Power diode is the most basic unit of power electronic circuit, and its unidirectional conductivity can be used for rectification, clamping and freewheeling of the circuit. The diode in the peripheral circuit mainly plays the role of anti-reaction to prevent the damage caused by the current backflow. Next, the editor will lead you to understand several major knowledge points about power diodes.
1. The forward rated current of the diode The rated current of the diode is the main nominal value of the diode, such as a 5A/100V diode, 5A is the rated current. Usually the definition of rated current is the rated average current that the diode can pass. But some test premise is square wave, that is, square wave current with an average value of 5A can be passed. Some have to test the premise is DC, that is, the DC current that can pass 5A. Theoretically speaking, for silicon diodes, a diode tested with a square wave can pass a larger DC current, because a square wave with the same average current will bring greater losses to the diode than a DC current. So is a 5A diode necessarily able to pass a 5A current? This is not necessarily because it is related to temperature. When the heat dissipation conditions are not good enough, the current that the diode can pass through will be limited by the junction temperature.
2. The reverse rated voltage of the diode When the diode is reversely cut off, it can withstand a certain back pressure, so the highest back pressure it can withstand is the rated voltage. For example, a 5A/100V diode has a rated back pressure of 100V. Diode manufacturers will leave a certain margin. 100V diodes are usually used with 110V without problems, but it is not recommended to use them, because if the rated value is exceeded, the manufacturer will not guarantee its reliability.
3. The forward voltage drop of the diode The diode will generate a voltage drop when the current flows in the forward direction. This voltage drop is related to forward current and temperature. Generally, the larger the silicon diode current, the larger the voltage drop; the higher the temperature, the smaller the voltage drop. However, the higher the temperature of silicon carbide diodes, the greater the voltage drop.
4. The reverse leakage current of the diode When the diode is cut off in the reverse direction, it is not completely ideal cut off. Under back pressure, some tiny current leaks from the cathode to the anode. This current is usually small, and the higher the back pressure, the greater the leakage current; the higher the temperature, the greater the leakage current. Especially in high-voltage applications, large leakage current will bring large losses.
5. The reverse recovery time and reverse recovery current of the diode When the diode is switched from forward bias to reverse bias, a large reverse recovery current will flow from the cathode to the anode, and the reverse current will first rise to the peak value and then drop. to zero. Then its rise and fall time is the reverse recovery time, and the peak current is the reverse recovery current. This will bring a lot of losses in high frequency applications. The reverse recovery time is positively related to the rate of decline of the forward current when the current and the diode are turned off. To solve this problem, on the one hand, use a diode with a faster recovery time, and on the other hand, use the ZCS method to turn off the diode.
6. Junction capacitance of diodes
Junction capacitance is a parasitic parameter of the diode and can be seen as a capacitance in parallel across the diode.
7. Parasitic inductance of diodes
The parasitic inductance of the diode is mainly caused by the lead wire, which can be regarded as the inductance connected in series with the diode.
8. Transient process when the diode is conducting forward
For the transient process of diodes, the reverse recovery characteristics are usually more concerned. But in fact, the process of diode from reverse deflection to forward conduction is also worth noting. When the diode is just conducting, the forward voltage drop will first rise to a maximum value before falling to the steady state value. And this maximum value increases with the increase of di/dt. That is to say, when the diode band is turned on, a forward peak voltage will be generated, and the voltage will be greater than the steady-state voltage. The forward peak voltage of the fast recovery tube is relatively small, and the slow recovery tube will be very serious.
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