Basic knowledge: Common problems with power diodes

Aguilera

Basic knowledge: Common problems with power diodes [Copy link]

1. What is the forward current rating of a diode? The rated current of a diode is the main nominal value of the diode. For example, for a 5A/100V diode, 5A is the rated current. Usually, the rated current is defined as the rated average current that the diode can pass. However, some are square waves before testing, that is, they can pass a square wave current with an average value of 5A. Some have to be tested under the premise of DC, that is, they can pass a DC current of 5A. Theoretically, for silicon diodes, diodes tested under square waves can pass a larger DC current, because the square wave with the same average current will cause greater losses to the diode than the DC current. So can a 5A diode definitely pass a current of 5A? Not necessarily. This is related to temperature. When your heat dissipation conditions are not good enough, the current that the diode can pass will be limited by the junction temperature. 2. What is the reverse rated voltage of a diode? When a diode is reverse cutoff, it can withstand a certain reverse voltage, so the highest reverse voltage it can withstand is the rated voltage. For example, a 5A/100V diode has a rated reverse voltage of 100V. Although all diode manufacturers will leave a certain margin, 100V diodes can usually be used up to 110V without any problems, but it is not recommended to use it this way, because if it exceeds the rated value, the manufacturer will not guarantee its reliability, and if there is a problem, it is your problem. In addition, many power supply design companies will also derating the design to ensure reliability. 3. What is the forward surge current of the diode? When the switching power supply is turned on or in other transient conditions, the diode needs to be able to withstand a large surge current without being damaged. Of course, this surge current should be non-repetitive or have a long interval. Usually the diode data sheet has a definition of this surge current, and its test conditions are often a single waveform surge current, such as a single sine wave or square wave. Its current value can often reach hundreds of A. 4. What is the forward conduction voltage drop of the diode? When the diode is forward-conducting and current flows through it, a voltage drop will be generated. This voltage drop is related to the forward current and temperature. Usually for silicon diodes, the larger the current, the greater the voltage drop. The higher the temperature, the smaller the voltage drop. However, for silicon carbide diodes, the higher the temperature, the greater the voltage drop. 5. What is the reverse leakage current of the diode? When the diode is reverse-biased, it is not completely ideally cut off. When it is subjected to reverse voltage, some tiny current will leak from the cathode to the anode. This current is usually very small, and the higher the reverse voltage, the greater the leakage current, and the higher the temperature, the greater the leakage current. Large leakage current will bring greater losses, especially in high-voltage applications. 6. What is the reverse recovery time and reverse recovery current of a diode? This is an important indicator of a diode, and the so-called fast recovery and slow recovery diodes are based on this standard. When a diode switches from forward bias to reverse bias, a large reverse recovery current will flow from the cathode to the anode, and its reverse current will first rise to a peak value and then drop to zero. Then the time it takes to rise and fall is the reverse recovery time, and the peak current is the reverse recovery current. This will bring great losses in high-frequency applications. The reverse recovery time and current are positively correlated with the rate of decline of the forward current when the diode is cut off. To solve this problem, one is to use a diode with a faster recovery time, and the other is to use the ZCS method to turn off the diode. 7. What is a soft recovery diode? When a diode is reverse-biased, the reverse current drops relatively slowly, which is called a soft recovery diode. Soft recovery has certain benefits in reducing EMI. 8. What is the junction capacitance of a diode? Junction capacitance is a parasitic parameter of a diode and can be regarded as a capacitor connected in parallel to the diode. 9. What is the parasitic inductance of a diode? The parasitic inductance of a diode is mainly caused by the lead wire and can be regarded as an inductance connected in series to the diode. 10. What is the transient process of a diode when it is forward-conducting? For the transient process of a diode, the reverse recovery characteristics are usually of great concern. However, there are also things worth noting about the process of a diode switching from reverse bias to forward conduction. When the diode is just turned on, the forward voltage drop will first rise to a maximum value before it drops to a steady-state value. This maximum value increases with the increase of di/dt. In other words, a forward spike voltage will be generated at the moment the diode is turned on, and the voltage must be greater than the steady-state voltage. The forward spike voltage of a fast recovery tube is relatively small, while that of a slow recovery tube will be very serious. This leads to another question: 11. In the RCD clamping circuit, should a slow tube or a fast tube be selected for the diode? RCD circuits are often used in some situations where clamping is required, such as the voltage clamping of the primary side MOS of flyback and the voltage clamping of the secondary rectifier. Some technical literature says that slow recovery tubes should be used. The reason is that due to the long reverse recovery time of slow recovery tubes, part of the energy in the clamping capacitor will be fed back to the circuit during the reverse recovery process of the diode, so that the loss of the entire RCD circuit can be reduced. However, this is only suitable for small current and low di/dt situations. For example, the primary side clamping circuit of a low-power flyback. But it is not suitable for large current and high di/dt clamping situations, such as the secondary clamping circuit of a power supply with a large current output. Because the slow recovery tube will generate a very high conduction voltage drop spike when it is turned on, resulting in the voltage on the clamping capacitor being very low, but it cannot clamp the spike voltage. So Schottky diodes should be selected. 12. What is a Schottky diode? Schottky diodes are diodes that use Schottky barrier technology. Compared with ordinary PN junction diodes, their advantages are: faster reverse recovery time, many call it 0 reverse recovery time. Although it is not really 0 reverse recovery time, it is much faster than ordinary diodes. Its disadvantage is that the reverse leakage current is relatively large, so it cannot be made into a high-voltage diode. At present, Schottky diodes are basically below 200V. Although some companies can provide high-voltage Schottky silicon diodes, they also connect several diodes in series and package them together. Of course, there are also companies that claim to have unique processes that can manufacture high-voltage Schottky diodes, but it is not known what kind of process. 13. What is a silicon carbide diode? Usually, the diodes used by everyone are basically made of silicon as raw material, but the recently popular silicon carbide diodes are diodes made of silicon carbide as raw material. At present, most common Schottky silicon carbide diodes are high-voltage Schottky silicon carbide diodes. Its advantages are: good reverse recovery characteristics, comparable to Schottky silicon diodes. But it can be made into high-voltage diodes. It has been widely used in PFC. Disadvantages: The forward conduction voltage drop is relatively large. Another difference from silicon diodes is that its conduction voltage drop increases with the increase of temperature. Early silicon carbide diodes also had disadvantages such as small impact current and low reliability. But it has been greatly improved. 14. What is a gallium arsenide diode? To be honest, I heard that gallium arsenide materials were earlier than silicon carbide, but I heard less about it later. At present, gallium arsenide seems to have some applications in LEDs, but it is still relatively rare in power devices. 15. Are diodes suitable for parallel connection? Theoretically, silicon diodes are not suitable for parallel connection because the on-state voltage drop decreases with the increase of temperature. However, many diodes now package two single tubes together, so that the temperature rise is relatively uniform, which brings benefits to parallel connection. However, the voltage drop of silicon carbide increases with the increase of temperature. In theory, it is suitable for parallel connection, but I have not done any experiments, so I don’t know what problems there will be.