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Power diode these 15 knowledge points you must know!

2019-10-15 16:24

1. What is the forward rated current of the diode?


The rated current of the diode is the main nominal value of the diode, such as 5A/100V diode, 5A is the rated current. The normal rated current is defined as the rated average current through which the diode can pass. But some tests are preceded by square waves, which can pass through a square current with an average value of 5A. Some have to be tested if they have direct current, which is a current that can pass through 5A dc. In theory, a silicon diode tested on a square wave can pass through a larger DC, because the square wave of the same average current will cause a larger loss to the diode than the DC. So can 5A diode pass through 5A current? Not necessarily, it depends on the temperature, when you don't have good enough cooling conditions, the current that the diode can go through will be limited by the junction temperature.

2. What is the reverse rated voltage of the diode?

When the diode reverse cut off, it can withstand a certain reverse voltage, so the highest withstand reverse voltage is the rated voltage. For example, for a 5A/100V diode, the rated backpressure is 100V. Although all diode manufacturers will leave a certain margin, 100V diode usually used 110V will not be a problem, but it is not recommended to use this, because, beyond the rated value, the manufacturer will not guarantee its reliability, a problem is your problem. And many power design companies, to ensure reliability, will also reduce the design.


3. What is the forward impact current of the diode?


Switching on power or other transient conditions, the diode needs to be able to withstand a large impact of the current is not bad, of course, the impact of the current should be not repeated, or the interval is very long. This impulse current is usually defined in the diode datasheet, and the test condition is usually the impulse current of a single waveform, such as a single sine wave or square wave. The current is often several hundred.


4. What is the direct voltage drop of the diode?


When the diode is conducting, it will generate a voltage drop when it flows through the current. The pressure drop depends on the forward current and temperature. In general, silicon diode, the greater the current, the greater the pressure drop. The higher the temperature, the smaller the pressure drop. But the higher the temperature of the silicon carbide diode, the greater the pressure drop.


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


Diodes in reverse cut off, not completely ideal cut off. Under the reverse pressure, there will be some small current leakage from the cathode to the anode. This current is usually small, and the higher the backpressure, the greater the leakage current, the higher the temperature, the greater the leakage current. Large leakage current may cause large losses, especially in high voltage applications.


6. What is the diode reverse recovery time and reverse recovery current?


This is an important indicator of the diode, the so-called fast recovery, slow recovery diode is the standard. When the diode is converted from positive bias to negative bias, a large reverse recovery current will flow from the cathode to the anode. The time it takes to rise and fall is the reverse recovery time, and the peak current is the reverse recovery current. This can be very costly in high-frequency applications. However, the reverse recovery time is positively correlated with the rate of decline of the forward current when the current and diode are cut off. To solve this problem, one is to use the diode with faster recovery time, and the other is to use ZCS to turn off the diode.


7. What is a soft recovery diode?


The diode in reverse recovery, reverse current decline is relatively slow, called 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 the diode and can be regarded as the capacitance in parallel on the diode.


What is the parasitic inductance of a diode?


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


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


For the transient process of the diode, the reverse recovery characteristic is usually concerned. However, there are also some noteworthy aspects of the process of the diode from reverse deflection to direct pass. When the diode is first switched on, the forward pressure drop will first rise to a maximum value before falling to a steady-state value. And this maximum, as di/dt increases. That is to say, the diode band conduction moment will produce a positive peak voltage, and the voltage is greater than the steady-state voltage. This forward peak voltage of the fast recovery tube is relatively small, and the slow recovery tube will be very serious. This leads to another question:


11. In the RCD clamp circuit, which is the diode, slow or fast?


RCD circuits are often used in situations where clamping is required, such as flyback original MOS voltage clamping and secondary rectifier tube voltage clamping. Some technical literature suggests that a slow recovery tube should be used, because the reverse recovery time of the slow recovery tube is relatively long, so that part of the energy in the clamp capacitor will be returned to the circuit during the reverse recovery of the diode, so that the loss of the whole RCD circuit can be reduced. But this is only suitable for low current, low di/dt. Like the low-power flyback's original side clamp circuit. But not suitable for high current, high di/dt clamping occasions, such as high current output power supply secondary clamping circuit. Because, when the slow recovery tube is on, it will produce a high conduction voltage drop peak, which means that although the voltage on the clamp capacitor is very low, it cannot clamp the peak voltage. So choose something like a Schottky diode.


12. What is a Schottky diode?


Schottky diode is a kind of diode that utilizes the Schottky barrier technology. Compared with ordinary p-n junction diode, it has the advantage of faster reverse recovery time, which is called 0 reverse recovery time in many cases. It doesn't have a 0 reverse recovery time, but it's much faster than a normal diode. Disadvantages: the reverse leakage current is relatively large, so it can not be made into a high voltage diode. Current Schottky diodes are basically under 200V. Although some companies can provide high-voltage Schottky silicon diodes, several of them are packaged in series. Other companies claim to have a unique process for making high-voltage Schottky diodes, but don't know what it is.


What is a silicon carbide diode?


Silicon diodes are generally used as raw materials, but the latest silicon carbide diodes are silicon carbide diodes. At present, the common high - voltage Schottky silicon carbide diode, its advantages: reverse recovery characteristics are very good, comparable to Schottky silicon diode. But you can make high voltage diodes. There are many applications in PFC. Disadvantages: a high-pressure drop of the forward guide. Another difference with silicon diodes is that the conduction pressure drop increases with the increase of temperature. Early silicon carbide diode, but also can withstand the impact of the current, reliability is not high. But there has been a lot of improvement.


What is a gallium arsenide diode?


To tell you the truth, I heard gallium arsenide material before silicon carbide but later heard less. Gallium arsenide currently appears to have some applications in LEDs, but less in power components.


15. Are diodes suitable for parallel connection?


In theory, silicon diodes are not suitable for parallel connection because the conduction pressure drop decreases with the increase in temperature, but many diodes now package two single tubes together so that the temperature rise is relatively uniform, which is beneficial for parallel connection. But silicon carbide's pressure drop rises with temperature and is theoretically suitable for parallel connections.
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