Introduction: The output voltage of the switching power supply will decrease when the load is short-circuited, and the output voltage will increase when the load is open or no-load.
The switching power supply will cause the output voltage to decrease when the load is short-circuited, and the output voltage will increase when the load is open 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 load, 40W or 60W bulbs are generally selected as fake loads (large-screen color TVs can be used as fake loads with more than 100W bulbs). The advantage is intuitive and convenient. According to whether the light bulb is illuminated and the brightness of the light, it is possible to know whether the power supply has voltage output And the level of output voltage.
But the shortcomings are also obvious. For example, the hot state resistance of a 60W bulb is 500Ω, while the cold state resistance is only about 50Ω. According to the following table, it can be seen: assuming that the main voltage output of the power supply is 100V, when a 60W bulb is used as a fake load, the current when the power supply is 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 false load will easily make the power supply difficult to start. Because the larger the bulb power, the smaller the cold resistance. Therefore, the high-power 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, the formula I=U/R can be used to calculate: the load current when the power is started is 100V/50Ω=2A, and the load current when the power supply is working is 100V/500Ω=0.2A, but you need to pay attention Yes: The above are theoretical calculations, and there may be discrepancies in practice. In order to reduce the starting current, a 50W electric soldering iron can be used as a fake load (both cold and hot resistance values are 900Ω) or a 50W/300Ω resistor, which is more accurate than using a 60W bulb.
Some power supplies can be directly connected to a fake load, and some power supplies cannot. Specific problems need to be analyzed in detail. The following is a detailed explanation of the three situations.
The first type is a separately excited switching power supply.
For the 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 separately excited switching power supplies with line pulse frequency lock and indirect sampling (such as Panda 2928 color TV), when directly connected to a dummy load (especially when connected to a higher power bulb such as 150W), the output voltage may drop more or 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, the line synchronization pulse can advance the switch tube conduction time, 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. The power supply's ability to carry load will inevitably be reduced. The sensitivity of the power supply with indirect sampling will be lower, and the output voltage will inevitably be reduced. 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), the voltage stabilization sensitivity is high, and the line load can be disconnected and the dummy load can be directly connected to the dummy load or even no-load for maintenance.
The second type is the switching power supply with line pulse synchronization, which can disconnect the line load and directly connect to the dummy load.
This kind of switching power supply is purely a self-excited switching power supply. The purpose of introducing forward and backward pulses at the base of the switching tube is to synchronize the switching tube self-excited oscillation with the horizontal pulse, so that the pulse radiation of the switching power supply will interfere with the diagonal bars of the screen. It is limited to the reverse travel of the line scan, so no interference can be seen on the screen. The line pulse added to the base of the switching tube only makes the switching 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 method of judging whether it belongs to this kind of power supply is that when the line-reverse pulse is disconnected, the switching power supply only makes a scream (because the oscillation frequency becomes lower), and the output voltage does not drop. Therefore, this kind of power supply can disconnect the line scan circuit and use the dummy load method to repair it.
The third category is the switching power supply with line pulse auxiliary excitation.
The 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 switching tube feedback network. The working process of this kind of switching power supply is: the switching tube generates self-excited oscillation after power-on, and its feedback network can only make the output terminal produce a voltage lower than 40% of the normal output under rated load. This voltage makes the line scan start, and the line pulse The feedback to the switch tube for auxiliary excitation can achieve the rated voltage output. This has two purposes: First, there is a step-down protection function. Once the line scan circuit fails, whether it is open or short, the output voltage of the switching power supply drops to 60% of the original value, reducing the damage range. The second is that 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 drops by 40% to 60% at this time, and the output voltage is even very low. Obviously, this kind of power supply cannot be directly disconnected for line scanning and repaired by the dummy load method, because at this time, even if the power supply circuit is normal, it is impossible to output the rated voltage. The method to distinguish power supply and line scanning circuit failure is to use an external power supply to supply power to the line scanning circuit alone. If the line scanning circuit works normally, it means that the switching power supply is not good.
