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 a switch-off event of an IBGT. The blue line shows the voltage overshoot at the IGBT terminals. The purple line shows total current.
- A snubber circuit – the traditional solution
- Reduce stray inductance using:
- Integrating DC link capacitors directly onto the bus plates
- 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 – 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:
- Integrating a DC link capacitor directly onto a DC link bus
- 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.
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
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)|
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.
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