Tutorial: Mitigating parasitic turn-on effect in IGBT-output drives to improve drive performance

03/29/2007


discrete control; motors, drives & motion control

Bottom IGBT parasitic turn-on due to Miller capacitor

One of the common problems faced in a majority of industrial motor applications is the cross-conduction phenomenon caused by parasitic Miller capacitance in the insulated gate bipolar transistor (IGBT) output transistors. The following tutorial examines the technical and economic trade-offs of using four different techniques to mitigate the affects of a parasitic turn-on due to a Miller capacitor.

1,200 V IGBTs are commonly used in a majority of 3-phase motor inverter applications. These industrial products not only require safety insulation and noise isolation, but also control and special protection functions to ensure reliable operation. In a typical industrial motor application, the Miller capacitor causes a dV/dt shoot-through during IGBT switching. This effect is noticeable in single supply gate drivers (0 to +15V). Due to this gate-collector coupling, a high dV/dt transient created during IGBT turn-off can induce a parasitic turn-on effect that is potentially dangerous. This effect will lead to an IGBT shoot-through across both IGBTs, which could damage them.

When turning on the upper IGBT, a voltage change dVCE/dt occurs across the lower IGBT. Current flow through the parasitic Miller capacitor the upper IGBT, the gate resistor and the internal driver gate resistor creates a voltage drop across the gate resistor. If this voltage exceeds the IGBT gate threshold voltage, a parasitic turn-on occurs. Designers should be aware that rising IGBT chip temperature leads to a slight reduction of gate threshold voltage, usually in the range of millivolts per °C. A lower IGBT voltage threshold translates to lesser driver voltage required to turn-on the IGBT. As a result, IGBT shoot-through would be more frequent and a parasitic turn-on can easily affect the IGBT.

There are three classical solutions to the above problem; the first is to vary the gate resistor, second is to add a capacitor between gate and emitter, and third is to use a negative gate drive. A fourth approach that can prove to be simple and effective is the active clamp technique.

Separate gate resistors for turn-on and turn-off

Adding a gate-on resistor influences the voltage and current change during IGBT turn-on. Increasing this resistor reduces the voltage and current changes, but also increases switching losses.

machine control & discrete sensors

Separate on and off gate resistor

discrete control; motors, drives & motion control

Additional capacitor between gate and emitter

machine control & discrete sensors

Negative supply voltage

discrete control; motors, drives & motion control

Active Miller clamping using additional transistor

Parasitic turn-on can be prevented by reducing the gate-off resistor. The smaller gate resistor will also reduce switching loss during IGBT turn-off. However, the trade off of switching off faster is a higher over-shoot and oscillation during turn-off due to stray inductances. Higher over-shoot voltage and oscillation is a negative behavior as it could make the required for IGBT maximum voltage rating higher. As a result, some design optimization between lower parasitic Miller voltage, switching losses, over-shoot voltage and voltage oscillation of both on and off gate resistors would be required. Hence, this is not an ideal solution.

Additional gate emitter capacitor

Adding a capacitor between the gate and emitter will influence the switching behavior of the IGBT. Its job is to take up additional charge originating from the Miller capacitance. The gate charge necessary to reach the threshold voltage, however, increases. This increases the required driver power and the IGBT exhibits higher switching losses for the same gate resistor.

Negative power supply

The usage of negative gate voltage to safely turn-off and block an IGBT is typically used in applications with nominal current above 100 A. Due to cost, negative gate voltage is often not utilized in IGBT applications below 100 A. The addition of a negative supply voltage increases design complexity.

Active Miller clamp

Another measure to prevent unwanted IGBT turn-on is proposed by shorting the gate to emitter path. This can be achieved by an additional transistor between the gate and emitter. This 'switch' shorts the gate-emitter region after a certain gate-emitter voltage is reached. The currents through the Miller capacitance are shunted by the transistor instead of flowing through the output driver pin. This technique is called active Miller clamp.

Unlike the earlier gate resistor and capacitor solutions, the Miller clamp will pull the gate voltage to a low value until a fixed gate threshold is reached. Hence this can be viewed as a general solution that will work at different operating conditions compared to the fixed gate and capacitor design. The only drawback is the addition of the transistor and passive components that would increase design size.

The gate-resistor and gate-emitter capacitor solutions are typically used in smaller power application (IGBT rating less than 25 A) and for designs that are more cost sensitive. The negative supply and active Miller clamp are more suitable for medium to higher power applications, such as industrial motor control, uninterruptible power supplies and industrial inverters, where protection and safety outweigh cost.

The active Miller solution is a lower cost alternative to adding a negative voltage supply. However, for applications with nominal IGBT current above 120 A, a gate driver with dual power supply (negative supply voltage included) can be used, as cost sensitivity is significantly reduced. Also, the requirement for a higher current Miller clamp transistor for larger IGBT rating should be taken into consideration.

In recent years, integrated IGBT gate drivers have included the active Miller clamp solution along with de-saturation protection and under-voltage lock-out. This approach has helped to reduce design complexity and product size for many power designers and industrial/consumer manufacturers.

References:
1. Avago Gate Optocoupler Datasheet ACPL-332J / ACPL-331J
2. Active Miller Clamp , Avago Application Note AN5315
3. Semikron Application Manual , Chapter 3.5 Driver
4. Semikron Application Manual , Chapter 1 Power Semiconductor Basics

Related Control Engineering knowledge base: ' Harmonic Mitigation in AC Drives ' explains the need to suppress voltage distortions from adjustable speed drives before they create problems throughout your plant.

Gary Aw, Avago Technologies ,
Control Engineering Machine Control eNewsletter
(Read the latest; subscribe for free using the link at the top of the page.)





No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.