Switching power supply genuine and fake load inspection

I believe many people know the power supply, so how to judge the load of the power supply? Under what circumstances should the switching power supply be connected to what kind of load, when is it a real load, when is it a fake load, and why is it connected to a fake load? This is what we need to understand. The switching power supply will cause the output voltage to decrease when the load is short-circuited, and the output voltage will also increase when the load is open-circuited or no-load.

During 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, a 40W or 60W light bulb is generally selected as a dummy load (large-screen color TVs can use a light bulb of more than 100W as a dummy load). The advantage is that it is intuitive and convenient. According to whether the light bulb emits light and the brightness of the light, it can be known whether the power supply has a voltage output. and the level of output voltage.

But the shortcomings are also obvious. For example, the hot resistance of a 60W light bulb is 500Ω, but the cold resistance is only about 50Ω. It can be seen from the table below: Assume that the main voltage output of the power supply is 100V. When a 60W bulb is used as a dummy load, the working current of the power supply is 200mA, but the main load current at startup reaches 2A, which is 10 times the normal operating current. , Therefore, using a light bulb as a dummy load will easily make it difficult to start the power supply. Since the greater the power of the light bulb, the smaller the cold resistance, the starting current of the high-power light bulb will be larger, making it more difficult to start the power supply.

When calculating the starting current and operating current of the power supply, you can use the formula I=U/R to calculate: the load current when the power supply is starting is 100V/50Ω=2A, and the load current when the power supply is working is 100V/500Ω=0.2A. However, it needs to be noted. Yes: The above is a theoretical calculation, and the actual calculation may be different. In order to reduce the starting current, you can use a 50W soldering iron as a dummy load (both hot and cold resistances are 900Ω) or a 50W/300Ω resistor, which is more accurate than using a 60W light bulb.

Some power supplies can be directly connected to dummy loads, but some power supplies cannot. Specific problems need to be analyzed in detail. The following is a detailed explanation of the three types of situations.

The first type is a separately excited switching power supply.

For separately excited power supplies without line pulse synchronization (such as Changhong N2918 color TV), the line load can be disconnected and directly connected to the dummy load. 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 forward row and reverse pulses 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. The above is the method for judging the power load. I hope it will be helpful to everyone.