MOS controlled Thyristors (MCTs)

MOS controlled Thyristors (MCTs) have the combination of thyristor current and voltage capability with MOS gated turn-on and turn-off. Various sub-classes of MCTs can be made: P-type or N-type, symmetric or asymmetric blocking, one or two-sided off-FET gate control, and various turn-on alternatives including direct turn-on with light.

All of these sub-classes have one thing in common; turn-off is accomplished by turning on a highly interdigitated off-FET to short out one or both of the thyristor’s emitter-base junctions. Harris Company is the only present (1977) supplier of MCTs, however ABB has introduced a new device called “Insulated Gate Commutated Thyristor” (IGCT) which is the same family of devices.

The MCT turns on simultaneously over the entire device area giving the MCT excellent di/dt capability. The MCT offers a lower specific on-resistance at high voltage than any other gate-driven technology.

Figure depicts the MCT equivalent circuit. MCT closely approximates a bipolar thyristors (the two transistor model is shown) with two opposite polarity MOSFET transistors connected between its anode and the proper layer to turn it on and off. Since MCT is a NPNP device rather than a PNPN device and output terminal or cathode must be negatively biased.

Driving the gate terminal negative with respect to the common terminal or anode turns the P channel FET on, firing the bipolar SCR and thus MCT turns on. Driving the gate terminal positive with respect to the anode turn on the N channel FET on shunting the base drive to PNP bipolar transistor making up part of the SCR, causing the SCR to turn off and thus MCT turn off. It is obvious from the equivalent circuit that when no gate to anode voltage is applied to the gate terminal of the device, the input terminals of the bipolar SCR are un-terminated. Operation without gate bias is not recommended.

Gate Driver mounted on IGBT module

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Insulated Gate Bipolar Transistor (IGBT)

IGBT is turned on by applying a positive voltage between the gate and emitter and is turned off by making the gate signal zero or slightly negative.

IGBT has low on-state voltage drop and high off-state voltage characteristics with the excellent switching characteristics, simple gate-drive circuit, peak current capability and high input impedance.

IGBTs are available in current and voltage ratings well beyond what is normally available for power MOSFETs.

IGBTs are replacing power MOSFETs in high-voltage applications where conduction losses must be kept low. The IGBT has a much lower voltage drop than a MOSFET of similar ratings.

The switching speed of an IGBT is faster than a BJT but inferior to that of MOSFETs.

IGBTs do not have second breakdown problem due to its physical structure is similar to that of power MOSFETs. SOA is similar to that OSFET.

Characteristics of IGBT

 

The i-v characteristics appears qualitatively similar to that of a logic level BJT except that the controlling parameter is an input voltage.
The transfer curve i_c-v_GE is identical to that of the power MOSFET except V_GE(th) and the slope values.

Structure of IGBT

In general, IGBTs can be classified as punch-through (PT) and nonpunch-through (NPT) structures. Fig. below shows the PT structure here and n⁺ buffer layer is normally introduced between the p⁺ substrate and the n^- -epitaxial layer, so that the whole n^- drift region is depleted when the device is blocking the off-state voltage, and the electrical field shape inside the n^- drift region is close to a rectangular shape. Because a shorter n^- region can be used in the punch-through IGBT, a better trade-off between the forward voltage drop and turn-off time can be achieved. PT IGBTs are available up to about 1200 V.

Circuit Model of IGBT and Operating Principle

Operation is similar to that of MOSFET except the resistance offered by the top n region, which is very much similar during on-state.

The decrease in resistance occurs because of the injection of holes from the top p⁺ zone into the n zone. This effect is called conductivity modulation of the n region.

R_RR- body region spreading resistance.

R_MOD- modulated resistance due to carrier injection from the top p⁺ zone.

Channel (drain) current of MOSFET is the base current of pnp transistor.

Collector current of pnp transistor is the base current of npn transistor.

Operating Principle of IGBT

 

If C is positive with respect to E and if G is positive with respect to E greater than the threshold level, n channel is created, current flows through the channel (D to S) of the MOSFET.

The current flowing through the channel serves as the base current for pnp transistor, which causes emitter current to flow in this transistor resulting in the large-scale injection of holes across the top pn junction-these holes are responsible for the conductivity modulation of the middle n zone.

Saturation voltage characteristics of IGBT

R_on of an IGBT is usually 10 times smaller than that of a power MOSFET of the same size and voltage capability. This is because of the conductivity modulation process in the drift region.

The major current flow through the drive MOSFET again because of the conductivity modulation of the drift region.

V_CE(sat) = V_J1 + V_drift + I_DR_channel

Where V_J1 ~ 0.7 – 1.0 V,

V_drift < that of power MOSFET, and I_DR_channel ~ that of power MOSFET

Normally, the on-state or saturation voltage drop is used is instead of on-state resistance.

Even in IGBTs with the same structure, the IGBT with a fast switching speed has a larger on-state voltage drop, and vice-versa.

The on-state voltage changes little between room temperature and the maximum junction temperature. This is because of the combination of positive temperature coefficient of the MOSFET section and the negative temperature coefficient of the voltage drop across the drift region.

Safe Operating area (SOA) of IGBT

The FBSOA defines a maximum V-I region in which the device can be commanded to operate with simultaneous high voltage and current. The device current can be controlled through its gate (or base) and the length of the operation is only limited by its thermal limitation. Device with FBSOA normally have an active region in which the device current is determined by the control signal level. Example IGBT….

 

Forward Biased Safe operating Area (SOA) of IGBT

SOAs

1. The FBSOA is identical to that of the power MOSFET.
2. The RBSOA (for turn-off) is different from the FBSOA. The reapplied dv_CE/dt is limited to avoid the latch-up of IGBT (or latch-on of the parasitic thyristor). But the values are quit large which can be easily controlled by the gate. If latch-up occurs, it must be terminated quickly, otherwise the IGBT will be destroyed.
3. The allowable maximum temperature, T_J(max) is 150°C.
4. The maximum collector current can be 4 to 10 times the normal rated current for 5-10µs depending on the value of V_CE.

Commercial individual IGBTs and IGBT modules

1. Commercial individual available IGBTs have nominal current ratings as large as 200-400 A and voltage ratings as 1700V. voltage rating up to 2-3kV are projected. (The voltage ratings of IGBTs are higher than those of BJTs due to the small current gain of the pnp BJT.)
2. For a 1kV device, the on-state voltage is 2-3 V at rated current.
3. The turn-on and turned-off times are less than 1µs.
4. IGBTs are available in module in which 4 to 6 individual IGBTs are connected in parallel. Hence, the current ratings are in the ranged of 1000 to 1500 A.

Typical Gate drive Circuit for IGBT modules

IGBT Characteristics: a comparison

The IGBT characteristics in comparison with BJT, MOSFET with similar size and ratings.

Features	BJT	MOSFET	IGBT
Drive method	Current	Voltage	Voltage
Drive circuit	Complex	Simple	Simple
Input impedance	Low	High	High
Drive power	High	Low	Low
Switching speed	Slow (µs)	Fast (ns)	Medium
Operating frequency	Low (< 100 kHz)	Fast (<1 MHz)	Medium
SOA	Narrow	Wide	Wide
Saturation voltage	Low	High	Low

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Power MOSFETs

n-Channel enhancement type MOSFET

V-MOSFET

 

MOSFETs Switching

Parasitic Model

Switching Model

Cross Section of COOLMOS

On-state Resistance

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GTO

1. The GTO is a power switching device that can be turned on by a short pulse of gate current and turned off by a reverse gate pulse. This reverse gate current amplitude is dependant on the anode current to be turned off. Hence there is no need for an external communication circuit to turn it off.
2. Because turn-off is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more capability for high-frequency operation than thyristors. The GTO symbol and static characteristics (similar to that of SCR) are shown in Fig. below.

GTO Structure

A high degree of interdigitation is required in GTOs in order to achieve efficient turn-off. The most common design employs the cathode area separated into multiple segments (cathode fingers) and arranged in concentric rings around the device center. The internal structure is shown in Fig. A common contact disk pressed against the cathode fingers connects the fingers together. It is important that all the fingers turn off simultaneously; otherwise the current may be concentrated into fewer fingers, with damage due to overheating more likely.

The high level of gate interdigitation also results in a fast turn-on speed and high di/dt performance of GTOs. The most remote part of a cathode region is no more than 0.16mm from a gate edge and hence the entire GTO can conduct within 5 ms with sufficient gate drive and the turn-on losses can be reduced. However, interdigitation reduces the available emitter area end therefore the low-frequency average current rating is less than for a standard thyristor with an equivalent diameter.

 Switching Characteristics of GTO

Switching phases of GTO

Turn-on: A GTO has a highly interdigited gate structure with no regenerative gate. Thus it requires a large initial gate trigger pulse. Minimum (I_GM) and maximum values of gate pulse can be derived from the device data sheet. The rate of rise of gate current di/dt will affect the device turn-on losses. The duration of the I_GM pulse should not be less than half the minimum for time given in data sheet rating. A longer period will be required if the anode current di/dt is low such that I_GM is maintained until a sufficient level of anode current is established.

On-state: The GTO operates in a similar manner to the thyristor. If the anode current remains above the holding current level then positive gate drive may be reduced to zero and the GTO will remain in condition. However, as a result of the turn-off ability of the GTO, it does posses a higher holding current level than the standard thyristor and, in addition, the cathode of the GTO thyristor is subdivided into small finger elements to assist turn-off. Thus, if the GTO thyristor anode current transiently  dips below the holding current level, localized regions of the device may turn-off, thus forcing a high anode current back into the GTO at a high rate of rise of anode current after this partial turn-off. This situation could be removed during conduction but held at a value at least 1% of the turn-on pulse to ensure that the gate does not unlatch.

Turn-off: The turn-off performance of a GTO is greatly influenced by the characteristics of the gate turn-off circuit. Thus the characteristics of the turn-off circuit must match with the de-ice requirements. The gate turn-off process involves the extraction of the gate charge, the gate avalanche period, and the anode current decay. The amount of charge extraction is a device parameter and its value is not affected significantly by the external circuit conditions. The device data sheet gives typical values for I_GQ.

·         For reliable operation of GTOs, the critical aspects are proper design of the gate turn-off circuit and the snubber circuit. A GTO has a poor turn-off current gain of the order of 4 to 5. For example, a 2000-A peak current GTO may require as high as 500 a of reverse gate current. Also, a GTO has the tendency to latch at temperature above 125˚C.

·         GTOs have the I²t withstand capability and hence can be protected by semiconductor fuses. GTOs are available up to about 4500 V, 2500A.

GTO Gate Drive Requirement

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