The power diode is an uncontrollable switch capable of conducting very high currents and withstanding very high voltage levels - thousands amperes and thousands of volts. It operates in the same way as a small power diode, and has similar characteristics.
A typical electrical diode circuit is shown in the figure. The current level in a circuit depends on both a supply voltage and a load. The diode only determines the direction of a current flow.
The bipolar power diode is based on a PN semiconductor junction, as shown in the figure. Hence, it is a minority carrier device. The simplest structure of a power diode includes an additional n- layer. It increases the reverse voltage ratings.
Diode Reverse Characteristics
A real diode conducts very little current when it is reverse-biased. The maximum reverse current is IRRM (Reverse Repetitive Maximum current). It determines the rated value URRM of the reverse voltage. For a bipolar diode, this voltage is typically 10÷12 hundreds of volts, but there are diodes with higher voltages.
Diode Forward Characteristics
When a positive voltage is applied, the diode is turned on. A forward current, IF, flows through the circuit. The forward voltage drop UF is typically (1 ÷ 1,5) V. The knee (threshold) voltage UFO and the forward resistance rF are given in the datasheets. For safe operation, IF should not exceed the rated average current, IF(AV)M, given in the datasheets.
Switching Characteristics - Switching ON
Switching characteristics show the behavior of a power diode during its transition from an off-state to an on-state and vice-versa. In an ideal circuit, an ideal diode switches on and off without delay.
Switching Characteristics - Switching OFF
To switch off the diode, a negative voltage is applied. This is called reverse biasing. During the process, the current decreases, and for a while it even flows in a negative direction. It reaches a Reverse maximum level IRmax and returns to zero. The diode stops conducting in a reverse direction. The duration of the time that the diode is conducting in reverse is called the reverse recovery time - trr. The lower trr is, the higher the frequency application diode.
A reverse voltage spike which appears across the diode could be very high and could damage the diode. If this occurs, the improper reverse current flows through the circuit.
The time, trr, is a very important diode parameter. The illustration shows its influence on the electrical circuit. At low frequencies, the diode functions perfectly. Increasing the frequency reveals a small reverse recovery period - the real electronic circuit operation. At very high frequencies, the time, trr, approximately equals the duration of the negative half-cycle. The rectifying effect disappears. The diode is unsuitable for this high frequency. Schottky Diodes have a low trr. Depending on the trr value, the power bipolar diodes are divided into Rectifier Diodes (very high trr) and fast recovery diodes (low trr).
The thyristor is a power electronic device. It acts as a controllable switch, that is either open (off-state of the thyristor) or closed (on-state of the thyristor). It has three terminals: anode (A), cathode (K), and gate (G), as shown in the figure.
Typical Thyristor Circuit
Thyristors are used for power conversion or power control. A typical electrical circuit for such applications consists of a Power Supply (voltage source), Load, Thyristors, and Gate Control Circuit. The voltage supplied could be AC or DC.
A thyristor is a device with a four-layer, semiconductor structure: p1, n1, p2, and n2. It has three PN junctions: J1, J2, and J3. J3 is also known as the gate junction.
The thyristor can conduct very high currents - typically hundreds of amperes. Hence, the device's semiconductor structure has to dissipate high power levels - hundreds of watts. For this reason, thyristors are manufactured in many different packages, capable of dissipating these high power losses. Some of these are shown above.
Mode of Operation - OFF-State
When a positive voltage is applied to the anode with respect to the cathode of a junction, J2, it is reverse-biased. No current flows from the anode to the cathode. The device is in the forward blocking state (forward off-state).
The four-layer structure can be visualized as the interconnection of two three-layer devices. A two-transistor SCR model is illustrated in the figure. According to the model, if the base current of Q2 increases, the collector current of Q2 increases. Hence, the base current and the collector current of Q1 also increase. The two transistors drive each other harder and harder. They get locked into deep saturation in a few microseconds. This process is called theregenerative process.
Triggering the Thyristor
The usual, and strongly recommended, way to trigger a thyristor to the on-state is by using a gate pulse. A small gate current from an external circuit initializes the base current of Q2, and starts the regenerative process. In a few microseconds, the thyristor goes to its switched-on state and remains in this state even after the gate current is annulled.
Thyristor - Device Check
A thyristor can be tested to see if it's in good working order with an ohmmeter. A normal, trouble-free device shows high resistance (several hundred kiloOhms) in both connections ("plus" from the ohmmeter to the anode, and "minus" to the cathode). A damaged thyristor is somewhat shorted-circuited in both directions.
Ideal Thyristor I-V Characteristics
An idealized thyristor I-V characteristic has three important regions: reverse blocking, forward blocking, and forward on-state.
Real Thyristor I-V Characteristic - Off-State
In reverse-biasing, the real thyristor has the same characteristics as a power diode. The maximum reverse current which the device can safely sustain is IRRM.
Let's suppose that a thyristor is in its off-state, forward biased, and there is a gate current present. In this case, the thyristor switches on at voltages lower than the rated voltage, UDRM. This mode of operation is undesirable! A properly designed gate control circuit should not supply positive current when the thyristor is in its off-state.
Real Thyristor I-V Characteristics - On-State
To switch on a thyristor, two basic conditions should be met:
The forward voltage drop (on-state voltage) on the thyristor UT is between 1 and 3V.
The current through the device must not exceed the rated values specified in the data sheets.
There are two important, but not so apparent, parameters: holding current, IH, and leading current, IL. They can be explained by focusing on the very beginning of the on-state.
The current, IL, shows the lowest required current through the thyristor during its switching on. This is the third requirement to turn on the device.
The relationship between the gate-cathode voltage (UG) and the gate current (IG) is known as the gate characteristic. The gate characteristic is temperature dependant. For all devices of a given thyristor type, there exist a family of characteristics. This range of possible gate junction operating points has its upper and lower limits. The upper limits are the maximum gate current and gate power dissipation. The lower limits are the minimum gate current and the voltage under which the device cannot be turned on under the worst conditions.
The output stage of the gate control circuit is called the gate pulse amplifier (driver). The pulse amplifier has two purposes: (1) to produce a short (ten microseconds) pulse with optimal shape, and (2) to provide an electrical separation between the power thyristor and the control circuit. At the beginning of the pulse, the gate current (IG) is higher in order to ensure the fast switching of the thyristor. Next, the current is reduced to minimize power dissipation, PG.
Dynamic Characteristics - Turn On
Until now, we have assumed that the thyristor turns on immediately. The actual device needs a finite time in order to switch from a forward off-state to an on-state.
At the beginning of the switching on process, only a small slice (closest to the gate terminal) conducts the current. If, at this moment, the thyristor is forced to conduct the rated current, an excessively high current density occurs in this small device slice. It can overheat the structure and can destroy the thyristor. The spreading of the conductivity must be ahead of the rate of the actual rise in thyristor current. If so, the current will be distributed uniformly in the whole semiconductor structure. The critical rate of rise of the anode current is provided in the datasheets as parameter di/dt. For example, 100 A/µs, 500 A/µs, 1000 A/µs.
Dynamic Characteristics - Turn Off
During the switching off of a real thyristor, a negative current flows through it. This current decreases to zero after a time interval called the reverse recovery time (trr). If, at this moment, a positive voltage bias is applied, the device will turn on without a gate pulse. To avoid this undesirable effect, the negative bias should be held for a time known as the turn-off time (tq). According to this parameter, the devices are divided in two groups: rectifier thyristors (tq over 100 µs) and inverter thyristors (tq = 8 ÷ 40 µs).
When the thyristor is switched-off, applying positive anode-cathode voltage causes an additional current to enter the gate junction. This current is proportional to the voltage rising rate and the space capacitance, CJ2. If this rate is too high, then the induced current is high enough to turn on the thyristor without a gate pulse. This usually provokes a short circuit in the power conversion system, or another kind of improper mode of operation. Therefore, the voltage rising rate should not exceed certain critical values, called the parameter du/dt, as provided in the datasheets.
In summary, there are two ways to switch-off a thyristor. The first is based on reducing the thyristor current ,iT, under the holding current, IH. This has few practical uses. The second, requires applying negative voltage across the device. The time interval during which the negative voltage is applied is called the schematic off-time, tOFF. It should be longer than the thyristor off-time, tq.
When the thyristor is used in DC circuits, a method of forced commutation is used. A specially designed auxiliary circuit turns off the conducting thyristor. It is assumed that the capacitor is initially charged to the voltage, UC0.
Many practical applications require the commutation of AC line voltage. Since a thyristor is a unidirectional device, it is possible to use a pair of thyristors connected in reverse parallel. A device with the same behavior and characteristics is called a triac. It is a bi-directional device which operates as an AC circuit breaker. It controls the power supplied to the load by switching the connection to the AC source off and on.
The triac semiconductor structure originates from the structure of two thyristors in reverse parallel. The triac terminals are called the Main Terminal 1 (MT1) and the Main Terminal 2 (MT2).
The triac I-V characteristic is symmetrical as shown above.
The triac can be used in power controlling not only as an AC breaker (to switch on and off the load to the AC source), but also to realize a phase shift control (switching on the device at a controlled moment during the half-cycle of the supply voltage). Due to this phase control, the part of the sinusoidal voltage that is applied to the load, changes. Hence, it varies the rms value of the output voltage.
The diac is a device similar to the triac but without a control gate. The diac can be switched on only by increasing the anode voltage. The switching voltage is, typically, several tens of volts.
The GTO (Gate Turn-Off Thyristor) is one of the newest devices in the thyristor family. The GTO operates as a thyristor. The main difference between them is the manner of switching off the GTO. To turn off the device, it's enough just to apply a negative gate pulse. There is no need for a negative anode voltage, such as the one required by a thyristor. Therefore, it simplifies the power electronic circuits.
Power Devices - Cooling
High currents in all power devices produce very high internal power losses, PLOSS, which must be dissipated from the device. Transferred into heat, PLOSS increases the semiconductor's temperature and may overheat the device.
The rate of heat transfer between two bodies with a temperature difference of ΔΘ depends on the thermal resistance, RΘ. The resistance, RΘ is defined as RΘ = ΔΘ / PLOSS.
Play and Learn: Capacitor Influence on toff Time
Connect the power supply to its appropriate polarity. Activate the gate control circuit. Observe the waveforms on the scope screen. Change the capacitor value by clicking on it. Observe the differences in the shape of the voltages across the thyristor on the oscilloscope display. Explain the influence of the capacitor value on the schematic toff time.
Play & Learn: toff Parameter
Observe the different voltage shapes in the circuit by clicking on the red blinking symbol. Each symbol indicates the point where the oscilloscope will be connected. Explain how the toff parameters will change if several green and red lamps are connected in parallel to the shown lamps.