The Gate characteristics of scr refer to the relationship between the gate current and the anode current or voltage. In this article, we will discuss the thyristors gate characteristics,Gate Characteristics of SCR,the gate characteristics of thyristor is a plot of and gate characteristics of a thyristor.
The forward gate characteristics of scr or thyristor are shown in Figure in the form of a graph between gate voltage and gate current.
The gate characteristic of scr or thyristor can be divided into three regions:
- Blocking region: In this region, the thyristor is in the off state and there is no anode current flowing through the device. The gate current required to maintain this state is very small and is typically in the range of microamps.
- Forward region: In this region, the thyristor is conducting and there is a significant anode current flowing through the device. The gate current required to turn on the thyristor in this region is much higher than the gate current required to maintain the off state.
- Saturation region or the forward conducting region: In this region, the thyristor is fully conducting and the anode current has reached its maximum value. The gate current required to maintain this state is very small, but it must be maintained at this level to keep the thyristor conducting.
Here positive gate to cathode voltage Vg and positive gate to cathode current Ig represent dc values.As gate-cathode circuit of a thyristor is a p-n junction, gate characteristics of the device are similar to that of a diode. For a particular type of SCRS, Vg-Ig characteristic has a spread between two curves 1 and 2 as shown in Figure. This spread, or scatter, of gate characteristics is due to difference in the low doping levels of P and N layers. The gate trigger circuitry must be suitably designed to take care of this unavoidable scatter of characteristics. In Figures, curve I represents the lowest voltage values that must be applied to turn-on the SCR Curve 2 gives the highest possible voltage values that can be safely applied to gate circuit.
Each thyristor has maximum limits as Vgm for gate voltage and Igm for gate current. There is also rated (average) gate power dissipation Pgav specified for each SCR. These limits should not be exceeded in order to avoid permanent damage of junction J3. There are also minimum limits for Vg and Ig for reliable turn-on, these are represented by oy and ox respectively in Figure. As stated before, if Vgm Igm and Pgav are exceeded, the thyristor can be destroyed. This shows that preferred gate drive area for an SCR is bcdefghb as shown in Figure.
Where oy,ox = Minimum gate voltage and current to trigger an SCR.
Vgm,Igm = Maximum permissible gate voltage and current.
Oa = Non-triggering gate voltage.
A non-triggering gate voltage is also prescribed by the manufacturers of SCRs. This is indicated by oa in Figure. If firing circuit generates positive gate signal prior to the desired instant of triggering the SCR, it should be ensured that this unwanted signal is less than the non-triggering gate voltage oa. At the same time, all spurious or noise signals should be less than the voltage oa.
In this figure is shown in a trigger circuit feeding power to gate-cathode circuit. for this circuit
Es = Vg + IgRs
Where Es = gate source voltage
Vg = gate cathode voltage
Ig = gate current
Rs = gate source resistance
The internal resistance Rs of trigger source should be such that current (Es/Rs) is not harmful to the source as well as to the gate circuit when SCR is turned on. In case Rs is low, an external resistance in series with Rs must be connected.
A resistance R1 is also connected across gate-cathode terminals, Figure(b), so as to provide an easy path to the flow of leakage current between SCR terminals. If Igmn and Vgmn are the minimum gate current and gate voltage to turn-on SCR, then it is seen from Figure(b) that current through R1 is Vgmn/R1 and the trigger source voltage Es is given by
Es = (Igmn + Vgmn/R1)Rs + Vgmn. ---------eqn(1)
Here OD trigger circuit short circuit current =Es/Rs. Let us consider a thyristor whose Vg-Ig characteristic is given by curve 3. Intersection of load line AD and Vg-Ig curve 3 gives the operating point S. Thus, for this SCR, gate voltage = PS and gate current OP. In order to minimise turn-on time and jitter (unreliable turn-on), the load line and hence the operating point S, which may change from S1 to S2 must be as close to the Pgav curve as possible. At the same time, the operating point S must lie within the limit curves 1 and 2. The gradient of the load line AD (=OA/OD) will give the required gate source resistance Rs.The minimum value of gate source series resistance is obtained by drawing a line AC tangent to Pgav curve.
Gate drive requirements in terms of continuous dc signal can be obtained from show in figure.
%20Pulse%20gating%20and%20(b)%20high-frequency%20carrier%20gating%20of%20SCRS,%20(c)%20Thyristor%20protection%20against%20reverse%20overvoltages.webp "Gate Characteristics of a Thyristor")
However, it is common to use a pulse to trigger a thyristor. For pulse widths beyond 100microsec, the dc data apply. For pulse widths less than 100microsec, magnitudes of gate voltage and gate current can be increased.
As stated before, thyristor is considered to be a charge controlled device. Thus, higher the magnitude of gate current pulse, lesser is the time to inject the required charge for turning-on the thyristor. Therefore, SCR turn-on time can be reduced by using gate current of higher magnitude. It should be ensured that pulse width is sufficient to allow the anode current to exceed the latching current. In practice, gate pulse width is usually taken as equal to, or greater than, SCR turn on time. If T is the pulse width as shown in Fig(a), then
T greater than or equal to ton
With pulse triggering, greater amount of gate power dissipation can be allowed; this should, however, be less than the peak instantaneous gate power dissipation Pgm as specified by the manufacturers. Frequency of firing (or pulse width) for trigger pulses can be obtained by taking pulse of
(i) amplitude Pgm
(ii) pulse width T
(iii) periodicity T1
Therefore,
Pgm/T1 greater than or equal to Pgav
Pgm T f greater than or equal to Pgav
Pgav/fT less than or equal to Pgm. -------eqn(2)
Where f =1/T1 = frequency of firing,or pulse repetition rate, in Hs,
and T = pulse width in sec.
In the limiting case,Pgav/fT = Pgm or f = Pgav/T.Pgm
A duty cycle is defined as the ratio of pulse-on period to periodic time of pulse. In Fig(a), pulse-on period is T and periodic time is T1. Therefore, duty cycle δ is given by
δ = T / T1 = fT
From Eqn(ii),Pgav/ δ less than or equal to Pgm or Pgav/ δ =Pgm
Sometimes the pulses of Fig(a) are modulated to generate a train of pulses as shown in Fig(b). This technique of firing the thyristor is called high-frequency carrier gating.
The advantages offered by this method of firing the SCR are lower rating, reduced dimensions and therefore an overall economical design of the pulse transformer needed for isolating the low power circuit from the main power circuit. For an SCR, Vgm and Igm are specified separately. If both of these are used for pulse firing, then Pgm may be exceeded and the thyristor would be damaged.
There is also prescribed a peak reverse voltage (gate negative with respect to cathode) that can be applied across gate-cathode terminals. Any voltage signal, given by the trigger circuit (or by any interference), exceeding this prescribed limit of about 5 to 20 V may damage the gate circuit. For preventing the occurrence of such hazards, a diode is connected either in series with the gate circuit or across the gate-cathode terminals as shown in Fig(c) Diode across the gate-cathode terminals, called clamping diode, prevents the gate-cathode voltage from becoming more than about 1 V. Diode in series with gate circuit prevents the flow of negative gate source current from becoming more than small reverse leakage current.
The magnitude of gate voltage and gate current for triggering an SCR is inversely proportional to junction temperature. Thus, at very low temperatures, gate voltage and gate current must have high values in order to ensure turn-on. But Pgm should not be exceeded in any case.
The resistor R1 connected across gate-cathode terminals also serve bypass a part of the thermally-generated leakage current across junction J2 when SCR the forward blocking mode; this improves the thermal stability of SCR.
How does a thyristor switch on and off
What is the condition for thyristor to turn on?
Note:-The gate current required to turn on the thyristor is much smaller than the current required to keep the device on. Therefore, once the thyristor is triggered, the gate current can be removed, and the device will continue to conduct current until the anode current drops below the holding current or the device is turned off by reducing the voltage across it.
What are 4 applications of SCR?
Conclusion
The gate characteristics of or scr thyristor describe the relationship between the gate current and the resulting anode current, and the gate current required to trigger the thyristor is typically very small.
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