[Repost] Several commonly used anti-reverse connection protection circuits
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1. Normally, the DC power input reverse connection protection circuit uses the unidirectional conductivity of the diode to achieve reverse connection protection. As shown in Figure 1: This connection method is simple and reliable, but the power consumption is greatly affected when a large current is input. If the input current rating reaches 2A, if Onsemi's fast recovery diode MUR3020PT is selected, the rated tube voltage drop is 0.7V, then the power consumption must at least reach: Pd = 2A × 0.7V = 1.4W, which is inefficient and generates a lot of heat, so a heat sink must be added. Alternatively, you can use a diode bridge to rectify the input so that the circuit always has the correct polarity (Figure 2). The disadvantage of these schemes is that the voltage drop across the diode consumes energy. With an input current of 2A, the circuit in Figure 1 consumes 1.4W, and the circuit in Figure 2 consumes 2.8W. Figure 1. A series diode protects the system from reverse polarity. The diode has a 0.7V drop. Figure 2 is a bridge rectifier, which can work normally regardless of polarity, but two diodes are turned on, and the power consumption is twice that of Figure 1. MOS tube type anti-reverse protection circuit 25)]Figure 3 uses the switching characteristics of the MOS tube to control the conduction and disconnection of the circuit to design an anti-reverse connection protection circuit. Since the internal resistance of the power MOS tube is very small, the current MOSFET Rds (on) can reach the milliohm level, which solves the problems of voltage drop and excessive power consumption in the existing diode power supply anti-reverse connection solution. The polarity reverse connection protection connects the protection field effect tube in series with the protected circuit. The protection field effect tube is a PMOS field effect tube or an NMOS field effect tube. If it is a PMOS, its gate and source are connected to the ground terminal and the power terminal of the protected circuit respectively, and its drain is connected to the substrate of the PMOS element in the protected circuit. If it is an NMOS, its gate and source are connected to the power terminal and the ground terminal of the protected circuit respectively, and its drain is connected to the substrate of the NMOS element in the protected circuit. Once the power polarity of the protected circuit is reversed, the protective field effect transistor will form a short circuit to prevent the current from burning the field effect transistor components in the circuit and protect the entire circuit. The specific N-channel MOS tube reverse connection protection circuit is shown in Figure 3. ![]() Figure 3. NMOS tube type reverse connection protection circuit The N-channel MOS tube is connected in series between the power supply and the load through the S pin and the D pin. The resistor R1 provides a voltage bias for the MOS tube. The switching characteristics of the MOS tube are used to control the conduction and disconnection of the circuit, thereby preventing the reverse connection of the power supply from damaging the load. When connected in the forward direction, R1 provides the VGS voltage and the MOS is saturated and turned on. When connected in the reverse direction, the MOS cannot be turned on, so it plays a role in preventing reverse connection. The Rds (on) of the power MOS tube is only 20mΩ, and the actual loss is very small. For a current of 2A, the power consumption is (2×2)×0.02=0.08W, and no external heat sink is needed. This solves the problems of voltage drop and excessive power consumption in the existing diode power supply reverse connection protection solution. ![]() [color=rgb(25, 25, VZ1 is a voltage regulator to prevent the gate-source voltage from being too high and breaking down the MOS tube. The on-resistance of NMOS tube is smaller than that of PMOS, so it is best to choose NMOS. NMOS tube is connected to the negative pole of the power supply, and the gate is turned on at a high level. The PMOS tube is connected to the positive pole of the power supply, and the gate is turned on at a low level. In general, it is commonly used for high-end drive MOS,When turned on, the gate voltage needs to be greater than the source voltage. When the high-side drive MOS tube is turned on, the source voltage is the same as the drain voltage (VCC), so the gate voltage must be 4V or 10V greater than VCC. If you want to get a voltage greater than VCC in the same system, you need a special boost circuit. Many motor drivers integrate charge pumps. It should be noted that appropriate external capacitors should be selected to obtain sufficient short-circuit current to drive the MOS tube. MOS tube is voltage driven. In theory, as long as the gate voltage reaches the turn-on voltage, DS can be turned on. No matter how large the gate string resistance is, it can be turned on. But if the switching frequency is required to be high, the gate to ground or VCC can be regarded as a capacitor. For a capacitor, the larger the string resistance, the longer the gate takes to reach the turn-on voltage, and the longer the MOS is in the semi-conducting state. In the semi-conducting state, the internal resistance is large, and the heat will also increase, which is very easy to damage the MOS. Therefore, at high frequencies, the gate string resistance should not only be small, but generally a pre-drive circuit should be added. Let's first understand the basic knowledge of MOS tube switches. MOSFET is a kind of FET (the other is JFET), which can be manufactured into enhancement type or depletion type, P channel or N channel, there are 4 types in total, but only enhancement type N channel MOS tube and enhancement type P channel MOS tube are actually used, so usually NMOS or PMOS refers to these two types. As for why depletion-type MOS tubes are not used, it is not recommended to dig into the details. Of these two types of enhancement-type MOS tubes, NMOS is more commonly used. The reason is that it has a small on-resistance and is easy to manufacture. Therefore, NMOS is generally used in switching power supplies and motor drive applications. In the following introduction, NMOS is also the main focus. There is parasitic capacitance between the three pins of the MOS tube, which is not what we need, but is caused by the limitation of the manufacturing process. The existence of parasitic capacitance makes it more troublesome when designing or selecting the driving circuit, but there is no way to avoid it. I will introduce it in detail later. In the schematic diagram of the MOS tube, you can see that there is a parasitic diode between the drain and the source. This is called a body diode. This diode is very important when driving an inductive load (such as a motor). By the way, the body diode only exists in a single MOS tube, and it is usually not present inside an integrated circuit chip. 2. MOS tube conduction characteristics Turning on means acting as a switch, which is equivalent to closing the switch. The characteristics of NMOS, when Vgs is greater than a certain value, it will turn on, which is suitable for the situation when the source is grounded (low-end drive). As long as the gate is 25)]The source voltage can reach 4V or 10V. PMOS has the characteristic that it will be turned on when Vgs is less than a certain value, which is suitable for the situation when the source is connected to VCC (high-end drive). However, although PMOS can be easily used as a high-end driver, NMOS is usually used in high-end drivers due to its large on-resistance, high price, and few replacement types. 3. MOS switch tube loss Whether it is NMOS or PMOS, there is an on-resistance after conduction, so the current will consume energy on this resistance. This part of the consumed energy is called conduction loss. Choosing a MOS tube with a small on-resistance will reduce the conduction loss. The on-resistance of low-power MOS tubes is generally around tens of milliohms, and some are several milliohms.MOS is not turned on and off instantly. The voltage across the MOS has a process of decreasing, and the current flowing through it has a process of increasing. During this period of time, the loss of the MOS tube is the product of the voltage and the current, which is called the switching loss. Usually the switching loss is much larger than the conduction loss, and the faster the switching frequency, the greater the loss. The product of the voltage and current at the moment of conduction is large, and the loss caused is also large. Shortening the switching time can reduce the loss during each conduction; reducing the switching frequency can reduce the number of switches per unit time. Both methods can reduce switching losses. 4. MOS tube drive Compared with bipolar transistors, it is generally believed that no current is required to turn on the MOS tube, as long as the GS voltage is higher than a certain value. This is easy to do, but we also need speed. In the structure of the MOS tube, it can be seen that there is a parasitic capacitance between GS and GD, and the driving of the MOS tube is actually the charging and discharging of the capacitance. Charging a capacitor requires a current, because the capacitor can be regarded as a short circuit at the moment of charging, so the instantaneous current will be relatively large. When selecting/designing a MOS tube driver, the first thing to pay attention to is the size of the instantaneous short-circuit current that can be provided. When selecting a MOSFET, there are two major types of MOSFET: N-channel and P-channel. In a power system, a MOSFET can be regarded as an electrical switch. When a positive voltage is applied between the gate and source of an N-channel MOSFET, its switch is turned on. When turned on, current can flow from the drain to the source through the switch. There is an internal resistance between the drain and the source, which is called the on-resistance RDS(ON). It must be clear that the gate of the MOSFET is a high impedance terminal, so a voltage must always be applied to the gate. This is the resistor connected to the ground by the gate in the circuit diagram described later. If the gate is floating, the device will not work as designed and may turn on or off at inappropriate times, causing potential power loss in the system. When the voltage between the source and the gate is zero, the switch is closed and the current stops passing through the device. Although the device is turned off at this time, there is still a small current, which is called leakage current, or IDSS. Step 1: N-Channel or P-Channel Selection The first step in choosing the right device for your design is to decide whether to use an N-Channel or P-Channel MOSFET. In a typical power application, when a MOSFET is connected to ground and the load is connected to the rail voltage, the MOSFET forms a low-side switch. In the low-side switch, an N-channel MOSFET should be used due to the voltage required to turn the device off or on. When the MOSFET is connected to the bus and the load is grounded, a high-side switch is used. P-channel MOSFETs are usually used in this topology due to voltage drive considerations. Step 2: Determine the rated current The second step is to select the rated current of the MOSFET. Depending on the circuit structure, this rated current should be the maximum current that the load can withstand under all conditions. Similar to the voltage case, the designer must ensure that the selected MOSFET can withstand this rated current, even when the system generates a peak current. The two current conditions considered are continuous mode and pulse spike. In continuous conduction mode, the MOSFET is in steady state, where current flows continuously through the device. A spike is when a large surge (or spike current) flows through the device. Once the maximum current under these conditions is determined, it is straightforward to select a device that can withstand this maximum current. After selecting the rated current, the conduction losses must also be calculated. In practice, MOSFETs are not ideal devices, as there is energy loss during the conduction process, which is called conduction losses. When the MOSFET is "on", it acts like a variable resistor, which is determined by the device's RDS(ON) and varies significantly with temperature. The power loss of the device can be calculated as Iload2×RDS(ON), and since the on-resistance varies with temperature, the power loss will also vary proportionally. The higher the voltage VGS applied to the MOSFET, the smaller the RDS(ON); conversely, the higher the RDS(ON). For the system designer, this is where the tradeoffs are made, depending on the system voltage. For portable designs, it is easier (and more common) to use lower voltages, while for industrial designs, higher voltages can be used. Note that the RDS(ON) resistance increases slightly with current. The various electrical parameter variations for RDS(ON) resistance can be found in the manufacturer's data sheet. Step 3: Determine Thermal Requirements The next step in selecting a MOSFET is to calculate the system's thermal requirements. The designer must consider two different scenarios, the worst case and the real world. It is recommended to use the worst case calculation because it provides a greater safety margin to ensure that the system will not fail. There are also some other measurements on the MOSFET data sheet that need attention; such as the thermal resistance between the semiconductor junction of the packaged device and the ambient, and the maximum junction temperature. The junction temperature of the device is equal to the maximum ambient temperature plus the product of thermal resistance and power dissipation (junction temperature = maximum ambient temperature + [thermal resistance × power dissipation]). This equation can be solved for the maximum power dissipation of the system, which is defined as I2 × RDS (ON). Since the designer has determined the maximum current that will pass through the device, the RDS (ON) at different temperatures can be calculated. It is worth noting that when dealing with simple thermal models, the designer must also consider the heat capacity of the semiconductor junction/device case and case/environment; that is, the printed circuit board and package do not heat up immediately. 25)]Usually, a PMOS tube will have a parasitic diode, which is used to prevent the source and drain terminals from being reversely connected. The advantage of PMOS over NMOS is that its turn-on voltage can be 0, and the voltage difference between DS voltage is not much. The conduction condition of NMOS requires that VGS must be greater than the threshold, which will cause the control voltage to be greater than the required voltage, causing unnecessary trouble. There are two applications of using PMOS as the control switch: In the first application, PMOS is used to select the voltage. When V8V exists, the voltage is all provided by V8V, PMOS is turned off, VBAT does not provide voltage to VSIN, and when V8V is low, VSIN is powered by 8V. Note the grounding of R120. This resistor can stably pull down the gate voltage to ensure the normal opening of PMOS. This is also the hidden danger caused by the high impedance of the gate described above. The role of D9 and D10 is to prevent the voltage from flowing back. D9 can be omitted. It should be noted here that the DS of this circuit is actually connected in reverse, so the function of the switch tube cannot be achieved due to the conduction of the attached diode. Please pay attention to this in actual application. ![]() Look at this circuit, the control signal PGC controls whether V4.2 supplies power to P_GPRS. In this circuit, the source and drain are not connected in reverse. The significance of R110 and R113 is that R110 controls the gate current so that it is not too large, and R113 controls the normal state of the gate, pulling R113 high to turn off PMOS. It can also be regarded as a pull-up of the control signal. When the internal pin of the MCU is not pulled up, that is, when the output is open drain, it cannot drive PMOS to turn off. At this time, an external voltage is required to pull up, so the resistor R113 plays two roles. R110 can be smaller, up to 100 ohms. In addition, let's look at the switching characteristics of the MOS tube. 1. Static characteristics As a switching element, the MOS tube also works in the cut-off or on state. Since the MOS tube is a voltage-controlled element, its working state is mainly determined by the gate-source voltage uGS. The working characteristics are as follows: [color=rgb(25, 25, ※ uGS※ uGS> turn-on voltage UT: MOS tube works in the conduction area, and the drain-source current iDS=UDD/(RD+rDS). Among them, rDS is the drain-source resistance when the MOS tube is turned on. Output voltage UDS=UDD·rDS/(RD+rDS), if rDS 2. Dynamic characteristics [color=rgb(25, 25, MOS tubes also have a transition process when switching between the on and off states, but their dynamic characteristics mainly depend on the time required for the stray capacitance related to the circuit to charge and discharge, while the time for the tube itself to accumulate and dissipate charge when it is turned on and off is very small. The following figures (a) and (b) respectively show a circuit composed of an NMOS tube and its dynamic characteristics schematic diagram. NMOS tube dynamic characteristics schematic diagram When the input voltage ui changes from high to low and the MOS tube changes from the on state to the off state, the power supply UDD charges the stray capacitance CL through RD, and the charging time constant τ1=RDCL. Therefore, the output voltage uo needs a certain delay to change from low level to high level; when the input voltage ui changes from low to high and the MOS tube changes from the off state to the on state, the charge on the stray capacitance CL is discharged through rDS, and its discharge time constant τ2≈rDSCL. It can be seen that the output voltage Uo also needs a certain delay to change to a low level. However, because rDS is much smaller than RD, the conversion time from off to on is shorter than the conversion time from on to off. Since the drain-source resistance rDS of the MOS tube when it is turned on is much larger than the saturation resistance rCES of the transistor, and the drain external resistance RD is also larger than the collector resistance RC of the transistor, the charging and discharging time of the MOS tube is longer, making the switching speed of the MOS tube lower than that of the transistor. However, in the CMOS circuit, since the charging circuit and the discharging circuit are both low-resistance circuits, the charging and discharging process are relatively fast, so that the CMOS circuit has a higher switching speed.
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