SCR | Tyristor Selection Tips

 
There are two categories of SCR, the fast turn-off inverter grade SCR and relatively slow turn-off converter grade SCR. Some of the properties of these are interdependent and designers have aimed to combine them in most suitable manner. The application therefore decides what category is to be chosen.
 
Once the category is chosen in each category a choice of SCR is available depending upon:

  • voltage rating
  • current rating
  • Type of package.

 

Voltage Rating Selection of SCR

 
SCR are rated for maximum repetitive forward and reverse voltage ratings. SCR may be operated upto these ratings. Designers however choose a safety factor and a safety factor 2 to 2.5 is used for transient voltages. Generally, some form of overvoltage suppression is used but complete protection is not cost effective.
 
Excessive dv/dt causes misfiring of SCR. Such transients may be suppressed by CR snubber circuits across SCR. Snubber circuit cost and complexity must be considered in selecting proper dv/dt rating.
 

Current Rating Selection of SCR

 
Selection of current rating decides the size of the SCR. Current rating chosen may be the normal operating current level or maximum expected overload current. In data books SCR are categorized according to their current ratings. Unlike voltage rating, the current rating for a chosen SCR is rarely utilized.
 
Indirectly SCR current rating may be chosen by selecting the SCR by its junction to heatsink thermal impedance. SCR thermal impedance is given by:
 
SCR thermal impedance, Rth(j – h) = (T/P)oC/watt
 where P = power dessipated in watts.
T = Temperature rise in oC.
 
Power dissipation in the SCR is caused by several factors such as conduction current and on-state voltage, leakage current under blocking voltage conditions, gate current and voltage, and switching losses.
 
SCR thermal impedance is an important characteristic. The larger the SCR smaller is its thermal impedance. Efficiency of the cooling system allows more power to be dissipated in a given SCR. In this case the cooling system cost may dictate use of larger SCR to reduce the cost of cooling system. However, efficient cooling system will reduce the size of the equipment.
 

Surge Current Rating Selection of SCR

 
In many applications a SCR may have withstand short circuit conditions due to short circuit until the circuit is interrupted by fuse or the circuit breaker. Surge current rating appropriate for the application has to be selected as the SCR may be subjected to such conditions several time in its lifetime.
 
dI/dt:  A fast rise high gate pulse allows higher dI/dt. Inverter grade SCR usually have high dI/dt ratings however in the class available SCR may have various magnitudes of dl/dt. Choosing a SCR with high dI/dt rating reduces the cost of required inductance for limiting the value of dI/dt to which the SCR is subjected during its operation.
 

Package Selection of SCR

 
At present SCR are available five distinct packages such as

  • Discrete plastic
  • Plastic module
  • Stud base
  • Flat base and
  • Press pack packages.

 
Discrete plastic package with planar type construction is the cheapest device available upto 25 A and 1000 V applications. Besides being cheap it is easily mounted in circuits with large number of other components.

SCR Selection Tips

Plastic modules have same advantages as in discrete plastic package except that it contains more than one device and is available in current ranges upto 100 A. These modules are attractive to user because they can be easily mounted into the circuit by bolting to the heatsink, without any insulation.
 
The stud base package is a good compromise between low thermal resistance and ease of mounting. This construction is used for current ranges between 5 to 150 A and full range of voltage. The disadvantage is that it is not easy to isolate this device from the heatsink.
 
Flat base devices have the same advantage as that of stud base plus advantage of isolating it from the heatsink by a thin layer of insulation. These devices are available from 10 to 400 A.
 
Press pack devices are used for very high currents exceeding 200 A and voltages exceeding 1200 V. The electrodes and SCR are contained in a ceramic envelope to provide the necessary isolation between anode and cathode. Clamping pressure is decided for safety of SCR assembly and minimum thermal and electrical contact resistance. As both the surfaces are clamped to heatsinks, minimum thermal resistance is achieved.
 

SCR | Thyristor | Ratings | Specifications | Parameters

 
There is a large number of ratings associated with thyristors. These ratings are in terms of voltage, current, power, temperature and derivatives of voltage and current and time to switch on and off. In practice, it may not be necessary to give consideration to each and every rating to make choice of a thyristor.
 
A thyristor can be chosen on the basis of manufacturer’s recommendations depending on maximum voltage and current rating and grade (converter or inverter).
 
The complexity of ratings is due to thermal characteristics of silicon and behavior of junctions. Normal symbols are used for currents and voltages followed by subscripts, which denote direction or state as:
 
D = off state
R = reverse
P = forward
T = on-state
 
Except the gate, second letter defines whether the rating is working, repetitive, or non-repetitive value, where:
 
W = working value,
R  = repetitive and
S = Non-repetitive.
 
The third letter is M indicate peak value when required.
 

Anode Voltage Rating of Thyristor | SCR

 
A.C. mains voltage is not smooth voltage and some transient component occurs regularly and others occurs only sometimes. Figure indicates such components. It is difficult to observe all such transient components on ordinary C.R.O. if high intensities are not used for the beam.
 
VRWM  – The crest working reverse voltage, neglecting transients. In practice it is the peak negative value of sine wave supply voltage.

SCR thyristor ratings specifications parameters

 
VTM: Maximum on-state or forward voltage. It is usually defined in terms of SCR rated average of RMS forward current, VTM is the peak or maximum on-state anode-cathode voltage. Some manufacturers refer to this specification as VF or VFM. For most SCRS, VTM is in the order of 1.6 V.
 
VDRM: Maximum forward blocking voltage. It is the peak repetitive off-state anode-cathode voltage. Beyond this value, the SCR will break down in forward conduction. This is referred to as the breakover voltage with no input gate current.
 
VRRM: Maximum reverse blocking voltage. The peak repetitive off-state reverse anode-cathode voltage. Beyond this value, the SCR will break down into reverse conduction.
 
VRSM – Non-repetitive peak reverse voltage. It is the peak value of surge voltage that lasts up to 10 ms but does not repeat.
 
VDWM – Crest working on state voltage, applied in the forward direction.
 
VDSM – Non-repetitive peak off state voltage applied in the forward direction.
 
Repetitive and non-repetitive values are determined by voltage limit and by instantaneous energy that can be dissipated within the device. Exceeding these voltage ratings may not damage the device but exceeding VRSM can damage the device.
 
V1 – Continuous off-state voltage. It is the voltage between anode and cathode with specified forward current and junction temperature.
 
dv/dt – The dv/dt characteristic of a thyristor is the maximum rate of rise of the anode voltage which will not trigger the thyristor. It is dependent on junction temperature. It is called Critical Rate of rise of off-state voltage.
 
Reapplied dv/dt – Rate of rise of forward voltage following turn-off commutation.
 

Current Ratings of Thyristor | SCR

 
ITAV – Average on state current. The limit to current rating is set by the junction temperature. Average current limit is however set by heat sink temperature.
 
IRMS – RMS value of on-state current. This is important at small conduction angles.
 
ITRM – Repetitive peak on-state current that can be drawn through the device when rms and mean current ratings are not exceeded.
 
ITSM – Non-repetitive peak on-state current. This value is chosen for selection of fuses.
 
I2t – This is the measure of maximum forward non-recurring over-current capability for very short pulse durations. The value is valid for specified pulse duration. I is in RMS amps and t is pulse duration in seconds. (This is used for fuse co-ordination).
 
di/dt – The di/dt ratings of a thyristor indicates maximum rate of rise of on-state current. Maximum current of a thyristor assumes steady state condition, when all the area of the device is carrying current. However, when a thyristor is first triggered ‘on’ condition starts at one or more places near the gate. Small area of conduction then spreads from these points to the whole crystal. Higher value of di/dt may cause ‘hot spots’ in junction and subsequent failure. It must be limited by additional inductance in series with thyristor.
 
IDRM: Peak forward blocking current. It is the maximum forward leakage current resulting from specified forward blocking voltage (VDRM) and operating temperature. In some cases, the maximum allowable case or junction temperature is specified. A typical value for IDRM ranges from less than 1 mA for low-power SCRs to about 100 mA for high-power SCRs.
 
IRRM: Peak reverse blocking current. It is the maximum reverse leakage current resulting from specified reverse blocking voltage (VRRM) and operating temperature. In some cases, the maximum allowable case or junction temperature is specified. A typical value for IRRM ranges from less than 1 mA for low-power SCRs to about 100 mA for high-power SCRs.
 
IL – Latching current. It is the minimum on-state current needed to keep the device in the on-state after the triggering pulse has been removed.
 
IH: Holding current. It is the minimum load current required to maintain the SCR in on-state. Typically values of holding current ranges from about 6mA for low power SCRs to 80mA for high power (65 A) SCRs.
 
It is the current below which a thyristor fails to conduct when the anode current is smoothly fails to conduct when the anode current is smoothly decreased. The holding current occurs when the current in a device in on-state is being decreased till it turns off. (Latching current occurs when the device is in off state and is being turned on).
 

Temperature Ratings of Thyristor | SCR

 
Ti – Junction temperature (Normally not possible to measure).
Tmb – Mounting temperature.
Tamb – Ambient temperature.
Zth – Transient thermal resistance.
 

Gate Ratings of Thyristor | SCR

 
PGAV –  Average gate power dissipation.
PGM – Peak gate power dissipation.
 
IGT: The minimum forward gate current to turn on the SCR. Since IGT varies with the operating temperature, load resistance and forward blocking voltage, many SCR manufacturers specify IGT in terms of these operating parameters. Low-power SCRs require an IGT of about 100 to 300 µA for turn on; the IGT for medium and high power SCRs normally ranges from about 5 to 150 mA.
 
IRG – Reverse gate current.
IFGM  –  Forward peak gate current.
VFGM –  Forward peak gate to cathode voltage.
VGD – Maximum continuous gate to cathode voltage not to trigger the device.
VRGM – Gate to cathode voltage to trigger the device.
 
VGT: The minimum positive gate voltage required to turn on the SCR. VGT varies with the operating temperature, load resistance, forward blocking voltage, and gate-cathode resistance. For temperatures of about 25oC, SCR gate trigger voltage is typically 0.7 to 0.8 V. For higher temperatures of about 100 to 125oC , VGT drops to approximately 0.2 V.
 
VRGM – Gate to cathode peak reverse voltage.
 
tq –  Turn-off time. The time interval between the instant when the principal current has decreased to zero after external switching of the principal voltage circuit and the instant when the thyristor is capable of supporting a specified anode to cathode voltage without turning on.
 
ton – Turn-on time. The time interval between a specified point at the beginning of the gate pulse (say 50% of maximum) and the instant when the principal voltage (current) has dropped (risen) to a specified low (high) value during switching of a thyristor from off-state to the on-state by a gate pulse.
 
td – Delay time. The time interval between a specified point at the beginning of a gate pulse and the instant when the principal voltage (current) has dropped (risen) to a specified value near its initial value during switching of a thyristor from the off-state to the on-state by a gate pulse.
 
tr – Rise time. The time interval between the instants at which the principal voltage (current) has dropped (risen) to a specified low (high) value during switching of a thyristor from the off-state to the on-state by a gate pulse.
 
Fast switching thyristors have very low values of turn-on and turn-off times. This is achieved by the presence of gold in silicon or by ‘gold killing’. Presence of gold in thyristors reduces minority carrier life-time but increases leakage current. For faster devices leakage current is more and hence relatively lower values or voltage VRWM and VDWM. A special variety of thyristors (pulse modulator thyristors) have been developed by optimizing for rapid turn-on. This is achieved by minimizing n and p bases and lowering the doping levels. Such thyristors operate at few thousand pulses per second.
 

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