Comparison of Controllable Power Electronic Devices

Voltage, current and frequency ranges

The figure (in log-log scale) shows the power-frequency capability of the current devices and their future trends. The power is given by V-I ratings product, that is, the product of the maximum blocking voltage and maximum turn-off current. Note that the BJT is completely removed from the figure.

Thyristors and triacs are essentially low-frequency (50/60 Hz) devices, and currently the thyristor has the highest power rating.

The future trend, as indicated by the dashed curve, also indicates that has the highest power rating

High-power thyristors are used in high-voltage dc (HVDC) systems, phase-control type static VAR compensators (SVC), and large ac motor drives.

GTOs and IGCTs have a higher frequency range (typically a few hundred hertz to one kilohertz) but their power limits are lower than that of a thyristor. Normally, with a higher power rating, the switching frequency becomes lower, and this is indicated by tapering of the areas at higher frequency.

However, an IGCT’s switching frequency is somewhat higher than that of a GTO (not shown).

IGBT intelligent power modules (IPMs) come next with higher frequency but lower power range.

The lower power end of GTO/IGCTs overlaps with IGBTs.

The discrete IGBTs have higher frequency and lower power range, as shown.

Power MOSFETs have the highest frequency and lowest power range.

	Power MOSFET	IGBT	GTO	IGCT
1. Voltage and current ratings (selected device for comparison)	100 V, 20 A* (dc)	1.2 KV, 50 A* (dc)	6 KV, 6000 A* (pk)	4.5 KV, 4000 A* (pk)
2. Present power capability	1.2 KV, 50 A	3.5 KV, 1200 A or higher	6 KV, 6000 A	6.5 KV, 3000 A
3.Voltage blocking	Asymmetric	Asymmetric*	Asymmetric/symmetric	Asymmetric/symmetric
4. Gating	Voltage	Voltage	Current	Current
5. Junc. Temp. range	-55 to 175	-20 to 50	-40 to 125	-40 to 125
6. Safe operating area (ºC)	Square	Square	2nd breakdown	Square
7. Conduction drop (V) at rated current	2.24	2.65	3.5	2.7
8. Switching frequency	106 Hz	1 kHz – 20 kHz	400 Hz	1.0 kHz
9. Turn-off current gain	-	-	4 to 5	1
10. Turn-off di/dt	-	-	500 A/µs	3000 A/µs
11. Turn-on time	43 ns	0.9 µs	5 µs	2 µs
12. Turn-off time	52 ns	2.4 µs	20a µs	2.5 µs
13. Snubber	Yes or No	Yes or No	Yes (heavy)	Yes or No
14. Protection	Gate control	Gate control	Gate control or very fast fuse	Gate control or very fast fuse
15. Application	Switching power supply, Low power motor drive	Motor drive, UPS, induction heating, etc.	Motor drives, SVC, etc	Motor drives, HVDC, SVC etc.
16. Comments	Body diode can carry full current but sluggish (trr = 150 ns) Ipk = 56 A	Large power range, very important device currently *Reverse blocking available	Dv/dt = 1000 V/µs High uncontrollable surge current	Built-in diode, High uncontrollable surge current Dv/dt = 4000 V/µs
	*Harris IRF 140	*Powerex PM50RVA 120 7-pack IPM	*Mitsubishi -FG6000AU-120D	*ABB 5SHY35L4512

 

previous Series and Parallel Connected Power Electronic Devices

Series and Parallel Connected Power Electronic Devices

Maximum voltage and current of a power semiconductor device limit its power-handling capability for power system applications. Hence, several devices of the same type can be connected

1. In series to increase the overall voltage rating or
2. In parallel to increase the overall current rating or
3. In series and parallel to increase overall power capability.

IGBTs require special care to match the characteristics due to the variations of the temperature coefficients with the collector current. IGBT modules (up to six in parallel) are commercially available.

However, the characteristics (i-v, β, Vth, etc) for the same type of devices must be similar and turn-on and turn-off must be simultaneously.

Voltage and current sharing elements similar to those for thyristors may be used (discussed in the next slide)

Normally, each sharing device is derated by almost 10% from its rated voltage or current for safety purpose.

Thyristors in Series: The problem of unequal voltage sharing ( during reverse and off conditions) between the thyristors in series is overcome through connecting balancing resistors (R) in parallel with the devices so that they equally share the reverse voltage. The current through the resistors is >> diode current

The RSCS networks have two functions: transient voltage sharing and dv/dt protection

Thyristors in Parallel: The problem of unequal load current sharing ( during on condition) between the thyristors in parallel series is overcome through connecting small resistors or magnetically coupled inductor in series with the device to force equal share of current.

 

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Protection of power electronic Devices

1. When the devices carry high currents they generate a lot of heat internally, which can destroy the device. A heat sink is a way of removing this heat. It is a metal plate with fins which act as radiators of the heat.
2. The device is bolted to the sink usually with some heat conducting paste smeared between them.
3. Cooling system available: air (natural convection), fan, liquid (water)

 

Two flat-pack thyristors mounted on a liquid-cooled heat sink

Power diode mounted on an air-cooled heat sink.

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Power losses in power electronic devices

Assuming ideal diode and no reverse recovery currents:

1. Io = iT + iD at all time
2. When turning the switch T on, the diode is reverse biased only after i_T rises to i_o. Until then, V_T = V_d.
3. When turning the switch off, the diode does not conduct until v_T rises to V_d. Until then i_T = I_o.

During transitions, the power loss in the switch is given by the product p = vT ⋅iT, given by the shaded areas.

Thus,


    
     
and so on.

During transitions, the power loss in the switch is given by the product p = v_T ⋅i_T, given by the shaded areas. If the turn-on and turn-off transients are not short compared to T_s, the power loss in the switching process, P_s, may become large compared to the loss during the ON time.

Note that Ps increase proportionately with f_s, while P_on does not, since T_on and T_s are proportional to each other.

Exercise-1

In the step-down converter circuit of Figure 8 below, Vd = 250 V and Io = 50 A.

The MOSFET parameters are listed below:

BV_DSS = 400 V, ID_,max = 80 A, VGS_,th = 5 V, r_DS(on) = 0.05 ohm,

T_j,max = 175 0 C, R_өj-a = 0.5 0 C/W, t_on = t_off = 200 ns

1. What is the power dissipation in the MOSFET assuming a switching frequency fs = 10 kHz and a duty cycle D = 50%?
2. What is the maximum average power that can be dissipated in the MOSFET? Assume an ambient temperature of 250C.
3. The duty cycle D will vary from 20% to 90%. What is the maximum permissible switching frequency fs? Assume that the period 1/fs is large compared to the switching times of the MOSFET.

   

Note that junction temperature at the device (Tj), assuming a power dissipation of P is thus given by

                        ΔT _ja = ΔT _j − ΔT _a = P (R _θjc + R _θcs + R _θsa)

              Or      ΔT _ja = P (R _θja)

The total thermal resistance is Rθja

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IGCT/GCT

1. IGCT (Integrated Gate Commutated Thyristor) is an ABB product, GCT ic by Mitsubishi, but the concept is the same. Here Turn-off time is very low.
2. Power consumption by the GCT driver is greatly reduced compared with that of a conventional GTO driver.
3. The key to achieving a hard-driven or unity-gain turn-off condition lies in the gate current commutation rate. If the gate driver of a GTO is very fast so the gate current can increased rapidly to the anode current level and the cathode current decreases to zero before the anode current begins to decay. A rate as high as 6KA/µs is required for 4-KA turn-off.
4. One method for the implementation of a hard-driven GTO, is to hold the gate loop inductance low enough (3 nH) that a DC gate voltage less then the breakdown voltage of the gate-cathode junction (18 to 22 V) can generate a slew rate of 6 KA/µs. This approach is used in the IGCT/GCT, where a special low-inductance GTO housing and a carefully designed gate driver meet this requirement. The power consumption by the GCT driver is greatly reduced compared with the conventional GTO driver, since the gate current is present for a much shorter period of time. Figure shows the external view of the two commercially available GCTs.
5. The key disadvantage of the GCT approach is the high cost associated with the low-inductance housing design for the GTO and the low inductance and high current design for the gate driver.

GCT operation principle and two GCTs developed by Mitsubishi and ABB (Photographs courtesy of Mitsubishi (top) and ABB (bottom).)

Impact of switching speed

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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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