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Q&A with Tony Armstrong, Director of Marketing for Power by Linear Group at Analog Devices, who discusses safety in power designs


Question: What are the safety problems frequently encountered in low- and high-power product design, and how can engineers address these?

Answer: One common problem of switching power supplies is “unstable” switching waveforms. Sometimes, waveform jittering is so pronounced that audible noise can be heard from the magnetic components. If the problem is related to the printed circuit board (PCB) layout, identifying the cause can be difficult. This is why proper PCB layout at an early stage of a switching supply design is crucial. Its importance cannot be overstated.

A good layout design optimises supply efficiency, alleviates thermal stress and, most importantly, minimises the noise and interactions among traces and components. To achieve these, it’s important for the designer to understand the current conduction paths and signal flows in the switching power supply.

To illustrate: a switching power supply circuit can be divided into a power stage circuit and a small signal control circuit. The first includes components that conduct high current and, in general, these should be placed first. The small-signal control circuitry is subsequently placed in a specific spot in the layout. Large current traces should be short and wide to minimise PCB inductance, resistance and voltage drop. This is especially critical for traces with high di/dt pulsating current flow.

In a design without external heat sinks for surface-mounted power MOSFETs and inductors, it is necessary to have sufficient copper area as a heat sink. For a DC voltage node, such as input/output voltage and power ground, it is desirable to make the copper area as large as possible. Multiple vias are helpful in further reducing thermal stress. For the high dv/dt switch nodes, the proper size of the switch node copper area is a design trade-off between minimising the dv/dt related noises and providing good heat sinking capability for the MOSFETs.

Finally, the control circuitry should be located away from the noisy switching copper areas. It is preferable to have the control circuitry located close to the Vout+ side for a buck converter, and close to the Vin+ side for a boost converter, where the power traces carry continuous current. Space permitting, the control IC should be populated a small distance (0.5-1”) from the power MOSFETs and inductors, which are noisy and hot. However, if the space forces the controller to be located close to power MOSFETs and inductors, special care must be taken to isolate the control circuitry from power components with ground planes.

For high-power device products, the safety design requirements are the same, except that power levels and voltages are higher here. Obviously, more care has to be taken with high voltages, such as those greater than 100V. Nevertheless, one should adhere to the following points:

1. Minimise the area of the high voltage end of any secondary winding on the magnetic components;

2. Provide sufficient spacing for all high-voltage nodes (MOSFETs’ drain, output voltage and any secondary winding of the transformer), to meet breakdown voltage requirements;

3. Keep the electrical path formed by capacitors, the primary of transformer and drain of the MOSFETs as short as possible. Lengthening this path effectively increases the leakage inductance of the transformer, resulting in potential overvoltage on the MOSFET drain.

Question:       What can IC manufacturers do to improve the safety of the end products?

Answer:          They should design their devices with safety consideration during the design process. There is an industry standard in the automotive market segment specifically for this purpose: ISO26262. It applies to all activates during the lifecycle of safety-related systems comprising electrical, electronic and software components.

There are three main phases for development in ISO 26262 that affect design: Concept, Design and Test.

The Design phases are divided between System Responsible and Component Responsible. [Note:An IC on its own can’t have an ASIL rating. A system can be shown to meet an ASIL rating only when based on the Hazard Analysis and Risk Assessment (HARA)].

This standard was developed as a top-down approach to design. Its intent is to address important challenges in today’s road vehicles: Safety of new electrical and electronic systems and software functionality deal with an increasing complexity of software content and mechatronics implementation.

ISO26262 is a process of design not just documentation, although that is a key part, as well. The process is designed to find and protect against failures prior to releasing a part, as well as have comprehensive testability to prevent systemic and random failures from causing harm.

Question:       What are the latest advances being made in the field of automotive electronics?

Answer:          A newly-proposed automotive standard, LV148, combines a secondary 48V bus with the existing 12V system. The 48V rail includes an integrated starter generator (ISG) or belt start generator, a 48V Lithium-Ion battery and a bi-directional DC/DC converter for delivery of up to 10kW of available energy from the 48V and 12V batteries combined. This technology is targeted at conventional internal combustion automobiles, as well as hybrid electric and mild hybrid vehicles, as auto manufacturers strive to meet increasingly stringent CO2 emissions targets.

For this new standard, the 12V bus will continue to power the ignition, lighting, infotainment and audio systems. The 48V bus will supply active chassis systems, air conditioning compressors, adjustable suspensions, electric superchargers/turbos and also support regenerative braking. The decision to use an additional 48V bus, which is expected to be available across production models soon, can also support starting the engine, which would make stop-start operation smoother. Moreover, the higher voltage means smaller cable cross-sections are needed, which reduces cable size and weight. Today’s high-end vehicles can have more than 4km of wiring.

Vehicles will become more like PCs, creating the potential for a host of plug-and-play devices. On average, commuters spend 9% of their day in an automobile. Thus, introducing multimedia and telematics into vehicles can potentially increase productivity, as well as provide additional entertainment.

The future for the 48V battery system is very much near-term. According to some auto manufactures, a 48V-based electrical system results in a 10%-15% gain in fuel economy for internal combustion engine vehicles, thereby reducing CO2 emissions.

Moreover, future vehicles that use a dual 48V/12V system will allow engineers to integrate electrical booster technology that operates independently of the engine load, thereby helping to improve acceleration performance. Already in its advanced development phase, the compressor is placed between the induction system and intercooler and uses 48V to spin-up the turbos.

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