Application Note 26 Fast charging batteries with Zetex high current PNP transistors and benchmarq controller ICs Neil Chadderton Introduction Fast charge controller ICs The advances of digital technology and a waiting market have created a huge demand for portable products including cellular telephones, PDAs, laptop computers, CD players, and gaming systems. By their nature, all of these products must derive their energy from an integral rechargeable cell/battery pack that must periodically be re-charged from a mains outlet. The type of battery employed, or its chemistry, depends on the overall pricing available to the product - this will depend on the perceived product lifetime, the required energy/weight ratio, and which marketable advantages there may be apparent from reducing the number of re-charge cycles. There are a number of fast charge controller ICs available that perform all the necessary monitoring and regulation control functions on one monolithic IC. The Benchmarq Microelectronics series of fast charge controller ICs monitor the terminal voltage to determine state of charge, and also the temperature of the battery pack (via a thermistor) to prevent deterioration of the separator material. Fast charge termination is effected by any of the following: For the latest generations of products, users are expecting higher performance, less re-charge cycles, longer battery pack life and faster charge times. All of these parameters are very dependent on the manner in which the cell or battery pack is treated during the charging process. This includes consideration of the preferred cell charging method, thermal concerns, and pre-charge conditions. Delta temperature/delta time (∆T/∆t) Negative delta voltage (-∆V) Maximum temperature Maximum time Maximum voltage To allow cost effective IC designs, typical fast charge controller ICs are manufactured on existing CMOS processes, that do not readily lend themselves to incorporating on-chip pass elements within system cost constraints. Therefore the IC controls an external discrete transistor that is used within a linear regulator, or a step-down frequency modulated DC-DC converter topology to provide a suitable current source for charging. ISSUE 2 - SEPTEMBER 2003 AN26 - 1 SEMICONDUCTORS Figure 1. Fast charge circuit (Benchmarq Microelectronics) using the ZTX788B ISSUE 2 - SEPTEMBER 2003 SEMICONDUCTORS AN26 - 2 In general, switched mode circuits are preferred due to the higher efficiencies possible, and the smaller packaged pass devices required to perform the sourcing function. For example, a comparison of the power losses for a four `C’ cell battery pack charged from a 12V DC source at 2A, leads to values of greater than 12W for the linear control system, but less than 2W for the switched mode option. The external pass transistor used within either circuit mode can be either a PNP Bipolar or a P-Channel MOSFET. For the linear regulator mode, the device must be capable of being heatsunk to remove the power dissipated, which will probably dictate that the product will be a large packaged, large silicon area device which will define a costly component. For the switched mode option, there is more choice available. Due to the higher silicon efficiency of the Bipolar technology compared with MOSFETs, high current fast switching PNP transistors are available that use a fraction of the silicon area required for comparably specified MOSFETs. This allows the bipolar chip option to be encapsulated within a smaller package, and therefore presents a lower cost to the designer. The PNP transistor chosen for this particular application was the ZTX788B - one member of the ZTX788B to ZTX796A series. These devices are manufactured using a high gain emitter process to produce a Super-β hFE range. The ZTX788B is a 15V, 3A continuous TO92 compatible part that exhibits very low VCE(sat), and can therefore replace much larger packaged components (such as TO220, TO126 and D-Pak) in switching applications. Figure 2 demonstrates the on-state voltage of the `788B as a function of load current with base drive current as parameter. Tables 1, 2 and 3 (Appendix A) summarize the pertinent parameters for the ZTX788B and other devices of interest for this application. Figure 2. VCE(sat) v collector current chart for the ZTX788B The circuit shown in Figure 1 is one example of a circuit topology developed by Benchmarq using Zetex E-Line PNP transistors. The circuit allows the fast charging of Ni-Cd or Ni-MH battery packs for laptop computers and similar powered portable systems at a charge current of 2.3A, (defined by the current sense resistor R9). The potential divider formed by RB1 and RB2 is used to feedback a cell corrected level to the IC voltage sense input, that is then used to define the battery pack voltage. Other ICs/circuits are available from Benchmarq for higher current charging and for other cell chemistries. ISSUE 2 - SEPTEMBER 2003 AN26 - 3 SEMICONDUCTORS The circuit employs a circuit modification to provide an active turn-off for the transistor. [This doesn’t however impact on the cost advantage over the MOSFET alternative]. Figure 3 shows the area around the pass transistor with the relevant level shift and turn-off components. The small signal switching transistor Q2, provides level translation from the “MOD” output of the controller IC, and its emitter resistor defines the base current for Q3, the ZTX788B. The inductor L2, Schottky diode (1N5820), and the 10µF capacitor are the Buck converter (step-down) components. The 1k base emitter resistor for Q3 would, in the absence of Q1, L1 and the signal diode (1N4148), provide passive turn-off for the pass transistor. Figures 4 and 5 help to demonstrate the difference in turn-off performance for the two methods. These waveforms were recorded from a charger circuit using the bq2004 and ZTX789A for a 5 cell battery (output voltage to charge to 6V), and an input voltage of 12V. Both figures show the output from the IC controller (from the “MOD” pin), and the voltage from the collector of the ZTX789A with respect to ground. Figure 3. Active turn-off circuit for Bipolar transistors, allowing high efficiency DC-DC conversion at high frequency ISSUE 2 - SEPTEMBER 2003 SEMICONDUCTORS AN26 - 4 Figure 4. Oscillograph of charger circuit employing passive turn-off for ZTX789A, showing: 1. Collector-to-0V waveform, and 2. IC “MOD” drive waveform. Note bipolar transistor storage and fall times. Channel 1: 5V/div, Channel 2, 2V/div, timebase at 2µs/div Figure 4 shows the turn-off produced with a passive turn-off circuit - a resistor. The storage time of the bipolar transistor, or the time from the falling edge of trace 2 to the start of the falling edge of trace 1 is shown to be 1.6µs, and the fall time is 340ns. This fall time could represent a major power loss contributer and therefore would (under some circumstances) serve to limit the maximum load current. ISSUE 2 - SEPTEMBER 2003 AN26 - 5 SEMICONDUCTORS Figure 5. Oscillograph of charger circuit employing active turn-off for ZTX789A, showing: 1. Collector-to-0v waveform, and 2. IC “MOD” drive waveform. Note bipolar transistor storage and fall times. Channel 1: 5V/div, channel 2, 2V/div, timebase at 2µs/div Figure 5 shows the turn-off produced with an active turn-off circuit as shown in figure 3. The storage time has reduced to less than 200ns, and the fall time to 90ns. These switching values are comparable to, or better than the large P-Channel MOSFETs that would otherwise be specified, and allow the bipolar device to be operated to much higher currents than possible with passive turn-off circuitry. ISSUE 2 - SEPTEMBER 2003 SEMICONDUCTORS AN26 - 6 Appendix A BVCEO IC(DC) ICM hFE @IC/VCE VCE(sat)(max.) @IC/IB FMMT717 Device 12V 2.5A 10A 275(typ) 2.5A/2V 140mV 1A/10mA Package SOT23 ZXT13P12DE6 12V 4A 15A 450(typ) 1A/2V 90mV 1A/10mA SOT23-6 ZXT13P20 20V 4A 10A 450(typ) 1A/2V 130mV 1A/10mA SOT23-6 Table 1. PNP transistors for fast chargers <1A. BVCEO IC(DC) ICM VCE(sat)(max.) @IC/IB Package ZTX788B Device 15 3 8 300 min hFE @IC/VCE 2A/2V 450mV 2A/10mA E-Line FZT788B 15 3 8 300 min 2A/2V 450mV 2A/10mA SOT23-6 ZTX789A 25 3 8 200 min 2A/2V 450mV 2A/20mA E-Line FZT789B 25 3 8 200 min 2A/2V 450mV 2A/20mA SOT23-6 ZTX90A 40 2 6 200 min 1A/2V 450mV 1A/10mA E-Line FZT90A 40 2 6 200 min 1A/2V 450mV 1A/10mA SOT23-6 ZTX949 30 4.5 20 140(typ) 5A/1V 320mV 5A/300mA E-Line FZT949 30 4.5 20 140(typ) 5A/1V 320mV 5A/300mA SOT23-6 ZTX951 60 4 15 140(typ) 4A/1V 300mV 4A/400mA E-Line FZT951 60 4 15 140(typ) 4A/1V 300mV 4A/400mA SOT23-6 Table 2. PNP transistors for fast chargers 1 to 4A. BVCEO IC(DC) ICM hFE @IC/VCE VCE(sat)(max.) @IC/IB Package FZT1147A Device 12V 5A 20A 340(typ) 2A/12V 130mV 1A/6mA SOT223 FZT1148A 25V 4A 10A 320(typ) 240mV 1A/7mA SOT223 2A/2V Table 3. PNP transistors for linear and switch mode high current (5A) fast chargers. ISSUE 2 - SEPTEMBER 2003 AN26 - 7 SEMICONDUCTORS Application Note 26 ISSUE 2 - SEPTEMBER 2003 © Zetex plc 2003 Americas Asia Pacific Zetex GmbH Streitfeldstraße 19 D-81673 München Zetex Inc 700 Veterans Memorial Hwy Hauppauge, NY 11788 Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 [email protected] USA Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 [email protected] Zetex (Asia) Ltd 3701-04 Metroplaza Tower 1 Hing Fong Road Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 [email protected] Europe Zetex plc Fields New Road Chadderton Oldham, OL9 8NP United Kingdom Telephone (44) 161 622 4444 Fax: (44) 161 622 4446 [email protected] These offices are supported by agents and distributors in major countries world-wide. This publication is issued to provide outline information only which (unless agreed by the Company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. The Company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. For the latest product information, log on to www.zetex.com SEMICONDUCTORS AN26 - 8