The secret of lithium battery protection against short circuit protection

by:CTECHi     2021-07-17

Presumably everyone is familiar with the lithium battery protection board, and everyone understands its function, which is to protect the lithium battery pack or single-cell lithium battery from overcharge, overdischarge and short circuit, thus avoiding some unnecessary losses for us , Today, the Chinese Chuangfa will let you know what is the secret of lithium battery protection against short circuit protection? Let's take the application in power bicycles as an example. What are the main ones?

(1) Large short-circuit current

In electric vehicles, the voltage of lithium iron phosphate batteries is generally 36V or 48V, and the short-circuit current varies with battery capacity, internal resistance, and line parasitics. The inductance and the contact resistance at the time of short-circuit change, usually several hundred 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, therefore, the short-circuit protection circuit has a certain delay Time. And due to the error of the current detection resistor, the current detection signal and the delay of the system response, usually, the short-circuit protection time is set between 200μS and 1000μS according to different applications. This requires the power MOSFET to be able to survive this time under high short-circuit current. Internal safety work, which also increases the difficulty of system design. Lithium battery short-circuit protection. When 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 driving 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).

1) Complete conduction phase

As shown As shown in 2(a), when the short circuit occurs, the MOSFET is in a fully conductive state, and the current quickly rises to the maximum current. In this process, the power MOSFET bears the power consumption of PONu003dISC2*RDS(on), so it has a small RDS(on) MOSFET has low power consumption.

The transconductance Gfs of the power MOSFET also affects 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 saturated conduction voltage drop will be large. As shown in Figure 3, the VDS(ON) of the MOSFET

is short-circuited. When it reaches 14.8V, the power consumption of the MOSFET will be very large, which will cause the MOSFET to fail due to over power consumption. If the MOSFET is not working in the saturation region, its turn-on voltage drop should be only a few volts, as shown in VDS in Figure 2(a).

2) Turn-off stage

As shown in Figure 2(b), after the protection circuit works, the MOSFET starts to be turned off. During the turn-off process The power consumed by the middle MOSFET is POFFu003dV*I. Because the voltage and current are high when it is turned off, the power is very large, usually reaching several kilowatts or more. Therefore, the MOSFET is easily damaged due to instantaneous overpower. At the same time, the MOSFET is in the saturation zone during the turn-off period, and thermal imbalance between the cells is prone to cause premature failure of the MOSFET.

Improving the turn-off speed can reduce the turn-off loss, but this will cause another problem. The equivalent circuit of the MOSFET is shown in Figure 4, which contains a parasitic triode. During the short-circuit period of the MOSFET, all current flows through the MOSFET channel. When the MOSFET is turned off quickly, part of its current will flow through Rb, which increases the base voltage of the transistor and makes the parasitic transistor turn on and the MOSFET fails prematurely.

Therefore, the appropriate turn-off speed must be selected. Since different MOSFETs withstand different turn-off speeds, it is necessary to set a proper turn-off speed through actual tests.

Figure 5(a) shows the fast turn-off waveform. When turning off, the gate charge is quickly discharged by the transistor to quickly turn off the MOSFET.

Figure 5(b) is a slow turn-off circuit. A resistor is connected in series to control the discharge speed. Increasing the resistance can slow the turn-off speed.

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