Tutorial: Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor

This tutorial installment is: Power Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor. This topic answers the following questions:

• What are the minority carrier power semiconductor switches used today?
• What are the typical applications for these switches?

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As shown in the last installment of this tutorial, power semiconductor switches can be classified by the type of charge carriers: minority carrier devices or majority carrier devices. This topic will discuss minority carrier devices. For a discussion of majority carrier devices, please see the topic Power Semiconductor Switches, MOSFETs and Schottky Diodes.

Minority Carrier Device Family

Minority carrier devices have charge storage times of a few hundred nanoseconds to a few microseconds. Consequently the switching time is limited to this order of magnitude, and the maximum practical switching frequency of a power supply with minority carrier active switches is limited by the storage time to about 50 to 100 kHz. Some device types have greater minority charge storage and/or more limited methods of minority charge removal, resulting in further frequency limitations. The minority carrier devices used today are the PIN diode, BJT, IGBT, and the thyristor.

PIN Diode

The power versions of the PN junction diode have broad applicability in switching power supplies. Besides the usual breakdown voltage and rated current, the other main consideration in selecting a diode for rectification use is the reverse recovery time. The PIN diode is the same as a PN diode but with the addition of an intrinsic layer between the P and the N layers. This intrinsic layer provides the breakdown voltage capability of the device since under reverse bias conditions, it is devoid of charge carriers.

Line frequency rectifiers can use standard recovery time diodes. Switching power supply applications using minority carrier transistors such as IGBT’s or BJT’s require diodes with fast recovery times of less than 500nS. Switching power supply applications using majority carrier devices such as power MOSFETs require ultra fast recovery times of less than 100nS.

Advantages of the PIN Diode:

• Inexpensive
• Does not require control

Disadvantages of the PIN Diode:

• Has a reverse recovery time which contributes to power loss

PN Diode Maximum ratings available from Digi-Key as of 3/2011:

• Maximum breakdown voltage, (V_RRM):  4500 volts
• Maximum current, (I_F):  600A
• Maximum apparent power (V_RRM * I_R):   2137kVA¹

Bipolar Junction Transistor (BJT)

The BJT is an older technology. Switching power supply applications for BJTs include switching voltages over 600V at frequencies less than 20kHz. Due to conductivity modulation effects, the BJT can exhibit lower on state collector-emitter voltage than the drain-source voltage across a comparable MOSFET.  The BJT has three terminals: collector, base, and emitter. The BJT conduction state is controlled by the level of current injection into the base terminal. The governing relationship is:

The current gain, h_FE, for a power BJT may be in the range of 20-100 for lighter collector currents. However, as maximum rated collector collector current is approached, hFE, degrades to the range of  5-10 making control of the device more difficult. Additionally, V_CEsat increases causing much greater conduction losses.

The BJT also has substantial storage charge which limits its ability to turn off quickly. Typical storage time and collector fall time is in the range of 1-5 microseconds. This turn-off time limits the maximum practical switching frequency of power supplies using BJT’s as the power semiconductor switch.

Advantage of the power BJT:

• In low power and lower switching frequency applications, the slower switching speed can be used to minimize EMI generation, reducing or eliminating the requirements for input EMI filtering. Power supply cost and size reductions result.
• Lower on state voltage relative to a comparable power MOSFET’s for devices with ratings over 600 volts.

Disadvantages of the power BJT:

• Substantial control current required, resulting in efficiency issues,
• Substantial turn-off storage time resulting in slow switching times and therefore relatively slow maximum switching frequencies.

Power BJT Maximum ratings available from Digi-Key as of 3/2011:

• Maximum collector to emitter breakdown voltage, (V_CEO):  1200 volts
• Maximum collector current, (I_C):  60A
• Maximum apparent power (V_CEO * I_C):   28.8kVA¹

Insulated Gate Bipolar Transistor (IGBT)

The IGBT is a recent technology. Switching power supply applications are for switching voltages over 600V at frequencies less than 100kHz and for higher power levels than either the BJT or power MOSFET can support.  The IGBT, is a three terminal hybrid of the BJT and power MOSFET. Internally, the drain of an N-channel power MOSFET sinks current from the base of a PNP BJT. Therefore, the IGBT has a high impedance input similar to a MOSFET. It also has a collector-emitter output which has a large breakdown voltage and a low on state voltage as in a BJT. IGBT technology has been greatly improved, resulting in shorter turn off delay times compared to the BJT.

Advantages of the IGBT:

• Voltage controlled, high input impedance device, easier than current control of BJT.
• For devices with higher voltage ratings, the on state voltage is much lower than that of a comparable MOSFET.
• Power handling capability is ten times better than power MOSFETs or BJT’s
• Shorter delay times relative to the BJT, about 300nS to 1500nS.

Disadvantages of the IGBT:

• Have a current tail turn-off characteristic which results in longer delay times relative to the power MOSFET.

IGBT Maximum ratings available from Digi-Key as of 3/2011:

• Maximum collector to emitter breakdown voltage, (V_CEO):  4000 volts
• Maximum collector current, (I_C):  400A
• Maximum apparent power (V_CEO * I_C):   340kVA¹

IGBT Modules

IGBT modules (parallel-serial combinations of  IGBTs in the same package) with VA ratings of over 4000kVA are available from Digi-Key.

 

Thyristors

The thyristor family includes the silicon controlled rectifier, and the gate turn-off thyristor.

Silicon Controlled Rectifier (SCR)

The SCR is the oldest technology used in power switching applications.  SCR applications primarily involve either phase control of AC line power or electronic crowbars where it is desired to rapidly dump energy stored in a large capacitor. Because an SCR has a large turn-off time, it is not suitable for switching applications  greater than 400Hz.

The SCR has three terminals: anode, gate, and cathode. Conduction from the anode to the cathode begins when current is injected into the gate. A conducting SCR looks like a diode, and once conduction begins, the SCR latches on so than removal of the gate drive does not turn the SCR off. The SCR can only be turned off when the anode to cathode current falls below the holding current specification or reverses. Before the SCR can turn off, the minority charge carriers must be removed by the reverse current or recombination effects.

Advantages of the SCR:

• Operating characteristics lends the SCR to phase control of AC power and for crowbar applications.
• Lowest cost per kVA of all power semiconductor switches.²
• Large power handling capability.

Disadvantages of the SCR:

• Substantial minority carrier charge storage time, limiting upper switching frequency to 400 Hz or less.

SCR Maximum ratings available from Digi-Key as of 3/2011:

• Maximum anode to cathode breakdown voltage, (V_AK):  3800 volts
• Maximum anode current, (I_A):  3360A
• Maximum apparent power (V_AK * I_A):   6726kVA¹

Gate Turn-Off Thyristor (GTO)

The GTO thyristor is similar to the SCR but can be turned off through application of negative current to the gate electrode. This turn off capability expands the areas of applications to include not only phase control of AC line power, but also variable speed motor drive and inverters as well. Additionally, the GTO thyristor is able to switch somewhat faster than the SCR.

As of this time (3/2011), Digi-Key is not stocking GTO thyristors. See the Thomas Register for suppliers and distributors, but expect long lead times for these specialty components.

Next Topic

The next tutorial installment is Power Semiconductor Switches, MOSFETs and Schottky Diodes. This topic answers the following questions:

• What are the majority carrier power semiconductor switches used today?
• What are the applications for these switches?

If you need assistance with power electronics design, call or email us today for help with your requirements. You can also go to our power electronics consultant website for more information about our services for business clients. Thank you for reading this tutorial article entitled “Power Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor”

Next topic: Power Semiconductor Switches, MOSFETs and Schottky Diodes
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Notes:

¹ As used in this article apparent power is a relative figure of merit for power handling comparison. It does not indicate the absolute power through put capability of the device.

² See page 88 of Reference 4.

 

References

[1]    Reid L. Sprite, “Power Semiconductors: The BJT, MOSFET, and IGBT”, December, 2004.

[2]    Ned Mohan, Tore M. Underland, and William P. Robbins, Power Electronics, 2^nd Edition, New York, NY, John Wiley & Sons, 1995.

[3]    Yong “Perry” Li, “Why Consider a BJT over a MOSFET?”, EETimes, 10/25/2010.

[4]    Robert W. Erickson, Dragan Maksimovic, Fundamentals of Power Electronics, 2nd Edition, Norwell, MA, Kluwer Academic Publishers, 2001.