Chapter 9 Practical Application Issues of Power Semiconductor Devices Power Electronics Chapter 9 Practical Application Issues of Power Semiconductor Devices

Outline 9.1 Gate drive circuit 9.2 Protection of power semiconductor devices 9.3 Series and parallel connections of power semiconductor devices

9.1 Gate drive circuit Basic function of gate drive circuit: Generate gate signals to turn-on or turn-off power semiconductor device according to the commanding signals from the control circuit. Other functions of gate drive circuit: Reduce switching time (including turn-on time and turn-off time) Reduce switching loss (including turn-on loss and turn-off loss) and improve efficiency Improve protection and safety of the converter Gate drive circuits provided by power semiconductor manufacturers and Integrated gate drive chips are more and more widely used.

Electrical isolation in the gate drive circuit Gate drive circuit usually provides the electrical isolation between control circuit and power stage. Two ways to provide electrical isolation Optical Optocoupler, fiber optics Transformer Magnetic Schematic of an optocoupler

Thyristor gate current pulse requirments Shape of gate current pulse waveform: Enhanced leading part Magnitude requirement (for the enhanced leading part and the other part) Width requirement (for the enhanced leading part and the whole pulse) Power of the triggering signal must be within the SOA of the gate I-V characteristics Ideal gate current pulse waveform for thyristors

Typical thyristor gate triggering circuit

Typical gate signal and gate drive circuit for GTO

A typical gate drive circuit for IGBT based on an integrated driver chip M57962L integrated driver chip

9.2 Protection of power semiconductor devices Protection circuits Overvoltage protection Overcurrent protection Snubber circuits—specific protection circuits that can limit du/dt or di/dt Turn-on snubber Turn-off snubber

Causes of overvoltage on power semiconductor devices External reasons Overvoltage caused by operation of mechanic swithes Overvoltage caused by thunder lightening Internal reasons Overvoltage caused by the reverse recovery of diode or thyristor Overvoltage caused by the turning-off of fully-controlled devices

Measures to protect power semiconductor devices from overvoltage Lightening arrestor RC or RCD snubbers (will be discussed later) Zener diode, Metal Oxide Varistor (MOV), Break Over Diode (BOD)

Measures to protect power semiconductor devices from overcurrent Fuse Circuit breaker Protection with current feedback control in the control circuit Protection with overcurrent detection in the gate drive circuit—the fastest measure

Functions and classifications of snubbers Limiting voltages applied to devices during turn-off transients Limiting device currents during turn-on transients Limiting device current rising rate (di/dt) at device turn-on Limiting the rate of rise (du/dt) of voltages across devices during device turn-off Shaping the switching trajectory of the device Classifications According to different switching transients Turn-off snubber (sometimes just called snubber) Turn-on snubber According to the treatment of energy Power dissipating snubber Lossless snubber

Operation principle of typical snubbers Circuit configuration Switching trajectory

Other turn-off snubbers

9.3 Series and parallel connections of power semiconductor devices Object To increase the capability to deal with voltage or current Issues and solutions Series connection Issue: even voltage sharing Solutions: Selection of devices that are closer to each other in the characteristics Voltage sharing circuit Parallel connection Issue: even current sharing Current sharing circuit and symmetrical circuit layout

Series Connection of thyristors Voltage sharing circuit Steady-state voltage sharing circuit Dynamic voltage sharing circuit

Parallel Connection of Power MOSFETs Easy to realize because of the positive temperature of their on-state resistance Need a small damping resistor in series with the individual gate connections Still need to select devices that are closer to each other in the characteristics Circuit layout should be symmetrical