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Retrofitting Wide Band Gap Devices to classic Power Modules using Silicon RC Snubbers
Abstract
Parasitic inductance causes voltage overshoot and oscillations in classic hard-switched commutation cells and power modules. Newly introduced silicon-based resistive capacitors (SiRC) can short the switching cell inside the power module itself and thereby unlink external parasitic inductance. Classic and mechanically robust power modules are now capable of switching large currents with unlimited turn-on and turn-off speed. This approach minimizes voltage spikes and parasitic oscillations without use of integrated or external attached pulse capacitors. Additionally, cutting down switching losses reduces chip area, saves energy and gains power rating of these SiRC-based power modules.
... 1200 V [3]). This overshoot and the high frequency ringing, can be significantly reduced by an RC snubber circuit [4][5][6]. In contrast to the use of a single capacitor as snubber [1], the RC snubber is not only reducing the overshoot, but also damping the high frequency oscillations. ...
... The snubber design of [1,2,[4][5][6] relies mostly on the switching behavior of the power module itself. In contradiction, this research takes the power module in a three-phase two-level VSI application operating with continuous space vector modulation (SVPWM) into account. ...
The maximum voltage overshoot of SiC MOSFETs in power modules limits the switching speed of voltage source inverters in electric vehicles. As the efficiency increases with faster switching transitions, decreased commutation loop inductances allow higher switching speeds at same voltage overshoot level and extend the vehicle’s range. In this research, a SiC power module including an integrated RC-snubber is designed to increase the switching speed at same voltage overshoot. The RC-snubber acts as a damped low inductance commutation loop filtering the switching transients and suppressing the ringing, whereas the low frequency components are filtered by the dc-link capacitor. To meet the target of increased system efficiency, the losses of all inverter components are analyzed for space-vector modulation (SVPWM)
Due to the increased switching speed and reduced device capacitance, the parasitic oscillation of the switching device has become the primary high-frequency EMI noise source in SiC converters. The DC-link snubber offers a potential to mitigate the noise in a cost-efficient way. However, the temperature limits and thermal de-ratings of the existing ceramic capacitor hinder the exploration of module-level integration. In this work, a silicon-based
RC
snubber is designed to fit the working temperature of the power module. With the developed four-order damping and thermal coupling models, the iterative method is proposed to optimize the integration performance. The experiments validate the temperature stability of the integrated snubber. With the aid of the proposed
RC
snubber, the EMI noise induced by the parasitic oscillation in the prototype module is reduced by 7 to 22 dB compared with the non-snubber module with ceramic decouplers.
The dynamic current imbalance between paralleled SiC
mosfet
s will cause unbalanced losses and reduce current capacity. The existing current balancing methods will make the circuit more complex or hard to design and implement. Hence, this article proposes a dynamic current balancing method by connecting monolithic Si-
RC
snubber between paralleled
mosfet
s, which maintains a simple circuit structure and is easy to implement. Balanced dynamic currents can be achieved by adjusting the
RC
snubber connecting location. Meanwhile, low voltage spike and low switching oscillation can be achieved by using
RC
snubber. First, an analytical current sharing model is established for paralleled SiC
mosfet
s. The key parameter affecting the balance of the drive circuit and then affecting the dynamic current sharing is obtained for the first time. Based on the conclusions of the model, this article then presents the current balancing method using
RC
snubber. Then, a power module with monolithic Si-
RC
snubber is designed, fabricated, and tested. Experimental results verify the current sharing model and the current balancing method. The current difference of the optimized module is reduced by more than 50% under different conditions. Finally, the effectiveness of this method in the module with more paralleled
mosfet
s is verified.
The development of low-inductance components suitable for fast-switching wide bandgap inverters has been a major focus of the last years. To realise fast and clean switching without ringing, most power modules are equipped with integrated high-frequency capacitors. In order to damp the oscillation, it is quite common to integrate additional resistances. This paper introduces a DC-link inverter concept that requires no internal high-frequency capacitor due to a
small stray inductance in the current commutation loop. The prototype is composed of a hybrid DC-link with optimised DClink capacitor technologies, which can be used for a costefficient and compact inverter concept. We discuss the effects of the parasitic elements of the different DC-link capacitor technology and investigate into a low ESL capacitor interconnection with two different capacitors in order to realise a current commutation loop (CCL) inductance below 6 nH.
Monolithically integrated RC-snubbers were realized by metal-insulator-semiconductor capacitors on a silicon substrate also serving as a series resistor. These devices provide a promising alternative to passive surface-mounted device components that are commonly used for snubber applications in power electronic circuits. The surface area of the substrate was enlarged with circular trench structures to increase the integration level of the capacitor, and a silicon nitride layer with a thickness of 1.05 μm was deposited on top of a 20 nm-thin silicon oxide layer as a potential dielectric for applications up to 600 V. With the trench geometry, a capacitance per surface area of 0.6 nF/mm2 was achieved, which is more than ten times the capacitance of a planar device using the same dielectric layers. However, combining the thick silicon nitride layer with the trench geometry caused an excessive wafer bow of nearly 800 μm, so deposition and structuring of a surface passivation layer, such as polyimide, was not feasible. Therefore, inert oil had to be used as a surface passivation for high voltage measurements. The silicon nitride
dielectric exhibits a leakage current density lower than 0.3 nA/mm2 at the requested 600 V operating voltage, while dielectric breakdown of the devices is observed at 1050 V. A low deviation in capacitance and series resistance across the wafer and a high yield regarding the high voltage stability is achieved because of the good quality and homogeneity of the silicon nitride
dielectric layer.
Silicon RC-snubber for ringing suppression in 900 V applications
- N Boettcher
N. Boettcher et al. "Silicon RC-snubber for ringing
suppression in 900 V applications", 31st IEEE International Symposium on Power Semiconductor
Devices and ICs ISPSD, Shanghai, China, 2019