A power diode is a p-n junction device which has some constructional changes compared to low power signal diodes and are rated for kilo volts, kilo amperes and kilo watts.
The common application of power diodes are freewheeling didoes used for recovery of trapped energy, rectifier circuits.
A p-n junction diode is formed when p type semiconductor physically contacts n type semiconductor. The p-region has high concentration of holes (majority carriers) and low electron concentration (minority carrier). The n-region has high concentration of electrons (majority carrier) and low concentration of holes (minority carriers).
Generally p+ indicates highly doped p-region and n– indicates lightly doped n-region.
Formation of Depletion Layer
When p-region and n-region comes in contact with each other, electrons from n-region diffuse in to p-region leaving a positive charge behind. Similarly holes from p region diffuse in to n-region leaving behind negative charge. Due to this diffusion n region has positive charge and p region has negative charge near the junction. These charges across the junction produce electric field. This field stops further diffusion. So, each side of junction remains with immobile ions. This region extends in both p and n semiconductor layers called depletion region or space charge region. The potential difference across depletion region is called barrier potential (VT).
For Si, V = 0.7 V and for Ge, VT = 0.3 V.
Forward Biased diode: When p region connected to positive terminal and n region to negative terminal. Forward Bias voltage is inversely proportional to the width of depletion region.
Reverse Biased diode: When n region connected to positive terminal and p region to negative terminal. Reverse Bias voltage is directly proportional to the width of depletion region.
Note: As junction temperature increase, width of depletion region/ barrier potential decreases.
Note: If the region is lightly doped then depletion region extends more in it and it has large breakdown voltage. If the region is highly doped then depletion region extends less in it and it has small breakdown voltage.
Construction of Power Diode
It has highly doped n+ region. On which there is, a lightly doped ‘n–‘ layer. As this ‘n–‘ layer is lightly doped (nearly intrinsic) and lightly doped region has high reverse blocking capability. The thickness of n-layer is proportional to the breakdown voltage. Because of this ‘n–‘ layer or called ‘i’ layer the diode is also known as p-i-n diode.
V-I Characteristics of Power Diode
During forward bias from Va = 0 to , VT forward anode current is very small. Beyond Cut-in voltage or threshold voltage or turn on voltage (VT= 1V for power diode) anode current rises rapidly and diode starts conducting. For signal diodes anode current increase first exponentially then linearly while in case of power diode anode current rises linearly.
During reverse bias, small reverse anode current flow called leakage current up to breakdown voltage (VBR). After VBR reverse current increase abruptly. It may destroy diode and must be avoided.
For ideal diode forward drop is zero, reverse leakage current is zero, i.e. VBR = ∞, VT = 0.
Manufacturer indicates peak inverse voltage (PIV) which is same as VBR.
Power Diode Reverse Recovery Characteristics
When diode is reversed biased, the anode current starts decreasing linearly from If to zero. After the forward diode current decays to zero the diode continues to conduct in reverse direction because of pressure of stored charges in depletion region and semiconductor layers. The diode regains its blocking capability until reverse recovery current decays to zero.
trr = time for which reverse current flows from the instant when anode current becomes zero to reverse recovery current decays to 25% of its reverse peak value IRM. This time decides the switching frequency of any circuit.
ta = time between zero crossing of forward current and IRM reached. During this time stored charges in depletion layer are removed.
tb = time between IRM reached to reverse reached 0.25 IRM. During this time charges from semiconductor layer are removed.
Qr = stored charges or reverse recovery charge which is represented by shaded area. It must be removed in time trr.
S =tb/ ta = softness or S-factor. It is measure of voltage transients that occur during the time diode recovers. If S is small, then diode has large oscillatory voltage.
If S = 1; soft recovery diode
If S < 1; snappy recovery diode or fast recovery diode.
Using equation (1), (2) and (3) we get;
Classification of Power Diode
- General Purpose diode: trr = 25 micro-sec, rated kilo-Ampere, 50 kV, used in rectifiers.
- Fast Recovery Diode: trr = 5 micro-sec, rated few 100 Ampere, 3 kV, used in chopper.
- Schottky Diodes: trr = nano-seconds, rated few 300 Ampere, 100 V, used in SMPS (switch mode power supply). They are fastest diodes.