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Posted on 01 November 2019

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High-frequency gate drive module features ultra-flat design

A new family of versatile 1.7kV gate driver cores has been specifically tailored to the needs of fast switching and high-power systems. With their innovative high-voltage planar transformers, these 20W, 60A modules of the 1SC2060P series greatly outperform conventional ring-core solutions in terms of power density and flatness.

By Sascha Pawel and Jan Thalheim, CT-Concept Technologie AG, Switzerland

 

The driver core is literally the central building block of power electronics systems. It comprises all signal transmission functions between the primary “digital” side and the secondary “power” side. It is also here that the circuit’s “brain” acquires its “muscles”, as the incoming digital signals are translated into powerful gate drive patterns that directly control the IGBT or MOSFET gate.

Form Factor

A single type of driver core can serve a multitude of different applications. This versatility is associated with application-specific circuitry that can be connected between the driver core and power switch, such as gate resistors or active clamping devices. An adaptor board often holds both the driver core and the application-specific components. The form factor with respect to total height is usually dictated by the bulky transformer housing sitting on top of the gate driver board. Considerable space is made available in the overall power system by ensuring that bus bars, phase outputs or capacitors do not reach the full height of the gate drivers.

The new 1SC2060P flat driver core with planar transformers adds as little as 6.5mm in total height to the system, compared to some 15mm in conventional designs.

Power Density

Planar DC/DC converters are known to transfer significantly more power per board area thanks to their low-profile transformer. The magnetic core extends over a larger area, thus providing more cooling surface per volumetric unit of ferrite. Figure 1 shows the planar DC/DC transformer and planar signal transformer integrated into the driver board.

1SC2060P 20W, 60A planar gate driver core

A comparison of the driver output power per unit board area in Figure 2 reveals a significant difference between conventional solutions and the planar 1SC2060P.

Output power per board area for different gate driver cores

Low Stray Inductance

Stacked planar windings on circuit boards have an established reputation for low stray inductance. This benefit is widely acknowledged by designers of low-voltage DC/DC solutions, such as point-of-load (POL) converters. Not all the usual advantages of planar designs can be fully exploited in a gate driver, due to the strict 1.7kV insulation requirements. A dedicated layer structure has been developed and successfully applied, thus maintaining a very satisfactory 92% converter efficiency for the 1.7kV insulated DC/DC converter. The signal transmission can take even greater advantage of the low stray inductance. A lower stray magnetic flux is directly linked to a higher di/dt tolerance, so that magnetic fields originating from the power switches and wiring cannot affect the integrity of the driver signal.

High-Frequency Gate Drive

The 1SC2060P is the first planar transformer driver based on the highly integrated SCALE-2 chipset of ASICs. Along with all the benefits of monolithic integration, it offers extremely fast signal processing with typical driver delay times of below 80ns. Figure 3 shows the attainable drive power versus switching frequency. The data refers to free air cooling with no fans or heat sinks attached to the driver and is valid over the full ambient temperature range from –40°C to 85°C. The gate resistor parameter in Figure 3 refers to the resistance loop consisting of both turn-on as well as turn-off resistors. All internal gate resistances of the IGBT module are also included in the loop. The gate resistor value has some influence on the maximum output power for loop resistances lower than 4,8 Ω. At low frequencies, the full power of 20W and higher can be utilized. This is ideal for the parallel driving of multiple IGBT modules. Paralleling is also aided by a low delay-time jitter, thus assuring reproducible and uniform switching across parallel IGBT modules. Figure 4 shows a measurement over 185,000 typical turn-on cycles. The 6-Sigma value for the delay-time jitter is less than +/- 0.75ns.

Output power vs. switching frequency (free convection cooling)

Even at the lowest permissible gate loop resistance of 2.0Ω, the rated output power remains constant up to 130kHz, thus leaving ample reserve for even the most demanding hard-switching applications.

Delay time and jitter measurement

At the high-frequency end of the spectrum, the driver core may be used for resonant switching topologies with 8W of usable output power at 350kHz. The ability to utilize the high-frequency switching resources of today’s IGBTs and MOSFETs allows power systems to be made smaller, lighter, and more efficient (Efficiency is, of course, the single most important building block for sustainable energy in future). Figure 5 shows the output voltage swing versus the output power of the driver. The turn-on voltage is held constant over the whole power range to ensure an optimum trade-off between IGBT conduction losses and short-circuit manageability.

Output voltage swing vs. gate drive power

The drive current rating of the 1SC2060P is 60A. The driver platform can easily be scaled up to even higher currents. However, this will limit the maximum switching frequency. The dimensions of the driver board are unaffected by any increase in the output current.

Reliability

The FR4 board substrate is a composite material that lends itself quite naturally to HV insulation. It consists of woven glass-fiber strands embedded in an epoxy resin matrix. The planar architecture of the FR4 has undergone rigorous reliability testing to make sure that its inherent thermal and electrical benefits are accompanied by the long-term reliability expected of power system components.

Its insulation stability has been proven over more than 2600 slow thermal cycles between –40°C and 125°C (Figure 6). Thermal shocking between –55°C and 150°C confirmed the extreme stability of its insulation characteristics and mechanical properties. Long-term storage testing in a warm and humid environment (85°C and 85% relative humidity, RH, over 1000h) has been successfully completed. Finally, repeated mechanical shock tests were performed at 200g (2000m/s2) with absolutely no visible or measured degradation.

Insulation performance (partial discharge extinction voltage) vs. thermal cycling

Withstanding Conductive Anodic Filaments

Every circuit board has to prove its ability to withstand conductive anodic filaments (CAF). These are a well-known phenomenon in fiber-reinforced board material, such as FR4. Conductive paths, called filaments, may grow when an electric field is applied in humid environments.

A number of tests are run throughout the electronics industry to ensure long product life. The most popular ones involve 500h (Sun Microsystems) and up to 1000h (IPC) of testing under a DC bias and climatic conditions that strongly accelerate the mechanism. A criterion for more than 20 years service life is derived from a dedicated IPC standard (TM-650-2.6.25) and its associated document (IPC9691A). The CAF test is performed under a DC bias and 85°C, 85% RH (relative humidity) for a maximum of 1000h. The failure rate after 1000h can be correlated to an expected field failure rate as specified in IPC9691A.

Figure 7 shows the results of CAF testing to IPC-TM-650-2.6.25. The diagram is a three-parameter Weibull plot linking the test time to accumulated test failure rates. The further right on the graph the curve runs, the better the CAF resistance is. Starting from the left border, we see the very high failure rate of a straightforward planar design such as that used for low-voltage inductors and DC/DC converters. Both the standard FR4 material and the board design are responsible for this unacceptably high level of CAF activity.

CAF test data- Weibull plot of THB

The green group of curves illustrates the measured performance of tried-and-tested non-planar boards. In such designs, the insulation is implemented by lateral spacing between the primary and secondary sides of the driver. An absolute minimum of a 6mm spacing between conductors is permissible in 1.7kV systems as stipulated by EN50178. Such designs typically failed after between 500 and 800h in the CAF test (though this does not establish a fundamental limit).

This fact shows the severity of the test criteria, since CAF failures are not observed frequently – if at all – over the product lifetime of the widely used 6mm-spacing designs.

The blue curve to the right shows the CAF withstand capability achieved by planar transformers in the newly developed high-voltage (HV) design when supported by appropriate material selection. The driver boards use readily available low-CTE and halogen free FR4 of specified origin.

The last curve to the very right shown in orange illustrates the additional improvement of the dedicated HV planar design achieved by optimizing the chemical, mechanical and thermal parameters of the board-production process.

The CAF test data show clearly that there is no lifetime limitation compared to conventional power designs when carefully optimized planar technology is used. This finding is also supported by applying the IPC test criterion. Figure 7 shows three points marking the IPC test criterion at 900V, 1300V, and 1500V, each representing 5% accumulated failures over 20 years in worst case operation. No CAF failures are to be expected at any of these voltages, since the Weibull curve for the final planar design does not cross the criteria locations on the time axis. These tests thus show that the expected CAF failure rate over 20 years is 0% on the basis of IPCTM650.2.6.25.

In real-life applications, the maximum DC link voltage of 1.7kV power systems may reach 1500V only in extreme cases and is usually some 1200V. Moreover, the maximum average voltage across the driver insulation is even lower than the DC link voltage because the respective IGBTs or MOSFETs are switched on for some time during the cycle. All these cases are reliably handled by the HV planar transformer driver 1SC2060P.

Summary

A new gate driver core is introduced that has been specifically designed for both high-power systems and high-frequency gate drives. The 20W driver module features CONCEPT’s novel planar transformer technology and the second generation SCALE-2 chipset. This combination allows extremely fast switching up to 350kHz.The driver is truly in perfect shape for today’s power applications with its high power density and ultra-flat form factor.

 

 

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