Transistors, thyristors and power electronics

Diodes conduct whenever they are forward biased; transistors and thyristors can be switched on and off by a small control signal. These controllable semiconductors are the building blocks of every electronic control and power conversion device an electrician meets: the transistor outputs of PLCs and sensors, solid-state relays, dimmers, soft starters, battery chargers, UPS inverters, solar inverters and variable frequency drives. Power electronics uses them as fast switches to convert electrical energy efficiently from one form to another. This lesson explains how bipolar transistors, MOSFETs and IGBTs work as switches, how thyristors such as SCRs and triacs control AC power, how converters are built from these devices, how switching losses and heat sinking limit them, and how wide-bandgap devices are changing power electronics.

The transistor as a switch

A transistor has three terminals; a small signal at one terminal controls a larger current between the other two. Transistors can amplify signals, but in control and power work they are used almost entirely as switches, fully on (saturated, with a small voltage drop) or fully off (blocking), because a switch dissipates little power in either state while a partly-on transistor wastes energy as heat.

• Bipolar junction transistors (BJTs) have a base, collector and emitter. A small base current controls a collector current about 50 to 300 times larger. NPN transistors switch the load to the negative side (sinking); PNP transistors switch to the positive side (sourcing). BJTs are used in small signal circuits and in many sensor and PLC outputs.
• MOSFETs (metal-oxide-semiconductor field-effect transistors) have a gate, drain and source. A voltage on the gate (typically 5 to 15 V), with almost no steady gate current, controls the drain-source channel, which acts like a small resistance, R_DS(on), when on. MOSFETs switch fast and efficiently at lower voltages and dominate switch-mode power supplies, DC motor controls, battery systems and low-voltage drives.
• IGBTs (insulated-gate bipolar transistors) combine a MOSFET-like gate with a bipolar output stage, handling high voltages (600 to 6,500 V) and large currents with modest on-state voltage drops of about 1.5 to 3 V. IGBTs are the workhorse switches of variable frequency drives, UPS inverters, solar inverters and traction drives.

Sinking and sourcing outputs

The terms sinking and sourcing, central to PLC and sensor wiring (Lessons 4 and 6), come from transistor switching. A sinking output (NPN or N-channel) connects the load to the negative (0 V) side when on; the load's other end connects to +24 V. A sourcing output (PNP or P-channel) connects the load to +24 V when on; the load's other end connects to 0 V. A sinking output must be wired to a sourcing input and vice versa. Mixing them up is one of the most common wiring errors in control panels: nothing happens, or the input is always on.

Thyristors: SCRs and triacs

Thyristors are four-layer semiconductor devices that, once triggered, stay on until the current through them falls to nearly zero. This latching behaviour suits AC power control, where current naturally passes through zero twice each cycle.

• The silicon-controlled rectifier (SCR) conducts in one direction when a gate pulse is applied while it is forward biased, and turns off at the next current zero. SCRs are used in controlled rectifiers for DC drives, battery chargers, and in pairs (back-to-back) in soft starters and heater controllers.
• The triac conducts in both directions and can be triggered in either half-cycle, used in light dimmers, small motor speed controls and solid-state relays for AC loads.

Phase-angle control delays the trigger point in each half-cycle to reduce the average voltage delivered to the load, as in dimmers and soft starters. Burst firing (zero-cross switching) instead switches whole cycles on and off, as in heater controllers, producing less electrical noise but coarser control. Phase-angle control produces harmonics and radio-frequency noise, which is why dimmers and soft starters may need filtering and why soft starters are bypassed by a contactor once the motor reaches speed.

Power conversion

Power electronic converters are built from switches, diodes, inductors and capacitors:

• AC-DC (rectifiers): diode bridges, or SCR or active IGBT front ends that can regulate the DC voltage, reduce harmonics and return energy to the supply (regeneration).
• DC-DC (choppers): buck converters step voltage down, boost converters step it up, and isolated converters use high-frequency transformers. They are found in power supplies, battery chargers and solar optimizers.
• DC-AC (inverters): IGBTs or MOSFETs switch the DC bus to synthesize AC of variable voltage and frequency using pulse-width modulation (PWM), as in drives, UPS systems and grid-tied solar inverters.
• AC-AC: soft starters, cycloconverters and matrix converters control AC directly.

A variable frequency drive combines a rectifier, a DC bus with capacitors, and a PWM inverter. Its output is a train of fast voltage pulses at the DC bus voltage whose average follows a sine wave, with the motor's inductance smoothing the current. These steep-edged pulses cause the reflected wave, bearing current and electromagnetic interference issues covered in ELEC-310.

Pulse-width modulation

PWM controls the average output of a switch by varying the fraction of time it is on, the duty cycle. A 24 V supply switched with a 25 % duty cycle delivers an average of 6 V to a load that smooths it (such as a motor, heater or LED with filtering). Switching frequencies range from a few hundred hertz for heaters to 2 to 16 kHz in motor drives and 50 kHz to over 1 MHz in modern power supplies. Higher switching frequencies allow smaller inductors and capacitors and quieter motors, but increase switching losses and electromagnetic interference.

Losses and heat

Even as switches, semiconductors dissipate power: conduction losses (on-state voltage drop times current, or I² R_DS(on)) and switching losses during each transition from on to off and back, when voltage and current overlap briefly. Losses become heat that must flow through the device package to a heat sink and into the air. Every semiconductor has a maximum junction temperature (commonly 150 °C to 175 °C for silicon), and its life shortens rapidly as temperature rises. This is why drives and soft starters are derated at high ambient temperatures and altitudes, why their cooling fans and heat sink fins must be kept clean, and why enclosures need adequate ventilation or cooling. A blocked fan is one of the most common causes of drive failure.

Protection of power semiconductors

Semiconductors fail quickly when overloaded: unlike a copper conductor, a junction can be destroyed by an overcurrent lasting milliseconds. Protection includes fast-acting semiconductor fuses (marked aR or gR) that clear faults before the device fails, electronic current limiting and desaturation detection in drive gate drivers, snubber circuits (resistor-capacitor networks) and metal-oxide varistors (MOVs) that limit voltage spikes, and thermal sensors on heat sinks. When replacing fuses in drives, soft starters or rectifiers, use exactly the type specified by the manufacturer; an ordinary fuse of the same rating may not protect the semiconductors.

Wide-bandgap semiconductors

Silicon carbide (SiC) and gallium nitride (GaN) devices are replacing silicon in many applications. They can block higher voltages in smaller devices, switch much faster with lower losses, and operate at higher temperatures. SiC MOSFETs are now common in electric vehicle inverters and chargers, solar inverters and high-efficiency drives; GaN transistors are used in compact chargers and power supplies. For electricians, the effects include smaller, more efficient equipment, and faster voltage edges that can worsen reflected wave and electromagnetic interference problems unless filters and cable practices are followed.

Testing transistors and thyristors

Power semiconductors are usually tested in place by the equipment's own diagnostics, but some checks are possible with a meter in diode-test mode with power removed and capacitors discharged. On a drive, the rectifier diodes and the freewheeling diodes across each IGBT can be checked from the input and output terminals to the DC bus: each should read a normal diode drop (about 0.3 to 0.5 V on large devices) in one direction and open in the other. A reading near zero indicates a shorted device, which is the usual failure mode. Manufacturers publish test procedures for their equipment; follow them, and never apply power to a drive with a suspected shorted IGBT, which can cause an arc flash inside the enclosure.

Power electronics in everyday installations

Electricians meet power electronics everywhere: LED drivers in lighting, electronic ballasts, EV chargers, heat pump and air conditioner inverters, solar inverters, battery storage systems, UPS units, and the switch-mode supplies in every computer and control panel. All of these draw non-sinusoidal current from the supply, contributing to harmonics (ELEC-320), and many include filters and surge protection. Many also include their own ground-fault and residual current detection, which can interact with upstream ground-fault protection. Reading the installation instructions for each device, including requirements for dedicated circuits, conductor sizes, overcurrent protection types and grounding, prevents nuisance trips and premature failures.