Summary report of fifteen power diodes

What is a diode? How many types of diodes are there in total? Here is a summary of some power diodes, hoping to provide some help to everyone.

1、 What is the forward rated current of a diode?

The rated current of a diode is its main nominal value, for example, for a 5A/100V diode, 5A is the rated current. The usual definition of rated current is the capability of the diode.

Rated average current passing through. But some tests are conducted with square waves, which means that a square wave current with an average value of 5A can be used. Some tests require direct current, which means they can pass a direct current of 5A. In theory,

For silicon diodes, diodes tested with square waves can pass through larger DC currents because square waves with the same average current will cause greater losses to the diode compared to DC currents. So can a 5A diode necessarily pass a current of 5A? Not necessarily, it's related to temperature,

When your heat dissipation conditions are not good enough, the current that the diode can pass through will be limited by the junction temperature.

2、 What is the reverse rated voltage of a diode?

When a diode is turned off in reverse, it can withstand a certain amount of back voltage, and the highest back voltage it can withstand is the rated voltage. For example, a 5A/100V diode has a rated back voltage of 100V. Although all diode manufacturers leave a certain margin, and 100V diodes usually have no problem using 110V, it is not recommended to use them in this way because if they exceed the rated value, the manufacturer will not guarantee their reliability. If there is a problem, it will be your fault. Moreover, many power design companies will downgrade their designs to ensure reliability.

3、 What is the forward surge of a diode?

Switching power supplies require diodes to withstand large surge currents without breaking during startup or other transient situations. Of course, these surge currents should be non repetitive or have long intervals. Usually, the data manual for diodes defines this surge current, and its testing conditions are often a single waveform surge current, such as a single sine wave or square wave. Its current value can often reach several hundred.

4、 What is the forward conduction voltage drop of a diode?

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

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

When a diode is turned off in reverse, it is not completely ideal. When subjected to back pressure, there will be some small currents leaking from the cathode to the anode. This current is usually very small, and the higher the back pressure, the greater the leakage current. The higher the temperature, the greater the leakage current. A large leakage current can cause significant 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 diodes, and the so-called fast recovery and slow recovery diodes are based on this standard. When a diode transitions from forward bias to reverse bias, a large reverse recovery current flows from the cathode to the anode, and its reverse current first rises to the peak and then drops to zero. So the time of its rise and fall is the reverse recovery time, and the peak current is the reverse recovery current. This will cause significant losses in high-frequency applications. The reverse recovery time is positively correlated with the rate of decrease in forward current when the current and diode are turned off. To solve this problem, one is to use a diode with faster recovery time, and the other is to use ZCS to turn off the diode.

7、 What is a soft recovery diode?

A diode that experiences a slower decrease in reverse current during reverse recovery is called a soft recovery diode. Soft recovery has certain benefits in reducing EMI.

8、 What is the junction capacitance of a diode?

The junction capacitance is a parasitic parameter of a diode, which can be seen as a parallel capacitance on the diode.

9、 What is the parasitic inductance of a diode?

The parasitic inductance of a diode is mainly caused by the leads and can be regarded as an inductance connected in series with the diode.

10、 What is the transient process when a diode is conducting in the forward direction?

For the transient process of diodes, the reverse recovery characteristics are usually of more concern. However, there are also noteworthy aspects to the process of a diode transitioning from reverse deflection to forward conduction. When the diode is first turned on, the forward voltage drop will first rise to a maximum value before dropping to the steady-state value. And this maximum value increases with the increase of di/dt. That is to say, the diode will generate a forward spike voltage at the moment of conduction, and the voltage must be greater than the steady-state voltage. The forward spike voltage of the fast recovery diode is relatively small, and the slow recovery diode will be very severe.

11、 In RCD clamp circuits, should we choose slow diodes or fast diodes?

RCD circuits are commonly used in situations that require clamping, such as voltage clamping for flyback primary MOS and voltage clamping for secondary rectifiers. Some technical literature suggests the use of slow recovery transistors, as they have a longer reverse recovery time. This allows a portion of the energy in the clamp capacitor to be fed back to the circuit during the reverse recovery process of the diode, thereby reducing the overall losses of the RCD circuit. However, this is only suitable for situations with low current and low di/dt. For example, the primary clamp circuit for low-power flyback. But it is not suitable for high current, high di/dt clamping scenarios, such as secondary clamping circuits for high current output power supplies. Because slow recovery transistors generate high conduction voltage drop spikes during conduction, even though the voltage on the clamp capacitor is low, it cannot clamp the spike voltage. So we should choose Schottky diodes and the like.

12、 What is Schottky diode?

Schottky diode is a type of diode that utilizes Schottky barrier technology. Compared with ordinary PN junction diodes, it has the advantage of faster reverse recovery time, which is often referred to as zero reverse recovery time. Although the reverse recovery time is not really zero, it is much faster compared to 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, most Schottky diodes are below 200V. Although some companies can provide high-voltage Schottky silicon diodes, they also package several diodes in series together. Of course, some companies claim to have unique processes for manufacturing high-voltage Schottky diodes, but it is not known what kind of process it is.

13、 What is silicon carbide diode?

Usually, the diodes used by most people are made of silicon as the raw material, but the recently popular silicon carbide diodes are those made of silicon carbide as the raw material. Currently, the commonly used high-voltage Schottky silicon carbide diodes have the advantage of excellent reverse recovery characteristics, comparable to Schottky silicon diodes. But it can be used as a high-voltage diode. It has been widely used in PFC. Disadvantage: The forward conduction voltage drop is relatively large. Another difference from silicon diodes is that their conduction voltage drop increases with temperature. Early silicon carbide diodes had disadvantages such as low ability to withstand surge currents and low reliability. But there has been significant improvement so far.

14、 What is gallium arsenide diode?

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

15、 Are diodes suitable for parallel connection?

In theory, silicon diodes are not suitable for parallel connection because their conduction voltage drop decreases with increasing temperature. However, many diodes now package two single diodes together, which results in relatively uniform temperature rise and benefits for parallel connection. However, the pressure drop of silicon carbide increases with temperature, and theoretically it is suitable for parallel connection.