Power diodes | its structure, types and characteristics

# Power diodes | its structure, types and characteristics

### The characteristics of semiconductor power diodes & Transistors in power electronics

Power diodes device is a very important part of power electronics. The power semiconductor diode and transistor both are the same. Their function is the same as low power diode and transistor but structure and procedure control large power. Power diode and transistor are very large type power and signal device than low power diode and transistor in voltage current and power rating. In the power devices, lower switching speed diode and power transistor are compared. Power diode preserves energy by conversion AC to DC like a freewheeling diode. The power transistor is used as a switching device. Besides, the power transistor is used in transistor DC chopper and inverter.

## Power Diodes Structure

The power diode structure diagram and circuit symbol drawing is shown below-

Power diode has two ends like a normal diode. One is the anode or positive end and the other is the cathode or negative end. In the above figure, an anode end is created by light dropping a P-type material on the N-type substrate which works as a positive end of diode and whose thickness is very less. On the other hand cathode end is created by high doping of N type material whose thickness is so high and worked as the negative end of diode. The middle layer of anode and cathode called drift region whose thickness is more than anode but less than cathode. The breakdown voltage and ohmic loss of diode are dependent on this layer. If the layer increases, the breakdown voltage is also increased. The main problem of this layer is its high ohmic loss that produces high heat in diode, as a result diode burns or ruins. Usually, the different size of the layer is changed with the capacity of current diode.

#### Power semiconductor diodes I-V characteristics curve

When a positive voltage is given in anode end and a negative voltage is in cathode end of diode then the diode is called forward biased and goes to diode conduction. During this time, the little amount of forwarding current flows till VBF through the diode. The value of forwarding current increases quickly if the forward voltage is applied more than VBF. Again, when a negative voltage is applied in the anode and a positive voltage is in the cathode, the diode is called reverse biased. At that time, for minority carriers in diode a little reverse flows. But when the value of reverse voltage is more than VBR then reverse breakdown in diode and high quantity reverse current flows which have shown in the above figure. Here, VBF of the diode is called forward breakdown voltage or knee voltage and VBR is called reverse breakdown voltage or Zener breakdown voltage.

## Types of power diode and Identification

An ideal diode does not have any time of reverse recovery. The production cost of such kind diode is much. In many cases, the effect of reverse recovery time is not a matter of discussion, or it can be avoided, so available diode can be used there. Power diode can be divided into three based on recovery characteristics and production process. This characteristics and usage limitation is related to their application.

1. Standard or general-purpose diodes
2. Fast recovery diodes
3. Schottky diodes

General-purpose diodes: The reverse recovery time of the general-purpose rectifier diode is comparably much. Generally, at this time 25 μsec and in low-speed work it is used. Where recovery time is not critical, such as in diode rectifier till 1 kz frequency and line commutated converter etc is used. The current rating is half an ampere to a few hundred an ampere and voltage rating is from 50 V to 5 kilovolts of such kind of diode. Usually, such kind of diode consists of diffusion.

Fast recovery Diodes: The recovery time of the first recovery diode is usually less than 5 μsec. Such kind of diode is used where recovery time is important, such as- DC-DC and DC- AC charging circuit. This diode is made in the current rating of less than 1 ampere to hundreds ampere and from 50 volts to 3 kilovolts. For the voltage rating of 400 volts, the first recovery diode usually consists of diffusion and its recovery time can be controlled with platinum and gold diffusion. For voltage rating less than 400 voltage the switching speed is quicker in epitaxial diodes than diffusion diode. The base wideness of epitaxial diode is less, therefore its first recovery time is less than 50 ηsec. Different types of first recovery diodes are shown in the following figure-

Schottky diodes: Schottky diode removes the problem of storing charge in PN junction. It is done by setting a barrier potential between metal and a semiconductor connection. A metal layer consists of the thin epitaxial layer of n-type silicon. The potential barrier simulates the behavior of PN junction. Rectifying functions depend on minority carrier, therefore any extra minority carrier does not recombine in it. For having self-capacitor in semiconductor diode, recovery is very slow. The value of the recovery charge of the Schottky diode is very less than the same PN junction diode. Since it is for junction capacitance, so it does not depend on di/dt. The forward voltage drop of its comparatively less. The linkage current of the Schottky diode is more than PN junction diode. The conduction voltage of the Schottky diode is comparatively less and the linkage current is very high. Therefore, the limit of the maximum allowable voltage is 100V. The current rating of its from 1 ampere to 300 amperes. Schottky diode is the ideal for conducting high current and low voltage DC power.

The reverse recovery characteristics of semiconductor diodes:

Minority charge carrier of a power diode takes some time to neutralize with a retroactive charge carrier, this time is called reverse recovery time. A forward bias current is the attainment of the majority of the forward-biased junction and minority charge carrier. After the diode forward current is empty, some current is seen to flow through the diode. It happens for the minority charge carrier of the diode. Minority charge carrier needs some time to neutralize with a retroactive charge carrier, this time is called reverse recovery time. The reverse recovery time is shown in the following figure. Reverse recovery time is expressed with trr. The time from crossing zero of reverse current to 25% reverse current is called reverse recovery time (IRR) consists of this two times (trr). trr, ta and td the time that needs to make IRR zero and to reach in negative highest value that is ta and the time which needs to reach in zero from the highest value that is tb. ta/tb is the ratio of the softness or wetness of diode.

∴ trr= ta + tb Peak reverse current. IRR = ta di/dt, here di/dt is reverse current conversion rate. The amount of charge that flows through the diode during the time of passing from diode’s forward blocking condition to reverse blocking condition is called the reverse recovery charge (QRR) of the diode. Since the reverse recovery curve of the diode is almost triangle-shaped, so-

\begin{aligned}Q_{RR}=\dfrac {1}{2}I_{RR}t_{a}+\dfrac {1}{2}I_{RR}t_{b}\\ =\dfrac {1}{2}\left( ta+t_{b}\right) I_{RR}\\ =\dfrac {1}{2}I_{RR}t_{rr}\ldots \ldots \left( i\right) \\ \left( \because t_{rr} =ta+t_{b}\right) \\ \therefore I_{RR}=\dfrac {2QRR}{t_{rr}}\end{aligned}\\ \begin{aligned}Now,t_{a}\dfrac {di}{dt}=\dfrac {2Q_{RR}}{t_{rr}}\\ \Rightarrow t_{rr}t_{a}=\dfrac {2Q_{RR}}{\dfrac {di}{dt}}\\ \Rightarrow t_{rr}t_{rr}=\dfrac {2Q_{RR}}{\dfrac {di}{dt}}\\ \Rightarrow t_{11}=\left( \dfrac {2Q_{RR}}{\dfrac {di}{dt}}\right) ^{\dfrac {1}{2}}\end{aligned}\\ \begin{aligned}Again,I_{RR}=ta\dfrac {di}{dt}\\ \Rightarrow I_{RR}=\left( \dfrac {2Q_{RR}}{\dfrac {di}{dt}}\right) ^{\dfrac {1}{2}}\dfrac {di}{dt}\\ \Rightarrow \left( 2Q_{RR},\dfrac {di}{dt}\right) \dfrac {1}{2}\ldots \cdot \left( 3\right) \\ \therefore I_{RR}=\left( 2Q_{RR}\dfrac {di}{dt}\right) ^{\dfrac {1}{2}}\end{aligned}

So, from the equation of (2) and (3) it is seen that pic reverse recovery current and time of diode, both depend on the changing rate of stored charge and current of diode. Again, from the equation (1) it is seen that reverse recovery charge or stored charge of diode depends on reverse recovery current (IRR) and time (trr) of diode. So it can be said that the pic reverse current and reverse recovery time of diode both depend on the value of diode’s forward current.

The V-I Characteristics of series-connected diodes:

Some times in case of applying high voltage necessary voltage rating can’t be found by an available commercial diode. As a result, diodes are connected in series, therefore reverse blocking power increases. Suppose, two diodes are connected like figure. In the case of reality, the V-1 characteristic line of such kind of diode can’t be matched due to their production fault. In the figure, V-1 characteristic line of a kind of diode is shown. At the forward bias condition, both diodes conduct the same amount of current and forward voltage drop of each diode is almost the same. On the other hand, at reverse blocking conditions, each diode carries the same leakage current, as a result, blocking voltage shows the difference in unexpected rate.

The solution to this problem is to use two resisters so that voltage is divided equally. For same voltage sharing leakage current of every diode must be different, which is shown in the following figure-

Since total leakage current must share by a diode and resister, so it can be said that- Is = Is1+IR1 = Is2 + IR2

If the resistances are the same, then R=R1=R2 and two diode voltage will be different at a little rate. These two different voltage depends on the inconsistency of V-1 characteristic line. VD1 and VD2 these two voltage values get from the above equation. That is VD1 + VD2 = VS

A transient condition, voltage sharing is found by a connecting capacitor in the crossword of each diode, which is shown in the following figure. In this circuit, the increasing rate of blocking voltage is confined with Rs.

The V-I Characteristics of parallel-connected diodes: In case of using or applying high power, the diodes are connected parallel, as a result, current conductivity power increases. The current sharing of diode will match with their forward voltage drop. By using equal inductance or current sharing resister balanced current sharing is possible. It can be possible by using equal forwarding voltage drop or same kind of diode. As the diodes are connected in parallel, so the reverse blocking voltage of each diode must be equal.

The resisters of the figure help to share current in a steady-state condition. In dynamic conditions, current sharing is found by connecting coupled inductor. If the current flowing through D1 is increased, L1 in the crossing of L di/dt will increase and therefore the voltage of opposite polarity will attract in L2 inductor. Through D2 impedance way of the flow rate will create and the current will transfer in D2. If the inductors produce voltage spikes, then it will must costly and for conducting high current.

#### Omar Faruk

I am Omar Faruk, the owner of Engineers Advice. He is a person who always tries to invent something new & share that others. He loves to write about Mechanical, Electrical and Electronics related content. He has completed his diploma from the Department of Mechanical and graduation from the Department of EEE.

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