Basic structure of Power Diode:
Power diode consists of three layers. Top layer is a heavily doped P+ layer. Middle layer is lightly doped n– layer and the last layer is a heavily doped n+ layer.
The heavily doped p+ layer act as an anode. The thickness of this layer is around 10 μm and doping level is 1019 cm-3.
Last layer of the heavily doped n+ act as a cathode. The thickness of this layer is around 250 to 300 μm and doping level is 1019 cm-3.
Middle layer of lightly doped n– is known as a drift layer. The thickness of the drift layer depends on the required breakdown voltage. The breakdown voltage increases with an increase in the width of the drift layer. Resistivity of this layer is high because of the low level of doping. If the width of the drift layer increased, then the on-state voltage drop increase therefore power loss is more. The doping level of the drift layer is 1014 cm-3.
The junction is form between the anode layer (p+) and drift layer (n–). The cross-section area of the diode depends on the magnitude of current to be handled. Higher the current to handle, more the area required.
Operating Principle of Power diode:
The operating principle of power diode is same as the conventional PN junction diode. A diode conducts when the anode voltage is higher than the cathode voltage. The forward voltage drop across the diode is very low around 0.5V to 1.2V. In this region, the diode works as a forward characteristic.
If the cathode voltage is higher than the anode voltage, then the diode works as blocking mode. In this mode, diode works according to the reverse characteristic.
I-V characteristic of Power Diode:
The I-V characteristic of power diode is as shown in the figure. The forward current increase linearly with an increase in forward voltage.
A very small amount of leakage current flows in the reverse bias (blocking mode). The leakage current is independent of the applied reverse voltage. The leakage current flows due to the minority charge carriers. When the reverse voltage reaches the reverse breakdown voltage, avalanche breakdown occurs. Once the reverse breakdown occurs, the reverse current increase drastically with small increase in reverse voltage. The reverse current can control by an external circuit.
Application of Power Diode in Power Electronics:
- High voltage rectifier
- As freewheeling diode
- As feedback diode
In the case of reverse breakdown, As the voltage and current of the diode are large, the power dissipation is dangerously high and it can destroy the device.
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