Power Diode

In this Article, We will be going to see the Power Diode, the V-I characteristics of a Power Diode, Reverse Recovery Characteristics, the Need for a power Diode, the Construction of power diodes, Working of Power diodes with their types. At last, we will conclude our article With Advantages, Disadvantages, and FAQs.

Power Diode

What is a Power Diode?

A power diode metal-semiconductor interface device is different from a normal standard pn-junction diode, in terms of its structural features and dimensions. The power diode primarily conducts current in one direction by blocking the current flow in the reverse direction, with lower resistance in that direction and high resistance in the reverse direction. In power electronics power diode is of much importance since it can be used in many applications such as power supply and rectification purposes.

It operates on the principle of a semiconductor junction; in power diodes P-type semiconductor is connected to an N-type semiconductor material. When a positive voltage is applied to the P-type side and a negative voltage to the N-type side (forward bias), it allows current to flow easily through the diode with a lower voltage drop. However, when the polarity of the applied voltage is reversed (reverse bias), the diode blocks most of the current flow due to the creation of a depletion layer that significantly increases resistance. This characteristic is vital in preventing undesired reverse currents in circuits and maintaining the desired direction of current flow.

Power diodes come in various forms and are capable of handling high voltages and currents efficiently. They play a crucial role in converting AC (alternating current) to DC (direct current) in power supplies, protecting circuits from reverse voltage, and managing power flow in various electronic devices and systems.

Power Diode

Symbol of the Power Diode

Symbolic representation of power diode

V-I characteristics of Power Diode

The V-I (voltage-current) characteristics of a power diode describe its behavior in terms of voltage and current when operating in different modes.

V-I Characteristic curve of power diode

• With increase of the source voltage Vs from zero to cut- in voltage, the forward- diode current is veritably small.
• Cut- in voltage is also known as threshold voltage or turn- on voltage.
• The current of the diode rises rapidly for the values lying beyond the cut-in voltage and the diode starts conducting.
• For silicon diode, the cut- in voltage is around 0.7 V. When diode conducts, there’s a forward voltage drop of the order of 0.8 ~1V.
• For high power diodes, the forward current grows linearly with increase in voltage.
• For low power diodes, the forward current first increases exponentially with the voltage and then becomes linear with respect to change in voltage.
• In the reverse biased condition, a small reverse current called leakage current, arises.
• The leakage current is nearly independent of the magnitude of reverse voltage until this voltage reaches breakdown voltage. At this breakdown point, voltage remains nearly constant but reverse biased current becomes relatively high-limited only by the external circuit resistance.
• By operating the diode below the peak reverse repetitive voltage V_RRM .
• Peak Inverse Voltage is the largest reverse voltage to which a diode may be subordinated during its working. PIV is the same as V_RRM

Reverse Recovery Characteristics

Reverse Recovery Characteristics shows the behavior of power diode, when transitioning from the conduction state to the non-conducting state. Given below is the graph fo Reverse Recovery Characteristic of power diode.

Reverse Recovery Characteristic curve of power diode

The behavior of the power diode when it is switched from conducting state of a non-conducting state is known as reverse recovery characteristics of the power diode.

• When a diode is forward-biased, it conducts current easily. However, when the polarity reverses and the diode become reverse-biased, there’s a brief period where the diode doesn’t immediately stop conducting. During this time, known as the reverse recovery time ( trr), carriers (electrons or holes) that were previously injected into the diode take some time to get extracted.
• This reverse recovery time (trr), is essential because during this interval, the diode is still conducting, which might cause issues in certain applications, especially in high-frequency circuits or power electronic systems.
• The reverse recovery characteristic is typically depicted on a graph showing the reverse recovery current ( I_RR), which is the residual current flowing momentarily during the transition period, and the reverse recovery time, the time it takes for the diode to fully stop conducting after the polarity reverses.
• To minimize the impact of reverse recovery, engineers use diodes specifically designed for fast recovery times or select diodes with lower reverse recovery times for applications where rapid switching or minimizing power losses is crucial.
• As soon as the forward current IF decays to zero, the diode is still conducted in the reverse direction due to the pressure of charges in the depletion region and the layers of semiconductors.
• The diode regains its blocking capability unit reverse recovery current decays to zero.
• Reverse recovery time- ( trr)- The reverse recovery time is defined as the amount of time during the interval in which the forward current is zero and the reverse recovery current reaches 25% of its maximum value.
• During the time ta, the charge stored in depletion layer is removed. During tb, charge from the semiconductor layers is removed.

Need for Power Diode

Power diodes are the essential semiconductor devices which are required in various circuits. They’re crucial in electronic circuits for various purposes, especially in converting alternating current (AC) to direct current (DC) in power supplies and rectification circuits.

The key reasons for the necessity of power diodes include:

• Power diodes rectifies the AC signals by allowing current flow in one direction only while blocking the current in reverse direction and converts AC to pulsating DC. This conversion is important for various electronic devices that requires DC power supply.
• Power Diodes in combined with other components can regulate voltages, preventing reverse current flow and controlling the voltage level within a circuit.
• Power diodes can be used as switch in various applications like inverters, converters, and motor control for switching purposes. By turning it on or off, they control the flow of current enabling efficient energy management and reduced power losses in electronic circuits.
• They are to protect many sensitive components because they allow the flow of current only in one direction.

Construction of Power Diodes

Power Diodes are the Semiconductor devices which are used for handling high vltages and currents in electronic applications.Given below is the diagram of the construction of power diodes

Power Diode Intrinsic Structure

NOTE: ‘+’ Indicates heavily doped, ‘-’ indicates lightly doped

• The n- layer acts as a middle layer/drift layer, the thickness of this layer mainly depends on the breakdown voltage & the level of doping is 10¹⁴ cm^-3. Once this layer width increases then breakdown voltage will be increased.
• The n+ layer acts as a cathode and p+ layer acts as an anode with thickness of the layer is 250-300 μm & 10 μm respectively and the level of doping is 10¹⁹ cm^-3 for both the layers.
• In heavily doped regions carrier density is more, but thickness of that layer is less, hence in both p+ and n+ region carrier density is 1019 cm^-3 .
• In lightly doped regions carrier density is less, hence in n- region carrier density is 10¹⁴ cm^-3 .
• Here the diode construction is known as vertical structure manufacturing.
• This vertical structure manufacturing increases the surface area, increases heat dissipation and also increases the voltage handling capacity.
• Here the purpose of n- substrate or layer is to increase the voltage handling capability unde reverse bias condition. Also, to increase peak inverse voltage (PIV), n- substrate is used.
• It also increases the depletion width, which in turn increases the voltage capacity.

Working of Power Diodes

The working principle of power diodes is that they conduct electricity primarily in one direction by blocking the flow of current in other direction. Given below is the Circuit diagram of the power Diode.

Voltage Supply to the power diode

Now we will see step by step Explanation of the working of power diode

• The power diode consists of two region, P-type semiconductor region and an N-type semiconductor region joined together to form a P-N junction interface.
• It starts operating in forward bias, when a positive voltage (anode) is connected to the P-side and a negative voltage (cathode) is connected to the N-side, due to which the barrier potential across the junction reduces and the current starts flowing easily across the diode.
• Due to this application of voltage the holes start to flow from p-side to n-side and the electrons flows from n-side to p-side enabling the diode to conduct effectively and act as closed switch for current flow.
• On applying a reverse voltage (positive to the N-type material and negative to the P-type material) the barrier potential across the junction is increased. This blocks the flow of current in the reverse direction, and the diode acts as an open switch, allowing only a tiny leakage current due to minority carriers.
• The diode enters breakdown region, at a specific reverse voltage called the breakdown voltage. This phenomenon can be interpretated as either Zener breakdown or avalanche breakdown, which allows the diode to conduct in the reverse direction beyond this threshold without damaging the diode.
• The recombination of charges takes place Due to dual injection of positive and negative charge carries, hence the diode starts to conduct.
• The n+ region is responsible for the recombination. Thus, due to this recombination the resistance in n- drift region decreases, and the diodes begins to conduct.

Types of Power Diode

Diodes are classified on the basis of their reverse recovery characteristics, speed of operation, current and Voltage handling capacity. There are mainly three types of power diode

• General Purpose Diodes
• Fast Recovery diodes
• Schottky Diodes

General Purpose Diodes

General Purpose Diodes are one of the types of the power Diodes. They are designed to handle high power levels in the electronic circuits. They are used for Handling higher currents and Voltages which makes them suitable for power supply and Control Applications. Given below is the Circuit diagram and Symbol of the General Purpose Diodes.

General purpose power diode

• Low frequency applications (1 kHz) especially for Diode Rectifier. They allow the flow of current only in one direction by converting the AC to DC, and they achieve this by limiting the flow of current in the reverse direction.
• High Reverse Recovery time (25 μς).
• Capable of handling high Voltage & Current.
• Current Rating: 1 A to 1000 A.
• Voltage Rating: 50 V to 5 kV.
• They require a minimum certain voltage to start conducting since they have a lower forward drop voltage.
• Applications: Battery charging, electric traction, electroplating and welding.

Fast Recovery Diodes

Fast Recovery diodes which are also known as fast-recovery epitaxial diodes (FREDs).It Has Short Recovery time(trr) which is mainly in the range of nanoseconds to microseconds. This makes this diodes to rapidly switch off when applied voltage polarity changes which reduces switching losses and improve efficiency. Given below is the circuit diagram of Fast recovery diode.

Fast Recovery diodes

• Low Reverse Recovery time (Less than 5 μs).
• These diodes can have a faster recovery time after being forward biased. Thus, they can be turned off quickly and hence can be used in high frequency applications such as AC-DC converters.
• Current Rating: 1 A to 100 A.
• Voltage Rating: 50 V to 3 kV.
• These diodes have higher efficiency due to their lower reverse recovery time, which allows them to save energy and minimize power dissipation during transition from one state to other.
• Fast Recovery Diodes ensure more precise control over the flow of current and are suitable for high-frequency circuits where rapid switching is required.
• Applications: These diodes are used in circuits requiring high-speed switch transitions and high frequency optimization devices, such as power supplies, inverters, motor drives, and various rectification devices.

Schottky Diodes

Schottky diode is another type of power diode which is also known as Schottky barrier diodes (SBDs).SBDs are used due to its fast switching speed, low forward voltage drop, and high efficiency. These diodes are formed by the junction of a metal electrode (usually made of platinum or tungsten) and a semiconductor material (typically silicon) instead of a traditional p-n junction .given below is the circuit diagram of the Schottky Diodes.

Schottky diode

• Unlike a P-N junction like regular diodes, Schottky diode consists of Metal-Semiconductor interface junction.
• They have a lower forward voltage drop (0.3V to 0.4V) which is responsible for their reduced power loss and faster switching. Their low forward voltage drop improves the efficiency, making them suitable in power supply and rectification circuits.
• They have a faster recovery time as compared to other types of power diodes.
• Very less ON state voltage drop (0.3 to 0.4 V).
• High Reverse Leakage Current.
• Lower peak inverse voltage (PIV): 50 V to 100 V.
• Due to their construction, Schottky diodes have a faster switching speed, making them suitable for high-frequency applications.
• Applications: It is commonly used in higher power applications where good efficiency is required such as high precision rectifiers, SMPS (Switch Mode power supply) circuits, Radio-Frequency mixers, detector circuits.

Applications of Power Diodes

• It acts as an important rectification device in converting alternating current (AC) to direct current (DC) and also supplies power to various devices like TV, Laptops, AC etc.
• It is used in freewheeling diodes to protect the SCR (Silicon-controlled rectifier) by protecting it from the damaged caused due to large reverse recovery voltage generated by the inductive load.
• It enhances the input power factor and also helps to decrease the ripple components in the output response.
• It is used in many voltages regulated circuits to stabilize the effect of voltage change in the output.
• It is used for mixing (multiplexing) of the signals and controlling the size of the signal.

Selection of Power Diodes

For Selecting the appropriate power diode for a particular application several factors need to be consider:

• Consider the diode with suitable voltage and current ratings in order the ensure the safety of the circuit, so that the power diode withstands higher amount of current and protects the other parts of the circuit from getting damaged.
• Ensure the diode must have faster recovery time and order to be used for fast switching transition, and minimize the losses due to transitions, and high frequency applications.
• Consider the diode with minimum power losses.
• The forward voltage dropout of the diode should be minimum in order to improve the performance efficiency and use them in rectification purposes.
• Consider the diode with least temperature sensitivity to withstand high temperature and high-tolerance level of the power diodes.
• To withstand the high temperature, consider the diode with high thermal resistance.

Advantages of Power Diodes

• The Power diodes have the larger voltage and current handling capacities, making them appropriate for applications requiring higher power systems.
• Power diodes have faster switching speed and faster recovery time which makes them suitable for high-frequency applications the required rapid transitions.
• Power diodes are used for supplying power to power devices and are suitable for converting AC signals to DC providing large rectification efficiency.
• due to their lower forward voltage drops, power dissipation in power dioses is minimum and have high energy efficiency in circuits.
• The have the ability to protect the components from high temperature variations and also stabilizes the output signal.

Disadvantages of Power Diodes

• Power diodes have a reverse recovery time due to which they take more time to switch off after being forward biased, which is responsible for the power loss and decrease in efficiency during high-speed switching operations.
• Power diodes are generally expensive than normal standard diodes.
• they have significant power losses due to Forward Voltage Drop
• These diodes are temperature sensitive which reduces the operation efficiency and reliability.
• Power diodes possess a larger reverse leakage current compared to standard pn-junction diodes, which is a great matter of concern for the applications which requires high accuracy.

Comparison Between Power Diodes and Signal Diodes

PARAMETER	POWER DIODE	SIGNAL DIODE
Purpose	Used in high-power applications, rectification, etc.	Used in low-power applications, signal processing.
Current handling Capacity	Higher current handling capabilities.	Lower current handling capabilities.
Voltage handling capacity	Typically, higher voltage ratings.	Lower voltage ratings.
Reverse Recovery Time	Longer Reverse Recovery Time.	Shorter Reverse Recovery Time.
Switching speed	Slow as compared to signal diode.	Faster as compared to power diode
Forward Voltage Drop	Higher as compared to signal diode.	Lower as compared to power diode.

Conclusion

Power diodes proves to be an essential component which is used in modern power electronics. These diodes cover a wide range of applications finding their use in household to industry purposes. these diodes are highly efficient devices which tends to consume less power and also reduces the power loss. They’re also used in many circuitries to protect them from large current leakages and higher thermal shocks. These didoes are also known for their high-frequency operations which allows them to faster switch speeds and applicable in various domains like rectification, RF transmission etc.

FAQs on Power Diodes

What is S-factor in power diode? Explain its significance.

The S-factor in power diodes represents the softness of the recovery characteristics during the diode’s turn-off process. Softness factor determines the state of recovery of the power diode. It is denoted by the symbol ‘Ω’ or ‘s’. Mathematically it is defined as the ratio of t_b to t_a.

Where, t_a is the recovery time due to the charge stored in the depletion region and t_b is the recovery time due to the charge stored in the bulk of semiconductor.

i.e., s = t_b/t_a

Case 1: tb = ta, Means that s = 1. This S-factor indicates a softer recovery characteristic, meaning a smoother transition during turn-off.

Case 2: tb<ta, means that s<1. This indicates a snappy recovery.

• Diodes with a lower S-factor tend to experience less stress on the device and the surrounding circuitry during the switching process.
• By controlling the rate of change of reverse recovery current, a lower S-factor contributes to higher efficiency in power conversion applications.
• The S-factor is also used to determine the reverse recovery time (trr) , trr = ta + tb.
• it is also used to find the reverse recovery charge (Qrr) stored in the diode during turn off state. Qrr = 1/2* (di/dt) *ta*trr

What key role does reverse recovery play in the performance of power diode?

The reverse recovery time in power diodes determines the transition state of the diode from conducting in the forward direction to blocking in the reverse direction. This influences the diode’s ability to minimize switching losses, improve efficiency, and reduce stress on the circuitry during operation.

How does the switching speed of power diodes get affected with the addition of impurities?

By addition of impurities of the diode the switching speed is affected since doping level is responsible for the switching speed current of the power diodes. Higher doping levels tend to increase switching transitions and decreases the carrier lifetime. The higher concentration of impurities results in more readily available charge carriers that can respond readily to the change in the applied voltage of the electrical signal. Doping impurities also affects carrier mobility (The speed at which charge carriers move through the semiconductor material), by influencing the diode’s response time during switching.