Design of lithium battery protection circuit based on power MOSFET

　　Lead-acid batteries are safe, cheap, and easy to maintain, so they are still widely used in electric bicycles. However, lead-acid batteries are highly polluting, bulky, and have a low number of cycles. As countries around the world have increasingly stringent environmental protection requirements, the use of lead-acid batteries will become increasingly restricted. As a new type of environmentally friendly battery, lithium iron phosphate batteries have begun to be gradually applied to electric vehicles and will become a development trend. Usually, due to the characteristics of lithium iron phosphate batteries, their charging and discharging processes need to be protected in applications to avoid overcharging, overdischarging, or overheating to ensure the safe operation of the battery. Short-circuit protection is an extremely harsh working condition during the discharge process. This article will introduce the characteristics of power MOSFETs in this working state, as well as how to select power MOSFET models and design appropriate drive circuits.

　　Circuit structure and application characteristics

　　The simplified model of the discharge circuit of the lithium iron phosphate battery protection board of the electric bicycle is shown in Figure 1. Q1 is a discharge tube, using an N-channel enhancement MOSFET. In actual work, according to different applications, multiple power MOSFETs will be used in parallel to reduce the on-resistance and enhance the heat dissipation performance. RS is the equivalent internal resistance of the battery, and LP is the battery lead inductance.

　　During normal operation, the control signal controls the MOSFET to turn on, and the battery pack terminals P+ and P- output voltage for the load. At this time, the power MOSFET is always in the on state, and the power loss is only conduction loss, without switching loss. The total power loss of the power MOSFET is not high, and the temperature rise is small, so the power MOSFET can work safely.

　　However, when the load is short-circuited, since the loop resistance is very small and the battery's discharge capacity is very strong, the short-circuit current suddenly increases from tens of amperes in normal operation to hundreds of amperes. In this case, the power MOSFET is easily damaged.

　　Difficulties in short-circuit protection of lithium iron phosphate batteries

　　(1) Large short-circuit current

　　In electric vehicles, the voltage of lithium iron phosphate batteries is generally 36V or 48V. The short-circuit current varies with the battery capacity, internal resistance, parasitic inductance of the circuit, and contact resistance during short circuit, and is usually hundreds or even thousands of amperes.

　　(2) The short-circuit protection time cannot be too short

　　In the application process, in order to prevent transient overload from causing the short-circuit protection circuit to malfunction, the short-circuit protection circuit has a certain delay. In addition, due to the error of the current detection resistor, the delay of the current detection signal and the system response, the short-circuit protection time is usually set at 200μS to 1000μS according to different applications. This requires the power MOSFET to be able to work safely within this time under high short-circuit current, which also increases the difficulty of system design.

　　Short circuit protection

　　When the short-circuit protection works, the power MOSFET generally goes through three working stages: fully turned on, turned off, and avalanche, as shown in Figure 2, where VGS is the MOSFET drive voltage, VDS is the MOSFET drain voltage, and ISC is the short-circuit current. Figure 2(b) is an enlarged view of the turn-off period in Figure 2(a).

　　Figure 2: Short-circuit process. (a) Full conduction phase; (b) turn-off and avalanche phase.

　　(1) Full conduction stage

　　As shown in Figure 2(a), when a short circuit just occurs, the MOSFET is in a fully on state and the current quickly rises to the maximum current. During this process, the power consumption of the power MOSFET is PON = ISC2 * RDS(on), so the MOSFET with a smaller RDS(on) has lower power consumption.

　　The transconductance Gfs of the power MOSFET will also affect the conduction loss of the power MOSFET. When the Gfs of the MOSFET is small and the short-circuit current is large, the MOSFET will work in the saturation region, and its saturation on-voltage drop is large. As shown in Figure 3, the VDS(ON) of the MOSFET reaches 14.8V when short-circuited, and the MOSFET power consumption will be large, causing the MOSFET to fail due to excessive power consumption. If the MOSFET does not work in the saturation region, its on-voltage drop should be only a few volts, as shown by VDS in Figure 2(a).

　　Figure 3: Turn-on phase of low transconductance MOSFET 　　(2) Turn-off phase

　　As shown in Figure 2(b), after the protection circuit works, it starts to turn off the MOSFET. During the shutdown process, the power consumed by the MOSFET is POFF = V * I. Since the voltage and current are very high during shutdown, the power is very large, usually reaching more than several kilowatts, so the MOSFET is easily damaged by instantaneous overpower. At the same time, the MOSFET is in the saturation zone during the shutdown period, and it is easy to cause thermal imbalance between the units, resulting in premature failure of the MOSFET.

　　Increasing the turn-off speed can reduce the turn-off loss, but this will cause other problems. The equivalent circuit of MOSFET is shown in Figure 4, which contains a parasitic transistor. During the short circuit of MOSFET, all current flows through the MOSFET channel. When the MOSFET is turned off quickly, part of its current will flow through Rb, thereby increasing the base voltage of the transistor, turning on the parasitic transistor and causing the MOSFET to fail prematurely.

　　Therefore, it is necessary to select a suitable turn-off speed. Since different MOSFETs have different turn-off rates, it is necessary to set the appropriate turn-off speed through actual testing.

　　Figure 4: MOSFET equivalent circuit

　　Figure 5(a) is a fast turn-off waveform. When turned off, the gate charge is quickly discharged through the transistor to quickly turn off the MOSFET. Figure 5(b) is a slow turn-off circuit. A resistor is connected in series in the loop to control the discharge speed. Increasing the resistance can slow down the turn-off speed.

　　Figure 5: Power MOSFET turn-off circuit. (a) Fast turn-off circuit; (b) Slow turn-off circuit.

　　Figure 6: AOT266 shutdown waveform. (a) Fast shutdown waveform; (b) Slow shutdown waveform

　　AOT266 is a new generation of medium voltage MOSFET from AOS. Its withstand voltage is 60V and RDS(ON) is only 3.2 milliohms. It is suitable for application in lithium iron phosphate battery protection. Figure 6(a) shows the waveform of AOT266 fast shutdown when the design is incorrect. AOT266 fails during the fast shutdown process. When it fails, its voltage spike is 68V. After failure, the current cannot return to zero. The root cause of its failure is that it is shut down too quickly. Figure 6(b) shows the waveform of slow shutdown when the correct design is used and the discharge resistor is 1K. The MOSFET shutdown time reaches 13.5us and the voltage spike is 80.8V, but the MOSFET does not fail. Therefore, slow shutdown can improve the short-circuit capability in this application.

　　(3) Avalanche stage

　　At the end of the MOSFET turn-off process, the MOSFET usually enters an avalanche state, as shown in the avalanche stage in Figure 2(b). At the end of the turn-off, the MOSFET drain voltage spike is VSPIKE = VB + LP * di/dt. The lead inductance LP and di/dt of the loop are too large, which will cause the MOSFET to overvoltage, resulting in premature failure of the MOSFET.

　　Selection principles of power MOSFET

　　(1) Determine the number of MOSFETs required in parallel and the appropriate RDS(ON) through thermal design;

　　(2) Try to choose a MOSFET with a smaller RDS(ON) so that fewer MOSFETs can be used in parallel. Multiple MOSFETs in parallel are prone to current imbalance. The parallel MOSFETs should have independent and equal drive resistance to prevent oscillation between the MOSFETs;

　　(3) Select a MOSFET with appropriate gFS based on the maximum short-circuit current, the number of MOSFETs in parallel, the drive voltage, etc.;

　　(4) Considering the voltage spike at the end of the shutdown phase, the avalanche energy of the MOSFET cannot be too small.

　　summary

　　In the application of lithium iron phosphate battery protection in electric vehicles, short-circuit protection design is directly related to the reliability of the entire system. Therefore, it is necessary not only to select a suitable power MOSFET, but also to design a suitable drive circuit to ensure the safe operation of the power MOSFET.