Semiconductor Diode Applications: Biasing, Rectification, and Regulation
Diode Biasing Fundamentals
What is Diode Biasing?
Diode biasing is the process of applying an external DC voltage to a PN junction diode to control its operation and determine whether it will allow or block the flow of electric current.
Types of Diode Biasing (Forward and Reverse)
1. Forward Biasing
- Connection: The positive terminal of the external voltage source is connected to the P-type material, and the negative terminal is connected to the N-type material.
- Operation: This applied voltage opposes the built-in potential barrier of the diode. The depletion region (the insulating area at the junction) becomes very narrow.
- Result: The potential barrier is overcome, and majority charge carriers (holes from P-side, electrons from N-side) cross the junction. A large current flows through the diode. The diode acts like a closed switch.
2. Reverse Biasing
- Connection: The negative terminal of the external voltage source is connected to the P-type material, and the positive terminal is connected to the N-type material.
- Operation: This applied voltage aids the built-in potential barrier. It pulls majority carriers away from the junction, making the depletion region much wider.
- Result: The high potential barrier blocks the flow of majority carriers. Only a very small current (called reverse saturation current, I₀) flows due to minority carriers. The diode acts like an open switch.
Half-Wave Rectifier (HWR) Analysis
Rectifier Definition and HWR Working Principle
A rectifier is an electronic circuit that converts an alternating current (AC) input signal into a pulsating direct current (DC) output signal. Its primary function is AC-to-DC conversion.
A Half-Wave Rectifier consists of an AC source (often with a transformer), a single diode (D), and a load resistor (R_L).
HWR Working Cycle
(Circuit Diagram and Waveforms are relevant for visualization.)
- Positive Half-Cycle (0 to π): The diode (D) is forward-biased. It acts like a closed switch, allowing current to flow through the load resistor (R_L). The output voltage (V_out) across R_L follows the input voltage.
- Negative Half-Cycle (π to 2π): The diode (D) is reverse-biased. It acts like an open switch, blocking the flow of current. The output voltage (V_out) across R_L is zero.
This process repeats, resulting in an output that consists only of the positive half-cycles of the input AC, producing a pulsating DC.
HWR Efficiency and Ripple Factor Derivation
Average and RMS Current Derivation
Let the input current be i = I_m sin(ωt), where I_m is the peak current.
- Average DC Current (I_DC):
I_DC = (1 / 2π) ∫₀^π I_m sin(ωt) d(ωt) = I_m / π
- RMS Current (I_RMS):
I_RMS = sqrt((1 / 2π) ∫₀^π (I_m sin(ωt))² d(ωt)) = I_m / 2
Efficiency (η)
Efficiency is the ratio of DC output power (P_DC) to AC input power (P_AC).
η = P_DC / P_AC
Substituting the power expressions:
η = ((I_m / π)² R_L) / ((I_m / 2)² R_L) = 4 / π² ≈ 0.406 or 40.6%
Ripple Factor (γ)
The ripple factor measures the purity of the DC output.
γ = sqrt((I_RMS / I_DC)² – 1)
Substituting the current expressions:
γ = sqrt(( (I_m / 2) / (I_m / π) )² – 1) = sqrt((π / 2)² – 1) ≈ 1.21
Thus, the ripple factor of an HWR is 1.21.
Full-Wave Rectifier (FWR) Circuits
Full-Wave Rectifiers utilize both the positive and negative half-cycles of the AC input to produce a smoother DC output compared to HWRs.
Center-Tapped FWR Operation
This configuration uses two diodes (D₁, D₂) and a center-tapped transformer.
- Positive Half-Cycle: Diode D₁ conducts, and D₂ blocks. Current flows through R_L.
- Negative Half-Cycle: Diode D₂ conducts, and D₁ blocks. Current through R_L flows in the same direction as the positive half-cycle.
Full-Wave Bridge Rectifier (FWBR)
The FWBR uses four diodes (D₁, D₂, D₃, D₄) and does not require a center-tapped transformer.
- Positive Half-Cycle: Diodes D₁ and D₂ conduct; D₃ and D₄ block.
- Negative Half-Cycle: Diodes D₃ and D₄ conduct; D₁ and D₂ block.
In both cases, the current flows through the load resistor (R_L) in the same direction.
FWR Efficiency and Ripple Factor Derivation
Average and RMS Current
For both Center-Tapped and Bridge FWRs:
- Average DC Current (I_DC):
I_DC = (1 / π) ∫₀^π I_m sin(ωt) d(ωt) = 2I_m / π
- RMS Current (I_RMS):
I_RMS = sqrt((1 / π) ∫₀^π (I_m sin(ωt))² d(ωt)) = I_m / √2
Efficiency (η)
η = P_DC / P_AC
Substituting the current expressions:
η = (I_DC² R_L) / (I_RMS² R_L) = (2I_m / π)² / (I_m / √2)² = 8 / π² ≈ 0.812 or 81.2%
Ripple Factor (γ)
γ = sqrt((I_RMS / I_DC)² – 1)
Substituting the current expressions:
γ = sqrt(( (I_m / √2) / (2I_m / π) )² – 1) = sqrt((π² / 8) – 1) ≈ 0.48
The ripple factor of a Full-Wave Rectifier is 0.48.
Zener Diode Voltage Regulation
Zener Diode Regulator Circuit and Working
A Zener diode is used as a shunt voltage regulator by exploiting its unique property: when reverse-biased into its breakdown region, the voltage across it (V_Z) remains nearly constant, even if the current flowing through it changes significantly.
Circuit Configuration
To function as a regulator, the Zener diode is connected in reverse-bias (cathode to positive, anode to ground) in parallel with the load (R_L). An unregulated DC input (V_in) is fed through a series resistor (R_S).
Principle of Operation
The Zener diode operates in breakdown, ensuring the output voltage is maintained at V_L = V_Z. The series resistor R_S drops the excess voltage (V_in – V_Z). The total current I_S splits into Zener current (I_Z) and load current (I_L): I_S = I_Z + I_L. The Zener automatically adjusts I_Z to absorb changes in V_in or I_L, ensuring V_L remains stable.
Zener Regulator Performance Metrics
The performance of a Zener regulator is measured by its ability to maintain a constant output voltage despite variations in input or load.
Source Effect (Line Regulation)
This measures the change in output voltage (V_L) for a given change in input voltage (V_in), assuming the load (R_L) is constant.
- If V_in increases, the extra current flows through the Zener (I_Z increases).
- Due to the Zener’s small dynamic resistance (r_z), V_Z only increases slightly.
- Performance: A good regulator shows only a few millivolts change in V_Z for several volts change in V_in.
Load Effect (Load Regulation)
This measures the change in output voltage when the load current (I_L) changes, assuming V_in is constant.
- If R_L decreases, I_L increases. Since I_S is nearly constant, the Zener current I_Z must decrease.
- As long as I_Z remains within its specified limits (I_Z(min) < I_Z < I_Z(max)), V_Z remains nearly constant.
Diode Logic Gates and Power Supply Structure
Diode Logic Circuits (AND and OR Gates)
Diodes can be used to implement basic binary logic functions.
(a) OR Gate Implementation
The output is HIGH if any input is HIGH.
- Circuit: Diodes D₁ and D₂ have anodes connected to inputs A and B. Their cathodes join at output Y, which is connected to ground via resistor R.
- Operation: If A or B is HIGH (Logic 1, e.g., 5V), the corresponding diode conducts, pulling the output Y HIGH (≈ 5V). If both are LOW (Logic 0, 0V), no diode conducts, and Y is pulled to 0V.
(b) AND Gate Implementation
The output is HIGH only if all inputs are HIGH.
- Circuit: Diodes D₁ and D₂ have cathodes connected to inputs A and B. Their anodes join at output Y, which is connected to a positive supply (e.g., +5V) via resistor R (pull-up).
- Operation: If A or B is LOW (0V), the corresponding diode conducts, pulling the output Y LOW (≈ 0V). Only when both A and B are HIGH (5V) do both diodes block, allowing the pull-up resistor R to pull Y HIGH (≈ 5V).
Block Diagram of a DC Power Supply
A DC power supply converts high AC mains voltage into a constant, regulated DC voltage. The major functional blocks are:
- Transformer: Steps down the high AC input voltage (e.g., 230V) to a lower, safer AC voltage (e.g., 12V). It also provides electrical isolation.
- Rectifier: Converts the low AC voltage into a pulsating DC voltage using diode circuits (e.g., Half-Wave or Full-Wave Bridge).
- Filter: Smooths the pulsating DC output, typically using a large capacitor. The capacitor reduces the ripple voltage, making the DC output flatter.
- Regulator: Maintains a constant DC output voltage regardless of fluctuations in the input voltage or changes in the load current. This is often achieved using Zener diodes or specialized regulator ICs.
- Load: The final device or circuit that utilizes the stable, regulated DC voltage.
Flow Summary: AC Input → Transformer → Rectifier → Filter → Regulator → Load