Introduction to Power Electronics:
Many applications in electronics and electrical engineering involve diodes.
Power diodes are crucial in power electronics circuits for electric power conversion.
Functions of Diodes in Power Electronics:
Act as switches in rectifiers.
Provide freewheeling in switching regulators.
Reverse charge on capacitors and facilitate energy transfer.
Ensure voltage isolation.
Allow energy feedback from the load to the power source.
Assist in trapped energy recovery.
Characteristics of Power Diodes:
Often idealized as switches.
Practical diodes have limitations and differ from ideal characteristics.
Possess higher power, voltage, and current-handling capabilities compared to signal diodes.
Have lower frequency response or switching speed than signal diodes.
Components Used in Power Electronics Circuits:
Inductors (L) and capacitors (C) are commonly utilized as energy storage elements.
Power semiconductor devices control energy transfer in circuits.
Understanding Switching Behaviors:
Clear comprehension of RC, RL, LC, and RLC circuits is essential.
Diodes connected in series with switches demonstrate power device characteristics.
Analysis of switching circuits involves components R, L, and C.
Diode enables unidirectional current flow, while the switch manages on/off functions.
Power Diode Characteristics:
A power diode is a two-terminal pn-junction device.
The pn-junction is formed by alloying, diffusion, and epitaxial growth.
Modern control techniques in diffusion and epitaxial processes ensure desired device characteristics.
Forward Biasing:
When the anode potential is positive with respect to the cathode, the diode is forward biased and conducts.
A conducting diode has a relatively small forward voltage drop across it, influenced by manufacturing process and junction temperature.
Reverse Biasing:
When the cathode potential is positive with respect to the anode, the diode is reverse biased.
Under reverse-biased conditions, a small reverse current (leakage current) flows, typically in the range of micro- or milliampere.
The leakage current increases slowly in magnitude with reverse voltage until the avalanche or zener voltage is reached.
For most practical purposes, a diode can be regarded as an ideal switch
- \[\boxed{I_D=I_S\left(e^{V_D / n V_T}-1\right)}\]The v-i characteristics of practical diode can be expressed by an equation known as , and it is given under dc steady-state operation by\[\begin{aligned} I_D & = \text{current through the diode, A } \\ V_D & =\text{diode voltage with anode positive with respect to cathode,} ~ \mathrm{V} \\ I_S & = \text{leakage (or reverse saturation) current,}~ 10^{-6} - 10^{-15} \mathrm{~A} \\ n & = \text{empirical constant known as emission coefficient,}\\ & \text{ or ideality factor , value varies from 1 to 2 } \end{aligned}\]
- . Boltzmann’s constant: ; absolute temperature in Kelvin coulomb (C); electron charge: where \(V_T\)\[V_T=\frac{k T}{q}\]
- , At a junction temperature of\[V_T=\frac{k T}{q}=\frac{1.3806 \times 10^{-23} \times(273+25)}{1.6022 \times 10^{-19}} \approx 25.7 \mathrm{mV}\]
At a specified temperature, the leakage current \(I_S\) is a constant for a given diode.
The diode characteristic a can be divided into three regions:
Forward-biased region \(\Rightarrow V_D>0\)
Reverse-biased region \(\Rightarrow V_D<0\)
Breakdown region \(\Rightarrow V_D<-V_{B R}\)
Forward-biased region:
In the forward-biased region, \(V_D > 0\).
The diode current \(I_D\) is very small if the diode voltage \(V_D\) is less than a specific value \(V_{TD}\) (typically \(0.7 \mathrm{~V}\)).
The diode conducts fully if \(V_D\) is higher than this value \(V_{TD}\), known as the threshold voltage, cut-in voltage, or turn-on voltage.
Thus, the threshold voltage is a voltage at which the diode conducts fully.
Let’s consider \(V_D = 0.1 \mathrm{~V}\), \(n = 1\), and \(V_T = 25.7 \mathrm{mV}\).
- as: The corresponding diode current\[I_D = I_S \left(e^{V_D / n V_T} - 1\right) = I_S \left[e^{0.1 / (1 \times 0.0257)} - 1\right] = I_S(48.96 - 1) = 47.96 I_S\]
Approximately \(I_D \approx I_S e^{V_D / n V_T} = 48.96 I_S\), with an error of \(2.1\%\).
As \(V_D\) increases, the error decreases rapidly.
For \(V_D > 0.1 \mathrm{~V}\), usually the case, \(I_D \gg I_S\).
- error to: Diode current equation can be approximated within\[I_D = I_S \left(e^{V_D / n V_T} - 1\right) \approx I_S e^{V_D / n V_T}\]
Reverse-biased region:
In the reverse-biased region, \(V_D < 0\).
If \(V_D\) is negative and \(\left|V_D\right| \gg V_T\), which happens for \(V_D < -0.1 \mathrm{~V}\).
- The exponential term in the diode current equation becomes negligibly small compared to unity. Then,\[I_D = I_S \left(e^{-\left|V_D\right| / n V_T} - 1\right) \approx -I_S\]
This indicates that \(I_D\) in the reverse direction is constant and equals \(I_S\).
Breakdown region:
The reverse voltage is typically high, often exceeding 1000 V in magnitude.
The reverse voltage may surpass a specified voltage known as the breakdown voltage \(V_{\text{BR}}\).
A small change in reverse voltage beyond \(V_{\text{BR}}\) results in a rapid increase in reverse current.
Operation in the breakdown region is non-destructive if the power dissipation remains within a "safe level," as specified in the manufacturer’s data sheet.
However, it’s often necessary to limit the reverse current in the breakdown region to keep the power dissipation within a permissible value.
Reverse Recovery Characteristics:
Diode reverse recovery refers to its behavior when transitioning from conducting to blocking state after switching from forward to reverse bias.
During regular operation, a diode conducts in forward bias and blocks in reverse bias.
However, there’s a brief reverse conduction period when switching abruptly from forward to reverse bias, known as reverse recovery.
Reverse recovery is primarily determined by the time needed to remove stored charge carriers in the diode’s depletion region, called reverse recovery time (trr).
During this time, the diode conducts in reverse direction, causing a temporary reverse current.
Reverse recovery time includes three components:
trr: Time to remove stored charge carriers from the depletion region, crucial for switching speed.
Qrr: Total charge flowing during reverse recovery, representing stored charge from forward conduction.
Irr: Peak reverse current during recovery, occurring when stored charge carriers are rapidly depleted.
Manufacturers specify these characteristics in datasheets for applications like power supplies and converters to minimize power loss and enhance efficiency.
At time \(t_2\):
Current becomes negative and excess charges are removed.
Junction becomes reverse-biased and rapidly adopts the applied negative voltage.
Equilibrium charge distribution of a non-biased junction is reached.
From \(t_2\) to \(t_3\):
Charge distribution approaches that of a reverse-biased junction.
Diode behavior:
Begins to withstand the reverse voltage.
Diode current drops rapidly to zero.
Practical diode: Current reversal delayed due to parasitic inductance.
Reduction rate determined by lead inductance.
\(t_{rr}\) (reverse recovery time):
Time taken for current to reverse.
Crucial parameter in switching applications.
Interval between \(t_1\) and \(t_2\):
Sometimes termed as storage time.
- The reverse recovery time is calculated as:\[\begin{aligned} t_{rr} & = t_a+t_b \\ t_{rr} & = \sqrt{\dfrac{Q_{rr}}{(di/dt)}} \quad \text{If}~t_b << t_a \\ I_{rr} & = \sqrt{\dfrac{di}{dt}2Q_{rr}} \end{aligned}\]
Problem:
The reverse recovery time of a diode is \(t_{rr}=3~\mathrm{\mu s}\) and the rate of fall of the diode current is \(di/dt=30~\mathrm{A/\mu s}\). Determine
the storage charge \(Q_{RR}\),
the peak reverse current \(I_{RR}\).