Power diode analysis

What is a power diode? What is its function? Power diodes are one of the components that electronic engineers inevitably deal with. What is your opinion on power diodes? If you are unclear, don’t panic. This article mainly summarizes the knowledge about power diodes. Click here to learn together:

1. What is the forward current rating of a diode?

The rated current of the 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. But some tests use square waves, which means they can pass a square wave current with an average value of 5A. Some tests require DC, which means they can pass a DC current of 5A. Theoretically, for silicon diodes, diodes using square waves as test conditions can pass larger DC currents, because square waves with the same average current will cause greater losses to the diodes than DC currents. 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 voltage rating of a diode?

When the diode reversely cuts off, it can withstand a certain amount of back pressure, so the highest back pressure it can withstand is the rated voltage. For example, a 5A/100V diode has a rated back voltage of 100V. Although all diode manufacturers will leave a certain margin, 100V diodes usually have no problem when used up to 110V, but it is not recommended to use it because if it exceeds the rated value, the manufacturer will not guarantee its reliability. If there is a problem, you will be the one problem. Moreover, many power supply design companies will derate the design in order to ensure reliability.

3. What is the forward inrush current of a diode?

When the switching power supply is turned on or under other transient conditions, the diode needs to be able to withstand a large inrush current without breaking down. Of course, this inrush current should be non-repetitive or have a long interval. Usually the diode data sheet defines this inrush current, and the test condition is often a single waveform inrush current, such as a single sine wave or square wave. Its current value can often reach several hundred.

4. What is the forward voltage drop of a diode?

The diode conducts in the forward direction and produces a voltage drop when current flows. This voltage drop is related to forward current and temperature. Usually for silicon diodes, the greater the current, the greater the voltage drop. The higher the temperature, the smaller the pressure drop. However, the higher the temperature of silicon carbide diodes, the greater the voltage drop.

5. What is the reverse leakage current of a diode?

When the diode is blocking in reverse, it is not completely ideal. When subjected to back pressure, some tiny current will leak from the cathode to the anode. This current is usually very small, and the higher the back pressure, the greater the leakage current, and the higher the temperature, the greater the leakage current. Large leakage current will cause large losses, especially in high-voltage applications.

6. What are the reverse recovery time and reverse recovery current of a diode?

This is an important indicator of the diode. The so-called fast recovery and slow recovery diodes are based on this standard. When the diode switches from forward bias to reverse bias, a large reverse recovery current will flow from the cathode to the anode. The reverse current first rises to the peak value and then drops to zero. Then its rising and falling time is the reverse recovery time, and the peak current is the reverse recovery current. This will cause great losses in high-frequency applications. The reverse recovery time is positively related to the current and the rate of decline of the forward current when the diode is turned 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 reversely recovers, the reverse current decreases 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 the diode and can be regarded as a parallel capacitance on the diode.

9. What is the parasitic inductance of a diode?

The parasitic inductance of the diode is mainly caused by the lead and can be regarded as the inductance connected in series on the diode.

10. What is the transient process when the diode is conducting forward?

For the transient process of the diode, the reverse recovery characteristics are usually more concerned. But in fact, there is something worth noting about the process of the diode changing from reverse deflection to forward conduction. When the diode first turns on, the forward voltage drop will first rise to a maximum value and then drop to a steady-state value. And this maximum value increases with the increase of di/dt. That is to say, a forward spike voltage will be generated when the diode band is turned on, and the voltage will be greater than the steady-state voltage. The forward peak voltage of fast recovery tubes is relatively small, but it will be serious for slow recovery tubes. This leads to another question:

11. In an RCD clamp circuit, which diode should be chosen as a slow diode or a fast diode?

RCD circuits are often used in situations where clamping is required, such as the voltage clamping of the flyback primary MOS and the voltage clamping of the secondary rectifier. Some technical documents say that slow recovery tubes should be used. The reason is that the slow recovery tube has a relatively long reverse recovery time, so part of the energy in the clamping capacitor will be fed back to the circuit during the reverse recovery process of the diode, thus reducing the loss of the entire RCD circuit. can be lowered. However, this is only suitable for low current and low di/dt situations. For example, the primary side clamping circuit of a low-power flyback. But it is not suitable for high current and high di/dt clamping situations, such as the secondary clamping circuit of a power supply with high current output. Because the slow recovery tube will generate a very high conduction voltage drop spike when it is turned on, resulting in the inability to clamp the spike voltage even though the voltage on the clamping capacitor is very low. So you should choose Schottky diodes and the like.

12. What is a Schottky diode?

Schottky diode is a diode that uses Schottky barrier technology. Compared with ordinary PN junction diodes, its advantages are: faster reverse recovery time, which many call it zero reverse recovery time. Although the reverse recovery time is not really zero, it is much faster than ordinary diodes. Its disadvantage: the reverse leakage current is relatively large, so it cannot be made into a high-voltage diode. Current Schottky diodes are basically below 200V. Although some companies can provide high-voltage Schottky silicon diodes, several diodes are connected in series and packaged together. Of course, some companies claim to have a unique process that can manufacture high-voltage Schottky diodes, but they don't know what kind of process it is.

13. What is a silicon carbide diode?

Usually, the diodes we use are basically made of silicon, but the recently popular silicon carbide diodes are diodes made of silicon carbide. Currently, most of the common ones are high-voltage Schottky silicon carbide diodes. Their advantages are: very good reverse recovery characteristics, comparable to Schottky silicon diodes. But it can be used as a high voltage diode. It has many applications in PFC. Disadvantages: The forward voltage drop is relatively large. Another point that is different from silicon diodes is that the conduction voltage drop increases as the temperature rises. Early silicon carbide diodes also had shortcomings such as small inrush current they could withstand and low reliability. But there has been a lot of improvement.

14. What is a gallium arsenide diode?

To be honest, I heard that gallium arsenide material preceded silicon carbide, but I heard less about it later. At present, gallium arsenide seems to have some applications in LEDs, but there are still relatively few applications in power devices.

15. Are diodes suitable for parallel connection?

Theoretically speaking, silicon diodes are not suitable for parallel connection because the conduction voltage drops as the temperature rises. However, many diodes now package two single tubes together so that the temperature rise is relatively even, which brings benefits to parallel connection. However, the voltage drop of silicon carbide increases with temperature, so it is theoretically suitable for parallel connection. The above is the technical analysis of power diodes, I hope it can help you.