As the core component of power electronic system, power semiconductor device has been an indispensable electronic component in modern life since it appeared in the 70s of last century. Especially in recent years, the face of the global energy shortage and environment deterioration test, in order to meet the demand of energy saving and new energy development, power electronic system power conversion and processing is more and more widely used, all kinds of power electronic devices is toward large capacity and high reliability and modular direction. As an important component, power diodes are widely used in household electronics and industrial electronic systems, automotive and power train electronic systems, smart grid, ship and aerospace fields. With the development of power semiconductor device design level and manufacturing technology, the performances of power diodes, such as withstand voltage level, conduction current, switching loss and dynamic characteristics, have been greatly improved.
Because of the high cost and easy destruction of the power semiconductor devices, the computer simulation is usually used in the design of the system.
The accuracy of the power electronic system simulation is determined by the model and model parameters used by the simulation. To get accurate, reliable and practical guidance results, we must have accurate physical model parameters, and only have accurate physical model parameters, so the power semiconductor device model is meaningful.
However, due to the technical blockade of device manufacturers, the accurate model parameters of power semiconductor devices are difficult to get through manufacturers and conventional testing methods, which limits the use of simulation models and the improvement of device application level. For many years, how to accurately extract the key parameters within the power and electronic devices has been a hot topic in the field of power electronics. The dynamic characteristics of the power diode opening and closing can reflect the internal physical structure, the working mechanism and the distribution of the carrier in the base area. Firstly, in the analysis of the internal structure and dynamic characteristics of PIN power diode based on the key parameters to determine their dynamic characteristics were determined; then using the method of combining dynamic simulation and optimization algorithm to optimize the identification of the key parameters of power diode; the effectiveness of the proposed method for parameter identification of power diode is verified.
1 Basic structure and dynamic characteristics of PIN power diode
Figure 1 shows the principle diagram of the internal structure of the PIN type power diode and the carrier concentration distribution. The PIN diode consists mainly of the P region and the N zone and the low doping concentration of the I region (N- region). Due to the addition of I region, PIN diodes can withstand higher blocking voltage. The conduction resistance of diodes can be greatly reduced by conductance modulation when injected at large base area. The dynamic characteristics of power diodes, including the turn-on and turn off characteristics, are determined by the carrier distribution and the change process in the I area, which is manifested in the forward and reverse recovery characteristics of power diodes.
1.1 Opening characteristics
The guide with a diode transient conduction period will be accompanied by a peak of anode voltage overshoot, after a period of time to stabilize, and has a very small voltage drop (see Figure 2). The forward recovery process of the diode is mainly influenced by the length of the lead, the package of the device and the effect of the conductance modulation in the internal N region.
Under large injection conditions, the concentration of excess carrier determines the conductance modulation in the drift region. The excess carrier concentration in the injection drift region is determined by the continuity equation.
式中 n——Excess carrier concentration；
Jn——Electronic current density；
q——Unit charge amount；
τ——Excess carrier life。
The forward overshoot voltage only occurs when the current changes very fast, and the duration is less than the compound life. The current is mainly determined by the diffusion process, and the composite process can be ignored, so the electron current density is
Excess carrier concentration
In the formula, Dn is the electron diffusion coefficient.
In the transient process of forward recovery, the current density increases with the rate of a, and the excess carrier concentration in the drift region is obtained.
The total electron concentration in the drift region is
At a distance of X from the PN junction, a small section of the thickness of DX is considered, and the resistance of the drift region is the same.
The positive recovery voltage can be obtained.
Type TM——Diffusion passing constant；
VT——Temperature and voltage equivalent，VT =k T/q；
among kThe Boltzmann constant， k=1.38×10-23J/K ；
1.2 Turn off characteristic
When the diode in the conduction state suddenly applies a reverse voltage, the diode's reverse blocking capability will take a period of time to recover, which is the reverse recovery process. The diode is equivalent to a short circuit state before the blocking ability is restored. As shown in Figure 3, from t=tf, the forward current IF of the diode is reduced at the rate of dif/dt under the effect of the applied reverse voltage. The rate of change of IF is from the external reverse voltage E And the inductance L in the loop is determined,
When t=t0, the current in the diode is equal to zero. Before this, the diode is on the forward bias, and the current is positive current. After the t0 time, the forward voltage drop is slightly decreased, but it is still positive bias, and the current begins to reverse circulation and forms the reverse recovery current IRR. At t=t1 time, the charge Q1 in the drift region is pumped away, the reverse current reaches the maximum value of IRM, and the diode begins to recover the blocking ability. After the T1 time, for the PIN diode, the carrier concentration at the PN- junction in the recovery phase is higher than that in the other regions. Once the space charge layer is set up, it spreads rapidly in the N region, rapidly sweeping out the residual carrier, causing a sudden drop in the reverse current. Because the dirr/dt of the current descending speed is larger, the inductor voltage of the line will generate higher induction voltage. This inductive voltage is superimposed with the applied reverse voltage to the diode, so that the diode will withstand a high reverse voltage VRM.
After t=t2, the dirr/dt gradually decreases to zero, the inductance voltage drops to zero, the diode restores the reverse block and enters the phase of the static reverse voltage. The main factor affecting the reverse recovery process is the reverse recovery charge, that is, the total amount of charge Qrr is removed during the reverse recovery process.
Assuming that the free carrier concentration in the drift region can be linearized, a reverse recovery process can be established when the power diode is turned off at a constant rate of current change, as shown in Figure 4.
The catenary carrier concentration distribution based on the current state can be approximately replaced by the linear variation between the mean value of the middle part of the drift region and the concentration of x=0 n (-d) to the average carrier concentration Na at x=b. The concentration of these carriers is
Mean carrier concentration in drift region
Type τHL——Large injection surplus carrier life；
JT——Total current density of diode anode；
JF——Forward current density of diode；
La——Bipolar diffusion length。
At the first stage of the turn off process, the current density of the PIN rectifier changes from the pass state current density (JF) to zero at the t0 moment. At the end of the first stage, the carrier distribution becomes flat because the current is zero at the end of the t0 time. The change in the charge stored in this phase drift region is
Type a——Rate of change of current density。
The t0 moment of the current change to zero is expressed as
The second stage of the turn off process is the T1 time from the t0 moment of the current to zero to the P+N junction to begin to withstand the voltage. Time T1 can be obtained by analyzing the charge extracted from t=t0 to t=t1 during the shutdown of the transient process. The charges extracted during this period are
Time T1 is
When the third phase of the transient process is turned off, the voltage under the PIN diode begins to increase. Initially, the space charge area WSC (T) expands outward as time goes on. In this process, the charge stored in the drift region is further extracted, resulting in the decrease of the reverse current after T1. It is assumed that the current is approximately constant when the storage charge is extracted, and when the P+N junction is reversed at the T1 moment, the storage charge extracted at the t moment is the same.
Space charge zone voltage
The space charge area can be expressed as
The reverse recovery voltage is peak at the end of t=t2 at the third stage.