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Increasing the efficiency of high power inverters

High power IGBT-type inverters are being pushed to work at high frequencies e.g. >20kHz to avoid significant power losses and improve power density.  The stray inductance and high di/dt at each turn off cycle leads to a voltage overshoot.  The headroom required to allow for these overshoots significantly reduces the operating voltage at which the inverter can be rated.  Using a snubber circuit is the traditional solution to suppress the overshoot, however, there are component packaging technologies with exceptionally low stray inductance which address the problem without the size, cost and efficiency penalties of a snubber-based design.

Fig 1 - graph showing an IGBT

Fig 1 – Graph showing a switch-off event of an IBGT.  The blue line shows the voltage overshoot at the IGBT terminals. The purple line shows total current.

Potential solutions

  1. A snubber circuit – the traditional solution
  2. Reduce stray inductance using:
    1. Integrating DC link capacitors directly onto the bus plates
    2. Low inductance component package technology e.g. LinPak from ABB

1. The traditional solution – a snubber circuit

The classic solution for reducing voltage overshoot is the use of a snubber circuit to suppress the voltage spike – the most common topology being a resistor and capacitor in series across the switch. A snubber circuit dampens the overshoot since the voltage across a capacitor cannot change instantaneously.  Current flows for a fraction of a second, reducing the rate of voltage increase.

Fig 2 - snubber circuit

Fig 2 – diagram of a snubber circuit

However, on the down side, snubber circuits:

  • Are difficult to design for optimal performance – in particular, the voltage rating can be difficult to identify because of the complexity of transient waveforms
  • Reduce efficiency even when there is no load, since they are designed to dissipate energy
  • Increase the physical size of the design, not least because the snubber circuit must usually be mounted on a heat sink

2. Reducing stray inductance

The main benefit of reducing stray inductance stems from the fact that a snubber circuit can hopefully be removed entirely. This avoids the disadvantages listed above and leads to a smaller, more efficient and lower cost design requiring less thermal management.

There are two main options for reducing the stray inductance in a high power inverter:

  1. Integrating a DC link capacitor directly onto a DC link bus
  2. Using an IGBT module with low stray inductance and high contact area

Reducing the stray inductance of the module means less headroom required for the voltage overshoot which in turn means greater power density i.e.

  • Smaller, lower-rated components
  • Physically smaller design
  • Lower cost

Also, no snubber circuit to absorb energy means increased efficiency and reduced power loss.

2.1 Integrated DC link capacitor or DC link bus

SB Electronics (SBE) have shown that fully integrating a DC link capacitor with very low stray inductance onto the link bus means the voltage overshoot at each turn-off cycle can be improved by 20%.  This approach allows an increase in the operating voltage by 20% and therefore a significantly improved power density. Locating the elements closer together reduces resistance.  Lower resistance and lower inductance means improved efficiency, therefore, better power density and less thermal energy to dissipate.

SBE-test-kits

Fig 3 – Horizontally integrated DC link capacitors

Test kits of horizontally and vertically integrated DC link capacitors are available from PPM with a custom voltage and capacitance specification. Existing designs range in voltage from 450-1100V and capacitance 375-1500nF.

2.2 Packaging technology

In 2015, ABB announced LinPak, a new, open-standard phase-leg IGBT module topology for medium power levels. This module concept is designed to support fast and high current-density chipsets from 1200V up to 3300V. The exceptionally low stray inductance of 10nH allows a DC connection of very low inductance with sufficient area for high current densities.  LinPak offers several performance improvements over HiPak, including:

  • 65% improvement in voltage overshoot
  • Current density improvement >10%
  • Very smooth switching characteristics

LinPak medium power phase leg IBGT module

Fig 4 – the LinPak medium power phase leg IBGT module

Current rating per screw terminal – LinPak v HiPak

Module type Nominal current Phase current Current/M8 screw (phase terminals) Current/M8 screw (DC terminals)
LinPak 1000A 707A 354A 250A
HiPak 3600A 2546A 600A 600A

Current density – LinPak v HiPak v PrimePACK

Module type Current rating Footprint Current/Area
LinPak 2 x 1000A 100 x 140mm sq. 14.3A/cm 2
HiPak 3600A 140 x 190mm sq. 13.5A/cm sq.
PrimePACK TM 2 x 1400A 89 x 250mm sq. 12.6A/cm sq.

The total module inductance (including DC link) is 22nH compared to 27.5nH for HiPak – an improvement of 20%.  Moreover, the gates and auxiliary emitters are connected in parallel which means LinPak modules are easily paralleled with just one gate unit and without any significant de-rating.  Four LinPak modules in parallel can reduce the total inductance to only 5Nh, enabling a voltage overshoot reduction of up to 80% compared to a single HiPak module.

Click to see High Power Semiconductors 

Please contact us for more information or to order samples of IGBT modules or an SBE integrated DC link bus test kit 

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62Pak series of 1700V IGBT modules released for production.

Manufacturer ABB have announced that the 62Pak series of IGBT modules  is now officially released for mass production.   The 62Pak line-up consists of phase-leg modules in the 1,700V class available in either 300A, 200A or 150A current ratings. The state of the art SPT++ IGBT  diode chip-set offers best-in-class switching losses and ruggedness as well as a long lifetime in demanding applications thanks to high power cycling performance. These improvements have  been achieved by an optimized wire-bonding process, stamped spacers in the baseplate and main terminals plus a homogenous solder layer thickness.

Full switching performance up to 175°C

The 1,700V SPT++ chipset offers full switching performance up to 175 °C which allows higher over-load capability and/or improved safety margin to unexpected current surges.

Applications

The 62PAK is well suited for most power electronic applications such as:

  • Low-voltage and medium voltage drives
  • Static VAR compensators
  • Uninterruptable power supplies
  • Induction heating
  • Traction auxiliary converters.
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Thermal modelling now easier and quicker for IBGT modules

IGBT HiPak modules

Thermal modelling data now available for IBGT modules from ABB

Thermal descriptions for ABB’s IGBT power modules can  now be downloaded for use in the PLECS simulation platform. Engineers can save time and reduce the risk of errors by using preassembled thermal descriptions created by the manufacturer, replacing the need for time-consuming manual input of the product specification data.

New ABB IBGT 62Pak 1700V module with SPT++ chipset

ABB’s newest IBGT module is a 62Pak, 300A phase leg module  The SPT++ chipset offers a highly rugged and reliable solution, specified to 175 degrees C.

The ABB SEMIS web-based simulation tool

Engineers who don’t currently use PLECS can use ABB’s own web-based simulation tool SEMIS.  Available via the PPM High Power Semiconductors web page, using a series of simple steps, users can evaluate the performance of ABB products for common converter applications and make preliminary component selections for their design.

thermal modelling image

PLECS image

ABB semis image

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Dean MOV modules now UL 1449 certified

The TPM series of thermally protected MOV modules from Dean Technology are now certified according to the UL 1449 4th edition standard which defines requirements related to testing, marking and construction. The standard helps ensure that suppression devices include a thermal fail-safe to provide extra assurance against risk of fire or similar negative events.

The TPM series have high surge current capability and provide floating remote signalling contact for fault indication. They are available with surge ratings from 22 to 50kA and voltages from 150V. The whole series is now approved for UL 1449 4th edition for Type 1 and 2, and CSA for Type 2. Specifications can be found here on the PPM Power website. All of the TPM series are available immediately.

“The updated approval of the TPM series not only means that DTI has some of the best thermally protected suppression products, it also lays the groundwork for the next protection products in our extensive roadmap,” says Raj Maharaj, product manager for the CKE product line. “We work very hard to ensure we are up to date with all of our product offerings, pushing the boundaries with new suppression technology, and maintaining products that meet historic needs. The TPM series is a perfect example of that commitment.”

TPM-25 (2016.05.04)
TPM-50 (2016.05.04)
SS-P-DRS-Series
TPM-Series (2016.05.04)

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Choosing a programmable power supply (part 2)

In part one of choosing a programmable power supply we discussed voltage, current, cooling and power requirements for your application. Here, we explore some of the more subtle aspects of the specification. We discuss parameters such as accuracy and repeatability, the different between ripple and noise and show how power supplies can be connected in series or parallel to increase the maximum voltage or current whilst delivering the same technical performance.

Accuracy, stability & repeatability

Accuracy of display is how close to the voltage and current display reflect the actual voltage and current being supplied. Typically, these range from 0.001% through to 1% of full scale. Note that the analogue read-back accuracy won’t be the same as the display or RS-232 because different circuitry is used to present the value. e.g. LCD readouts will be limited by the number of digits it can display.
Accuracy of set point is the difference between the “demand”, to what is actually delivered. Again, these range from 0.001% to 1%. Power supplies that offer multiple programming interfaces, will specify different accuracy figures for each interface.
Stability is often quoted as the short-term drift of output voltage and current. A stable output will resist changes in ambient or internal temperatures and other aging effects over time e.g. stability over 8hrs = <0.5%.
Repeatability is the degree to which a user can leave one set point, perhaps due to a power cycling event, and achieve the same output values at a later point in time. The built-in monitoring of programmable power supplies makes this relatively easy to check.
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PLECS 3.7 – improved thermal modelling + processor-in-the-loop

PLECS – specialists in simulation software for power electronics – have released version 3.7 of their Blockset and Standalone products.

The updated products now feature:

    • Improved Thermal Modelling
      Semiconductor losses can now be described using functional expressions in addition to lookup tables. It is also possible to define custom parameters (such as gate resistance) and describe their influence on the device losses.

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Low cost, high accuracy digital high voltage meter available

HVM40B Digital High Voltage Meter

 

Manufactured by Dean Technology, the new HVM40B high voltage voltmeter can be relied upon for highly accurate measurement of positive or negative voltages up to 40,000 volts.
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New 30 kV Discrete Diodes

PPM Power has added two new Dean Technology discrete diodes – the UX-F30B and CL03-30 – to its comprehensive range.  The new axial lead diodes have a peak inverse voltage rating of 30kV and are higher voltage additions to the existing UX and CL03 series.
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IGBT HiPak Modules Enhanced – Improved Reliability

IGBT HiPak modules

ABB’s high power IGBT ‘HiPak module’ semiconductors have recently been revised to incorporate a number of benefits. These include:

  • Improved reliability
  • Enhanced processes
  • Better package design.

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Genesys Programmable Power Supplies now available up to 1500V

TDK Lambda Genesys Series programmable power supply

The TDK Lambda Genesys series of 10kW and 15kW programmable DC power supplies is now available with output voltages of 800V, 1000V, 1250V and 1500V. These new models have the same dimensions as the existing 7.5V to 600V models – 19” wide and 3U high.  The units can operate in either constant current or constant voltage modes and accept either three-phase 400V AC or 440V AC inputs.
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More Information?

Telephone +44 (0)1793 784389 or email: sales@ppm.co.uk