Soft and Hard Switching & Losses of semiconductors
Soft and Hard Switching:
- Semiconductors utilized in Static Power Converters operate in the switching mode to maximize efficiency. Switching frequencies vary from 50 Hz in a SCR based AC-DC Phase Angle Controller to over 1.0 MHz in a MOSFET based power supply.
- The switching or dynamic behavior of Power Semiconductor devices thus attracts attention specially for the faster ones for a number of reasons: optimum drive, power dissipation, EMI/RFI issues and switching-aid-networks.
- With SCRs’ 'forced commutation' and 'natural (line) commutation' usually described the type of switching. Both refer to the turn-off mechanism of the SCR, the turn-on dynamics being inconsequential for most purposes.
- A protective inductive snubber to limit the turn-on di/dt is usually utilized. For the SCRs’ the turn-off data helps to dimension the 'commutation components' or to set the 'margin angle'. Conduction losses account for the most significant part of total losses.
- Present day fast converters operate at much higher switching frequencies chiefly to reduce weight and size of the filter components. As a consequence, switching losses now tend to predominate, causing the junction temperatures to rise. Special techniques are employed to obtain clean turn-on and turn-off of the devices.
- This, along with optimal control strategies and improved evacuation of the heat generated, permit utilization of the devices with a minimum of deration.
Losses in Power Semiconductors:
- Losses can be segregated as follows:
- A converter consists of a few controlled and a few uncontrolled devices (diodes). While the first device is driven to turn-on or off, the uncontrolled device operates mainly as a slave to the former.
- Power loss in the converter is the aggregate of these losses. Occasionally the diode and the controlled device are housed in the same module.
- The losses corresponding to each contribute to the temperature rise of the integrated module.