The electrical schematic diagram is shown in figure 1.
Figure 1? Local electrical schematic diagram
(Image from the Internet)
Is the product in GB? Safety of household and similar electrical appliances? 4706. 1-2005? Part 1: general requirements, article 19. 1 1. In the short-circuit diode D test, the lithium battery caught fire, which did not meet the requirements of 19. 13 "Electrical appliances should not emit flame".
Why is it on fire?
case analysis
Personally, one of the main reasons is that the product does not use its own anti-short circuit battery.
Circuit principle
The main circuit in the juicing state is battery positive electrode → motor M→MOS transistor Q3→MOS transistor Q 1→ current sampling resistor R7→ battery negative electrode.
Small signal control main circuit flow:
The user presses and holds the power button 2 of the control panel? S, the output signal of the main control IC drives the MOS transistor Q 1 to turn on, and stops driving by pressing the power button for a short time, and Q 1 turns off.
If the product is charging through USB and/or the main current is too large, the main control IC stops the MOS transistor Q3 driving signal, and Q3 is turned off; On the other hand, if the product is not in the charging state, the battery voltage is normal and the main current is normal, the main control IC drives Q3 to conduct.
D 1 is connected in reverse parallel at both ends of the positive and negative poles of the brushed DC motor, which provides a tracking path for the overvoltage pulse generated at the moment when the motor is powered off, and prevents the breakdown of the driving circuit. When the motor is running normally, D 1 is in the reverse cut-off state and does not work.
Products include main control chip IC, charging control IC, display circuit and battery voltage monitoring circuit, which will not be described here.
Abnormal fault test of electronic circuit
Press GB? 4706. 1-2005 standard 1 9.1.2, analog short-circuit freewheeling diode d1,the main current is sampled by sampling resistor R7 and sent to the main control IC through R3. After internal judgment, it is overcurrent. Start the overcurrent protection program, stop driving Q3, and cut off the main current. The product has no obvious temperature rise, which meets the requirements of judgment clause 19. 13.
As the "protection electronic circuits" such as R7, ic and Q3 play a role in the above tests, it is necessary to add the "double fault" test in clause19.1.3, namely:
19. 1 1.3 = protection electronic circuit 19. 1 1.2 simulated fault (e.g. short circuit at S-D pole of Q3)+repeat the previous test (freewheeling diode D/kloc-
Figure 2? Short circuit state circuit diagram
At this time, the main current returns from the battery anode to the battery cathode via the short-circuited D 1, the short-circuited Q3, the MOS transistor Q 1 and the sampling resistor R7, which means that the load is short-circuited and the overcurrent protection circuit fails at the same time, as shown in Figure 2.
The juice cup test results of the case problem show that the battery is on fire, and it is estimated that there is no protective circuit board inside the lithium-ion battery.