Abstract: The mobile communication terminal product was already popular in ours daily life, therefore, its design’s secure question appears especially important. On the mobile termination product safe hidden danger biggest place - - power source management design, proposed some design idea enhances the product the security.
Key word: Mobile communication terminal; Power source management; May charge the lithium ion battery
Introduction
The mobile communication terminal product like GSM handset, the CDMA handset and the PHS Personal Access Phone System telephone already penetrated popularize to ours daily life, promoted China telecommunications’ development, also has brought the convenience for ours life and quickly. But simultaneously, as a result of some mobile termination manufacturer’s design defects, many times presented the handset to explode the offending somebody event, but caused the detonation the primary cause to lie in the power source management part design to have the flaw or the design existence imperfect place.
Compares with other existing batteries, may charge the lithium ion battery to have many superiority, this causes them to become suits in the portable application power source. They may provide the higher energy density (to reach 200W · h/kg or 300~400W · h/L high, respectively is Ni/Cd or the Ni/MeH battery 2.5 times and 1.5 times) and the higher battery voltage (carbon anode battery is 4.1V, graphite anodes battery is 4.2V). They have the non-memory effect, the self-discharge rate are young, but fast charging and discharging and merits and so on higher charging and discharging number of times.
The lithium ion battery’s higher chemistry energy density and the higher battery voltage causes us to be possible to make for the mobile termination product application is smaller and a lighter battery, but was light and a smaller power source most is light-sized to the present China mobile communication terminal product pursue is very important. Must want to use the battery capacity or the extension battery life fully, must control the charge parameter extremely strictly.
In view of the fact that the lithium ion may rechargeable battery’s above merit, this article introduce that in detail how to design highly effective, the safe lithium ion to be possible the rechargeable battery management electric circuit.
1 mobile communication terminal product lithium ion power source manages principle and design
The lithium ion power source manages the design mainly aims at the lithium ion battery’s characteristic to carry on. The lithium ion battery’s safety performance and the power supply performance mainly manifest in its charging and discharging parameter control. Figure 1 is the lithium battery power source management schematic diagram. This chart is composed of the control chip and the peripheral circuit. Then, do we on Figure 1 from the lithium battery electric discharge, how charge two aspects to discuss realize lithium battery’s management.
1.1 electric discharge principle of work
The battery has put will possibly bring the disastrous to the battery consequence, specially the big electric current has put or crosses repeatedly puts, the influence is bigger to the battery. Generally speaking, crosses the electric discharge to be able to cause the battery intrinsic pressure to elevate, the positive and negative extremely active material’s invertibility receives the destruction, even if charges also can only the part restore, the capacity will have the obvious weaken. The lithium ion power source manages one of electric circuit’s functions is to protect the lithium battery as for not to put.
Figure 1
Lithium battery’s normal work voltage is 2.575~4.2V. When the battery voltage within this scope, manages the electric circuit to manage MOSFET S4 to open, establishes the low resistivity channel between the battery (CELL) voltage and BATT , is advantageous flows to the handset load in the electric current from the battery. Under this situation, has put manifests is oversized for the output current. In the entire output procedure, the power source management electric circuit examines unceasingly from the battery outputs to the load electric current. When the battery output current surpasses usual protection value 3.5A time, the handset short circuit protection circuit starts to work, closes S4, shuts off the battery and the BATT connection.
When the battery discharges continually the battery voltage is lower than literature [1] the stipulation below electric discharge final voltage 2.375V, then belongs to the voltage to put. This time, Figure 1 the handset low voltage and the short circuit protection circuit starts to work, has put with the electric current is the same, closes S4, shuts off the battery and the BATT connection achieves protects lithium battery’s goal.
1.2 charge principle of work
The charge management electric circuit when carries on the charge to the lithium battery, is a complex process, both must guarantee that the lithium battery can fill, and must guarantee that lithium battery’s performance, needs to guarantee most importantly the lithium battery cannot be excessively sufficient. If the lithium battery the charging current is oversized in the charging-up, or the duration of charging is excessively long, the oxygen which produces is consumed without enough time, possibly causes the intrinsic pressure to elevate, the battery distorts, leaks the fluid and so on undesirable tendency. At the same time, its electricity performance will obviously also reduce.
The entire charging circuit should have the following several kind of charge pattern:
- - low voltage pre-charge pattern;
- - full speed charge pattern;
- - trickling charge pattern;
- - the peak cuts off, the pulse charge pattern;
- - charge closure pattern.
1.2.1 low voltage pre-charge pattern
When the battery voltage is lower than 3.0V, the power source management electric circuit enters the low voltage pre-charge pattern. When the battery extreme has put, to prevent the excessive charging current to create the damage to the battery performance, the charging circuit should adopt the evolution the charge way.
Regarding extreme has put together, the voltage has been lower than 0.7V the lithium battery, the power source management electric circuit will provide the pre-charge brook class to give the battery. This time the S1 shutting down, the battery charger provides the electric current through R1 to base pin Vdect, the battery charger provides the electric current the size to decide completely by R1 that the entire battery charger works nearly in the non-load situation. This kind of charge pattern even might already be the 0V battery carries on the charge to the voltage; When the battery voltage is higher than 0.7V is lower than 1.98V, exterior S1 and the S2 work, the power source management electric circuit may by a higher electric current to the battery carry on the charge. But, this time the triode S1 power loss examination electric circuit has not worked, must limit its power loss to be lower than 800mW, in order to avoid burns down S1; When the battery voltage is higher than 1.98V is lower than 3.0V, the entire power source management electric circuit normal work, this time the S1 control circuit causes S1 by the high electric current, but is lower than the full speed charging current to carry on the charge far to the battery, this electric current surpasses 100mA generally.
1.2.2 full speed charge patterns
When the battery voltage is higher than 3.0V, the pre-charge pattern ended, enters the full speed charge pattern. This time, the power source management electric circuit opens S1 and S2, and causes S1 to work in the saturated pattern, the battery charger provides the full speed charging current to the battery charge. But, the power source management electric circuit will limit the biggest charging current will be smaller than 1.5A.
This kind of charge pattern also has certain request to the battery charger, requests it to realize the current limiting output. Does the goal is advantageous for the mobile communication terminal manufacturer, when product design may act according to the product the definition, chooses the different charging current, realizes to the concrete lithium battery fast effective charge. In the model application, requests the output current which the battery charger provides to limit generally in 1A, the concrete electric current may act according to uses the lithium battery manufacturer recommendation use the charging current, so that the battery can have a high cycle life.
1.2.3 trickling charge patterns
This charge pattern is also one kind of constant pressure charge pattern actually, when the battery surface achieves termination charging voltage Vterm which the control circuit establishes, namely enters this kind of charge pattern. Because under the full speed charge pattern, the electric current is quite big, the battery surface voltage and the actual battery core’s voltage has the quite big dropping variance, the trickling charge pattern is uses for to reduce even eliminates this dropping variance. This time, the power source management electric circuit through controls S1 the make and break situation, will provide for battery’s maximum current limits in more than 100 mA. Because the battery imitates more and more full, therefore, brook class getting smaller, until closure.
1.2.4 peak closure pulse charge pattern
When the power source management electric circuit is in the trickling charge pattern, it meets periodically skips to the full speed charge pattern, forms the pulse current to carry on the charge to the battery. Big current pulse width general <100μs, like this is advantageous to the battery is filled quickly.
1.2.5 charge closure pattern
The power source management electric circuit can have a control pin, by handset’s CPU decided when stops the charge. Enters this kind of pattern, generally will have such several kind of situations: The handset examines the charging circuit including the lithium battery hyperpyrexia; It is not the original installation lithium battery; Already entered the trickling charge, does not require the duration of charging to be excessively long; The battery charger designs unreasonable and so on.
2 conclusions
Because the power source management electric circuit’s is important in mobile communication terminal product status, it not only involves to handset’s safety performance moreover relates user’s personal safety. This article proposed the solution may add the match through the special-purpose chip the peripheral circuit to be able to realize, but this kind of special-purpose chip many manufacturer like NationalSemiconductor, Motorola and so on have the very mature product.
