FEATURE COVER STORY Innovating size and power efficiency
Optimising design characteristics such as power efficiency and size are key requirements of many electronics systems. Here the latest power relay from Panasonic achieves just that
T
he miniaturisation of signal relays up to 2A and power relays up to 16A
capacity has long since been achieved. However there is now a compelling need for relays with higher breaking capacity to also reach higher density mounting to meet customers’ requirements for smaller PCB sizes. However, the aim is to achieve higher packing density (smaller size) without any significant compromises in key relay characteristics. As a good example, in the recently
developed Panasonic HE-S power relay the relative switching capacity (defined as the highest switching capacity of all contacts under specified conditions relative to the volume of the relay) that is considered to be a reliable indicator of the development trend has been boosted significantly. Crucial considerations for HE-S relay development included not only size requirements but also the requirement for minimal energy consumption. Much attention was focused on coil power
dissipation but also appreciating that power dissipation through the contacts should also be significantly improved. As an example of contact heat generation the relay is designed to be capable of conducting 35A through a make contact at an ambient temperature of up to 85°C. A standard power relay can have a constant heat loss of about 2Watts per contact assuming a typical contact resistance of 2Ohms. This results in a relay having to remove the heat generated from 4Watts for the two contacts. For a three- phase inverter that makes six contacts and therefore 12Watts. Together with the coil power of three relays with two contacts that comes to 15Watts. That is a considerable heat generation. The HE-S relay design indicates heating is much below this level. The HE-S relay is a circuit board relay
for a maximum switching power of 8.7KW at switching voltages up to 380V AC and switching currents up to 35A. The objective is 30,000 switching cycles at full load and an ambient temperature of 85°C. A small physical volume of 40cm3 still allows two normally open contacts designed as bridge contacts (this will make it easier too for the application to meet EN requirements and achieve a contact gap of more than 1.8mm). As a further special feature the relay is
12 JULY/AUGUST 2015 | ELECTRONICS
previous model and with conventional industrial relays. The compact layout of the relay is
possible thanks to a very efficient magnet system. The coil bobbin is very long and narrow with extremely close winding. This adds to the height but a very high level of efficiency is achieved which reduces control energy because leakage flux has been minimised. Together with the two movable contact
available with an auxiliary contact that monitors the make contact as a break contact. This auxiliary contact is switched by a separate actuator and is therefore electrically isolated from the contact set. The connections are externally directed in parallel to the coil terminals. This additional contact can be used to check welding of the main contacts however it does not meet the strict criteria of EN50205, which regulates conditions for relays with forcibly guided contacts.
Figure 1:
The HE-S power relay from Panasonic
springs the armature is over-moulded with plastic to form a single combined unit. This eliminates the otherwise standard actuator. This means that abrasion wear close to the contacts, which can adversely affect the reliability of contact making, is practically negligible. Once the armature is closed a high magnetic force is achieved which can be transferred directly via the contact springs to the contacts. Another advantage is the excellent resulting shock and vibration resistance. This magnetic circuit, with the switching contacts embedded, is then placed on a plastic body in which the stationary contacts are joined by high bond gluing. This is further strengthened by the fact that the two contact springs with the corresponding stationary contacts are arranged in contact chambers that are isolated from each other and from the coil. This makes it possible to achieve a breakdown voltage of 5000Vrms between the contact sets. A further improvement in this relay
design as a special feature is the arrangement of the terminals relative to the circuit board. While conventional 2 Form A relays run vertical to the coil terminals in the HES relay the conductor tracks are already unbundled in the relay. This can be seen in Fig. 2. For the user this means it will be easier to set-up the conductor tracks on the circuit board. As previously indicated the HES relay
SOLAR INVERTER APPLICATION As the HE-S relay was developed for applications in solar inverters nominal power is an especially important specification. To overcome the large contact gap >1.8mm, the armature travel is correspondingly large. This results in a high coil power requirement with 1.8W of power although this can be reduced to a nominal holding power of 230mW after about 100ms. This is a considerable decrease in required coil power compared with the switching capacity of the
Figure 2:
Example showing two Form A relays running vertical to the coil terminals in the HES relay with the conductor tracks already unbundled in the relay
has been developed initially for the solar market as an output relay for use in solar AC/DC inverters. However it also has many other possible markets since the relay works without a plastic actuator and is largely wear-free and so is also extremely resistant to shock and vibration. This makes it an ideal relay for use in charging cables for electric vehicles – an application that has much in common with solar inverter applications. Another promising area for new
applications is the storage batteries market. The relay can achieve amazing results for smaller DC loads.
Panasonic Electric Works UK
www.panasonic-electric-works.co.uk info.pewuk@
eu.panasonic.com
/ ELECTRONICS
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