In life, we often encounter problems with power failure, such as mobile phone adapters, PC power supplies, and some small appliances. When we encounter these problems and always lament the unreliability of the power supply, how can we design a stable and reliable power supply? Let's summarize the factors that affect the reliability of the power supply.
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 specified maximum withstand voltage values, such as: Vds and Vgs of the FET, reverse withstand voltage of the diode, maximum VCC voltage of the IC, and maximum withstand voltage of the input and output capacitors. Therefore, we must consider the maximum voltage that the device must withstand when designing. Then select the appropriate device according to the voltage, and finally carry out the actual test to verify. However, during testing we must test the voltage stress of all operating states of the power supply to ensure a safety margin of approximately 10% in the worst operating conditions.
2, current stress
The power supply current stress is often closely related to the thermal stress. For example, the maximum average current of the diode SK54 is 5A, but it is the limit parameter under the premise of satisfying the thermal stress derating. Therefore, we must meet the current stress and thermal stress of the device when selecting the device. Under the premise of satisfying the thermal stress of the device, the device with the appropriate rated current value can ensure the reliability of the power supply.
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. Therefore, we need to maintain a certain margin when designing the loop parameters; for example, the gain margin is generally maintained at about 20db, the phase margin is kept at about 45 degrees, and the crossover frequency is generally set at 1/6 of the switching frequency. The actual test is then performed to verify the stability of the loop.
4. Magnetic saturation of magnetic components
When designing flyback transformers and some energy storage inductors, setting the maximum flux Bm is especially critical. Since the maximum magnetic flux Bm is higher than the Bm during steady-state operation due to power supply start-up and short-circuit protection, I need to reserve enough margin when setting the Bm of the Transformer. As shown in Fig. 1, the core temperature is 100 °C. It can be seen that the core is nearly saturated when Bm=0.35T, so the transformer of ferrite P4 is considered for the limit of power supply, output overcurrent and short circuit. The steady state Bm is generally less than 0.25T.
figure 1
5, PWM dead time
For some H-bridge or half-H bridge topology power supplies such as half-bridge, full-bridge, and LLC resonance, the PWM dead time setting is critical to power supply reliability. In fact, the dead time is set to avoid the power supply of the upper and lower tubes, which means that the upper and lower tubes are turned off at the same time. After the upper tube is turned off, it is delayed for a while and then turned on. The tube or the upper tube is turned off after a period of time after the lower tube is turned off. As shown in Fig. 2, td is the dead time.
figure 2
6, the soft start of the power supply
The soft start of the power supply helps to reduce the peak voltage and peak current of the FET and output diode, thereby reducing its voltage stress and current stress. However, for LLC resonant power supplies, soft-start is critical to the reliability of the power-up; because the ICs of the LLC power supply are activated by high-frequency scanning. As shown in Figure 3, driving the PWM frequency at the start of the IC will slowly return to the normal frequency from the set maximum frequency, and the recovery time is also the soft start time. During this period, the power supply is in an unstable state. The longer the soft start time, the safer the startup. However, the long soft start time will also affect the power supply capacity and start-up time of the power supply.
image 3
7, protection circuit
In order to ensure the reliability of the power supply, in addition to the above six points, the corresponding protection circuit is also indispensable. The protection circuit specifically has input undervoltage protection, input overvoltage protection, output overcurrent protection, output short circuit protection, and output overvoltage 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, and the voltage stress exceeds the standard during the period, the power supply needs to add input overvoltage protection; to avoid output overcurrent and Short circuit causes overheating and magnetic saturation of the device. The power supply needs to add overcurrent protection and short circuit protection. To avoid damage to the power load terminal caused by excessive output voltage of the power supply, the power supply needs to add output overvoltage protection.
There are many factors affecting the reliability of the power supply. This article only talks about some of them. For example, EMC, safety regulations and overheat protection are all key factors affecting the reliability of the power supply. It may not be too difficult to design a power supply, but it is never too easy to design a stable and reliable power supply. We only consider all reliability factors and actually verify it when designing. Our power supply is a stable and reliable power supply.
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