2021.06.23 Si Power Device
In this article, recovery characteristics are considered on the basis of double-pulse tests using MOSFETs of various types. As the test circuit used in the evaluations, the basic circuit diagram presented in the previous article is employed. Operations used in confirmation are also similar to those described in the previous article, and so the previous article should be referred to as necessary.
Recovery Characteristic Evaluation Using Double-Pulse Tests
To evaluate MOSFET recovery characteristics, four types of MOSFET were used in execution of actual double-pulse tests. The MOSFETs were all Super Junction (SJ) MOSFETs; fast-recovery types and ordinary types were used for comparison.
We begin by presenting test results for the SJ MOSFET R6030JNZ4 (PrestoMOS™), provided with fast recovery characteristics, and the SJ MOSFET R6030KNZ4 with ordinary characteristics. Apart from the recovery characteristics, these SJ MOSFETs have closely similar electrical specifications; in the tests, the SJ MOSFETs were used for both Q1 and Q2.
Fig. 1 shows the waveforms of ID_L at turn-on in operation ③ of the previous article; Fig. 2 is the waveforms of the turn-on loss Eon_L.
Figure 1: Waveforms of the drain current ID_L of a fast recovery-type PrestoMOS™ and an ordinary SJ MOSFET
Figure 2: Waveforms of the power loss Eon_L of the fast recovery-type PrestoMOS™ and the ordinary SJ MOSFET
From Fig. 1, we see that the recovery current Irr and the recovery charge Qrr of the fast recovery-type R6030JNZ4 (PrestoMOS™), which is Q1, are much smaller than the values for the ordinary-type R6030KNZ4.
Fig. 2 shows that the turn-on loss Eon_L of the ordinary type R6030KNZ4 with large Qrr is greater than that of the fast recovery-type R6030JNZ4. Consequently, we see that the switching loss increases when Qrr of Q1 is largeｒ.
Next, the results of comparing, under the same conditions, the fast recovery-type R6030JNZ4 (PrestoMOS™) and other fast recovery-type SJ MOSFETs are shown. Fig. 3 compares ID_L waveforms similarly to Fig. 1; Fig. 4, similarly to Fig. 2, compares Eon_L waveforms.
Figure 3: Waveforms of the drain current ID_L of the fast recovery R6030JNZ4 and other fast-recovery SJ MOSFETs
Figure 4: Waveforms of power loss Eon_L of the fast-recovery R6030JNZ4 and the other fast-recovery SJ MOSFETs
As indicated in Fig. 3, Irr and Qrr for the R6030JNZ4 (PrestoMOS™) are smaller than for the other fast recovery-type SJ MOSFETs, and so the ID_L peak is smaller, and consequently, as seen in Fig. 4, Eon_L is smaller.
From these results, we see that a MOSFET with body diode characteristics characterized by a smaller recovery current Irr and smaller recovery charge Qrr has a smaller turn-on loss Eon_L. This is true even when comparing fast recovery-type SJ MOSFETs. Hence when aiming for reduced losses in the design stage, it is important that MOSFET recovery characteristics be evaluated using the method outlined here so that the optimum MOSFET can be selected.
Finally, one further matter requires attention. In the discussion of this article it was assumed that losses in a MOSFET with fast recovery characteristics can be reduced; but even when the recovery characteristic is fast, there are cases in which the turn-on loss cannot be reduced. One reason for this is the phenomenon known as self turn-on. This phenomenon arises from the gate capacitance of the MOSFET. Self turn-on will be explained in detail in the next article.
・A MOSFET having a small recovery current Irr and small recovery charge Qrr also has a small turn-on loss EON_L.
・This is also true in comparisons between MOSFETs with fast recovery characteristics.
・Evaluation of the recovery characteristics of a MOSFET is important for reducing losses.
・It should be noted that, due to the self turn-on phenomenon, it may not be possible to lower turn-on losses even when the MOSFET has a fast recovery characteristic.