Transistors｜Evaluation
Mechanism of Self Turn-on Occurrence
2021.09.01
Points of this article
・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.
table of contents
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.
Transistors
Basic
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Basics of Transistors
- Transistor Fundamentals: Structure, Types, and Operating Principles
- Bipolar Junction Transistor (BJT) Basics: Operation and Applications (NPN & PNP)
- NPN Transistor: Low-Side Switch Fundamentals
- PNP Transistor: High-Side Switch Fundamentals
- What is a Digital Transistor?
- Digital Transistor Selection
- ON Resistance
- Total Gate Charge
- How to select＜Selecting Transistors to Ensure Safe Operation＞
- Junction Temperature ＜Calculating Transistor Chip Temperature＞
- What is a Load Switch?
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Basics of MOSFETs
- What are MOSFETs? – MOSFET Parasitic Capacitance and Its Temperature Characteristic
- What are MOSFETs? – MOSFET Switching Characteristics and Temperature Characteristics
- What are MOSFETs? – MOSFET Threshold Values, ID-VGS Characteristics, and Temperature Characteristics
- What are MOSFETs? – Super-junction MOSFET
- What are MOSFETs? – Types and Features of High Voltage Super-Junction MOSFET
- What are MOSFETs? – Fast trr SJ-MOSFET：PrestoMOS™
- MOSFET Thermal Resistance and Power Dissipation: Packages Capable of Back-Surface Heat Dissipation
- Introduction
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Confirming the Suitability of a Transistor in Actual Operation – Introduction
- Confirmation of the Chip Temperature
- Confirmation of Suitability in Actual Operation and Preparations
- Confirmation that Absolute Maximum Ratings are Satisfied
- Confirmation that Operation is within the SOA (Safe Operating Area)
- Confirmation that Operation is within the SOA Derated at the Actual Operating Temperature
- Confirmation that Average Power Consumption is within the Rated Power
- Summary
- Summary
Evaluation
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The Importance of the Recovery Characteristics of Primary-side Switching Elements in LLC Converters －Introduction－
- Basic Configuration of an LLC Converter
- Features of LLC Converter Operation
- Basic Operation of LLC Converters
- Importance of MOSFET Recovery Characteristics for Off-Resonance of LLC Converters
- The Importance of the Recovery Characteristics of Primary-side Switching Elements in LLC Converters ーSummaryー
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The Importance of the Reverse Recovery Characteristics of Switching Elements in Inverter Circuits －Introduction－
- Types of Inverter Circuits and Energization Methods
- Basic Operation of 3-Phase Modulation Inverter Circuits
- Comparison of Losses in a PrestoMOS™ MOSFET and a Standard SJ MOSFET Using Double-Pulse Tests (Actual Measurement Results)
- Comparison of Efficiency of a PrestoMOS™ MOSFET and a Standard SJ MOSFET in a 3-Phase Modulation Inverter Circuit (Simulations)
- The Importance of the Reverse Recovery Characteristics of Switching Elements in Inverter Circuits －Summary－
- Mechanisms of MOSFET Destruction
- About Double-Pulse Tests
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Improving the Power Conversion Efficiency of Phase Shift Full Bridge Circuits – Introduction
- Basic Configuration of a PSFB Circuit
- Basic Operation of PSFB Circuits
- Guidelines Relating to Operation of Switching Elements Under Light Loading
- Guidelines Relating to Operation of Switching Elements Under Heavy Loading
- Evaluation of Efficiency
- Improving the Power Conversion Efficiency of Phase Shift Full Bridge Circuit – Summary –