Diodes are the simplest semiconductor device having only two layers, two terminals and one junction. The ordinary signal diodes have a junction formed by p type semiconductor and n type semiconductor, the lead joining p type is called anode and the other side lead joining the n type is called cathode. The figure below depicts the structure of an ordinary diode and its symbol.
Power diodes are also similar to signal diodes but have a little difference in its construction.
In signal diodes the doping level of both P and N sides is same and hence we get a PN junction, but in power diodes we have a junction formed between a heavily doped P+ and a lightly doped N– layer which is epitaxially grown on a heavily doped N+ layer. Hence the structure looks as shown in the figure below.
The N– layer is the key feature of the power diode which makes it suitable for high power applications. This layer is very lightly doped, almost intrinsic and hence the device is also known as PIN diode, where i stands for intrinsic. As we can see in the figure above that the net charge neutrality of the space charge region is still maintained as was the case in signal diode but the thickness of space charge region is quite high and deeply penetrated into the N– region.
This is due to its light doping concentration, as we know that the thickness of space charge region increases with decrease in doping concentration. This increased thickness of depletion region or the space charge region helps the diode to block larger reverse biased voltage and hence have a greater breakdown voltage. However adding this N– layer significantly increases the ohmic resistance of the diode leading to more heat generation during forward conduction state. Hence power diodes come with various mountings for proper heat dissipation.
V-I Charecteristics of Power Diodes
The figure below shows the v-i charecteristics of a power diode which is almost similar to that of a signal diode.
In signal diodes for forward biased region the current increases exponentially however in power diodes high forward current leads to high ohmic drop which dominates the exponential growth and the curve increases almost linearly. The maximum reverse voltage that the diode can withstand is depicted by VRRM, i.e. peak reverse repetitive voltage. Above this voltage the reverse current becomes very high abruptly and as the diode is not designed to dissipate such high amount of heat, it may get destroyed. This voltage may also be called as peak inverse voltage (PIV).
Reverse Recovery Charecteristics of Power Diode
The figure depicts the reverse recovery charecteristic of a power diode. Whenever the diode is switched off the current decays from IF to zero and further continues in reverse direction owing to the charges stored in the space charge region and the semiconductor region. This reverse current attains a peak IRR and again start approaching zero value and finally the diode is off after time trr. This time is defined as reverse recovery time and is defined as time between the instant forward current reaches zero and the instant the reverse current decays to 25% of IRR. After this time the diode is said to attain its reverse blocking capability.
From the figure we see that
ta → time when charge from depletion region is removed
tb → time when charge from semiconductor region is removed
Also from the figure we can say that
Where, is the rate of change of reverse current.
The area bounded by the triangular region in the above figure represents the total charge stored or reverse recovery charge, QR. Hence we can write
Now, for , putting in eq.1 and combining with eq.2, we get
Putting eq.3 in eq.1 for , we get
From eq. 3 and 4 we can see that trr and IRR depends on QR which in turn depends upon the initial forward diode current IF.
Another interseting parameter is defined for power diodes from its turn off characteristics known as Softness Factor (S-factor) defined as the ratio of times tb and ta.
If a diode has S-factor equals to unity it is known as soft-recovery diode and for S-factor less that unity it is known as fast or snappy-recovery diodes. S-factor indirectly indicates the voltage transient that occurs upon the turn off of the diode. Low S-factor implies high transient over voltage while high S-factor implies low oscillatory reverse voltage.
The total power loss during turn off is the product of diode current and voltage during trr. Most of the power loss occurs during tb.
In a typical data sheet of power diodes the most important parameters given are IF avg, IF RMS, VRRM, I2t rating, junction temp TJ, trr, S-factor, IRR. Apart form these many other parameters and graphs are also provided.
The power diodes can be classified into following categories, summarized in the table below, as per their properties:
|General Purpose Diodes||50-5000 V||1A to several thousand Amps||~25µs||UPS, battery chargers, welding, traction etc.|
|Fast Recovery Diode||50-3000 V||1A to several thousand Amps||SMPS, commutation circuits, choppers, induction heating||Doping done using platinum or gold|
|Schottky Diodes||Upto 100V||1-300 A||~ns||Very high frequency switching power supplies and instrumentation||Metal-semiconductor junction, usually Al-Si(n-type), majority carrier device, hence very low turn off time|