Light Activated SCR

Photothyristors are light-activated Thyristors. 
Two common photothyristors include the Light Activated Silicon Controlled Rectifier (LASCR) and the light-activated triac.

• A Light Activated SCR (LASCR) acts like a switch that changes states whenever it is exposed to a pulse of light. Even when the light is removed, the LASCR remains ON until the anode and cathode polarities are reversed or the power is removed.
• A light-active TRIAC is similar to a LASCR but is designed to handle AC currents.

LASCR:

• The LASCR is also known as Light Triggered Thyristor(LTT).
• It may be triggered with a light source or with a gate signal. Sometimes a combination of both light source and gate signal is used to trigger an SCR.
• In this, the gate is biased with voltage or current slightly less than that required to turn it on, now a beam of light directed at the inner p-layer junction turns on the SCR.
• The light intensity required to turn-on the SCR depends upon the voltage bias given to the gate. Higher the voltage(current) bias, lower the light intensity required.
• These devices are available up to the rating of 6kV and 3.5ka, with on-state voltage drop of about 2V and with light-triggering the requirements of 5mW.
• The symbol for a LASCR is shown below.
  

Basic Operation:

• When no light is present, the LASCR is OFF; No current will flow through the load.
• However, when the Light Activated SCR (LASCR) is illuminated, it turns ON, allowing current to flow through the load.
• The resistor in this circuit is used to set the triggering level of the LASCR.

How LASCRs Work
The equivalent circuit shown here helps explain how a LASCR works.

• When photons from a light source collide with electrons within the p-type semiconductor, they gain enough energy to jump across the pn-junction energy barrier—provided the photons are of the right frequency/energy.
• When a number of photons liberate a number of electrons across the junction, a large enough current at the base is generated to turn the transistors ON. The net result is a current that flows from the anode to the cathode.
• Even when the photons are eliminated, the LASCR will remain ON until the polarities of the anode and cathode are reversed or the power is cut.

LASCR Applications:

• The primary use of light triggered Thyristors is in high-voltage high- current applications, Static reactive-power compensation etc.
• The Light activated SCRs have complete electrical isolation between the light-triggering source and the high-voltage anode-cathode circuit.
• High Voltage Direct Current (HVDC) transmission systems, several SCRs are connected in series-parallel combination and their light-triggering has the advantage of electrical isolation between power and control circuits.

Voltage Regulation in SMPS using PWM technique

SMPS

Switched Mode Power Supply (SMPS):

It  is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. An SMPS is usually employed to efficiently provide a regulated output voltage, typically at a level different from the input voltage                            

Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions (which minimizes wasted energy). Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time.   

They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.

SMPS can be operated using a Error Amplifier, but latest development is done using Pulse Width Modulation.1

PULSE WIDTH MODULATION (PWM)

PWM is a powerful technique for controlling analog circuits with a processor’s digital outputs. PWM is employed in a wide variety of applications ranging from measurement and communications to power control and conversion. PWM uses a square wave whose  duty  cycle is modulated resulting in the variation of the average value of the waveform. PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.

High frequency PWM power control systems are easily realizable with semiconductor switch. The discrete on/off states of the modulation are used to control the state of the switch which correspondingly controls the voltage across or current through the load. The major advantage of this system is the switch are either off and not conducting any current, or on and have (ideally) no voltage drop across them. The product of the current and the voltage at any given time defines the power dissipated by the switch, thus (ideally) no power is dissipated by the switch. Realistically, semiconductor switches such as

MOSFETs or BJTs are non-ideal switches, but high efficiency controllers can still be built. 

Operation:

            Output of the transistor Q1 is sensed back to the Error Amplifier(EA) through sampling resistors. The other input to the EA is Reference voltage(Vref). The output voltage of EA is fed to the Pulse Width Modulator(PWM), other input to PWM is oscillator signal which can be Sawtooth waveform or Triangular waveform.

            Output of the PWM is a rectangular waveform. The width of the rectangular waveform is dictated by the output voltage of EA. This pulse can be used to drive the transistor Q1 through Driver.

            When the width of the pulse is varied, the ON time of transistor Q1 will also vary and consequently the amount of energy taken from the input voltage. So, by controlling Duty cycle, one can stabilize the output voltage.

            Let consider, the output is less than the required voltage then the output of EA will be more, therefore the ON time will be more and transistor Q1 will feed more power to the load .

·        In real-time scenario this total control is accomplished in an IC

Ø But, the transistor in the IC cannot drive the output. So, a driver is used to drive the pulse needed for the output. 

4G or Fourth Generation Networks

4G or Fourth Generation is future technology for mobile and wireless comunications. It will be the successor for the 3Rd Generation (3G) network technology. Currently 3G networks are under deployement. Approximatly 4G deployments are expected to be seen around 2010 to 2015.

The basic voice was the driver for second-generation mobile and has been a considerable success. Currently , video and TV services are driving forward third generation (3G) deployment. And in the future, low cost, high speed data will drive forward the fourth generation (4G) as short-range communication emerges. Service and application ubiquity, with a high degree of personalization and synchronization between various user appliances, will be another driver. At the same time, it is probable that the radio access network will evolve from a centralized architecture to a distributed one.

The evolution from 3G to 4G will be driven by services that offer better quality (e.g. multimedia, video and sound) thanks to greater bandwidth, more sophistication in the association of a large quantity of information, and improved personalization. Convergence with other network (enterprise, fixed) services will come about through the high session data rate. It will require an always-on connection and a revenue model based on a fixed monthly fee. The impact on network capacity is expected to be significant. Machine-to-machine transmission will involve two basic equipment types: sensors (which measure parameters) and tags (which are generally read/write equipment).

It is expected that users will require high data rates, similar to those on fixed networks, for data and streaming applications. Mobile terminal usage (laptops, Personal digital assistants, handhelds) is expected to grow rapidly as they become more user friendly. Fluid high quality video and network reactivity are important user requirements. Key infrastructure design requirements include: fast response, high session rate, high capacity, low user charges, rapid return on investment for operators, investment that is in line with the growth in demand, and simple autonomous terminals. The infrastructure will be much more distributed than in current deployments, facilitating the introduction of a new source of local traffic: machine-to-machine.

Key 4G technologies:

• Orthogonal Frequency Division Multiplexing (OFDM)
• Software Defined Radio (SDR)
• Multiple-input multiple-output ( MIMO )

Initially DoCoMo planned to introduce 4G services around 2010. Recently DoCoMo announced plans to introduce 4G services from 2006, i.e. four years earlier than previously planned. NTT DoCoMo, Inc. announced that high-speed packet transmission with 1 Gbps data rate in the downlink was achieved successfully in a laboratory experiment using fourth-generation (4G) mobile communication radio access equipments.

The key enablers for the 4G are:

• Sufficient spectrum, with associated sharing mechanisms.
• Coverage with two technologies: parent (2G, 3G, WiMAX) for real-time delivery, and discontinuous pico cell for high data rate delivery.
• Caching technology in the network and terminals.
• OFDM and MIMO.
• IP mobility.
• Multi-technology distributed architecture.
• Fixed-mobile convergence (for indoor service).
• Network selection mechanisms.

4G Resources/ Referances:

Alcatel 4G Mobile

Broadband and 4G Tutorial

4G Wireless technology (pdf)

4G news and info

4G faq