Soft switching of diode
Soft Switching of Diode:
- Hard switching and its consequences have been discussed above. Reduction of size and weight of converter systems require higher operating frequencies, which would reduce sizes of inductors and capacitors. However, stresses on devices are heavily influenced by the switching frequencies accompanied by their switching losses.
- It is obvious that switching-aid-networks do not mitigate the dissipation issues to a great extent. Turn-on snubbers though not discussed are rarely used. Even if used, it would not be able to prevent the energy stored in the junction capacitance to discharge into the transistor at each turn-on.
- Soft switching techniques use resonant techniques to switch ON at zero voltage and to switch OFF at zero current. There are negligible switching losses in the devices, though there is a significant rise in conduction losses. There is no transfer of dissipation to the resonant network which is non-dissipative.
- The switching trajectory in the voltage-current plane of a device is illustrated in Fig. 3.6 comparing the paths for that of a Hard-switched operation without any SAN, a Hard-switched with a R-C-D Switching-Aid-Network and a resonant converter. It is indicative of the stresses and losses.
- A designer would prefer the path to be as close as possible to the origin. A Zero Current Switch based converter is provided as illustration to the soft switching mechanism. It is equivalent to the topology shown above. The input capacitor and the one across the diode may be combined to arrive at this topology.
- The ZCS converter is considered to be in stable operation with Load current Itrans flowing through the diode and the inductor Lf. The Capacitor Cr is charged to Vs. On switching the transistor ON the current in it ramps up from zero but the diode continues conduction till this current reaches the load current Iout level.
- Subsequently, the load current and the resonating current flows through the transistor. This current reaches a natural zero when the negative magnitude of the resonating current equals the load current. The transistor thus switches in the Zero Current mode for both turn on and turn off.
- The diode, on the other hand switches in the Zero Voltage mode under both situations. It must be noted that the peak current stress on the transistor is high . The peak voltage stress on the diode is also about twice the supply voltage.
- Both these stresses are significantly higher than that in a comparable Hard switched buck converter. Consequently, while switching losses are practically eliminated in this resonant converter, conduction losses increase along with the device stresses. There is no scope of a SANs in resonant switching.