In today's electronic equipment, power Semiconductor And the electric resistance elements ( Capacitance And inductors) can be seen everywhere. In the normal course of their work will be online for two kinds of AC power supply undesirable side effects.
First, these devices will lead to a smaller power factor. Second, they will line current distortion, caused by electrical noise or the generation and phase offset between the line voltage.
Power factor is the actual use of power and communication line apparent power generated by the ratio of the two. Electrical equipment if there is a large capacitor or inductor will lead to apparent power is greater than the actual use of power, power factor of smaller.
Smaller power factor, power supply equipment in exchange for the wire the more power loss. If the power semiconductor device Switch Operation is very frequent, so that Switch Operation will cause the AC line current distortion and noise. In Switching Power Supply In particular.
Certain international standards (such as IEC61000-3-2) requirements for various types of electrical equipment that could allow the line current distortion and power factor of size. Achieve power factor compensation the simplest, most cost-effective way is to use enhanced - conversion circuit (Figure 1), this circuit can generate an input voltage higher than the output voltage.
Figure 1 to achieve power factor compensation for the simplest, most cost-effective way is to use to produce a higher output voltage than the input voltage increase - Conversion Circuit
Enhance the performance of diode Power to 300W for more than
equipment, usually operating in continuous conduction mode (ie CCM) under the enhanced converter. Converters required for the enhancement of two power semiconductor devices?? MOSFET and diode, which diode has a relatively high performance requirements, because it will affect the reverse recovery characteristics of MOSFET performance.
In continuous conduction mode, whenever the control IC Open the MOSFET, the diode will produce a higher forward current. As the increase in the full forward biased diode case occurs rapidly reverse bias, and the silicon diode closure will take some time, so when the flow back into the closed diode reverse recovery current of diode (IRR) would be very large (see the red curve in Figure 2).
Figure 2 four common enhanced diode (400V, 5A, 200A/ s, 125 ) in the reverse recovery waveform
Reverse current flow through MOSFET increase its operating temperature. This one designed with very low reverse recovery time (tRR) dedicated silicon diode, but they can reduce the IRR is usually very limited, often abrupt closure of the phenomenon (see Figure 2, black curve).
Low QRR and high softening coefficient
Schottky diode Than the behavior of PN junction devices like an ideal switch. Schottky diodes the two most important performance is its low reverse recovery charge (QRR) and its recovery softening coefficient.
For these two indicators are very important to increase converter. Low QRR in the diode will result in lower closing IRR. Coefficient will reduce the high softening diode EMI noise generated by the closure, the device produced on the anode voltage pulse peak, reducing the operations for the possibility of interference with PFC control IC.
Limitations of Schottky diodes Schottky diodes can greatly enhance the PFC converter enhanced performance, but the silicon Schottky diode with 250V reverse voltage of about restrictions. As the enhanced diode must be able to withstand 500 ~ 600V, so people started to use silicon carbide (SiC) devices, such compound can withstand a higher voltage. However, due to the high cost of SiC devices (similar silicon devices is 3 to 5 times), so very few applications can afford this kind of device used.
The past few years there have been a better performance of silicon diode, but they can not match the performance of SiC Schottky devices. Recently, it developed a new series of silicon rectifiers, their reverse recovery performance can be comparable with the SiC Schottky diode (see Figure 2, green curve).
Occurred in the PN junction silicon diode reverse bias must be eliminated before the QRR determined from its closed to generate the IRR size. QRR PN junction depends on minority carrier near the duration or life.
As Schottky diodes only by the N-type metal contacts Semiconductor materials Form, so they do not have minority carrier. When the Schottky diode reverse bias occurs when the IRR resulting from low-metal body contact with the diode capacitance discharge effect.
The design process in the silicon diode can be used in a variety of technical control devices in the minority carrier lifetime, so far, but can not match the SiC diode low QRR. The green curve in figure 2 shows, the latest silicon devices?? Qspeed Semiconductor Q series?? Able to reach the same low SiC Schottky devices IRR (blue curve in Figure 2 below).
Schottky diode is not a small number of carriers, because they only contacted by the metal composition of N-type semiconductor material.
Softening coefficient is a measure of maximum negative diode drop to zero when the speed of its IRR as an indicator. With fast recovery silicon diodes commonly used in the design process, minority carrier lifetime control technique can make the steep decline in IRR (black curve in Figure 2 below). This rapid shutdown process will produce a large number of anode diode EMI noise and large voltage spikes.
To offset that occurs when using fast diodes do not want to place these phenomena, we need to slow circuit well-designed. High softening coefficient means that the diode IRR zero rate of change (di / dt) is equal to or less than its maximum negative value up to speed. When the LED slowly closing, it arising in the diode anode less EMI noise, the resulting voltage spikes and lower, and less likely to interfere with control of IC work.
SiC Schottky diodes can match with silicon rectifiers now come out, so engineers should re-evaluate the design of PFC converters increase, take a look at the use of these has the same properties of SiC, after a new silicon device is to reduce design costs and / or improve the design performance.
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