Understanding the Role of Power Diode Devices
Power diodes are a type of diode, representing a straightforward semiconductor device. Like regular diodes, power diodes possess two terminals and conduct current in one direction. However, there are significant distinctions between power diodes and conventional diodes.
Differences Between Power Diodes and Standard Diodes
- Structural Variation:
- Standard diodes consist of 2 layers: P-type and N-type.
- Power diodes have 3 layers with a drift region between the P+ and N+ layers.
- Higher Rated Values:
- Power diodes have higher voltage, current, and power ratings.
- Standard diodes have lower voltage, current, and power ratings.
- Operating Speed:
- Power diodes operate at high speeds.
- Standard diodes operate at higher switching speeds.
- Power diodes are suitable for applications requiring high current and voltage, such as inverters.
- Standard diodes are used in small-signal applications.
Below is a comparative diagram illustrating the differences between power diodes and standard diodes.
The structure of a power diode consists of three layers, namely the P+ layer, the n- layer, and the n+ layer. The top layer, known as the P+ layer, is heavily doped. The middle layer is the n- layer, which is lightly doped, and the final layer is the n+ layer, which is heavily doped. The diagram below is a schematic representation.
The P+ layer serves as the anode, with a thickness of 10 μm and a doping level of Na, as depicted in the diagram with specific values.
The N+ layer functions as the cathode, with a thickness ranging from 250-300 μm and a doping level of Nd, as indicated by specific values in the diagram.
The N- layer serves as the middle layer or drift region, with its thickness primarily determined by the breakdown voltage and a doping level of Nd, as also specified in the diagram. Increasing the width of the N- layer will result in a higher breakdown voltage.
Principle of Operation of Power Diodes
The working principle of power diodes is similar to that of a standard PN-junction diode. When the anode voltage is higher than the cathode voltage, the power diode conducts. The forward voltage drop of a power diode is very small, typically around 0.5V – 1.2V. In this mode, the power diode operates with forward characteristics.
If the cathode voltage is higher than the anode voltage, the power diode enters the blocking mode. In this mode, the power diode behaves like a reverse-biased diode.
Power Diode Forward Bias
The operation of a power diode is somewhat similar to that of a standard diode. Consider the forward bias condition of a power diode, as shown below, where the positive terminal of the battery is connected to the anode, and the negative terminal forms a connection with the cathode.
In this scenario, the junction receives forward bias, and majority charge carriers (holes) from the p+ region start injecting into the n- drift region. When the injection rate is low, holes from the p+ region recombine with electrons in the n- region. However, as the injection rate increases, holes will penetrate and recombine with electrons in the n+ region. This is known as dual injection. Due to the flow and recombination of charge carriers in the drift region, once the threshold is exceeded, the diode begins to conduct heavily.
Reverse Bias of Power Diode
In the reverse bias condition, the negative terminal of the battery is connected to the anode, and the positive terminal is connected to the cathode.
In this situation, the junction becomes reverse biased, and, like a standard diode, the power diode also ceases to conduct in this condition. The depletion region extends into the drift region, causing it to be difficult for minority charge carriers to penetrate the junction.
It’s important to note that a sudden change in the applied voltage polarity does not immediately halt the flow of current. Additionally, the minority charge carriers stored in the junction lead to a small leakage current (approximately 100 mA) in the opposite direction through the diode. This reverse current exhibits dependence on junction temperature changes. Once the applied potential equals the breakdown voltage, impact ionization occurs.
The Characteristics of Power Diodes
The characteristics of power diodes primarily include three aspects:
- VI Characteristics
- Reverse Recovery Characteristics
- Switching Characteristics Next, we will analyze each of these three characteristics separately.
The VI Characteristics of Power Diodes
The following figure illustrates a comparison of the VI characteristics between a standard diode and a power diode.
In a standard diode used in the forward bias region, the current exhibits exponential growth in the bias region. However, in the case of a power diode, high forward currents lead to high ohmic voltage drops, which predominantly influence the exponential growth, resulting in a nearly linear increase in the curve.
The maximum reverse voltage that a power diode can withstand is expressed as VRRM, which stands for Peak Reverse Repetitive Voltage.
Beyond this voltage, the reverse current suddenly becomes very high, and because the diode’s design is not intended to dissipate such high heat, it may get damaged. This voltage can also be referred to as Peak Inverse Voltage (PIV).
Reverse Recovery Characteristics of Power Diodes
The following diagram illustrates the reverse recovery characteristics of power diodes. Whenever the diode turns off, the current decreases from IF to zero, and further continues in the reverse direction due to the charge stored in the space-charge region and the semiconductor region.
The characteristics of power diodes are the forward recovery time (tF) and the reverse recovery time (trr)
Forward Recovery Time (tF)
The forward recovery time is the time it takes for the diode to start conducting, known as the forward recovery time. In other words, it is the time it takes for the diode to switch from the off state to the on state, referred to as the forward recovery time (tF).
Reverse Recovery Time (trr)
After the forward diode current decays to zero, the diode continues to conduct in the reverse direction due to the charge stored in the two layers. The time during which the reverse current flows is called the reverse recovery time (trr). The diode maintains its blocking capability until the reverse recovery current decays to zero.
The time between the moment when the instantaneous forward diode current becomes zero and the moment when the instantaneous reverse recovery current decays to 25% of its reverse peak value (Irm).
It consists of two time periods, namely ta and tb, where trr = ta + tb.
The time ta is the time between zero forward current and the peak reverse current, during which the charge stored in the depletion region is removed. Time tb is measured from the instant of reaching Irm to the instant of reaching 0.25 Irm. During time tb, charge removal occurs in both semiconductor layers.
The ratio of ta/tb is known as the softness factor or S-factor. A diode with a softness factor of 1 is referred to as a soft recovery diode, while diodes with an S-factor less than 1 are called fast recovery diodes or fast-switching diodes.
Switching Characteristics of Power Diodes
Power diodes exhibit shorter conduction to reverse bias (turn-on) and reverse bias to conduction (turn-off) times. During these switching intervals, the behavior of the current flowing through the diode and the voltage applied across the diode is crucial for the following reasons.
Higher voltage/current may be the result of diode switching in various circuit applications that use the diode.
During diode switching operations, there is voltage and current involved. Diodes undergo slight losses for each switching time. At high switching frequencies, this can lead to overall power losses in the diode
Power diodes come in various IC package types. Examples include:
- Diode Outline (DO)
- Small Outline Diode (SOD)
- Transistor Outline (TO)
- Small Outline Transistor (SOT)
- Discrete Package (DPAK)
- Metal Electrode Leadless Face (MELF)
DO-4, DO-5, DO-8, DO-9, DO-15, DO-27, DO-34, DO-35, DO-41, and DO-201 are Diode Outline (DO) packages.
SOD-80, SOD-106, SOD-123, SOD-323, and SOD-523 are Small Outline Diode (SOD) packages.
TO-3, TO-66, TO-92, TO-202, TO-220, TO-237, and TO-247 are Transistor Outline (TO) packages.
SOT23, SOT26, SOT89, SOT143, SOT223, SOT323, SOT343, SOT346, SOT353, SOT363, SOT416, SOT457, and SOT523 are Small Outline Transistor (SOT) packages.
MELF packages for power diodes include QuadroMELF, MicroMELF, and MiniMELF.
D2PAK is a large surface-mount package that includes a heatsink. SC-59, SC-74, and SC-76 are plastic surface-mount packages with three leads
Parameters to consider when selecting power diodes:
- Typically used for the average rectified forward current for 50/60 Hz sinusoidal signals. It is the average when the diode conducts during one half of the AC signal, and the other half when the diode is non-conductive.
- The maximum current that the power diode can conduct without damage. This is commonly used in power supply line voltage rectification.
- The maximum continuous voltage that the power diode can handle. Any voltage spikes should be within this tolerance range. Rectifier diodes are often used in parallel with capacitors to smooth out high voltage spikes.
- The maximum reverse voltage for alternating current signals that the power diode can handle. Repetitive peak reverse voltage is always less than the maximum DC reverse voltage.
- The maximum reverse voltage that the diode can handle at any given time. The amplitude of any peak voltage for AC signals or the magnitude of a continuous signal should not exceed this value.
- The reverse current flowing through the power diode under reverse bias. This current is thermally induced and contributed by minority charge carriers. Leakage current in power diodes can be in the range of hundreds of milliamps.
- The time required for the current to drop from the forward current level to the leakage current level when the diode switches from forward bias to reverse bias. It is typically measured in nanoseconds and is an important parameter when power diodes are required for high-speed switching applications like SMPS.
- The highest temperature the diode can endure. In forward bias, diodes heat up due to the current passing through them. This is the diode’s junction temperature. The ambient temperature also heats the diode. As the temperature rises, the current through the diode increases, and diode temperature increases with current. This can ultimately damage the diode or lead to unpredictable behavior. Hence, the diode should only be used under appropriate operating conditions.
- There are many types of power diodes, including:
- High-current diodes
- High-voltage diodes
- PN power diodes
- PIN power diodes
- RF power diodes
- Switching power diodes
- Rectifier power diodes
- Common applications for power/rectifier diodes include:
- Half-wave rectification
- Full-wave rectification
- Battery charging circuits
- Inverter circuits
- DC power supplies
- Switching power supplies
- The PN junction of this diode is large, allowing for high current, but this junction’s capacitance can also be large, making it suitable for operation at lower frequencies, primarily used for rectification.
- It resolves AC electricity at high current and high voltage.
- Its size can be relatively large, and it may require attachment to a heat sink when conducting high currents.
- Special hardware is needed for installation and insulation from the available metal framework.
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