Since there are some unstable factors in the power supply circuit, the circuit designed to prevent such unstable factors from affecting the circuit is called the protection circuit. The protection circuits are common in all kinds of electronic products, such as: overcurrent protection, overvoltage protection, overheat protection, no-load protection, short circuit protection and so on. This paper has sorted out some common protection circuit.
Motor Overheating Protection Circuit
In the production of automatic lathe, electric heating oven, ball mill and other continuous operation of the mechanical and electrical equipments , as well as other unattended equipments, because accidents happen from time to timethe because of motor overheating or temperature controller failure , it’s necessary to take appropriate security measures. PTC thermistor overheat protection circuit can conveniently and effectively prevent the above accidents.
The following figure is an example of motor overheating protection, composed of PTC thermistor and Schmitt circuit control circuit. In the figure, RT1, RT2 and RT3 are three step PTC thermistors with the same characteristics, which are embedded in the windings of the motor stator respectively. Under normal circumstances, PTC thermistor is in normal state, the total resistance is less than 1 k Ω. At this time, V1 is cut off, V2 is conducted, relay K gets the normally open contact of electric absorption, and the motor is operated by the power supply of the municipal electric power.
When motor faultdue to local overheating , as long as there is a PTC thermistor heat exceeds the preset temperature, the resistance will be more than 10 k Ω. So V1 conduction, V2 cut-off, VD2 display red alarm, K loss of power release, motor stop running, to achieve the purpose of protection.
The selection of PTC thermistor depends on the insulation level of the motor. The Curie temperature of PTC thermistor is usually selected in a range about 40℃ lower than the limit temperature corresponding to the insulation grade of the motor. For example, for the B1 insulation of the motor, its limit temperature is 130℃, Curie temperature 90℃ PTC thermistor should be selected .
Protection Circuit In Inverter Power Supply
Inverters often need to carry out current conversion, if the current in the circuit exceeds the limited range, it will cause great damage to the circuit and key devices, so the protection circuit in the inverter power supply is particularly important.
Anti - Reverse Protection Circuit
If the inverter has no anti-reverse circuit, in the case of input battery anti-reverse will often cause disastrous consequences, light to burn fuse, heavy to burn most of the circuit. There are three kinds of anti-reverse connection protection circuits in the inverter: anti-reverse connection protection circuit composed of anti-parallel Schottky diode, as shown in the figure below.
From the figure, when the battery is reversed, the Schottky diode D is conducted but F is burned. If the back is Lord of the push-pull structure transform circuit, the two push-pull switches MOS tube parasitic diodes are equivalent and D in parallel, but the pressure drop is much bigger than Schottky, instantaneous current impact resistance ability and lower than the Schottky diode D, thus avoiding the large current through the parasitic diodes MOS tube, thereby protecting the two push-pull switches MOS tube.
This anti-reverse connection protection circuit structure is simple, it will not affect the efficiency, but after protection the fuse F will be burnt out, and it needs to be replaced to restore normal work.
Anti-reverse connection protection circuit of relay is adopted. The basic circuit is as follows:
As can be seen from the figure, if the battery is connected inversely and D inversely deflected, no current passes through the coil of relay K, the contact cannot be closed, and the power supply of the inverter will be cut off. This anti-reverse connection protection circuit has a better effect and will not burn out fuse F, but the volume of it is relatively large and the contact life of the relay is limited.
The anti-reverse connection protection circuit using MOS tube, the basic circuit is as follows:
In the figure, D is a parasitic diode with anti-reverse MOS connection, which is easy to draw the principle of analysis. When the battery polarity is not inversely connected, D is positively and partially conductive, and GS electrode of Q is positively and partially conductive by going through F, R1 and D to the negative electrode of the battery. The voltage drop after Q conduction is much smaller than that of D, so after Q conduction, D will not get enough forward voltage and stop.
When the polarity of the battery is reversed, D will end due to reverse bias, Q will also end due to reverse bias of GS, and the inverter cannot start. Due to the absence of mechanical contact switch, this anti-reverse connection protection circuit has a relatively long service life, and will not burn fuses like the anti-reverse connection protection circuit composed of anti-reverse Schottky diode. Therefore, it is widely used, but the disadvantage is that MOS has a certain loss when conducting.
LVD-Low Voltage Disconnect
In order to prevent the battery from being damaged by excessive discharge, we need to make the inverter stop working when the battery voltage is discharged to a certain voltage. It should be pointed out that the battery undervoltage protection is too sensitive to protect when the impact load is started. This makes it difficult for the inverter to start such loads, especially when the battery power is not very sufficient. See the battery undervoltage protection circuit below.
It can be seen that this circuit can quickly establish the sampling voltage of the battery and delay protection by adding D1 and C1.
Lithium Battery Charging Protection Circuit
Lithium battery over both charge and discharge will affect the battery life. In the design, we should pay attention to the lithium battery charging voltage, charging current. Then selecting the appropriate charging chip. Taking attention to prevent lithium battery overcharge, overdischarge, short circuit protection and other problems. At the same time, the design has to go through a lot of testing.
Design of Lithium Battery Charging Circuit
Charging Protection Circuit
Overcurrent Protection Circuit In Switching Power Supply
There are various forms of overcurrent protection, including resistance primary current-limiting circuit, base drive current-limiting circuit, power-free current-limiting circuit and 555 current-limiting circuit. The following diagram shows the comparison of these kinds of over-current protection circuits:
