Some discrete components used in power electronics

By -

Diodes
Schottky Diodes
Power Bipolar Junction Transistors
MOSFETs
Thyristors:
Silicon Controlled Rectifier (SCR)
Gate Turn-Off Thyristors
Insulated Gate Bipolar Transistors (IGBT)
Gate-Commutated Thyristors
Heatsink
Triacs
Motors and most other actuation devices are typically indirectly connected to the power supply through a power transistor which acts as a switch, either allowing energy to flow from the power supply to the motor, or disconnecting the motor from power. (The CPU, also connected to the transistor, chooses exactly when to turn it on or off).
When the switch is turned on, most of the power coming from the power supply goes to the motor. Unfortunately, some of the power is trapped by unwanted parasitic resistance in the power transistors, heating them up. Often a heatsink is necessary to keep the transistor from overheating and self-destructing. Practically all modern desktop or laptop PCs require a heat sink on the CPU and on the graphics chip. (A typical robot requires a heatsink on its power transistors, but not on its small CPU).
"Application note AN533: thermal management precautions for handling and mounting"  describes "how to calculate a suitable heatsink for a semiconductor device". It looks like this applies to all power semiconductors -- FET, BJT, Triac, SRC, etc. It gives thermal resistance for DPAK and D2PAK for FR4 alone, FR4 plus heatsink, Insulated Metallic Substrate (IMS), and IMS plus heatsink.
The principles behind heatsinking these power semiconductors are the same as the principles behind PC CPU heatsinking.
As long as the triac is on, the A1 and A2 pins act like they are shorted together. Current can flow in both directions through the triac -- whether A2 is higher or lower than the A1 voltage.Turning on a triac is easy: Typically digital logic is connected so its VCC is connected to the triac A1 pin. Some digital logic output pin is connected to a small resistor connected to the triac gate pin. When the digital logic pulls the gate pin low (towards VDD), the triac is triggered and turns all the way on.
Turning the triac off is a little more difficult. First the digital logic output drives the gate pin high (so it is the same voltage as the common A1 pin). But the triac remains on until the current through the triac drops to less than the holding current. Normally triacs are used with 50 Hz or 60 Hz AC, so the triac may stay on as long as 10 ms after the digital logic tries to turn it off.
After the triac is off, A2 acts like is disconnected and isolated from the rest of the triac (as long as the external circuit doesn't drive it outside its voltage rating, typically plus or minus several hundred volts).
Driving a triac is very similar to driving PNP transistor -- "gate" analogous to "base", "A1" analogous to "emitter", "A2" analogous to "collector".
By far, the most common silicon device to drive AC loads connected to mains voltage is the triac. A triac works better than a BJT or a FET transistor as a switch when controlling AC power. A triac can remain off while A2 can swing up and down hundreds of volts (relative to A1 and the gate). It's difficult to keep a BJT or FET transistor from spontaneously turning on in the "reverse" direction.
PNP transistors work better than triacs as switches when controlling DC power. A PNP transistor can quickly turn off even when full current is flowing through them. A triac will stay on indefinitely when DC current flows through it.
A "snubberless triac" or "logic level triac" will not turn on with positive gate current when the A2 lead is at a negative voltage (relative to A1), but they will turn on with a positive gate current when the A2 lead is at a positive voltage.  Some "standard" triacs will turn with a positive gate current (like a power NPN transistor) no matter if A2 is positive or negative, but such "positive gate current triggering" is not recommended. The "negative gate current triggering" is preferred. 

 AC-to-DC Conve
DC-to-AC Converter (Inverter)
DC-to-DC Converter (Chopper)

 AC-to-AC Converter(Cycloconverter)

Comments

Post a Comment

Popular posts from this blog

Bridge inverters

By -
The single-phase voltage source half-bridge inverters, are meant for lower voltage applications and are commonly used in power supplies. Figure 9 shows the circuit schematic of this inverter. Low-order current harmonics get injected back to the source voltage by the operation of the inverter. This means that two large capacitors are needed for filtering purposes in this design. As Figure 9 illustrates, only one switch can be on at time in each leg of the inverter. If both switches in a leg were on at the same time, the DC source will be shorted out. Inverters can use several modulation techniques to control their switching schemes. The carrier-based PWM technique compares the AC output waveform, v c , to a carrier voltage signal, v Δ . When v c  is greater than v Δ , S+ is on, and when v c  is less than v Δ , S- is on. When the AC output is at frequency fc with its amplitude at v c , and the triangular carrier signal is at frequency f Δ  with its amplitude...
Keep reading

DC/AC converters (inverters)

By -
DC to AC converters produce an AC output waveform from a DC source. Applications include adjustable speed drives (ASD), uninterruptable power supplies (UPS), active filters, Flexible AC transmission systems (FACTS), voltage compensators, and photovoltaic generators. Topologies for these converters can be separated into two distinct categories: voltage source inverters and current source inverters. Voltage source inverters (VSIs) are named so because the independently controlled output is a voltage waveform. Similarly, current source inverters (CSIs) are distinct in that the controlled AC output is a current waveform. Being static power converters, the DC to AC power conversion is the result of power switching devices, which are commonly fully controllable semiconductor power switches. The output waveforms are therefore made up of discrete values, producing fast transitions rather than smooth ones. The ability to produce near sinusoidal waveforms around the fundamental freq...
Keep reading

Source inverters

By -
Single-phase VSIs are used primarily for low power range applications, while three-phase VSIs cover both medium and high power range applications. Figure 5 shows the circuit schematic for a three-phase VSI. Switches in any of the three legs of the inverter cannot be switched off simultaneously due to this resulting in the voltages being dependent on the respective line current's polarity. States 7 and 8 produce zero AC line voltages, which result in AC line currents freewheeling through either the upper or the lower components. However, the line voltages for states 1 through 6 produce an AC line voltage consisting of the discrete values of Vi, 0 or –Vi. For three-phase SPWM, three modulating signals that are 120 degrees out of phase with one another are used in order to produce out of phase load voltages. In order to preserve the PWM features with a single carrier signal, the normalized carrier frequency, mf, needs to be a multiple of three. This keeps the magnitude of the ph...
Keep reading