How to ensure long-term reliability of the power supply?

At present, there are many kinds of power modules on the market, and the initial applications can meet the requirements, but with the test of time, they can not stand the test. Power system as the core of the system, absolutely does not allow this to happen, then how can we design a stable and reliable power supply it?

1, voltage stress

Power supply voltage stress is an important indicator to ensure the reliability of the power supply. Many devices in the power supply have high voltage specifications, such as: Vds and Vgs of the FET, reverse withstand voltage of the diode, large VCC voltage of the IC, and high voltage of the input and output capacitors. Therefore, we must consider the device to withstand the large voltage zui. Then choose the appropriate device based on voltage, zui and then verify the actual test. However, during the test, we must test the voltage stress of all working states of the power supply to ensure that we can also leave a safety margin of about 10% under the bad working conditions. ZLG input impulse voltage will make Zui a large amount of reservations, in order to cope with various industrial scenes, as shown in Figure 1 below.

Figure 1 ZLG power supply product limit characteristics table

2, current stress

Power supply current stress is often closely related to thermal stress, such as the diode SK54zui large average current of 5A, but it is the limit parameter under the premise of meeting the thermal stress derating. Therefore, we must choose the device to meet the current stress and thermal stress of the device at the same time; under the premise of meeting the thermal stress of the device, select the device with the appropriate rated current to ensure the reliability of the power supply. The ZLG input current will make a large reserve of zui to deal with the situation occurring in various industrial sites, as shown below.

Figure 2 ZLG power supply input characteristics table

3, feedback loop

The feedback loop is an important part of the power supply. We must ensure the stability of the feedback loop when designing the power supply, as shown in Figure 3 below. Therefore, we need to maintain a certain margin when designing the loop parameters; for example, the gain margin is generally kept at about 20db, the phase margin is maintained at about 45 degrees, the crossover frequency is generally set at 1/6 of the switching frequency, and the actual test is used to verify the loop. The stability of the road.

Figure 3 Power feedback loop

4. Magnetic saturation of magnetic elements

When we design a flyback transformer and some energy storage inductors, it is particularly critical to set a large flux of Bm. Since the large magnetic flux Bm is larger than the steady state Bm during power-on and short-circuit protection, I need to reserve sufficient margin for the Bm of the transformer. As shown in Fig. 4, when the core temperature is 100°C, the core is nearly saturated when Bm=0.35T. Therefore, for the consideration of the limit conditions such as power supply startup, output over-current, and short circuit, the ferrite P4 material transformer The steady state Bm is generally less than 0.25T.

Figure 4 Magnetic saturation curve of magnetic element

5, PWM dead time

For some H-bridge or half-H bridge topologies such as half-bridge, full-bridge, and LLC resonance, the PWM dead-time setting is critical to power reliability. Actually, setting the dead time actually avoids the upper tube and the lower tube from passing through and thus causes the power to burst. That is to say, setting the time for the upper and lower tubes to be turned off at the same time is delayed for a period of time after the upper tube is turned off and then turned on again. After the tube is turned off, the tube is turned on for a period of time and then turned on. As shown in Figure 5, td is the dead time.

Figure 5 PWM Dead Time Diagram

6, the soft start of the power

The soft start of the power supply helps to reduce the peak voltage and peak current of the FET and the output diode, thereby reducing its voltage stress and current stress. But for LLC resonant power supply, the soft start is very critical to the reliability of power supply start-up; because the IC of LLC power supply is started by high-frequency scanning. As shown in Figure 6, when the IC is started, the PWM frequency will be gradually restored from the set high frequency, and the recovery time is also the soft-start time. During this period, the power supply operates in an unstable state. The longer the soft start-up time, the safer the startup. However, if the soft start-up time is too long, it will affect the power supply capacity and start-up time of the power supply.

Figure 6 Schematic diagram of the soft start of the power supply

7, protection circuit

To ensure the reliability of the power supply in addition to the above six points, the corresponding protection circuit is also indispensable. Protection circuits include input under-voltage protection, input over-voltage protection, output over-current protection, output short-circuit protection, and output over-voltage protection. In order to avoid the abnormal operation of the power supply when the input voltage is too low, the power supply needs to add input undervoltage protection; in order to avoid the input voltage is too high, so that the voltage stress during the period exceeds the standard, the power supply needs to add input overvoltage protection; in order to avoid output overcurrent and Short-circuit causes the device to overheat, magnetic saturation and other phenomena. The power supply must add over-current protection and short-circuit protection. In order to prevent the output voltage of the power supply from being too high, the load end of the power supply is damaged, and the output of the power supply must be protected against overvoltage.

There are many factors that affect the reliability of power supplies. Only some of them are discussed here. For example, EMC, safety regulations, and overheat protection are all key factors that affect the reliability of power supplies. We may not be too difficult to design a power supply, but it is not easy to design a stable and reliable power supply. When designing only reliability factors and actually verifying it, the power supply can be considered as a stable and reliable power supply.