Supertex inc. AN-D09 Application Note Battery Back-Up Utilizing Low Threshold MOSFETs Introduction The simple battery backup circuit shown in Figure 1 utilizes Supertex low threshold DMOS devices to achieve excellent efficiency. In fact, one of the main reasons why MOSFETs are gaining popularity is that very low voltage drops, which surpass the performance of various kinds of diodes and bipolar transistors, can be achieved. Many other benefits of low gate threshold MOSFETs are explained in the text. Circuit Description and Operation The battery backup circuit has two modes: 1) Battery charging, and 2) Battery backup. 1) Battery charging mode The 120VAC is stepped down via transformer and full-wave rectified by D1, D2, and C1 to 7.5VDC. This 7.5VDC supplies power to RL as well as providing the charging current to the batteries. R1, D3, and D4 generate a 1.2V reference for U1a and U1b. D5, R2, R3, C2, and COMP2 keep Q1 and Q2 off when switch S is closed. The battery, consisting of 5 nickel cadmium cells in series, is being charged with a current set by R8 and the intrinsic drain to source diode of Q2. For fully discharged batteries, there will be a high charge current for a few seconds, rapidly decaying to a slow charge. As the battery becomes almost fully charged to 6.8V, the current is reduced to a trickle charge current of a few milliamperes. The trickle charge current is further reduced to microamperes when VBATT exceeds 7.0V. This is because the voltage across the diode of Q2 is 0.5V and will allow only a small amount of current flow. This maintains full charge of the battery, when not in use, over an extended period of operation. 2) Battery backup mode When switch S is opened, simulating power outage, unplugged equipment, or blown fuse, the circuit goes into battery backup mode. U1b turns on Q1 and Q2. As VBATT supplies the 60Ω load, U1a monitors the VBATT voltage keeping it from fully discharging, as complete discharge and subsequent cell voltage reversal can degrade the performance of the NiCd battery. The circuit is designed for the U1a to turn Q1 and Q2 off if VBATT is less than 5.5V and on if greater than 6.5V. The hysteresis is designed to avoid oscillation and is set by R4, R5, R6, and R7. Figure 1. Battery Back-up Circuit 0.1μF S D1 120VAC - 7.5V C1 1000μF D2 R1 5.1kΩ + VREF D5 R2 100kΩ D3 R3 150kΩ C2 0.1µF D4 R4 8.2kΩ U1a 3 + 2 - 8 4 1 R6 39kΩ R5 2.0kΩ R7 5.1kΩ + Eveready Nicad 500mAH RL 60Ω Q1 5 6 VBATT + - U1b R8 30Ω 7 Q2 Doc.# DSAN-AN-D09 A040213 Supertex inc. www.supertex.com AN-D09 Design Considerations and Component Selec- current of 125mA are used. Maximum voltage drop across Q1 and Q2 works out to only 375mV. In actual operation, this tion voltage drop is substantially lower because the typical value of RDS(ON) is 0.8Ω. The voltage drop across Q1 and Q2 was measured to be 200 mV. The design of this circuit utilizes standard, readily available components. The number and different types of components are minimized. Diodes D1 to D5 are 1N4001. All resistors are standard 1/4 watt, 5% tolerance. National Semiconductor’s dual comparator LM393N is used for its low biasing current for U1. The battery consists of 5 Eveready nickel cadmium cells in series. The cells are AA size, CH15 with a C rating of 500 mAH. Figure 2 is a discharge curve of VBATT vs Time showing battery backup operation of approximately 4 hours. Figure 3 is a charge curve of the battery. The component selection ensured that basic charging current guidelines for Nicad cells were not violated. Assuming the worst case, using fully discharged batteries, the maximum charging current will be 227mA. The most important factor to be considered in the design is the selection of the MOSFETs Q1 and Q2, which are configured as an analog switch. In the battery backup mode, the voltage drop across the MOSFETs must be low to minimize resistive voltage drop and power loss, consequently enhancing battery life. Supertex TN0604N3, low threshold N-channel DMOS transistors, are selected for their guaranteed low on-resistance at low gate drive. Another aspect considered was their costeffective TO-92 package, which saves board space. Device TN0604N3 Typical RDS(ON) Maximum RDS(ON) Test Conditions 0.9Ω 1.5Ω VGS = 5.0V, ID = 750mA 0.6Ω 0.75Ω VGS = 10V, ID = 1.5A Rectified D.C. voltage - diode drop 7.5 - 0.7 = = 227mA R8 30 This current will last only for a few seconds, and is completely safe for the battery as well as Q2. In the charging mode, the battery voltage will be between 6.5 to 6.7V for the majority of the time. The charging current will be from: Q1 and Q2 are easily turned on with a simple pull-up resistor, R7. For a “worst case” design, RDS(ON) = 1.5Ω and a load 7.5 - 6.5 - 0.7 = 10mA to 30 VBATT vs Time 6.5 VBATT (volts) VBATT (volts) 70 5.5 5.0 6.5 6.0 5.5 4.5 0 VBATT vs Time 7.5 6.0 7.5 - 6.7 - 0.7 = 3.3mA 30 0 60 120 180 5.0 240 Figure 2. VBATT Discharge Curve Doc.# DSAN-AN-D09 A040213 0 4.0 8.0 12.0 16.0 t (hours) t (minutes) Figure 3. VBATT Charge Curve 2 Supertex inc. www.supertex.com AN-D09 Conclusion The charge rate will be from: Very low drain to source voltage drops can be achieved with MOSFETs. Bipolar transistor performance is limited by VCE(sat) and diodes by VF , depending upon the semiconductor material used. This circuit utilized the following features of MOSFETs: 10mA 3.3mA = 0.02C to = 0.007C 500mAH 500mAH which is very safe for the Nicad cells. 1. 2. 3. 4. Low drain to source voltage drop. Complete turn-on/off of bidirectional currents. Turn-on with low biasing voltages. No biasing power compared to base current loss in bipolar transistors. 5. Utilization of the intrinsic drain to source diode for limiting charging currents to efficient and safe levels. Optional Features When space is at a premium, Supertex’s TN2504N8 provides performance almost identical to TN0604N3, in the SOT-89 (TO-243AA) surface mount package. Added features such as battery backup mode indicator, low battery voltage early warning, or battery shutdown indicator can be incorporated by using one or more of the optional circuits shown in Figure 4A through 4C. These can be easily modified to interface with a microprocessor in more complex systems. The battery backup circuit described demonstrates the benefits of Supertex N-channel low gate threshold devices. These are available in either surface mount (TN2504N8) or TO-92 (TN0604N3) packages. These are ideally suited for battery powered applications. Very often, circuit designs require low on resistance to prolong battery life, low gate drive to meet battery voltage limitations, and small packages to accommodate board space limitations. The Supertex low threshold DMOS discrete transistor family were designed to satisfy such requirements. Nickel cadmium batteries are quite rugged. However, they are prone to damage due to cell voltage reversal if fully discharged. Other kinds of batteries are more sensitive, and may be damaged below a certain voltage per cell, e.g., 1.75V for lead acid. The circuits shown can be modified to suit other kinds of rechargeable batteries, e.g. lead acid, lead calcium (gel), lithium, etc. For lead acid, the threshold voltage, to disconnect the load from the battery can be adjusted to 1.75V per cell. Figure 4. Optional Circuitry VBATT VBATT 300Ω PIN 6 + PIN 5 - C 36kΩ + VREF - VBATT 300Ω 300Ω C PIN 3 VREF + - C 9.1kΩ A) Battery Back-up Mode Indicator B) Low Battery Voltage Early Warning C) Battery Shutdown Indicator Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com) Supertex inc. ©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited. Doc.# DSAN-AN-D09 A040213 3 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com