Detailed explanation of three types of situations where the switching power supply is connected to a dummy load
The first type is a switch mode power supply with external excitation.
For a separately excited power supply without pulse synchronization (such as Changhong N2918 color TV), the operating load can be disconnected and directly connected to a dummy load. For externally excited switching power supplies 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 light bulb such as 150W), the output voltage may drop significantly or have no output. This is because for such power supplies, although the addition of line pulses only serves the purpose of synchronization and frequency locking, without participating in oscillation, the line synchronization pulses can advance the conduction time of the switch tube. At this time, 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 indirectly sampled power supply has lower voltage regulation sensitivity, and the output voltage will inevitably decrease. However, if the voltage regulator 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 sensitivity of the voltage regulator, it can be disconnected from the operating load and directly connected to a dummy load, or even unloaded for maintenance.
The second type is a switching power supply with row pulse synchronization, which can disconnect the running load and directly connect it to a dummy load.
This type of switching power supply is purely a self-excited switching power supply. The purpose of introducing forward and reverse pulse at the base of the switching transistor is to synchronize the self-excited oscillation of the switching transistor with the row pulse, and to limit the interference of the pulse radiation of the switching power supply on the diagonal bars of the screen to the reverse scan of the row, so there is no interference visible on the screen. The row pulse applied to the base of the switching transistor only causes the switching transistor to conduct ahead of the deadline, and does 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 pulse is disconnected, the switching power supply only makes a sound (due to the lower oscillation frequency), and the output voltage does not decrease. Therefore, this power supply can disconnect the scanning circuit and repair it using the false load method.
The third type is the switching power supply with row pulse assisted excitation.
The forward and 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 switching tube feedback network. The working process of this switching power supply is as follows: after starting up, the switching tube produces self-excited oscillation. Under rated load, its feedback network can only generate a voltage at the output terminal that is 40% lower than the normal output. This voltage causes the line scan to start, and the feedback of the line pulse is given to the switching tube to assist in excitation, in order to achieve the rated voltage output. This has two purposes: firstly, it has a voltage reduction protection function. Once 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 drop to 60% of its original value, reducing the scope of damage. Secondly, both the power supply and line scan have a very short soft start process, reducing the failure rate of the power supply and line scan. If the feedback pulse circuit is removed from this type of power supply, the output voltage of the power supply will drop by 40% to 60%, and even the output voltage will be very low. It is obvious that this type of power supply cannot be directly disconnected for scanning and maintenance 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 method to distinguish between power supply and row scanning circuit faults is to use an external power supply to supply power to the row scanning circuit separately. If the row scanning circuit works normally, it indicates that the switch power supply is faulty.
