Power Electronic Devices Meaning and Classification Examples

Power Electronic Devices (PED), also known as power semiconductor devices, are mainly used for high-power electronic devices in power conversion and control circuits of power equipment (usually refers to currents ranging from tens to thousands of amperes and voltages of several above 100 volts). They are used in almost all electronic manufacturing industries, including notebooks, PCs, servers, monitors and various peripherals in the computer field; mobile phones, telephones and other terminal and central office equipment in the communications field; home appliances and various other devices in the consumer electronics field; various instruments and control equipment in the industrial control category. In short, their applications are everywhere in modern life.

Classifications

1. According to the degree to which power electronic devices can be controlled by control circuit signals:

1) For semi-controlled devices, the control signal can only control its on, but not off. The shutdown of the device depends entirely on the voltage and current it withstands in the main circuit, like thyristor.

2) A fully controlled device, the control signal can control its on and off, also known as a self-shutoff device, like Insulated gate bipolar transistor (IGBT) and power field effect transistor (Power MOSFET).

3) Uncontrollable device, a representative device is diode.

2. According to the nature of the signal added by the driving circuit between the control terminal and the common terminal of the power electronic device:

1) Current driven type: Turning on or off by injecting or extracting current from the control terminal.

2) Voltage driven type: Turning on or off is achieved by applying a certain voltage signal to the control terminal and common terminal, also known as field control device or field effect device.

3. According to the waveform of the effective signal added by the driving circuit between the PED control terminal and the common terminal:

1) Pulse trigger type: Once it enters the on/off state and the main circuit conditions remain unchanged, the device can continue to maintain the original state without continuing to apply the control terminal signal.

2) Level control type: Apply voltage/current to the PED control terminal and common terminal to maintain its state.

4. According to the two types of carriers, electrons and holes inside the device, participating in conduction:

1) Unipolar device: A type of carrier participates in conduction, also called a multi-carrier device.

2) Bipolar device: Two carriers, electrons and holes in conduction, also called minority carrier devices.

3) Composite device: A device that is a mixture of unipolar devices and bipolar devices, also called hybrid devices.

Diode

PN junction

1. Add forward voltage:

1) The left is positive and the right is negative, which promotes multi-sub diffusion, forms a diffusion current, and narrows the space charge region.

2) The external circuit forms a current flowing in from P and flowing out from N, which is called forward current IF.

3) As the external voltage increases, the self-built electric field is further weakened and the diffusion current increases.

4) The PN junction is turned on.

2. Add reverse voltage:

1) The left is negative and the right is positive, which promotes minority carrier drift, forms a drift current, and widens the space charge region.

2) The external circuit forms a current flowing in from N and flowing out from P, which is called reverse current IR.

3) When the minority carrier concentration is low and the temperature is constant, the drift current tends to be stable, which is called the reverse saturation current IS, generally in the order of microamps.

4) The reverse-biased PN junction presents a high-resistance state with almost no current, which is called the reverse cut-off state.

* Main Parameters

1. Forward average current IF(AV)

The average value of the maximum power frequency sinusoidal half-wave current allowed to flow through the power diode under the specified case temperature Tc and heat dissipation conditions when the power diode is operated for a long time. The maximum effective current the diode allows to flow is 1.57IF(AV):

Average value:

Effective value:

And, average value : effective value =1:1.57

On the contrary, if it is known that the effective value of a certain waveform current that a diode needs to flow in the circuit is ID, then at least a power diode with a rated current (i.e. forward average current, on-state average current) of ID/1.57 must be selected. Of course there is some margin to consider.

Note: Definition of effective values

Let an alternating current and a direct current pass through resistors with the same resistance respectively, if the heat generated of them in the same time is equal, then the value of the direct current is called the effective value of the AC current.

2. Forward voltage drop UF

The corresponding forward voltage drop when a diode flows through a specified steady-state forward current at a specified temperature.

3. Reverse repetitive peak voltage URRM.

Refers to the highest reverse peak voltage that can be repeatedly applied to a power diode. When selecting, select this parameter based on twice the maximum reverse peak voltage that the diode in the circuit can withstand.

4. Maximum operating junction temperature TJM

The highest average temperature that the PN junction can withstand without being damaged.

Thyristor

1. Internal Structure

1) A is anode, K is cathode, and G is the gate level (control end).

2) The internal structure has four layers and three junctions. The forward voltage (A>K) is applied to both sides, the J2 junction is reverse biased, and AK is blocked; the reverse voltage (A<K) is applied to both sides, J1 and J3 are reverse biased, and AK is still blocked, only a small leakage current can flow.

2. Working principle (dual transistor model)

1) Take a cross-section of the device and regard it as a combination of two transistors V1 and V2.

2) First add a positive voltage, and inject the driving current IG into the gate level from the outside, and then flow into the base of V2.

3) According to transistors, the collector and base currents have a multiple relationship, and then the collector current Ic2 is generated, which also constitutes the base current of the V1 tube.

4) The base current of V1 is amplified into the collector current Ic1, which further increases the base current of V2, forming a strong positive feedback.

5) Finally, both tubes enter a fully saturated state, that is, the thyristor is turned on.

6) At this time, remove the current IG injected into the gate from the external circuit. Since a strong positive feedback has been formed inside the tube, it will continue to remain on. To turn it off, the forward voltage applied to the anode must be removed, or a reverse voltage must be applied, or the current flowing through the thyristor must be reduced below a value close to 0. Then it can be seen that the driving process is more commonly known as triggering, and the circuit that generates the trigger current IG is called a gate-level trigger circuit, which is a semi-controlled device.

3. Basic features

1) Static characteristics

A. When the thyristor is subjected to a reverse voltage, it will not conduct regardless of whether there is a trigger current at the gate level.

B. The thyristor is turned on only when it withstands forward voltage and triggers current at the gate level.

C. Once the tube is turned on, the gate level loses control. Whether there is trigger current or not, the tube will remain on.

D. To turn off the tube, the current flowing through the thyristor can only be reduced to a value close to 0 by using the effect of external voltage and external circuit.

Volt-ampere characteristics curve of thyristor:

It can be obtained from the picture:

A. The first quadrant is the positive characteristic, and the third quadrant is the reverse characteristic.

B. The gate-level trigger current IG is 0, if a forward voltage is applied to the tube, it is in a forward blocking state and only a small leakage current passes; when the forward voltage exceeds the critical limit (the forward turning voltage Ubo), the leakage current increases sharply and the tube is on.

C. As the gate level trigger current increases, the forward transition voltage will decrease.

D. The characteristics of the thyristor after conduction are similar to the forward characteristics of the diode.

E. When turned on, if the gate current is 0 and the anode current drops below IH, the thyristor returns to the forward blocking state. And IH is the holding current that close to 0.

F. Apply a reverse voltage to the transistor, and its volt-ampere characteristics are similar to the reverse characteristics of the diode.

2) Dynamic characteristics

Thyristors have switching losses.

4. Main parameters

The main parameters of thyristor include voltage rating, current rating and dynamic parameters.

1) Voltage rating

Off-state repetitive peak voltage UDRM: The forward peak voltage that is allowed to be repeatedly applied to the device when the gate level is open and the junction temperature is at the rated value. It is specified that UDRM is 90% of the off-state non-repetitive peak voltage (that is, the maximum instantaneous off-state voltage) UDSM. And it should also be lower than the forward transition voltage Ubo, and the margin is determined by the manufacturer.

2) Current rating

On-state average current IT (AV): The average value of the maximum power frequency sinusoidal half-wave current allowed when the stable junction temperature does not exceed the rated junction temperature. This is also the parameter nominally rated current. It is similar to the corresponding definition of a power diode, and is defined according to the heating effect of the on-state loss of the device itself caused by the forward current.

Holding current IH: The minimum current necessary to maintain conduction of the thyristor. The higher the junction temperature, the smaller the IH. That is, it keeps falling from the on-state, and the current drops below IH, and the tube is turned off.

Holding current IL: The minimum current required to maintain conduction after the thyristor has just transferred from the off-state to the on-state and removed the trigger signal. That is, it keeps rising from the off state, and the current rises above IL, and the tube is turned on. For the same tube, IL is generally 2 to 4 times that of IH.

Inrush current ITSM: The non-repetitive maximum forward overload current caused by abnormal circuit conditions that causes the junction temperature to exceed the rated junction temperature. The surge current has two levels: upper and lower. This parameter can be used as the basis for designing the protection circuit.

3) Dynamic parameters:

A. Turn-on time: tgt, turn-off time: tq

B. Off-state voltage critical rise rate du/dt: The maximum rise rate of the applied voltage that does not cause the thyristor to change from the off-state to the on-state under the conditions of rated junction temperature and gate-level open circuit. If a forward voltage is applied to the tube in the blocking state, and it becomes larger and larger, that is, it has a positive rising rate, then the J2 junction in the blocking state will be equivalent to a capacitor, and a charging current will flow:

This current is called a displacement current, when it passes through the J3 junction, it will play a role similar to the gate-level trigger current. If the voltage rise rate is too large and the charging current is large enough, the thyristor will be misdirected. Therefore, the actual voltage rise rate during use must be lower than this critical value.

C. Critical on-state current rise rate di/dt: Under specified conditions, the maximum on-state current rise rate that the thyristor can withstand without harmful effects. If the current rises too fast, as soon as the thyristor is turned on, a large current will be concentrated in a small area near the gate level, causing local overheating and damaging the tube.

Fully Controlled Devices

1. GTO gate-level turn-off thyristor

It is a derivative device of thyristor and a current-driven device. The multi-dimensional integrated structure of GTO is beneficial to turn-off, is faster to turn on than ordinary thyristors.

2. GTR power transistor

It is a bipolar junction transistor BJT that can withstand high voltage and high current, sometimes also called power BJT. Within the scope of power electronics technology, GTR and BJT are equivalent.

3. Power field effect transistor/power MOSFET/MOS tube

Just like low-power field effect transistors (FETs) used in information processing applications are divided into junction type and insulated gate type, power field effect transistors also have these two types, but they usually mainly refer to MOSFETs in the insulated gate type.

Main features:

1) MOSFETs use gate voltage to control drain current and are voltage-driven devices.

2) The driving circuit is simple and requires little driving power.

3) Fast switching speed and high working frequency.

4) Thermal stability is better than GTR.

5) Small current capacity and low withstand voltage value, mostly used in power electronic devices with power not exceeding 10kW.

5. IGBT (Insulated-gate Bipolar Transistors)

IGBT combines the advantages of GTR and MOSFET.

Conclusion

1) Unipolar and composite devices are both voltage-driven devices, and bipolar devices are current-driven devices.

2) Characteristics of voltage-driven devices: High input impedance, small driving power required, simple driving circuit, and high operating frequency.

3) Characteristics of current-driven devices: It has conductance modulation effect, reduced on-state voltage, and small conduction loss, but its operating frequency is low, requires large driving power, and the driving circuit is relatively complex.

4) Voltage-driven devices are all level-controlled devices, while current-driven devices are either level-controlled (GTR) or pulse-triggered (thyristor, GTO).

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