Explain in detail the three types of situations where switching power supplies are connected to false loads
The first type is a separately excited switching power supply.
For non line pulse synchronous excitation power supplies (such as Changhong N2918 color TV), the line load can be disconnected and directly connected to the false load. For separately excited switching power supplies with row pulse frequency locking and indirect sampling (such as Panda 2928 color TV), when directly connected to a fake load (especially when connected to a high-power light bulb such as 150W), the output voltage may decrease significantly or have no output. Although the addition of row pulses only serves as synchronization and frequency locking, and does not participate in oscillation, row synchronous pulses can advance the conduction time of the switch tube, At this point, the power supply has the strongest load carrying capacity. 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 sensitivity of indirect sampling power supply voltage stabilization 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 switch transformer), due to its high voltage stabilization sensitivity, it can be disconnected from the running load and directly connected to the fake load, or even unloaded for maintenance.
The second type is a switching power supply with line pulse synchronization, which can disconnect the line load and directly connect the false load.
This type 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 switch tube is to synchronize the self-excited oscillation of the switch tube with the forward and backward pulses, limiting the diagonal interference of the pulse radiation of the switching power supply on the screen to the line scanning reverse, so the interference cannot be seen on the screen. The row pulses added to the base of the switch only make the switch conduct ahead of the deadline and do not constitute an auxiliary excitation function. Therefore, it is called a row pulse synchronous switching power supply. The method to determine whether it belongs to this type of power supply is that when the reverse travel pulse is disconnected, the switching power supply only sounds (due to a decrease in oscillation frequency), and the output voltage does not decrease. Therefore, this power supply can disconnect the line scanning circuit and repair it using the pseudo load method.
The third type is a switching power supply with row pulse auxiliary excitation.
The reverse pulse of this switching power supply not only synchronizes the self-excited oscillation frequency of the switching power supply, but also forms an indispensable part of the switch transistor feedback network. The working process of this switching power supply is: after turning on, the switch tube generates self-excited oscillation. Under rated load, its feedback network can only generate a voltage at the output end that is 40% lower than the normal output. This voltage causes the line scan to start, and the feedback of the line pulse to the switch tube is used as auxiliary excitation to achieve the rated voltage output. This has two purposes: firstly, it has a voltage reduction protection function. If there is a fault in the scanning circuit, whether it is an open circuit or a short circuit, the output voltage of the switching power supply will be reduced to 60% of the original value, reducing the scope of damage. The second is that both the power supply and line scanning have a very short soft start process, reducing the failure rate of the power supply and line scanning. If the feedback line pulse circuit is removed from this type of power supply, the output voltage of the power supply will decrease by 40% to 60%, or even the output voltage will be very low. Obviously, this type of power supply cannot be directly disconnected for scanning and repaired using the pseudo load method, because even if the power circuit is normal at this time, it is impossible to output the rated voltage. The method to distinguish between power supply and line scanning circuit faults is to use an external power supply to separately power the line scanning circuit. If the line scanning circuit works normally, it indicates that the switch power supply is poor.
