Give a thorough explanation of each of the three situations in which a switching power supply is connected to a fictitious load.
The switching power supply will cause the output voltage to drop when the load is short-circuited, and the output voltage will increase when the load is open-circuited 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, generally choose 40W or 60W bulbs as dummy loads (large-screen color TVs can use bulbs above 100W as dummy loads). and the level of the output voltage.
But the disadvantages are also obvious. For example, the hot state resistance of a 60W light bulb is 500Ω, while the cold state 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 light bulb is used as a dummy load, the current of the power supply is 200mA when the power supply is working, 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 it difficult to start the power supply. Because the greater the power of the bulb, the smaller the cold resistance, therefore, the larger the starting current of the high-power bulb, the more difficult it is to start the power supply.
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 is 100V/50Ω=2A when the power supply is starting, and the load current is 100V/500Ω=0.2A when the power supply is working. Yes: The above is a theoretical calculation, and the actual may be different. In order to reduce the start-up current, a 50W electric soldering iron can be used as a dummy load (both cold and hot state resistance is 900Ω) 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 will be explained in detail.
The first type is his 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 directly connected to the dummy load. For the separately excited switching power supply with horizontal pulse frequency locking and indirect sampling (such as Panda 2928 color TV), when the dummy load is directly connected (especially with a large power bulb such as 150W), the output voltage may drop a lot or No output, because of this type of power supply, although the addition of horizontal pulses is only for synchronization and frequency locking, and does not participate in oscillation, but the horizontal synchronization pulse can advance the turn-on 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 ability of the power supply to carry the load will inevitably decrease. In addition, the sensitivity of the indirect sampling power supply regulation is low, and the output voltage must also decrease. However, if this kind of power supply uses direct sampling (the sampling voltage is taken from the secondary side of the switching transformer) for this kind of power supply, due to the high sensitivity of the voltage stabilization, it can be disconnected from the line load and directly connected to the dummy load or even no-load for maintenance.
The second type is the line pulse synchronous switching power supply, which can disconnect the line load and directly connect to the dummy load.
This 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 self-excited oscillation of the switching tube with the horizontal pulse, and to interfere with the oblique lines of the screen by the pulse radiation of the switching power supply. Limited to line scan retracement so no disturbances are visible on the screen. The horizontal pulse added to the base of the switching tube only makes the switching tube conduction in advance of the cut-off period, and basically does not constitute an auxiliary excitation function, so it is called a horizontal pulse synchronous switching power supply. The way to judge whether it belongs to this kind of power supply is that when the reverse travel pulse is turned off, the switching power supply only sounds (because the oscillation frequency becomes lower), and the output voltage does not drop. Therefore, this power supply can be disconnected from the line scanning circuit and repaired by the dummy load method.
The third type is the switching power supply for the auxiliary excitation of the horizontal pulse.
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 feedback network of the switching tube. The working process of this kind of switching power supply is: the switching tube produces self-excited oscillation after starting up, and its feedback network can only make the output terminal generate a voltage lower than 40% of the normal output under the rated load. Feedback to the switching tube for auxiliary excitation, in order to achieve the rated voltage output. This has two purposes: one is to have the function of step-down protection. Once the line scanning circuit fails, no matter it is open circuit or short circuit, the output voltage of the switching power supply will drop to 60% of the original value, reducing the damage range. The second is that both the power supply and the row scan have a very short-time soft start process, which reduces the failure rate of the power supply and row 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 kind of power supply cannot be directly disconnected and repaired by 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 way to distinguish the failure of the power supply and the line scanning circuit 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 bad.
