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Posted on 01 July 2019

Trench IGBTS Ensures Low VCE(sat)

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Low-loss sustain and energy recovery circuits for Plasma Display Panels

The latest generation of IGBTs are helping to improve the performance and efficiency of flat panel displays. The plasma display panel (PDP) is the technology of choice for large video display applications, from high-end TV and home cinema to exhibition and public entertainment systems.

By Dr. Georges Tchouangue, Toshiba Electronics Europe (TEE)

 

High clarity and brightness at large screen sizes are among the key advantages of PDP technology. However, market success also demands acceptable energy efficiency, as excessive heat dissipation adversely impacts upon reliability as well as operating costs. One energy saving technique that is used in PDP design is to implement an energy recovery circuit (ERC) in conjunction with the sustain circuitry that illuminates each cell in the PDP matrix once per full picture refresh cycle.

During the sustain discharge, very high peak currents occur. The magnitude depends on factors including the number of cells illuminated with any single sustain pulse, which in turn is influenced by the size of the panel itself. However, for a modern PDP of around 42-inches or larger, peak sustain currents above 100A can be achieved. Since the PDP presents a capacitive load of several tens of nanofarads, an ERC is implemented to recover the reactive power that would otherwise be lost. This can be as high as 4-500W in a 42-inch display.

Switching Characteristics and Efficiency

However, the sustain and recovery circuits must themselves be designed to minimise losses, as resistive losses contributed by conduction and switching inefficiencies in the power transistors of these circuits can contribute significantly to overall power consumption. Hence, when designing the sustain and ERC networks, engineers need a power switching solution that delivers an optimal combination of high peak current handling capability and high voltage operation at 300V or higher typical in PDP applications, with on-state and switching losses.

Image 1

MOSFET, or IGBT

These requirements pitch PDP designers into the thick of the territorial battle between power MOSFETs and IGBTs. The power MOSFET typically displays good high frequency characteristics, high peak current capability and easy gate drive requirements. On the other hand, on-losses (conduction losses) increase with voltage rating, firstly due to the relationship between Rds(on) and the drain-source blocking voltage (VDSS), and secondly due to ID2 (where ID is the drain current). Trench technology has allowed MOSFETs to improve RDS(ON), but the voltages present in the sustain and ERC networks of a PDP system are considered high when designing with MOSFETs.

By contrast, IGBTs have traditionally come into their own when a very high voltage rating of 600-1200V is required, well in excess of the typical operating levels in a PDP application. Despite this, the superior conduction characteristics of IGBT, arising from its minority carrier action, are desirable for PDP applications. In addition, the IGBT shares MOSFET benefits such as simplified drive requirements, as well as a wide safe operating area (SOA) and high peak current capability. In addition, IGBTs can be connected in series without requiring dV/dt snubbers.

During discharge mode, the IGBT is subjected to soft-switching, requiring, therefore, very low on-losses. In that case the product VCE IC is important, whereas in the case of a MOSFET the product RDS(ON) ID2 is essential. As a result, the MOS technology has, with some enhancements, tended to dominate the PDP application space, as far as sustain circuitry and ERC are concerned.

Best of Both Worlds

Still, engineers searching to achieve a convincing performance edge for new PDP product designs are keen to benefit from these advantages of IGBT technology, at lower voltages in the 250-400V range, and without exposure to the higher switching losses resulting from traditional IGBT construction. While MOSFET development focuses on increasing the maximum rated operating voltage, to serve PDP and other high voltage applications, new IGBT structures are emerging that reduce saturation voltage and conduction losses, and also enhance switching characteristics.

The IGBT, essentially, combines the drive characteristics of a small-signal MOSFET with the conduction mechanism of a bipolar power transistor. As such, IGBT development may focus on optimising one or the other side of its behaviour. Hence, when choosing an IGBT for a given application, it is important to consider the device properties carefully to fit in with the operation of the overall system. For example, an IGBT designed to perform well when soft switching is applied will perform differently in a hard switching application. The arrival in the market of IGBTs optimised for PDP sustain and recovery applications further highlights this trend.

Application-Optimised Technology

An example is the latest, 5th generation IGBT technology from Toshiba. Devices, such as the GT30G121 rated for 400V operation and the GT30F121 for 300V operation, combine trench semiconductor process technology with low injection enhancement mode N-channel technology to meet these requirements. The fabrication and construction of these devices is specifically optimised for PDP applications. Trench technology ensures low saturation voltage (VCE(sat)) resulting in a further reduction in typical conduction losses, while enhancement mode ensures fast switching characteristics. At the same time the maximum pulse collector current (ICP) is 120A, which is sufficient for emerging PDP designs featuring staggeredsustain- pulse design to limit I2R losses generally, while the gate-emitter voltage (VGE) is typically low for an IGBT, at 15V.

In the circuit of Figure 1 these enhanced IGBTs are used to relieve transistor drive requirements and reduce conduction and switching losses in the sustain and energy recovery circuits, at typical PDP sustain rates.

PDP drive circuits, showing replacement of MOSFETs with application-optimised IGBTs in sustain and energy recovery circuits

Caption - Figure 1

Over and above the chip technology, component designers are also paying careful attention to important package details, to further drive down losses and to optimise thermal performance for high efficiency and to ensure a reasonable isolation voltage. For instance, low thermal impedance paths between the chip and copper frame are essential to remove heat from the junction. The new Toshiba IGBTs feature electrically and thermally optimised packaging technology, including the use of copper connectors rather than traditional aluminium bonding wire. Both of the new IGBTs will operate with a junction temperature of up to 150ºC, and are packaged within the industry-standard, familiar and convenient TO-220 outline.

 

 

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