Si Power Device|Evaluation



This article summarizes the entirety of the present topic.

In “Evaluating MOSFET Recovery Characteristics Using Double Pulse Tests”, it was noted that inverter circuits, totem pole type power factor correction (PFC) circuits, and the like are bridge circuits in which two MOSFETs are series-connected, and consequently a phenomenon occurs in which a current that passes through the upper and lower arms causes the turn-on loss to increase.

This phenomenon is strongly affected by the recovery characteristics of the body diode (parasitic diode) of the MOSFET in the opposite arm to the MOSFET that is switching, and so a double pulse test was used, and the results of evaluations of the recovery characteristics of MOSFET body diodes were presented.

In double pulse tests, turn-on losses were evaluated and compared from two standpoints. One was a comparison of an ordinary SJ MOSFET and a PrestoMOS™, which is a SJ MOSFET with a fast recovery characteristic; the other was a comparison of SJ MOSFETs featuring fast recovery characteristics from different manufacturers.

The following two results were obtained in the evaluations, and it was confirmed that in order to reduce the turn-on loss, it is important that the recovery characteristics of MOSFET body diodes be evaluated, and that MOSFETs with excellent recovery characteristics be selected.

  • 1. In the comparison of the ordinary SJ MOSFET and the PrestoMOS™ featuring a fast recovery characteristic, the recovery current Irr and recovery charge Qrr that are the main causes of turn-on losses are far smaller for the PrestoMOS™, contributing to reduced switching losses.
  • 2. In the comparison of the SJ MOSFETs featuring fast recovery characteristics from different manufacturers, there was a difference in Irr and Qrr between the manufacturers; among the MOSFETs of the three companies considered, the PrestoMOS™ was superior, and was most able to reduce switching losses.

In addition to these findings, it was also shown that there are cases in which even with a fast recovery characteristic, turn-on losses cannot be reduced, and that it is important to suppress a “self turn-on” phenomenon that is a cause of this. Self turn-on is a phenomenon that occurs due to the gate capacitances (CGD, CGS) and the RG of a MOSFET: when a switching-side MOSFET in a bridge configuration turns on, the free-wheeling MOSFET, which ordinarily should be turned off, turns on unintentionally, so that a through-current flows and losses increase.

The diagram below was also shown in “Mechanism of Self Turn-on Occurrence”, and indicates that when self turn-on occurs, in addition to the recovery current of the body diode, a large through-current also flows.

As indicated by the above-described evaluation result 2, in a comparison of SJ MOSFETs featuring fast recovery characteristics from different manufacturers, the PrestoMOS™ was able to achieve lowest turn-on loss due to a design in which the ratio of the gate capacitances was optimized to suppress self turn-on, in addition to the fast recovery characteristic.

Hence switching losses in a bridge circuit are affected not only by the recovery characteristics (Irr, Qrr) of the MOSFET body diode, but also by the effect of a through-current caused by self turn-on. By choosing a MOSFET in which both of these are suppressed, switching losses can be greatly reduced.

Below are links to the articles and the key points of each.

About Double-Pulse Tests

  • ・Double-pulse testing is a widely used method for evaluating the characteristics of power switching elements such as MOSFETs and IGBTs.
  • ・In addition to the switching characteristics of a tested element, the recovery characteristics of the body diode and of external fast recovery diodes and the like can also be evaluated.
  • ・Double-pulse tests are extremely useful for circuits in which losses arise due to recovery characteristics while a driven-side element is turned on.

Recovery Characteristic Evaluation Using Double-Pulse Tests

  • ・In a bridge circuit, turn-on losses are higher when the recovery characteristics of the body diodes in the MOSFETs are poor.
  • ・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 in a bridge-configuration circuit.
  • ・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.

Mechanism of Self Turn-on Occurrence

  • ・Self turn-on of a MOSFET in a bridge circuit is a phenomenon in which the MOSFET is unintentionally turned on by a change in VGS due to a sharp change in the VDS of the MOSFET.
  • ・In a bridge-configuration circuit, when a through-current flows due to self turn-on, the turn-on loss is increased, and so there are cases in which an expected loss reduction is not possible even with an excellent recovery characteristic.

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