Fast Recovery Epitaxial Diodes
During the last 10 years, power supply topology has undergone a basic change. Power supplies of all kinds are now constructed so that heavy and bulky 50/60 Hz mains transformers are no longer necessary. These transformers represented the major part of volume and weight of a traditional power supply. Today they have been replaced with smaller and lighter transfomers, whose core materials now consist of sintered ferrites instead of iron laminations and which can operate up to 250 kHz. For the same power rating, high frequency operation significantly reduces the weight and volume of the transformer. This development has been significantly influenced by new, fast switching power transistors, such as MOSFETS or IGBTs, working at high blocking voltages (VCES > 600 V).
However, nearly all topologies equipped with these transistors also need ultrafast diodes to conduct the reactive load current and to rectify the AC output when DC voltage is required. The switching behavior of these diodes must be tailored to match the switching charcteristics of the transistors.
This is not only true for switch mode power supplies but also for inverter circuits. For these inverters, manufacturers have chosen PWM frequencies of about 8 kHz to create a smooth sinoidial waveform of the output current or have used a PWM frequency above 20 kHz in order to operate above the audible level.
1) MOSFET = Metal Oxide Semiconductor Field Effect Transistor
2) IGBT = Insulated Gate Bipolar Transistor
Apart from the characteristics of the transitor switches, the on-state and dynamic characteristics of the free wheeling diodes have a significant impact on the power loss, the efficiency and the degree of safety in operation o the whole equipment. They also play a decisive role when it comes to increasing the efficiency of a SMPS and to reduce the losses of an inverter, which clearly mandates that ultrafast diodes be used. The ultrafast diodes described here embrace all characteristics of modern epitaxial diodes, such as soft recovery, low reverse recovery current IRM with short reverse recovery times.
The abbreviation FRED (Fast Recovery Epitaxial Diodes) stands for a series of ultrafast diodes, which have gained wide acceptance during the last few years. There exist several methods to control the switching characteristics of diodes and each leads to a different interdependency of forward voltage drop VF, blocking voltage VRRM and trr values. It is these interdependencies (or compromises) that differeniate the ultrafast diodesavailable on the market today. Fig.1 shows a qualitative relationship of forward voltage VF and reverse recovery time trr. The most important parameters for the turn-on and turn-off behavior of a diode (Fig. 2) VFR, VF, tfr and IRM, trr will be influenced by different manufacturing processes.
But as shown in Fig. 1, each technology must come up with a its own compromise between forward voltage and recovery time to obtain a device that will operate satisatisfac-torily.
Figure 2 shows a typical switching cycle for the diode. During forward conduction, the resistivity of the n- epitaxial layer (see Fig. 4) is decreased by excess minority charges (in this case holes) being stored there. When this forward current is commutated to another switch, the diode cannot regain its reverse blocking capability until this excess stored charge is removed, which can only be done by recombination of the stored holes with the background electrons or by reverse current flow through the diode. Since the ideal diode has zero reverse recovery current, the recombination of stored charge must be accelerated, which is done by the introduction of recombination centers into the n- epitaxial layer.