POWER IGBT

Any type of control by the electric motor to the mechanical, passing through the engines of robotic applications depends on the power devices.

We speak in this case of Ilift (insulated-gate bipolar transistors),the IGBT require a very accurate description. The dynamic performance of these devices vary widely depending on the load that must be controlled. Their mechanism of switch is not linear, so it is essential to select the right of the gate control parameters and optimize the design process to make efficiente the switching.

The same can be said for Mosfet.

That's why proper description of the switch losses, the switching speeds, the stability of the circuit and the area of ​​operability in security is the fundamental step in the entire process of designing.

The use of IGBT It is greatly increased lately. Compared to the MOSFET, this device shows many advantages over control in industry. The IGBT They have lower conduction losses and require a lower ignition voltage (more reduced power consumption). The greater use made of it today, It is in control of the variable speed motors, in traction control, inverters, in power supplies, etc..

L’IGBT It is a device halfway between the bipolar transistor and the MOSFET. The output characteristics are equal to those of a bipolar transistor, however, it is the voltage controlled oscillator (15 Volt is recommended) such as MOSFET.

To simplify the mentally idea what it is, imagine it as a sort of darlington in which the first driver transistor has been replaced by a mosfet.

The operating scheme is simple. As the voltage is applied between the gate and emitter, the input equivalent capacitance is charged through a gate resistor up to a threshold voltage that turns on the’IGBT. The other way around, when the capacitance between gate and emitter is discharged, the IGBT returns to the off state. The charging time and discharging of the input capacitor is the factor that limits the speed of the device switch. As well as the dimensioning of the gate resistor. Over this resistor is small, faster is the time the charging and discharging and therefore the shorter the time of switching dell’IGBT. The problems that are encountered, But, require a compromise in dimensioning due to the possible increase in the oscillation caused by the capacitance between gate and emitter and loss inductive parasitic effects.

IGBT IGBT HOW TO IMPROVE PERFORMANCE

To improve the performance of’IGBT a different input circuit is then necessary in relation to the load to be controlled in different applications. The optimization of this process requires a deeper understanding of the mechanism of switching depending on the actual load. A preliminary analysis provides stimulation of the device with different gate signals and the subsequent measurement of the output variables, such as the collector current and the collector-emitter voltage.

By choosing a random generator of waves (AFG, arbitrary function generator) with high output amplitude, and an oscillator for the measurement of the output characteristics, It is able to produce graphics with the representation of the features of interest.

With these waveforms, it is possible to determine the switching energy, losses during the state of on, e se l’IGBT It is operating within the safe range. These measurements provide engineers an essential basis for the determination of the key design factors, such as the input signal of the switching frequency and the amplitude limit for a smooth transition between the two states of operation and the necessary adjustments to be within the design parameters.

MOSFET and IGBT A CONFRONTATION

Comparing the cross-sections of a MOSFET and an IGBT, These look very similar. The basic difference is the addition of a substrate p below the n substrate. In theory, IGBTs are preferred in applications with reduced duty cycle, from low frequency (below 20 KHz), and from small variations on the line or load. The addition Igbt can also operate in the presence of high junction temperature (over 100 ° C), other tense (more than 1000 V) and with output powers in excess of 5 kW.

MOSFETs are preferred in high-frequency applications (above 200 kHz), in the presence of large variations on the line or load, duty cycle long, low voltages (below 250 V) and a lower power 500 W. Some classic examples of applications include switching power supplies and battery chargers.

An IGBT is capable of supporting a current density 2 O 3 times higher than that of a typical MOSFET. This means that a single IGBT can replace multiple MOSFETs in parallel, or a higher dimensions Mosfet. The new Igbt commercially support higher current density, They provide a greater efficiency with smaller die and therefore lower costs compared to similar devices Mosfet. The IGBTs have for some years replaced the MOSFET in applications with frequencies below 75 kHz. The latest generation of IGBTs solutions have made them convenient to use even for operating frequencies up to 150 kHz

IGBT REAL

The extremely simplified behavior described up to now served as an introduction to what will be said below. We will be deepened some aspects referring to a real component.

First, you must read the component data, take for example a STGW80H65DFB, defined by the manufacturer as 650 V, 80 A high speed – Trench gate field-stop IGBT

The first table shows the absolute maximum values, above which the manufacturer no longer guarantees the survival of the component. They highlighted the ones that can be considered the basic parameters.

• The maximum VCE voltage and the maximum current IC are the main characteristics of the component and can vary greatly between the various abbreviations.

• The maximum voltage VGE is a common value to almost all the IGBTs

• With TJ junction temperature means the maximum (and also a minimum) temperature which can be reached inside the component

Generally it's good to stay away from all submitted limits.

This table shows some values “Recommended”, under static conditions, ie without rapid changes over time

• The threshold voltage VGE denotes the voltage between Gate and emitter just sufficient to conduct the IGBT. This is not the value for the ideal operation. If VGE is less, the IGBT is an open switch.

• to VCE(sat) It is the voltage between the collector and emitter when the IGBT conducts well, ie, the voltage between Gate and emitter is 15 V. This is the tension between Gate and Source recommended.

This table describes the characteristics of the IGBT during switching from on to off and vice versa.

The CIES capacity is to input, between Gate and emitter. It is important because each time the IGBT is turned on you should charge the capacitor and each time the IGBT is turned off you must download it.

OBSERVATIONS

• E’ well that charging times / download are small

• The CIES is strongly non-linear, not worth ie the linear relationship between voltage and charge accumulated. In particular, the relationship between voltage and time during the charging or discharging is therefore not an exponential curve

This graph shows what happens to the current collector (IC) and the voltage between the collector and emitter (VCE) to vary the voltage between Gate and emitter (VGE).

They are highlighted two areas:

• In the green conduction area, in correspondence VGE = 15 V. The bond between I and V is a straight line not passing through the origin, almost a closed switch.

• In the yellow area in which the current is zero, then the IGBT behaves like an open switch. VGE has to be small, preferably 0 V (It appears written, the smallest value is indicated 7 V).

The last graph shows the area in which the IGBT operation is “sure”: the operation is correct only if the point identified by IC and VCE falls within the area bounded. This area is often referred to as SOA. In particular that indicated is relative to only shut down and in practice indicates the absence of latchup and second breakdown, the biggest problems of the older IGBT, overcoming these problems has made it convenient to use since the beginning of the present century.

They have been widely used in plasma TV having the high consumption, now with the dominance of technology much less greedy energy have gradually eliminated them from such equipment.

Note how this area is regular, that translates into relative “easiness” of use of the component and the inherent “strength”.

The last table shows the thermal resistance: for every watt dissipated dall'IGBT, the temperature rises of the indicated value.

The amount of amplification obtained from the insulated gate bipolar transistor It is a relationship between its output signal and its input signal. For a conventional bipolar junction transistor (BJT) the amount of gain is approximately equal to the ratio between the output current and the input current, called Beta.

For a field effect transistor in metal oxide semiconductor or MOSFET, there is no input current since the port is isolated from the main current carrying channel. Therefore, the gain of a FET is equal to the ratio between the variation of output current and input voltage variation, making a device transconductance and this also applies to the IGBT. So we can treat the IGBT as a power BJT whose base current is provided by a MOSFET.

The bipolar transistor with insulated gate It can be used in small signal amplifier circuits in the same way of the BJT type transistor or MOSFET. However, since the IGBT combines the low conduction loss of a BJT with the high switching speed of a power MOSFET, It is similar to a switch Solid optimal state, ideal for use in power electronics applications.

Furthermore, the IGBT has a resistance “on-state” much lower than an equivalent MOSFET. This means that the fall of I²R through the bipolar output structure for a given switching current it is much lower. The locking operation of the transistor IGBT is identical to a power MOSFET.

When used as a controlled switch, the bipolar transistor of the insulated gate has values ​​of voltage and current similar to those of the bipolar transistor. However, the presence of an insulated gate in an IGBT makes it much easier to drive compared to BJT, as a lower transmission power is required.

An insulated gate bipolar transistor is simply set to “ON” O “OFF” activating and deactivating the associated terminal Gate. By applying a positive input voltage signal on the Gate respect to the emitter will keep the device in a state “ON”, while setting the input signal to zero or slightly negative will make him “OFF” more or less the same way as a bipolar transistor and / or MOSFET. Another advantage is that the IGBT has an on-resistance of the channel were much lower compared to a standard MOSFET.

Because the IGBT is a device at a controlled voltage, It requires only a small voltage on the gate to maintain the conduction through the device as opposed to BJT that require that the base current is continuously fed in an amount sufficient to maintain the saturation.

Even the IGBT is a unidirectional device, which means that the current flows in only one direction, that is, from collector to emitter unlike the MOSFET that have current switching capacity bidirectionally.

As for the power mosfet most igibt has its own diode which in normal operation is polarized inversely.

A general comparison between BJT, MOSFET and IGBT are shown in the following table.

With this I have finished this brief and non-exhaustive overview of a little-known component. There would still be a huge amount of things to say but, continuing further in the description would become a treatise too technical and deepened interest to a wide audience of readers.

Rimando so for those who are really interested in pursuing this component thorough academic documentation.

Amilcare Greetings