Analysis of abnormal conditions of switching power supply based on secondary voltage stabilization flyback

For power engineers, mastering the basic principles of switching power supplies is a basic requirement for power engineers. Being able to design power products based on these principles and related formulas has become a necessary ability to enter the power industry. After a period of time, each power engineer will accumulate a certain amount of practical experience, gradually deepen their understanding and improve their ability to analyze and solve problems in actual combat. We often face various power supply abnormalities. Analyzing and solving these problems can enhance engineers' understanding of power supply design. Among them, flyback is the most widely used. This article takes the flyback circuit of secondary voltage regulation as an example to share the following abnormal conditions and related analysis.

Practical question 1: Will the device explode if the primary Rsense resistor is short-circuited and then turned on?

The problem of machine explosion can be divided into several situations. Taking the 2843 series as an example, the short circuit of the sampling resistor directly causes the current loop to malfunction. Secondly, according to common sense, it should enter the voltage-controlled output feedback loop for voltage regulation, and the duty cycle is always opened to the maximum. But the actual situation is that after removing the current circulation loop, there is no cycle-by-cycle current detection for correction, and the speed of the voltage loop is relatively slow (it is impossible to achieve the work of input power following output power). It is impossible to effectively adjust the inertia link of such a large inductance, so the loop oscillates, the drive enters a disordered mode (no longer fixed frequency) and the input energy is excessive, the inductor CCM mode. Based on the previous statement that only the voltage loop works and the relationship between explosion (the loop must be oscillating).

1. No-load condition: Since the output load current is very small, the speed of the voltage loop will be relatively faster (relative to heavy load), and the module may not explode for a short time or even never explode.

2. The heavier the load, the slower the voltage loop responds, the more serious the excess input energy is, and the faster the machine explodes.

3. Heavy load will not necessarily cause the IC to burn out. Many ICs now have slope compensation and high and low voltage compensation. If the compensation is too much, the IC can still have a current limiting function.

Practical question 2: Primary RCD absorption (600V MOS), if D is lost or poorly soldered, will it explode when working at full load of 265V?

Cold solder joint or missing parts are equivalent to no RCD. At this time, the stress of MOS will increase, generally exceeding the MOS specification, making MOS work in avalanche state, and it is repetitive. If the margin is insufficient, MOS will explode immediately, or it may not explode temporarily, but further look: if the machine is continuously baked under full load, as the temperature rises, the MOS stress will increase, and MOS with weaker individual tolerance will also explode.

Practical question 3: Can the bulk electrolysis be operated without it? What are the effects?

Removing the bulk capacitor will cause the bus voltage to be unstable, but the machine can still be started normally. The main impacts are as follows:

1) In the process from no-load to full-load, the output voltage will drop, mainly because the primary overcurrent protection will be triggered when the bus voltage is low.

2) At this time, the transformer will be saturated and there may be noise.

3) If the starting circuit is connected before the rectifier

4) The efficiency will be very poor at this time. The greater the bus voltage fluctuation, the worse the efficiency. Usually, one way to improve efficiency is to increase the capacitance.

Practical question 4: In principle, can the following circuit diagram pass safety regulations?

Circuit Diagram

It is difficult to meet safety regulations. Because the AC L/N is connected to the circuit behind the fuse, it is difficult to meet safety regulations for the safety distance. Here are two main points:

1) Those who have done safety regulations know that there is an open/short test. For IC, any two pins are short-circuited. After the short circuit, it can be protected, no output or crash, but the following two points must be met:

A. There should be no fire or smoke;

B. Hi-pot test is OK. In the schematic diagram above, when the HV pin and FB pin of the IC are short-circuited, the high voltage of HV will damage the optocoupler connected to the FB pin. In this case, testing the Hi-pot will fail, which poses a safety hazard. So how to solve it? You can connect a voltage regulator to the 3rd and 4th pins of the optocoupler, but pay attention to the power of the voltage regulator. If you choose the wrong type, the optocoupler will still be damaged.

2) Connect a resistor in series with the VCC winding. In the abnormal test of safety regulations, such as the VCC winding unloading, if there is no series resistor, the transformer may overheat, damage the insulation of the transformer, and cause the Hi-pot to fail, which poses a safety hazard. In practice, many people may not add it, but it is recommended to add it, which can withstand a certain voltage drop and is beneficial to the VCC voltage, and can also be used as a jumper.