This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.
Portable electronic devices use internal batteries that need to becharged from an external power supply, most usually an AC/DC walladapter. Li-ion battery technology has decreased the total weight ofportable devices.
However, a bad charge sequence can induce Li+ temperature increase,thermal runaway and burst, and endanger people's lives. One of theprimary safety measures is to protect the internal charger, whichmanages the battery pack charge, from the outside.
Causes of overvoltage
Portable electronic devices come with a wall adapter that is compatiblewith its maximum input rating. In this case, the output AC/DC voltageis well regulated to limit output ripple. But, despite recommendationsfrom suppliers to always use original adapters, an after-market exists:second or third wall adapters are used for travel or just to replacethe original one after a failure.
Depending on the complexity of the adapter, its output voltage canhave output transients that far exceed the ratings of the sensitiveelectronic components required to make today's small portable products.
|Figure1: Using an overvoltage protection device protects systems fromdangerous wall adapter transient voltage increases.|
Another possible cause of wall adapter output voltage increase isthe loss of optocoupler feedback in a switch-mode power supply charger.This failure can occur even in the high end AC/DC market. In this case,the output voltage could increase up to 20V. Using an overvoltageprotection device (OVP) protects systems from this dangerous voltageincrease as shown in Figure 1 above.
Overvoltage can also occur if an AC/DC is hot plugged. This behavioris due to the serial inductance in the adapter cabling. The maximumripple voltage depends on the mobile system's input capacitance andparasitic inductance in the cable.
Protection by integration
Using an overvoltage protection device (OVP) can protect a mobiledevice against these causes of overvoltage. Compared with previousgenerations of OVPs, the pass element (N MOSFET or P MOSFET) has nowbeen integrated to save PCB area.
The PCB area calculation of a dual die solution must take intoaccount the package size and the layout between the two devices. Acomparison between the new generation of OVP's and old driver + MOSFETsolutions, gives up to 60 percent of saved space.
The PCB layout must be carefully designed to improve thermaldissipation. Extra copper surface must be added to reduce junctiontemperature connected to the background pad.
As this pad is connected to NMOS drain, the extra copper surfaceshall 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 overvoltage threshold definition.Overvoltage lockout (OVLO) and undervoltage lockout (UVLO) thresholdsare determined by internal comparators which switch off the passelement when an under- voltage or over-voltage event is encountered (Figure 2 below ).
|Figure2: OVLO and UVLO thresholds are determined by internal comparatorswhich switch off the pass element when an under-voltage or over-voltageevent is encountered.|
The OVLO level must be higher than the AC/DC maximum operatingoutput voltage and lower than the maximum rating of the first componentof the system. Figure 3 below shows the typical portable device architecture based on a fullyintegrated OVP such as the ON Semiconductor NCP347MTAE.
To guarantee stability, an input capacitor must be placed in frontof the OVP, as close as possible to the IN pins. The characteristics ofthe capacitor must be in line with those of protection device.
|Figure3: The typical portable device architecture based on a fully integratedOVP such as the ON Semiconductor NCP347MTAE is shown.|
The capacitor's DC bias curves should be checked to ensure thatactual capacitor's value is high enough regarding the UVLO to OVLOvoltage range. For example, let's assume that 1µF ceramiccapacitor is necessary in front of the protection device:
Taking into account that the breakdown voltage of a ceramiccapacitor (higher than 200V) is higher than the OVP's maximum rating(30 V); a 10- or 16V/1µF can be used for these products. Thebreakdown voltage depends on the quality of the ceramic material.
|Figure4: A capacitor's DC bias curves will be checked to ensure the actualcapacitor's value is high enough regarding the UVLO and OVLO voltagerange|
Figure 4 above shows anexample of DC bias and Figure 5 below shows breakdown DC voltage of 0603/ X5R/1µF/16 V.
It is now possible to conciliate very low Rdson with a low profilepackage. For example, the NCP347's has an Rdson of only 110m? for a 2mmx 2.5mm WDFN package. Such devices are able to sustain up to 2A DCcurrent.
The typical dropout between wall adapter and charger is only 52mV at25°C. Thanks to the very low losses, these products are able tosupport wall adaptors having low output voltage.
The lower the dropout, the lower the thermal dissipation in theportable device and higher is the capability to stand the wall adapterhaving bad load regulation. Innovative architectures have enabled veryfast internal switch turn-off time to be compatible with very lowcurrent consumption.
|Figure5: The breakdown voltage depends on the quality of the ceramic materialused.|
Downstream systems never encounter overvoltage transient in mostcases. In the example above, the typical turn-off time is 1µs,with a maximum of 5µs. An “enable” pin may be available to turnon the device, or to pull up to the battery if we wish to isolate thesystem from the wall adapter.
A status pin may be used to supervise the voltage level. When thispin is an open drain input, it must be pulled up to the battery,through a minimum of 10k? resistor. Connecting the status pin to amicrocontroller input and the “enable” pin to an output, the OVP devicecan be completely turned off in the event of a constant voltage faultpresent on input pins.
On the other hand, the microcontroller can take into account thestatus pin, to turn on the OVP.
New solutions, standards
IC manufacturers are providing innovative solutions to efficientlyprotect mobile devices against overvoltage.
The NCP347 and NCP348 from ON Semiconductor are perfectillustrations of this trend. These fully integrated solutions are ableto meet most application needs thanks to their 2A charge currentcapability and up to 28V protection, with very fast turn off transient.
To be compliant with the various AC/DC output voltages, severalversions are proposed with different OVLO thresholds. Rdson, turn offtime, current consumption are the most stringent requirements.
In particular, one of these versions is compatible with USB chargeand is suitable for the new Chinese charging standard. Indeed, more andmore portable devices are equipped with USB connectors and can besupplied with USB host or wall adapter with USB connectors.