POWER DIODE : Structure, Types, Characteristics & Working

Definition

A power diode is a type of diode that is commonly used in power electronics circuits. The power diode is a simple semiconductor device that includes three layers, two terminals , a single junction & conducts current in one direction. In order to increase the power handling capacity we use power diode by small changes in the structure of simple p- n junction diode.

A simple p-n junction semiconductor diode having two layers, two terminals & a single junction. The junction of normal diodes can be formed through semiconductors like p-type and n-type. The terminal at p-type is known as anode whereas the terminal at the n-type is known as the cathode.

It can be used as a rectifier in converter circuits, freewheeling / flyback diode, reverse voltage protection, voltage regulation circuits etc.

Classification Of Diode

Majorly there are two classifications in diodes.

1. Power Diode
2. Signal Diode

For power diodes, its an advantage that they handle high power but…. at low frequency. Ex. SCR’s

For signal diodes, they operate in very high frequencies (MHz, GHz ranges) but have very low power handing capacity. Ex: MOSFET’s

Finally to obtain both at an average level, IGBT’s came in to existence which have a comparably speed and power handling capacity.

So here is the relation in diodes…. Power inversely proportional to frequency . i.e., if u want to operate at high power, forget about frequency.

Power diodes are similar to signal diodes, although they vary slightly in their construction. The doping level in signal diode for both P-layer & N-layer is the same whereas, in power diodes, the junction can be formed among a heavily doped P+ layer & lightly doped N– layer.

Structure Of Power diode

Power diode having three layers like the P+ layer, n– layer and n+ layer. The upper layer is the P+ layer, it is heavily doped and the lower layer is n+ layer, and it is also heavily doped and the middle layer is n– layer, it is lightly doped. The heavily doped upper and lower (P+ layer and n+ layer ) is called terminal layers and terminal layers is always heavily doped to increase the conductivity of the device.

Here the upper layer p+ layer acts as an anode, the thickness of this layer is 10 μm & the doping level is 10¹⁹ cm^-3.

The lower layer n+ layer acts as a cathode, the thickness of this layer is 250-300 μm & the doping level is 10¹⁹ cm^-3.

The middle layer n- layer acts as a drift layer, the doping level is 10¹⁴ cm^-3 and the thickness of this layer mainly depends up on the breakdown voltage.

Once drift layer width increases then breakdown voltage will be increased. Due to its light doping concentration of drift layer, as we know that the thickness of the depletion region increases with a decrease in doping concentration. This increased thickness of the depletion region helps the diode to block larger reverse-biased voltage and hence have a greater breakdown voltage.

For explanation of Power Diode in Hindi refer the video given below

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V-I Characteristics of Power Diode

The V-I Characteristics of Power Diode is given below it is just similar to signal diode.

When we increase applied forward voltage the forward current increases linearly.

In forward biased condition when anode is positive w.r.t. cathode the forward current increase linearly with an increase in forward voltage.

In reverse biased condition when cathode is positive w.r.t. anode diode does not conduct and the diode then experiences a small current flowing in the reverse direction called the reverse leakage current or reverse saturation current. This reverse saturation current flow due to the moment of minority carrier. This reverse saturation current is independent of applied reverse biased voltage.

If this reverse applied voltage continuously increases after a certain reverse applied voltage a large reverse current flow through the device & the depletion layer will be destroyed. That reverse voltage is called reverse breakdown voltage (V_BR). Result large reverse current flow through the diode. This large revers current will destroyed the diode, So the diode is designed to dissipate such a high amount of heat.

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Reverse Recovery Characteristics of Power Diodes

The reverse recovery characteristics of the Power diode is shown in the following figure.

The AC diode is characterized by the Forward recovery time (t_F) and reverse recovery time (trr) .

Forward recovery time (t_F): Forward recovery time is the time required by a diode to start conduction is called forward recovery time. In other words time taken by a diode to switch from off state to on state is called Forward recovery time (t_F).

Reverse recovery time (trr) : After the forward diode current decays to zero, the continue to conduct in the reverse direction because of the presence of the stored charges in the two layers . The reverse flow for a time called reverse recovery time (trr). The diode retain its blocking capability until reverse recovery current decays to zero.

The reverse recovery time is define as the time between the instance forward diode current become zero and the instant reverse recovery current decays to 25% of its reverse peak value Irm.

The reverse recovery time is the combination of two segment of time ta and tb i.e. trr = ta + tb where time ta is the time between zero crossing of forward current and peak reverse current Irm. During the time ta, charged stored in depletion region is removed. Time tb is measured from the instant of Irm to the instant where 0.25 Irm is reached. During tb carge two semiconductor is removed.

The ratio of ta/tb is called softness factor or S-factor. The diode with softness equal to one is called soft recovery diode and diode with softness factor less than one is called snappy recovery diode or fast recovery diode.

For explanation of Power Diode refer the video given below