Switching high voltages and currents with PCB-mounted relays
The increasing automation and electrification of our daily lives is leading to a growing demand for safe, robust and energy-efficient ways to switch relatively large amounts of power. Domestic solar-power systems use relays to control the flow of current for local consumption via an inverter, to battery storage, or back into the power grid. And electric vehicle (eV) chargers use relays to control the flow of large amounts of energy into car batteries, as well as to protect users by implementing fail-safe charging mechanisms and protection against faults such as ground current leakage.
What is perhaps surprising is how much power is being switched by these relays. Domestic car chargers may charge with 7 or 22kW of AC current while Tesla’s Superchargers are rated to 250kW with a high voltage DC charger. Lotus’ Evija electric hypercar has an 800V battery pack which, the company says, has been engineered to charge at up to 800kW. This implies that Lotus will have to develop a power delivery system - including suitable switching methods, cables, connectors and safety features - that can handle 1kA at 800V DC. That will be a challenge.
The high-voltage, high-current design challenge with DC current
This kind of power engineering represents several challenges. At the same time as designers are being asked to deliver more electrical energy, they are also trying to increase the efficiency of delivery by moving to higher working voltages.
Higher voltage operation reduces IR losses and I2R heating caused by resistances in the current path. Other ways of increasing energy efficiency include minimising these resistances and reducing the thermal impedance, or ability to shed heat, of components so that they don’t increase in temperature, thus increasing their resistance.
The downside of working at higher voltages is that it increases the strength of the electrical fields involved, which can lead to arcing between narrowly spaced conductors. Designers therefore have to factor in two more key metrics: creepage, the distance an arc can travel over a surface; and clearance, the shortest distance an arc can travel through air. These figures vary and are dependent on factors including the ambient humidity and air pressure, application-related standards - e.g. to meet medical or industrial systems requirements - and, in PCB design at least, the choice of board and coating material and the likelihood of any surface contamination.
In relay design, high voltages can lead to arcing when the electric field strengths rise as contact gaps narrow and air in the gap ionises. This arcing can erode contacts, increasing their resistance as well as causing electromagnetic interference. In the worst case, arcing can cause relay contacts to weld together, making it impossible to break the flow of current without manual intervention. This is a safety issue that must be addressed.
The ideal high-voltage relay
The challenge for designers who want to use high-voltage relays is that to date many of them have been relatively large, standalone components, designed to be mounted as a separate unit with wired connections to a control board. This creates additional costs in terms of the supporting components such as connecting leads and their terminations, sockets, mountings, and board connectors, as well as extra processing including hand assembly, inspection and test. And every interconnection, from board to leads to socket to relay connection creates a potential reliability issue.
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The ideal high-voltage relay should therefore be robust, compact enough to offer good power density and be PCB-mounted. Clearances between its (very low resistance) contacts should meet the technical requirements for suppressing arcing, as well as the relevant safety standards. The creepage distances between the relay’s board connections should also be large enough to fulfil isolation requirements, while respecting the design margins required by general and application-specific standards. And the relays should be able to switch high voltages and handle high currents for sustained periods.
Safety concerns demand that PCB-mountable high-voltage relays can withstand large inrush and short-circuit currents, as well as high voltage spikes of more than 10kV. The relays should also offer strong isolation between the control circuit and the load circuit. Environmental concerns demand very low contact resistances, to minimise resistive losses, and require that the hold currents used to keep the contacts of normally open relays closed are also low.
Finally, PCB-mounted high-voltage relays also need to be compact, robust and reliable. A product such as Panasonic's HE-V relay (left) fits these requirements. Specifically designed to handle high power DC loads, it features 2 Form A contacts, connected in series, and can handle loads up to 20A at 1,000VDC.
EV charger design for AC charging stations
EV chargers are also creating demand for power relays. Domestic and lower-power public and commercial chargers deliver AC power, relying on the vehicle’s onboard electronics to rectify it to DC and charge the batteries. Charging EVs at power levels at around 50kW uses a DC feed from the charger, as the high-power rectification is most efficiently done within the off-board charger.
Safe battery charging involves using a number of relays to connect and isolate vehicles to, or isolate vehicles from, the charger, and to ensure user safety by managing potential hazard conditions such as ground faults or creepage currents. Most of the relays used on such chargers need to handle between 16A at 250V AC and 32A at 380V AC in a three-phase system. For safety reasons the main relay in this circuit should be a normally-open fail-safe design, so that if the charger fails the current to the EV is cut off.
Relay manufacturers are adding new features to their PCB-mount offerings that help designers meet standards and ensure safety while minimising complexity. For example, the Panasonic HE-S relay, shown below, below, has two 2 Form A (single-throw, normally-open) low-resistance contacts with the wide 3.2mm contact gap required to meet the requirements of the IEC 61851--11 standard. Two relays are required in a three-phase system with a neutral line. The relay can be supplied with an auxiliary contact switched by a separate actuator, and therefore electrically isolated from the main contacts. If the main contacts become welded together, the auxiliary contacts will still retain a 0.5mm gap, and can still switch up to 1A at 230V AC. This provides a means for circuit designers to build in features to mitigate contact welding and improve system safety.
Detail of the Panasonic HE-S relay
With careful material choices, intelligent design decisions and due respect for the physics of transmitting large amounts of energy at high voltages, it is possible to achieve ongoing improvements in the functional integration of relays even in such challenging circumstances. The result for designers is smaller, cheaper and more efficient power control circuits to serve a widening portfolio of applications. The advantage for consumers is the emergence of new ways to safely use electrical energy to achieve greater convenience and efficiency in their lives…significant benefits from a relay rethink.
If you are investigating high-voltage PCB-mounting relays for your power management system, then download this white paper from Panasonic for an in-depth look at integrating these components into your design. Alternatively, if you would like to speak to one of our technical specialists, click the Ask an Expert button to get in touch in your local language.
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