یکی از ویژگیهای IGBT که آنرا از ترانزیستورهای قدرت معمولی جدا می کند سرعت سوئیچینگ بسیار بالاتر آن است که به دلیل ساختمان ویژه آن حاصل شده است. پهنای باند پالسهای ایجاد شده توسط IGBT حدود 50 تا 150 نانو ثانیه است که بسیار کمتر از پهنای 2 تا 4 میکرو ثانیه مربوط به ترانزیستورهای قدرت است. این سرعت بالای IGBT ها و عملکرد مناسبشان به خاطر امپدانس بالای گیت ورودی سبب کاربرد وسیع آنها در مدارات الکترونیک قدرت شده است. یکسو سازهای متکی به IGBT به دلیل سرعت بالای سوئیچینگ قادرند در فرکانسهای خیلی بالاتر نسبت به تریستورها استفاده شوند ، افزایش فرکانس می تواند کاهش حجم ترانس واسطه را بدنبال داشته باشد.
Traditionally, Solid State Frequency Converters (SSFC) have used Silicon Controlled
Rectifiers (SCR) to convert AC power to DC power. SCR’s have the advantage of being easy to control and inexpensive. They just have to be turned on or off at the zero-crossing point of the AC sine wave. Unfortunately, they have several significant disadvantages. They create high harmonic distortions in the input current waveform. They lower the input power factor taking more reactive current and they induce commutation spikes into the utility power supply.
Isolated Gate Bipolar Transistors (IGBT) has several advantages over SCR’s. They do not have to be turned on and off at the zero crossing point, they do not change the input power factor or induce commutation spikes into the input power source. They also allow better control of the entire power and frequency conversion process. For these reasons, IGBT’s are now being used in high end SSFC’s just as they are used in state of the art solid state
Uninterruptible Power Supplies (UPS).
To better understand the above points, let’s look at how each of these devices is used in the typical SSFC. A low end SSFC will have a pair of SCR’s for each phase of the power source.
One SCR is turned on for the positive portion of the AC waveform and turned off at the next zero crossing. While the opposing SCR of the pair is turned on for the negative portion of the
AC waveform and off at the next zero crossing. This on/off pattern is used to transmit full power to the output load.
A 12-pulse SCR system follows the same basic pattern but it splits the utility power into two 6-pulse rectifiers. The second 6-pulse rectifier is typically fed through a 30 degree phase shifting transformer. This provides a smoother DC current which puts less stress on the DC capacitors and shifts the input current harmonic distortion to higher frequencies with less energy content, but the design requires more components.
For both rectifier types (including versions with diodes instead of SCR’s) the energy will follow one direction only, which means the energy is only fed in the direction of the load.
There is no way back.
Instead of turning on and off at the zero crossing point IGBT’s are fully controlled using Pulse
Width Modulation (PWM) control algorithm.
PWM systems typically operate at much higher switching frequencies (6 – 8 kHz instead of 120 Hz for SCR’s). The higher switching frequency allows much finer control of the power conversion process. It also does not produce the audible noise that is often found in low end systems. The width of each pulse determines the amount of power that is converted to DC in the rectifier. But, because there are thousands of pulses in each cycle the energy being transferred per pulse is small which gives the IGBT system significant advantages over an SCR system.
Since the transistor can conduct the current in both directions, it is possible to feed some power back to the utility in case the load creates some reverse power. This is another significant advantage over an SCR system (simple no break power transfer capability).
Reference: A brief summary of the advantages
& disadvantages of using igbt’s and
scr’s in rectifier power stages