Optimized Diodes for Switching Applications
Great efforts have been made to improve power switches – MOSFETs and IGBTs – to decrease forward voltage drop and as well as to decrease turn-off energy. In switching inductive loads, the turn-on losses depend strongly of the behavior of the companion free-wheeling diode and now form the major part of over-all power losses. New developments like series connected diodes in a single package can greatly improve a given design. This paper shows how to choose the optimum diode using the specific example of a PFC circuit.
In hard switching applications with an inductive load, the free-wheeling diode causes high losses during the turn-on transition of the power switch. Power factor correction in non resonant mode is a typical example for such hard inductive switching. A very common topology is the boost configuration (Fig. 1), using a MOSFET as the usual power switch at higher frequencies.
Fig. 2 shows idealized current and voltage waveforms during the diode’s turn-off and MOSFET’s turn-on. These waveforms are also valid in inverter designs, where the diode and the power switch are parts of a phase leg. So the results obtained with this example can also be used for designing drive inverters, switched mode power supplies, line inverters and other similar applications.
A known method to achieve better performance for rectifiers for a given blocking voltage is to connect lower voltage diodes in series . For equal voltage sharing, it is sometimes necessary to connect RC snubber networks in parallel to each single diode, thereby making this solution rarely used. A newly developed housing makes it possible to connect two or more diodes in series within one single package. Matching and testing the dice for voltage sharing allows the user to design in these diodes without any additional snubber circuits. It now depends on the application and its switching frequency if single die or series connected diode is the better choice.
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