Zoltán Kiss- Area Sales Manager - East Europe - Endrich GmbH.
Special combined and hybrid circuit protection solutions
5 December 2012
Summary :
In the first part of the article we introduced the PolySwitch solutions for over current protection. There are a number of specially designed protection elements in the delivery program of Tyco Circuit Protection, which were designed for special purposes, and providing combined solutions to meet the engineers expectations in the field of circuit protection.
As we discussed earlier, overcurrent usually arise from overvoltages caused by abnormal operation of the power supply or the voltage peaks and surges coming from the power line. Over-voltage and over-current events therefore should be handled at the same time.
Combined protection – Hybrid solutions
PolyZen devices
A wide variety of portable electronics are used in every households, and the constant connection - disconnection cycles expose them to circuit damage caused by user errors, wrong supply voltages, reverse bias or transients. Zener diodes as traditional clamping devices represent the simple circuit protection solution for portable electronics. However, in order to withstand the potential power output of an incorrect power source used, while still providing resettable protection, this diode must be capable of dissipating nearly all of the power that the power source could deliver. The resultant protection solution would therefore require both a large diode and significant heat-sinking infrastructure.
PolyZen devices are micro assemblies of active over-voltage and passive over-current protection elements, that help protect sensitive electronics circuits from damage caused by transients, inductive voltage spikes or use of incorrect power supplies, even with reverse polarity. The structure of the device incorporates a fast acting, precise voltage clamping semiconductor based over-voltage protection (Zener diode) and a resistively non-linear, polymeric PPTC layer, that acts as a current limiting device. When the Zener diode becomes effective against an over voltage event and shunts the excess power to the ground, the current flowing through the device will cause an I2R self heating on the PPTC layer, and the additional heat generated in the Zener diode will also warm up the PPTC layer. This double effect causes PolySwitch element transitioning to high resistance state, which would then limit the fault current.
This small element reduces design cost by making possible to use a single element for protection up to hold current of 2.3A and power handling up to 30W. Also reduces the maintenance cost, since the PPTC PolySwitch part provides resettable protection solution. Consumer electronics require strong protection in order to avoid costly warranty issues. The new low-profile PolyZen CE (consumer electronics) series (height :1 mm) is rated at 2.6A as an industry standard for tablets, and integrates a precision Zener diode as overvoltage protection with 5.6 and 13.2V zener voltage in a single SMD assembly.
2PRO devices
The 2Pro family of TE Circuit protection is also an integrated overcurrent and overvoltage protection solution, but instead of the Zener diode this device has a metal oxide varistor (MOV) thermally coupled with the PolySwitch PPTC device . The element provides an excellent combined protection on the AC input lines. When overvoltage caused huge overcurrent arises, the single MOV may burn down by shunting all excess power to ground. In this special configuration the incorporated PolySwitch element would not only protect against thermal runaway, but also protects the MOV from burning. Traditional solutions, like using MOVs with blowing or thermal fuses would be never as exact as the 2PRO solution, which provides resetting function , thermal stability and simpler design, all cost reduction factors. Especially used on inputs affected by dirty AC sources ( LED lighting, smart meters etc), the 2PRO family offers a reliable and space/cost-saving solution for circuit protection designers. The device is available in 150mA, 350 mA and recently in 750 mA (AC) hold-current versions.
Metal Hybrid PPTC (MHP) devices
The growing demand of LI-ion batteries used in power tools, e-bikes and backup power supplies has generated the need of a cost efficient circuit protection solution capable of handling over 30A hold currents at over 30V DC.
These applications require robust, reliable circuit protection, they used to be complex, and expensive due to using special protection ICs, MOSFETs. When using conventional bimetals, the contacts should be large enough to handle high currents, therefore the overall device size is quite large. The number of switching is limited by the contact damage caused by the constant arcing between the contacts. In comparison the solution of TE Circuit protection would replace or can reduce the number of FETs and their heat sinks, therefore saves costs To achieve the goals of safe operation and cost effectiveness, a bimetal protector should be used, which is extended with a PPTC parallel device to help the bimetal acting precisely. In normal operation, as shown on the figure, the current flows through the bimetal due to the low resistance of its contact. In case of an abnormal even, like stuck axis of an electric motor or other event causing overcurrent, the heat generated in the bimetal would decrease the contact pressure, its contact resistance would grow. At this point of operation the current start flowing through the parallel PolySwitch element, that has low resistance at the moment. The I2R self heating of the PolySwitch caused by the leakage current would also heat the bimetal, keeping it in open and it latched position, preventing arcing of the contacts. The device is a perfect solution for Li-Ion battery protection. Moor protection as well as UPS designs. Double make, double break contact design allows high current rating in a smaller device package. The MHP is able to withstand heavy vibration and its impacts, rated at 36V (DC) and 100A maximum at 30A hold current.
An even more advanced version is the MHP-SA series, which is incorporated with a smart activation, a third terminal as a signal line, which enables the device to take advantage of the protection control IC monitoring the battery . If abnormality is detected, the IC can send an activation signal to the MHP –SA.
In normal case the contact resistance of the bimetal is lower that the PPTC, current flows through without any problem. In case of abnormal battery conditions in temperature, voltage or current detected by the monitoring system, the FET is ON, the heater PPTC activates and heats up the bimetal, the contact opens and cuts the current of the main line.
The third sub family, the MHP-TA series represent a special MHP type created to be used in portable devices such as tablets and E-book readers and ultra thin notebooks. This is rated for 9V DC and 6A and 15A hold current.
Samples and technical support can be required at the author.
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References
This article has been published on the following locations:
| # | Media | Link |
|---|---|---|
| 1 | Elektronet 2012/8 | Elektronet : elektronikai informatikai szakfolyóirat, 2012. (21. évf.) 8. sz. 22-23. old. |