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Interview


A guide to MLC capacitors with regard to dual-use legislation


Ceramic MLCC’s are finding new applications in the power and automotive industry due to their inherent very low inductance and wide operating temperature range, but there is an unexpected non-technical issue that raises its head when these parts are designed-in. Steve Hopwood, senior applications engineer at Knowles Capacitors, talks to CIE about how the perfect part for your system may bring you into conflict with international export law


Steve Hopwood, senior applications engineer C


onsider the following scenario. A designer of an invertor for a new EV application needs smoothing capacitors - traditionally leaded plastic film installed into plated through holes. This is inefficient from an assembly point of view and SMD technology would be preferred. As an MLCC has much lower inductance than film, and a higher temperature rating, it can be moved closer to the critical circuits and a lower capacitance used. Unfortunately, MLCC’s


12 March 2017


cannot offer the very high (≥10µF) capacitance needed, so several will need to be mounted in parallel, but to minimise parts count, the highest possible value will be specified. A typical working voltage of around 450Vdc, with ac ripple on top, means a 1kV rated part is needed in order to give a margin of safety. Therefore, an AEC- Q200 certified 1µF 1kV rated MLCC, dielectric type X7R, case size 3640 with inductance of ~2uH, might be ideal. Unfortunately, under the international dual-use regulations, this part is potentially export restricted. Dual-use legislation, with regard to capacitors, cascades from the Nuclear Suppliers Group (NSG) and relates to components that could have a use in military hardware under classification 3a201a2. It is an international law and most, if not all, countries have it included in their own export regulations. The NSG guidelines include that (6.A.4) dual use status applies to Pulse discharge capacitors having either of the following sets of characteristics: 1. Voltage rating greater than 1.4 kV, energy storage greater than 10 J, capacitance greater than 0.5 µF, and series inductance less than 50 nH; or 2. Voltage rating greater than 750 V, capacitance greater than 0.25 µF, and series inductance less than 10nH. MLCC’s have the potential to fall into category two even if they’re not classified as a pulse discharge capacitor. That’s because NSG doesn’t define pulse


Components in Electronics


discharge and there’s no actual clear definition of what is meant - is a capacitor pulse discharge because it’s designed to be, or because it’s capable of being one? A standard X7R MLCC may be capable of being used as a pulse discharge, irrespective of whether it was intended to be used as one. Some countries have avoided this issue by translating the NSG ruling into law without the words “pulse discharge”, taking the definition supplied by the NSG to be a suitable way of defining a pulse-discharge capacitor itself. This makes legislation easier to apply as it relates to all capacitors of any type.


Even if a capacitor with a voltage rating of 750Vdc and <250nF is selected the situation isn’t 100 per cent clear - the ruling has been clarified that the capacitance value cut-off is 250nF when measured at 750Vdc. The capacitance value of type II ceramic dielectrics (e.g. X7R) drops considerably with applied voltage. The aforementioned 1kV 1µF MLCC may be perfectly OK – so long as the actual capacitance value drops below 250nF when 750V is applied. Equally one has to consider the opposite possibility. Whilst most MLCC’s have a negative voltage coefficient of capacitance, some positive coefficient


types are now available. These can have a much higher effective capacitance at 750V than their headline stated value at 0V, making it hard to determine if a part is export restricted from the part number alone. The manufacturer will be able to confirm if a part could be considered dual-use and be able to provide graphs to back-up the statement if necessary - parts with a rated voltage <750V will be OK, but anything that suggested use was acceptable at a higher voltage than the declared rating could be seen as a way of trying to avoid the law and could incur penalties. To avoid the issue, the circuit designer could consider a radial leaded MLCC. They are covered by the legislation but in a grey area as the inductance depends where on the lead you measure it – the leads are usually around 1” to 1.5” (25m to 37mm) long and have their own inductance. Measured at the end of the lead it will probably exceed 10nH, but next to the body be much lower. Also, reducing the individual component value below the threshold and mounting capacitors in parallel on the board is routine practice and will give the same result at the expense of real estate.


Most countries demand a licence for


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