnected and the dummy load can be directly connected, or even no-load maintenance can be performed.