Diode as semiconductor device
Summary on Diodes:
- A surge voltage occurs when the free-wheel diode recovers. Consider a converter leg. The lower device is off and that the load current is circulating through the free-wheeling diode of the upper device.
- Now if the lower device turns on, the current in the free-wheel diode of the upper device decreases during the overlap period and the load current begins to commutate to the lower device. It becomes negative during reverse recovery of the upper free-wheel diode. When the free-wheel diode recovers, the current in the circuit associated to the diode jumps to zero.
- The parasitic line inductance Lp develops a surge voltage equal to Lp di/dt in opposition to the decreasing current. This di/dt is dictated by the recovery characteristic of the free-wheel diode.
- Fast recovery “snappy” diodes can develop very high recovery di/dt when they are hard recovered by the rapid turn-on of a device in series with it in the same converter leg. These diodes take a smaller time to quench the reverse recovery current compared to a soft recovery diode.
- The off-state losses of the main device and the turn-on dissipation may be neglected for most cases. With an IGBT driven DC-DC chopper as an example, the dissipation can be estimated as:
IGBT dissipation = Conduction losses Switching losses
Diode dissipation = Conduction losses Reverse recovery losses
- The values of Eon , Eoff , Err are at the rated values only and have to be adjusted to the working values of voltage (DC bus), VCE (working) and load current, Ic.
- The power device in a converter mostly sees an inductive load. Corresponding ideal waveforms are also indicated. The free-wheeling diode FWD, across the load is essential for clamping the induced voltages across the inductance when the device switches off.
- However, its presence causes the supply voltage, Vs to appear across the transistor whenever it carries part of the inductor current in overlap mode with the FWD during both turn-on and turn-off modes. This causes the transistor switching dissipation to increase. An RCD Switching-aid-network connected across the device reduces turn-off dissipation.
- The controlled rise of the collector voltage of the transistor aids this process. However, turn-off energy is accumulated in the SAN, which is ultimately dissipated in the resistor.
- The RCD does not also help reduce turn-on dissipation when the reverse recovery current of the diode and the SAN current add up with the load current with Vs again appearing across the device.