2016.12.15 Si Power Device
This is our third section on Si diodes. This time we will explain the characteristics of fast recovery diodes (hereafter "FRDs") and improvements in these characteristics, and their applications.
Features of Si FRDs
Si FRDs are diodes based on a PN junction, and feature rapid operation, with a short Trr. On the other hand, a generally somewhat high VF can be called another feature of these devices. For example, the VF for a general-purpose 10 A-class device is a little under 2 V. This is because of the tradeoff between a rapid-acting Trr and the VF value. However, devices with a greatly reduced VF are also being developed, and the optimal Si FRD can be selected according to the application.
The following table summarizes Si FRD features and applications suitable for each. In the table, the symbol "×" should be understood to mean inferior or poor compared with other devices.
Ultra-high-speed devices have high VF values, but because the reverse current IR is low, they experience low losses in continuous current mode of PFC (power factor correction) applications, to which they are well suited.
Devices that balance fast operation with a low VF are extremely versatile, and the speed of FRDs can be utilized in diverse applications.
The IR characteristic of ultra-low VF devices is somewhat inferior, but conduction losses can be reduced due to its low-VF characteristics, making such diodes suited to boundary current mode PFC applications with large peak currents.
The graphs below illustrate the tradeoff between VF and Trr. The device represented by the orange curves is an FRD with a low VF but a long Trr, and the IR during this time is also large. The device represented by the red curves has the opposite characteristics. We see that faster devices have a high VF, whereas lower VFs mean that Trr/IR is large.
Si FRD noise
From the standpoint of EMC, noise originating in switching power supplies and other switching phenomena is an important matter for study. The fast Trr times of Si FRDs mean that noise occurs during reverse recovery, that is, during the Trr period. The following diagrams illustrate noise during reverse recovery and after improvement. IRp represents the peak of the reverse current when the FRD turns off. The recovery slope or gradient is indicated by dir/dt. Noise during reverse recovery can be reduced if the IRp can be made small and the dir/dt can be kept gentle.
In actuality, such a low-noise Si FRD, called a "soft recovery" type FRD, has been successfully developed. Here we briefly describe the approach used.
In order to keep the IRp small and make the dir/dt gentle, it was necessary to take separate measures to improve both of these parameters. Initially the impurity concentration of the P-type silicon (anode) was reduced in order to lower the IRp. By this means the leak current was reduced and IRp also fell, but dir/dt was still steep and noise remained, and there was the further problem of a higher VF, resulting from the tradeoff described above.
Next, in order to make the slope more gentle, platinum (Pt) was diffused as a "lifetime killer" to shorten the lifetime, and dir/dt was successfully decreased. The lifetime is the time in the PN junction from when the off/reverse-bias state is entered until there is a return to the original state through recombination of minority carriers. If this lifetime is long, the residual carriers cause phenomena such as the persistence of currents. A "lifetime killer" hastens the recombination to shorten the lifetime. In general, impurities are diffused as lifetime killers; in this case, Pt was used.
We turn to production processes. This may be a bit difficult for persons who are not familiar with production, but based on these two methods, ultimately Si FRDs were realized having greatly reduced reverse recovery noise, while maintaining the Trr and VF characteristics essentially unchanged. The actual waveforms shown below compare the soft recovery type (red) and standard type (blue) devices. Clearly the noise level is extremely low for the soft recovery type.
Applications of Si FRDs Drawing on Trr and VF
Finally, we describe applications that are a good fit to Si FRD characteristics, in the hope that this will be of use in design. As explained previously, devices with a fast Trr have a high VF, and those with a low VF have slower Trr; this basic tendency is considered when selecting devices suited to application characteristics.
Where PFC is concerned, devices with extremely low VF values in BCM (boundary current mode), and devices with extremely fast Trr values in CCM (continuous current mode), each contribute to lessen losses.
Moreover, devices with reduced noise and with an optimized tradeoff between VF and Trr are suited to use in an extremely broad range of applications.
・Si FRD characteristics differ depending on the impurities diffused in the silicon.
・There is a tradeoff between the VF and Trr values of Si FRDs.
・Noise during reverse recovery has adverse effects in switching power supply applications, and so improved versions are being developed.