Portable electronic systems require external supply from a wall adapter (AC/DC subsystem) to charge the internal battery. Now, battery packs are based on Lithium technology, which decreases the total weight of the portable solution on one hand, yet imposes additional challenges on another. For example, a bad charge sequence can induce a Li+ temperature increase, thermal runaway and burst, endangering lives.
One of the first safety measures is to protect the internal charger, which manages the battery pack charge–from the outside.
Over voltage root cause
Portable and mobile suppliers provide a wall adapter in order to be compatible with the input maximum rating of their system. In this case, the output voltage of AC/DC is well regulated to limit output ripple. Yet, in spite of the recommendation made by the suppliers to always use original devices, an after market exists: second or third wall adapters are used for travel or just to replace the original one after a failure. Depending on the complexity of the adapter, its output voltage can have output transients that far exceed the ratings of the sensitive electronic components required to make today's small portable products.
Another possible root cause of wall adapter output voltage increase is the loss of the opto-coupler feedback (SMPS charger). This failure can occur even in the high end AC/DC market. In this case, the output voltage could increase up to 20V. By using an OVP device, the system never sees this very dangerous voltage.
Over voltage can also occur if AC/DC is hot-plugged. This behavior is due to serial
inductance in the adapter cabling. The maximum ripple voltage depends on the mobiles system's input capacitance and parasitic inductance in the cable. By placing an Over Voltage Protection Device (OVP) on the mobile equipment, the hot-plugged consequences (overshoot) will be eliminated by OVP's soft start feature.
How to design
If we look at previous generations of over voltage protection devices, the pass element (N MosFet or P MosFet) is now integrated to save PCB area. The PCB area calculation of a dual die solution must take into account the package size and the layout between the two devices. A comparison between the new generation of OVP's and old driver plus MosFet solutions, gives up to 60% of saved space.
Nevertheless, PCB layout must be carefully designed when charge current justify it, to improve thermal dissipation. The curves of R junction to air are available in ON Semiconductor datasheet documents. Extra copper surface shall be added to reduce junction temperature connected to background pad. As this pad is connected to NMOS drain, the extra copper surface shall be connected to IN pins or to an isolated plane. In all cases, this area must not be connected to ground.
Another important point is over-voltage threshold definition. OVLO and UVLO thresholds are determined by internal comparators, which switch off the pass element when an under-voltage or over-voltage event is encountered.
The OVLO level must be higher than AC/DC maximum operating output voltage and lower than the maximum rating of the first component of the system. Figure 3 shows typical portable device architecture based on a fully integrated OVP device.
To guarantee the stability, an input capacitor must be placed in front of the device, as close as possible to the IN pins. The characteristics of the capacitor must be in line with those of protection device. Capacitor's DC bias curves will be checked to ensure that actual capacitor's value is high enough regarding the UVLO to OVLO voltage range. For example, let's assume that 1μF ceramic capacitor is necessary in front of the protection device:
Taking into account that the breakdown voltage of a ceramic capacitor (upper than 200V) is higher than protection device's maximum rating (30V); a 10 or 16V/1μF can be used for these products. The breakdown voltage depends on the quality of the ceramic material.
Following you will find an example of DC bias and breakdown DC voltage of 0603/X5R/1μF/16V.
It is now possible to conciliate very low Rdson with low profile package. As an example, in one device, Rdson is only 110mΩ for 2×2.5mm WDFN package. Such devices are able to sustain up to 2 Amperes DC current. The typical dropout between wall adapter and charger is only 52 mV at 25°C. Thanks to the very low losses, these products are able to support wall adaptors having low output voltage. The lower the dropout, the lower the thermal dissipation in the portable device and higher is the capability to stand the wall adapter having bad load regulation.
Thanks to innovative architectures, very fast internal switch turn off time is now compatible with very low current consumption. Downstream system never encounters over voltage transient in most cases. In mentioned example, the typical turn off time is 1μs, with a maximum of 5μs.
Additional features may be available. “Enable” pin may be available to turn on the device, or to pull up to the battery to isolate the system from wall adapter. A status pin may be used to supervise the voltage level. When this pin is an open drain input, it must be pulled up to the battery, through a minimum of 10kΩ resistor.
By connecting the status pin to a microcontroller input and the “enable” pin to an output, the OVP device can be completely turned off in the event of a constant voltage fault present on input pins. On the other hand, the microcontroller can take into account the status pin, to turn on the OVP.
New solutions for new standards
IC manufacturers are providing innovative solutions to efficiently protect applications against over-voltage. These fully integrated solutions are able to answer to most application cases thanks to 2 Amperes charge current capability and up to 28V protection, with very fast turn off transient. To be compliant with the various AC/DC output voltages, several versions are proposed with different OVLO thresholds. Rdson, turn off time, current consumption are matching the most stringent requirements.
In particular, one version is compatible with USB charge and is suitable for the new Chinese charging standard. Indeed, more and more portable devices are equipped with USB connectors and can be supplied with USB host or wall adapter with USB connectors.
About the Author
Bernard Remaury has 10 years experience in the electronic industry. He is currently an Application Engineer within ON Semiconductor's Low Voltage Power Management Group. His focus is Battery Management and Protections applications for portable power management IC's. Bernard previously worked for Motorola in the Wireless Group and was involved in the development of Motorola's Power Management Unit. Bernard can be reached at: Bernard.firstname.lastname@example.org