1. Freewheeling diode on the requirements and absorption
With the development of power electronics technology, power device switching speed and faster, therefore, the modern fast switching element requires a fast diodes as freewheeling diodes. Every opening of the switch process, there is the freewheeling diode conduction state is turned off. And this process requires the diode with soft recovery characteristics. However, let's hard to believe that in a very long period of time, we ignore the role of fast diodes. Thus the freewheeling diode limits the switching element functions - limits the switching element of the switching speed.
In recent years due to the pursuit of greater efficiency, the role of fast diodes by a high degree of attention, through continuous improvement of its reverse recovery characteristics and its performance has been greatly improved.
2. Peak reverse blocking voltage and forward voltage drop
By the reverse blocking voltage VR is defined, we can know the diode or thyristor voltage leakage current shall not exceed the critical value of IR.
In most of the parameter table device parameters are temperature equal to the value at 25 ℃. When the temperature decreases reverse blocking ability. That is, if when the component in operation under the conditions below room temperature should be given special attention. When the temperature is higher than room temperature, in a corresponding increase in the reverse voltage, while its drain current is also increased. So we will be given in the parameter list of the leakage current at high temperature (125 ℃).
Of particular note is the use of gold for the diffusion process components, in their leakage current at high temperatures rise much faster.
Therefore, if the power system due to component damage and make it work in the environment, will likely lead to temperature imbalances in the system was not working properly.
Forward on-state voltage drop VF expressed at a given current, the diode conduction voltage drop conditions should be less than a given threshold. Generally, this value is measured at room temperature. However, the components in the system is operating at a certain temperature, so we have high temperatures on the components to be considered forward-state voltage drop.
3. Opening characteristics
Fast diode into conduction process, voltage is first raised VFRM, you can repeat the positive peak voltage, then fell forward on-state voltage levels. Figure shows the relevant VFRM and opening time tfr definition.
However, for the like used in the GTO and IGBT and freewheeling diode in absorbing diode, this definition exists some problems, this is because:
⑴. In such a circuit, the opening rate of rise of the current di / dt will be very high, usually reach 200V to 300V, it will be more than 100 times of VF. This application is a very tricky thing.
⑵. Practical application process, the diode is conducting state by the deadline to enter, resulting VFRM than the zero voltage into conduction much higher. When we're designing, it must be given special attention.
For absorbing diode, because the absorption circuit only after absorbing diode conduction can play a role, so the lower VFRM is that it is one of the important indicators.
The reverse blocking voltage greater than 1200V-wheeling diode, this repeatable positive peak voltage of the role of the role. Because in GTO, IGBT turn-off process, the parasitic inductance of the line will produce a voltage spike, which is superimposed on the freewheeling diode VFRM on the sum of the two may lead to component damage caused by overvoltage.
It is very important for the voltage must be measured to determine the voltage generated by the parasitic inductance and VFRM value.
4. Shutdown feature
Then the diode enters the off-state conduction process, the charge stored within it must be released. This process will reverse the direction of the diode current flow. Internal carrier recombination process we can use to describe the reverse recovery characteristics
Figure S represents an ideal switch, IL as a current source, VK is the commutation voltage source, LK is the commutation inductance.
Then the switch S is closed, the soft recovery diode voltage and current curves shown in Fig. The figure is two rigid diode current switching characteristic curve.
Commutation speed di / dt is determined by the voltage and inductance of the
－di/dt＝VK / LK
At time t0, the current reaches zero, at a time tw, the as through the start withstand reverse voltages. At the moment, the pn junction of the diode, all the carriers have been cleared. At time tirm, the reverse current reaches its maximum IRRM. In tirm after the current gradually decays to the leakage current values. Waveform trace is completely determined by the diode. If the decay process is very steep, and we call rigidity recovery characteristics; Conversely, if the decay process is very slow, we are called soft-switching characteristics.
Reverse recovery time defined as follows: beginning from t0 to the current decays to 10% of IRRM time.
Softness factor S = tf / ts
According to induction law, dir / dt determines the overvoltages appearing
Vind =-LK (dir / dt) max (4)
Recovery characteristics are affected by the following factors:
①. Temperature: diode recovery characteristics affected by the temperature in most possessions cases of elevated temperature on recovery characteristics are adversely affected. But some fast diodes at room temperature or below its recovery characteristics rather poor.
②. Applied reverse voltage: applied reverse voltage is higher, the worse the recovery characteristics.
③. Rate of rise of commutating current di / dt: rate of rise of commutating current di / dt great impact on the recovery characteristics, but different diodes exhibit different effects, and some diodes with di / dt increases hardens, while others are soft.
These factors listed above diode recovery characteristics of difficult to use a simple, quantitative definition to summarize. The circuit shown in Figure 3, and formula (2) or (3) obtained can be used to define the parameters of a single explained. Complete the reverse recovery characteristic judgment should be close to the actual conditions obtained by measurement.
Figure 6 is a more appropriate measurement lines.
Commutating current di / dt of the switching element can be adjusted gate resistor. VK is the DC bus voltage, the capacitor, IGBT and a diode connected between the parasitic inductance of the wire. When the shutdown of the load current has IGBT IGBT switch to the diode, the waveform shown in Figure 7, Figure 8.
We can see from Figure 8, to achieve the maximum recovery current in IRRM after the slow decay to zero, so we will diode reverse recovery characteristics further broken down into two parts:
①. The first part of the current to rise to the reverse recovery current in accordance with the peak phase and the subsequent dir / dt of the rate of descent. Generally, soft recovery diode dir / dt values ??and di / dt value is almost, but it is worth noting that the peak reverse recovery current of the switching element IRRM impact is considerable.
②. The second part is the part of the tail current, the part after the time interval trr, during the reverse recovery current decays to zero, trr no obvious significance. Because the diode has a reverse voltage, the diode generates internal loss and a major part of the process of tailing. Rigidity without tail current diode switching losses while it is low, but in fact it can not be applied. This is because it is too large for dir / dt is not conducive to the application element. The overall loss on the impact of the diode, the most important parameter is its peak reverse recovery current IRRM be as small as possible.
5. Fast diode series and parallel
5.1. Series connection
During tandem device application, you must pay attention to both its static and dynamic pressure equalizing.
Application devices in series, in the static, the application of the series leakage current of each element is inconsistent, thus minimizing leakage of the components to withstand the highest voltage, even to its rated limit, and must therefore be both parallel resistors. For n series circuit of diodes, we can get a simplified formula for calculating resistance:
The above formula: n-series element number; Vr-diode rated voltage; Vm-series circuit voltage maximum; value Ir-run at the maximum operating temperature of the diode leakage current deviation.
Experience has shown that dynamic voltage problems are permanent unlike static pressure problems. If a pn junction diode carriers disappear faster than the other, it is the turn-off process is earlier withstanding voltage. The value of n for a given voltage Vr diode in series, we can use a simplified formula to calculate capacitance:
Q RR-series element between the maximum deviation of reverse recovery charge, if you are using a device from the same manufacturing lot, we can assume that QRR = 0.3 QRR.
Device applications in tandem, only when all devices are static and dynamic symmetry achieve very satisfactory state of equilibrium, in order to maximize the use of tandem rated parameters of each device.
5.2. In parallel
Usually in the power devices in parallel applications, first of all we should consider both streams. In no particular measures are current case, the devices in parallel with each state voltage deviation as small as possible.
Device on-state voltage dependence on temperature is a measure of the device in parallel applications is an important parameter. Some types of devices on-state voltage positive temperature coefficient, two somewhat negative temperature coefficient devices. When a device is a positive temperature coefficient, it is more suitable for parallel applications. But since there is always the diode manufacturing variations, so the application of the diode in parallel, a large negative temperature coefficient (> 2mV / K) operating temperature it is possible to balance. Thereby enabling the device permanently disabled....
Application Design Example:
Example 1: Control DC motor requires a three-phase thyristor rectifier bridge, set the line voltage is Vline = 380V (RMS), 50Hz, DC output current Id = 3600A, thyristors air cooling, the maximum ambient temperature TA = 40 ℃, the waves of hope Chung current value ITSM = 20kA. Optional components and radiator.
1) voltage design
Peak value of the rated line voltage
2) Select the components and radiator
Now we have all the device of 1400V to select the most suitable, the main difference is that these devices dimensions different sizes and current ratings.
Since Id = 3600A, then the average current value of each device IT (AV) = 1200A. From the "high-power semiconductor devices catalog" on the "ordinary thyristor" series has the following components may be found to meet the requirements, they are: KPA 1400-14, KPX 1900-14 and KPB 3000-14.
Check "High-power semiconductor devices Product Data Sheet", the data obtained are shown in Table 1 below:
Element with the above data-state losses, taking into account the presence of the other losses, take-state losses as 1.1 times the value of the total power loss component value, then the total power loss to calculate the required heat sink thermal resistance the value of the data shown in Table 2:
Motor soft start
Motor power P = 220kw
Motor input voltage U = 380V
Three-phase power supply power calculation formula P = 1.732 * U * I * cosα
U and I values ??are valid when the motor starts normally take cosα = 0.8
Each phase current average IT (AV) = 0.9 * I
Each element of the current average IT1 (AV) = 0.45 * I
Starting current is limited to four times the rated
Device voltage selection principle VDRM / VRRM = (2 ~ 3) * 1.414 * U
Get to work and start the motor when operating the device current average
Select the appropriate device, such as KP9 1200-16
Calculate the steady state when starting the device computing device power consumption
Shell temperature calculation: Tj - Tc = PT (AV) Rjc
Calculator pieces steady state maximum case temperature TC1
Calculator pieces starting maximum case temperature TC2
Press the shell temperature radiator TC1 select the desired steady state
Press the shell temperature TC2 select the desired starting radiator
Junction temperature 20 ℃ left margin requirements, ambient temperature TA = 40 ℃
Compromise choice radiator