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?

To view a different topic, go to the Power Supply Tutorial table of contents.
Last topic: Power Semiconductor Switches, Classification
Next topic: Power Semiconductor Switches, MOSFETs and Schottky Diodes

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
Back to Power Supply Tutorial table of contents.

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.

The post Tutorial: Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor appeared first on POWER ELECTRONICS CONSULTANT.

This tutorial installment is: Power Semiconductor Switches, Classification. This topic answers the following questions:

• What are the main power semiconductor switch classifications?
• What are the important properties of the main classifications?

To view a different topic, go to the Power Supply Tutorial table of contents.
Last topic: Power Semiconductor Switches, Ideal Switches
Next topic: Power Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor

Power Semiconductor Switch Classification

Power semiconductor devices can be classified in at least two ways:

• by type of charge carrier,
• by active vs. passive function.

Charge Carrier Classification

Power semiconductor devices can have two modes of charge carrier conduction:

• minority carrier, where charge is carried by electrons in a P-type semiconductor, or by holes in an N-type semiconductor, or
• majority carrier, where charge is carried by electrons in an N-type semiconductor, or by holes in a P-type semiconductor.

Understanding what type of charge carrier conduction a device has helps us to understand its most basic properties and suitability for different applications.

Minority Carrier Devices

Minority carrier devices excel for applications involving higher voltage levels. Minority carrier devices used for power semiconductors have a p-n^– junction where the n^– region is very lightly doped or intrinsic. Since, under reverse bias conditions, the n^– region is nearly devoid of charge carriers and therefore has low conductivity, it can sustain a large breakdown voltage.

When the device is forward biased, minority carriers are injected into this same n^– region, resulting in a great increase in conductivity, so that under forward bias conditions, the on-state voltage is relatively low. This change in the conductivity is referred as “conductivity modulation”. Comparably rated majority carrier devices for high voltage applications would actually have a higher on-state voltage due to the product of current and channel resistance.

However, minority carrier devices are slow to turn-on and turn-off due to a large stored minority carrier charge which must be inserted or removed prior to the device changing state. This effect limits the maximum practical switching frequency of these types of power devices to 50-100kHz.

Majority Carrier Devices

Majority carrier devices excel for applications involving lower voltage levels and for higher frequencies. Majority carrier devices have fast switching times because they do not store minority charge. They operate either by electrostatic control of conduction cross-sectional area or by employing a metal semiconductor junction. Neither method involves minority carriers.

Active vs. Passive Devices

Power semiconductor switches can further be classified according to whether they are passive responding devices or actively controlling devices.

Passive Responding Devices

Passive devices change conduction state based upon the external voltage and current conditions. They do not need to be controlled. For example, when the current through a diode attempts to change direction, the diode turns off. When the diode becomes forward biased, it turns on.

Active Controlling Devices

Active controlling devices are typically three terminal devices and change from a blocking state to a conducting state as commanded by a control input signal. Active devices control the operation of a switching power supply.

Next Topic

The next 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 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 | Classification”

Next topic: Power Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor
Back to Power Supply Tutorial table of contents.

References

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

The post Tutorial: Classes of Power Semiconductor Switches appeared first on POWER ELECTRONICS CONSULTANT.

This tutorial installment is: Power Semiconductor Switches, Ideal Switches. This topic answers the following questions:

• What is the basic function of a power semiconductor switch in a switching supply?
• What are the most basic properties of a power semiconductor switch?

To view a different topic, go to the Power Supply Tutorial table of contents.
Last topic: Power Supply Capacitors and Inductors
Next topic: Power Semiconductor Switches, Classification

Power Semiconductor Switch Introduction

All ideal switching supplies continually decompose input power at a given voltage and current into “energy packets”. The small energy packets are then continually reassembled into output power at a new desired voltage and current.

The following image illustrates a 2:1 step down converter where each small block could represent an energy packet:

Observe that the output power is equal to that at the input. This property is characteristic of an ideal switching converter. In practical applications there will be some energy loss due to waste heat.

The power semiconductor switches, perform the decomposition and reassembly process, directing the flow of energy from the input, through the inductor and capacitor energy storage components, to the output.

The power semiconductor switches, depending upon the topology and requirements, are either active transistors, thyristors, or passive diode rectifiers. A topology is an arrangement of switches, inductors, and capacitors chosen to achieve a desired input to output conversion ratio.

Ideal Power Semiconductor Switch Properties

The ideal switch has the following properties:

• infinite breakdown voltage,
• when the switch is off, there is zero current through the switch,
• when switch is on, there is zero voltage across the switch,
• the turn-on and turn-off transition times of ideal switches are zero,
• since the either the voltage or the current is always zero in an ideal switch, the instantaneous dissipation which is the product of instantaneous voltage and instantaneous current is always zero.

Non-Ideal Power Semiconductor Switch Properties

A well chosen practical switch will approximate an ideal on a first order basis. Even so, the non-ideal properties of practical switches must be evaluated for every design.will have parasitic elements which cause:

• finite breakdown voltage,
• leakage current in the off-state,
• non-zero voltage across the switch in the on-state,
• non-zero turn-on and turn-off transition times,
• since there is some voltage across the switch in the on-state, and the transition times are non-zero, there is non-zero dissipation which must be managed.

Next Topic

The next tutorial installment is: Power Semiconductor Switches, Classification. This next topic will answer the following questions:

• What are the main power semiconductor switch classifications?
• What are the important properties of the main classifications?

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 | Ideal Switches”

Next topic: Power Semiconductor Switches, Classification
Back to Power Supply Tutorial table of contents.

The post Tutorial: Power Semiconductor Switches | Ideal Switches appeared first on POWER ELECTRONICS CONSULTANT.