Basics of Some Solid State Switches

Switches are devices which allows the current in one direction and blocks the current in other. Solid state switches are devices made up of semi conducting elements with doping in them. Power electronics and most of the devices around us made up of these switching elements. They found application in every circuit of power electronics. Basics of some of the most commonly used switches are given below for better understanding of further concepts.

 

Diode:

Diode Symbol (solid state switches)
Fig 1. Diode symbol

A diode is semiconductor device for conduction of current in one direction only. It is basically a switch which conducts in one direction only. It is made of simple p and n regions by doping in semiconductor element. The n region has excess of free electrons and doped with element with one extra electrons in outer orbit. While the p region possess holes in abundant as it is doped with electron deficient element. By combination of both these regions, a junction is formed between them which allow to pass the electrons only when the n region is connected with negative of external power source and p region is connected with positive of source. This configuration is known as forward biasing of diode.

 In which the depletion region become narrow and allows the current to pass through. The opposite configuration is called as reverse biasing. And blocks the flow of electrons due to expansion of depletion region. So, the diode conducts only in forward biasing configuration.

 So it has two regions – forward and blocking. The diodes are categorized on basis of power handling ability, the maximum reverse voltage (which is known as peak inverse voltage-PIV) and switching time.

Two legged thyristor/ PNPN diode:

The next modified form of the simple diode is two wire thyristor or PNPN diode. This is also a switching element with two PN junctions. Or two PN junction diodes are joined back to back. It has three regions of operations

  1. Reverse blocking region
  2. Conduction region
  3. Forward blocking region
two wire thyristor
Figure 2: 2 Wire PNPN

  The first two are the same regions as that of simple diode and their characteristics are same as mentioned above. The third region is that which differentiate it with the simple diode.

   When a PNPN diode is forward biased, it does not start conduction until a specific voltage appear across it. After that voltage it starts conduction and allows current to pass through it. This voltage is called as break over voltage (Vbo).

Once the voltage across the PNPN diode in forward direction reaches it starts conduction and do so until the current flowing through it falls below the certain level. This specific current is called as holding current (ih). Below the holding current it stops conduction and block the forward voltage until again break over voltage reaches.

VI characteristics of PNPN
Figure3: VI Characteristic of PNPN

As form the figure of VI characteristics of PNPN diode it can be seen that it blocks the forward current until VBO (break over voltage) and then starts conduction. And blocks all the reverse voltage Vr.

 As a whole we can conclude that a PNPN diode:
  1. Conducts in forward biasing when applied voltage is greater than break over voltage.
  2. Stop conduction when current falls below holding current
  3. Do not conducts in reverse biasing until reverse voltage exceeds and it stop acting as diode permanently.

Three Wire Thyristor/SCR

SCR Symbol (a solid state switch)
Figure 4: SCR Symbol

    The next modification in PNPN diode is three legged thyristor. It has same basic structure as PNPN diode. It is also named as silicon Controlled Rectifier (SCR). In fact it is PNPN junction with another leg in it. The extra leg here called as gate. As per the name, it is controlled rectifier/diode.

     The three legs are adjusted such that one is anode and the other is cathode as simple diode and PNPN diode. The anode is P and cathode is N at the ends. The third leg is at the 2nd P of two junction. As of following figure:

two transistor model of SCR
Figure 5: Two transistor model of SCR

The break over voltage of this thyristor in forward biasing can be adjusted by adjusting the gate current. The phenomenon of gate working can be simply described as when the positive pulse (current) is given to the p of the 2nd junction, it reduces the depletion region and reduces the break over voltage for conduction. That is, the gate controls the on and off of the diode. When the current is applied to the gate, the break over voltage of the same thyristor reduces and it starts conduction at voltage less than its break over voltage. That is, with increase in gate current, the break over voltage reduces.

I.e. if
Ig1 < ig2 < Ig3 < Ig4

Then                        Vbo1 > Vbo2 > Vbo3 > Vbo4

Graph of VI Characteristics of SCR
Figure 6: V-I characteristics of SCR with different gate current to break over voltage.

If these switches are placed in circuit where the maximum current is less than the break over voltage of that SCR, then only the method to turn it on to let the current pass is the pulse to the gate. In that condition increased current at gate will reduce the break over voltage and it starts conduction. We use the word current pulse in above for gate because of the reason that once it is triggered and current starts flowing through it, it continuous to flow regardless of break over voltage of the SCR until the current falls below the holding current. But once this is triggered, it cannot be turned off by gate. Sometimes it falls in the disadvantage of the SCRs and required separate circuits to switch it off. Also it has forward voltage drop 1.2 to 1.5 greater than ordinary forward biased diodes.

From above it can be concluded that the SCR:

  1. Conducts in forward biasing when applied voltage is greater than break over voltage.
  2. Stop conduction when current falls below holding current
  3. Do not conducts in reverse biasing until reverse voltage exceeds and it stop acting as diode permanently
  4. The break over voltage can be controlled by the gate pulse current.

It has advantages over simple bipolar junction transistor like, it has higher voltage blocking capabilities and has high on state gain. Also only a pulse of gate signal is required for a short time to make it in conduction state.

This controlled rectifier/diode found its wide application in motor drives and many other power electronics devices which require switching at high power and low switching frequencies.

Gate Turn off Thyristor (GTO):

GTO Symbol
Figure 7: GTO Symbol

                The further modification in simple silicon controlled rectifier is the GTO or gate turn off thyristor. It’s the thyristor (solid state switch) with addition to that it can also be turned off through its gate. The turning off procedure by the gate require a large amount of negative current for a small moment. When the GTO finds the negative current at its gate (as pulse) it turns the conduction off no matter the current flowing through it is greater than the holding current. These thyristor are preferred over SCR thyristor as they eliminated the use of external circuit to turn the thyristor (SCR) OFF in DC circuits.

Typically, a GTO requires more gate current pulse to turn it on as compared to ordinary SCR. The negative pulse required to turn it off must be there for 20μs to 30us. And the magnitude of the negative pulse current must be 1/4th or 1/6th or the current passing through it during forward biasing.

DIAC:

Symbol of DIAC
Figure 8:DIAC Symbol

    Unlike the previously discussed thyristors the DIAC is five layered switching device. It has five layers such that PNPNP which is same as two PNPN diodes are connected back to back.It has ability to conduct in both direction. There is no reverse blocking region in DIAC and it allows the current to pass through it in both direction when the break over voltage exceeded. That is, no matter the polarity of the voltage applied to it. When the break over voltage exceeded, it start conduction. Once the break over voltage exceeded and current started flowing, it continuous until the current passing falls below the holding current. As in the figure 9 it can be seen that once break over voltage (VbO) exceeded, it conducts in either direction.

 It founds its application in AC circuits where the controlled current has to pass into the circuit after delay. Like phase shift circuits. Mostly it is used in triggering circuit of TRIAC in fan dimmer circuits of phase shift of AC.

Five Layer pn junctions
Figure 10: Five layer

Graph of VI Characteristics of DIAC

TRIAC

TRIAC Symbol
Figure 11: TRIAC Symbol

    It is same as the DIAC with the third leg of gate in it. Or it is two SCRs connected in back to back direction having a common gate. It starts conducting the current when the voltage across it goes greater than the break over voltage of it in either direction just like the DIAC. But it has a gate and the current pulse to the gate reduces the break over voltage to start conduction the current just like the SCR but the difference from the SCR is that it can conduct in both directions where the voltage is greater the break over voltage.

    It is used in many switching devise to control AC power like fan dimmer etc. but it has disadvantage that it cannot handle very high power and the switching is very low so it founds its application in devices operates at domestic AC frequency.

VI Characteristics of TRIAC
Figure 12: VI characteristic of TRIAC

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Asad Ullah

I am MSc Electrical Scholar under a fellowship program. Working with Modeling and Simulation software related to my field are my activities in leisure.

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