2021.09.01 Si Power Device
In the previous article, it was explained that, through double-pulse tests of ordinary-type and fast recovery-type SJ MOSFETs, losses in a bridge circuit could be reduced by using MOSFETs with fast recovery characteristics, and on the other hand there are cases in which turn-on losses cannot be reduced even when using fast recovery-type MOSFETs. In this article, the self turn-on phenomenon that is one cause of this behavior is explained.
About the Self Turn-on Phenomenon
Self turn-on is a phenomenon that occurs due to the gate capacitances (CGD, CGS) and RG of a MOSFET; in a circuit with a bridge configuration in which two MOSFETs are connected in series, when the switching-side MOSFET turns on, the free-wheeling MOSFET, which originally should be turned off, turns on unintentionally, so that a through-current flows and losses increase.
Mechanism of Occurrence of Self Turn-on
This figure is the same as one that was used in explanations in "About Double-Pulse Tests" and shows the basic operation of a double pulse test.
When transitioning from operation ② to operation ③, the drain-source voltage VDS_H of the high-side MOSFET Q1 changes sharply from 0V to Vi. Due to the rate of change dVDS_H/dt (change in voltage per unit time) that occurs at this time, current flows through in CGD_H, CGS_H, and RG_H. This current causes the voltage across CGS_H to rise, and when VGS_H exceeds the MOSFET gate threshold value, the MOSFET is unintentionally turned on. This is called self turn-on; when this self turn-on occurs, a through-current flows between the high-side Q1 and the low-side Q2. Below is a schematic diagram showing the body diode recovery current and the through-current occurring due to self turn-on.
Because inverter circuits and totem pole PFC circuits and the like are bridge circuits in which two MOSFETs are connected in series, in addition to recovery losses, there are cases of increased turn-on losses due to through-currents caused by self turn-on.
Results indicated that turn-on losses are lower for the R6030JNZ4 (PrestoMOS™) used in evaluations in the previous article than for the other fast recovery-type SJ MOSFETs. In addition to the superior recovery characteristic, this is because the ratio of the gate capacitances is optimized in a structure that can suppress self turn-on.
・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.
・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.