What is diode reverse recovery time (trr)?
Even if the voltage applied to the diode is changed from the forward direction to the reverse direction, it will not turn off immediately. A certain time is required for the transition from ON to OFF. Of the time required for this transition, the time during which the current flows in the opposite direction is called the reverse recovery time (trr).
Toshiba has the fast recovery diodes (FRD) and some of switching diodes designed to reduce this time. Toshiba's FRDs include ultra-fast rectifier diodes (S-FRD) and high-efficiency rectifier diodes (HED). Also, Schottky barrier diodes are unipolar devices that do not use minority carriers, so in principle there is no reverse recovery time.
In a bipolar device such as a pn junction diode, when the device is on, electrons, which are minority carriers in p-type semiconductors, and holes, which are minority carriers in n-type semiconductors, act as carriers of the current. ( For details, please refer to the e-learning Basics of Schottky Barrier Diodes 1-3. pn junction )
Basics of Schottky Barrier Diodes 1-3. pn junction
In addition, in pn junction diodes, the on-resistance is lowered by accumulating excess carriers in a low-impurity-concentration layer when the diode is on. ( For details, please refer to the e-learning Basics of Schottky Barrier Diodes 2-3. Conductivity modulation )
Basics of Schottky Barrier Diodes 2-3. Conductivity modulation
When transitioning from this on state to off, the stored charge (carriers) must be released. A reverse recovery time is required as the time required for these carriers release.
At time t0 the diode begins to transition from the on state to the off state. Since the diode is in a conductive state, the minority carriers return to their original region or recombine, and the charge accumulated in the parasitic capacitance is discharged, the current decreases to zero, further it turns negative. This reversed current is called reverse recovery current. The peak value of this current is Irr. The voltage of the diode also becomes zero at the timing of this peak current. At this time, the diode is in a non-conducting state. Minority carriers still remain. Since these minority carriers cannot return beyond the depletion layer, they have no choice but to disappear through recombination. Therefore, the time after the peak depends on the lifetime.
In FRDs and HEDs, the reverse recovery time is shortened by implanting heavy metals such as platinum (Pt) into the drift region to create crystal defects and trap minority carriers when a reverse bias is applied.
The Schottky barrier diode (SBD), a unipolar device that does not use minority carriers for current transmission provides virtually zero reverse recovery time. Silicon SBDs are available with a withstand voltage of only up to 60 V whereas SiC SBDs, which are wide-bandgap semiconductor devices, are available with a withstand voltage of up to 650 V because of their high electric breakdown field.
Nowadays, demand is growing for progressively smaller and lighter electronic devices. One of the solutions to this is to increase the switching frequency of a power supply. This helps reduce the size of the transformer.
Therefore, it is required to reduce their switching loss, making it more important than ever before to improve the reverse recovery characteristics of diodes.
Toshiba provides 650-V SiC SBDs suitable for this application.
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Please refer to the information below regarding the reverse recovery time.
