Detailed explanation of the switching power supply connected to the false load of the three types of cases
The first type is a separately excited switching power supply.
For separately excited power supplies without row pulse synchronization (such as Changhong N2918 color TV), the row load can be disconnected and the dummy load 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 it is directly connected to a false load (especially when connected to a larger 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 only plays a role in synchronization and frequency locking, and does not participate in oscillation, the horizontal synchronization pulses can advance the conduction time of the switch tube, and the power supply has the strongest load capacity at this time. , if the row load is disconnected, the row synchronization pulse will lose its effect, the power supply's ability to carry the load will inevitably be reduced, the power supply voltage stabilization sensitivity coupled with indirect sampling will be low, and the output voltage will also be reduced. However, if the voltage stabilizing circuit of this type of power supply adopts direct sampling (the sampling voltage is taken from the secondary side of the switching transformer), due to the high voltage stabilization sensitivity, it can be disconnected from the line load and directly connected to a false load or even no-load for maintenance.
The second type is a switching power supply with row pulse synchronization that can disconnect the row 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 row and 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 that the pulse radiation of the switching power supply interferes with the oblique bars of the screen. Limited to the line scan inverse path, so no interference is visible on the screen. The horizontal pulse applied to the base of the switching tube only causes the switching tube to 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 horizontal pulse synchronization. The way to judge whether it belongs to this kind of power supply is that when the reverse pulse is turned off, 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 row scanning circuit and use the false load method to repair it.
The third category is the switching power supply with line pulse auxiliary excitation.
The reverse pulse of this kind of switching power supply not only completes the synchronization of the self-oscillation frequency of the switching power supply, but also forms an indispensable part of the switching tube feedback network. The working process of this kind of switching power supply is: after the power is turned on, the switching tube generates self-excited oscillation. Under the rated load, its feedback network can only make the output terminal generate a voltage lower than 40% of the normal output. This voltage starts the row scan, and the row pulse The feedback provides auxiliary excitation to the switch tube to achieve the rated voltage output. This has two purposes: First, it has a voltage reduction protection function. Once the row scanning circuit fails, whether 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 scope of damage. Second, both the power supply and line scan have a very short soft-start process, which reduces the failure rate of the power supply and line scan. For this kind of power supply, if the feedback row 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 the line scan is inspected using the false load method, because even if the power circuit is normal at this time, it is impossible to output the rated voltage. The way to distinguish between power supply and row scanning circuit faults is to use an external power supply to power the row scanning circuit separately. If the row scanning circuit works normally, it means that the switching power supply is defective.
