MOS field-effect transistor (MOSFET), as a key component in modern electronic equipment, has been widely used. MOS tubes not only shine in the field of power conversion and signal amplification, but also play an indispensable role in the design of switching circuits. Here, INFINITECH will introduce nine simple MOS switch circuit diagrams to help engineers and electronics enthusiasts better understand and master these basic and practical circuit designs for more efficient and stable power control.

Fundamentals of MOS switch circuits

MOS tube, full name metal oxide semiconductor field effect tube (MOS), is a voltage control device with the advantages of high input impedance, low power consumption and high speed. In switching applications, the MOS tube can act as a fast-responding electronic switch that determines its on-off or on-off state by controlling the gate voltage, thus achieving precise control of the current.

Detailed explanation of nine kinds of simple MOS tube switching circuit diagram

The first: mos tube switch circuit diagram

Switching characteristics of MOS tubes

Static characteristic

MOS tube as a switching element, is also working in the cut-off or on two states. Since MOS is a voltage control element, its working state is mainly determined by the gate-source voltage uGS.

The working characteristics are as follows:

uGS< On-voltage UT: The MOS tube works in the cut-off zone, the drain-source current iDS is basically 0, the output voltage uDS≈UDD, the MOS tube is in the "off" state, and its equivalent circuit is shown as the following figure.

uGS> On-voltage UT: The MOS tube works in the on-zone, drain-source current iDS=UDD/(RD+rDS). Where, rDS is the drain-source resistance when the MOS tube is on. Output voltage UDS=UDD·rDS/(RD+rDS), if rDS "RD", uDS≈0V, the MOS tube is in the "on" state, its equivalent circuit is shown in Figure (c) above.

Dynamic characteristic

There is also a transition process in the switching between the on-off and the on-off states of the MOS tube, but its dynamic characteristics mainly depend on the time required for the charge and discharge of the stray capacitor related to the circuit, and the charge accumulation and dissipation time of the tube itself is very small. The following figures (a) and (b) respectively show a circuit composed of an NMOS tube and its dynamic characteristics.

Dynamic characteristic diagram of NMOS tube

When the input voltage ui changes from high to low and the MOS tube changes from on-state to off state, the power UDD charges the stray capacitor CL through RD, and the charging time constant τ1=RDCL. Therefore, the output voltage uo changes from low level to high level through a certain delay; When the input voltage ui changes from low to high and the MOS tube changes from the cut-off state to the on-state, the charge on the stray capacitor CL is discharged through rDS, and the discharge time constant τ2≈rDSCL. It can be seen that the output voltage Uo can also be converted into a low level after a certain delay. But because rDS is much smaller than RD, the transition time from cutoff to conduction is shorter than the transition time from conduction to cutoff.

Because the drain source resistance rDS of the MOS tube is much larger than the saturation resistance rCES of the transistor triode, and the drain external resistance RD is also larger than the transistor collector resistance RC, the charging and discharging time of the MOS tube is longer, so that the switching speed of the MOS tube is lower than the switching speed of the crystal triode. However, in CMOS circuits, because the charging circuit and discharge circuit are low-resistance circuits, the charging and discharging process is relatively fast, so that the CMOS circuit has a higher switching speed.

MOS tube conduction characteristics

On means as a switch, equivalent to the switch closed.

The characteristics of NMOS, Vgs greater than a certain value will be switched on, suitable for use when the source is grounded (low-end drive), as long as the gate voltage reaches 4V or 10V.

The characteristics of PMOS, Vgs less than a certain value will be on, suitable for the case of the source connected to VCC (high-end drive). However, although PMOS can be conveniently used as a high-end driver, NMOS is usually used in high-end drivers due to large on-resistance, high price, and few replacement types.

MOS switch tube loss

Whether it is NMOS or PMOS, there is an on-resistance after the on-resistance, so that the current will consume energy on this resistance, and this part of the energy consumed is called the on-loss. Choosing MOS with small on-resistance will reduce on-loss. At present, the on-resistance of the low-power MOS tube is generally about tens of milliohm, and a few milliohm are also available.

When MOS is on and off, it must not be completed in an instant. The voltage at both ends of the MOS has a declining process, and the current flowing through it has a rising process. During this period of time, the loss of the MOS tube is the product of voltage and current, which is called the switching loss. Usually the switching loss is much larger than the on-off loss, and the faster the switching frequency, the greater the loss.

The product of voltage and current at the on-moment is large, and the loss caused is also large. Shorten the switching time, can reduce the loss of each conduction; Reducing the switching frequency can reduce the number of switching times per unit time. Both methods can reduce the switching loss.

The second: mos tube switch circuit diagram

In the figure, the positive charge of the battery is connected to the 2-pin source of the field effect tube Q1 through the switch S1. Since Q1 is a P-channel tube, its 1-pin gate provides a positive potential voltage through the R20 resistance, so it cannot be energized, and the voltage cannot continue to pass. The 3v voltage regulator IC input pin cannot get voltage, so it cannot work and does not turn on! At this time, if we press the SW1 power button, the positive charge through the key, R11, R23, D4 added to the base of the triode Q2, the base of the triode Q2 gets a positive potential, and the triode is on (mentioned earlier when talking about the triode), because the triode emitter is directly grounded, the triode Q2 is equivalent to the gate of Q1 is directly grounded. The voltage added to it through the R20 resistance is directly into the ground, the gate of Q1 changes from high potential to low potential, and the energizing of Q1 is added from Q1 to the input pin of the 3v voltage regulator IC, the 3v voltage regulator IC is the U1 output 3v working voltage vcc supply to the main control, the main control through reset to clear 0, read the firmware program detection and a series of actions. A control voltage at the output is sent to PWR_ON and then sent to the base of Q2 through R24 and R13 voltage, keeping Q2 always in the on-state, even if you release the power button to disconnect the base voltage of Q1, this time the control voltage sent by the main control is maintained, Q2 will always be in the on-state. Q1 can continuously provide working voltage to 3v regulator IC! SW1 also sends control signals of different duration and frequency to the master PLAYON through the voltage division of R11 and R30 resistors at the same time. The master sends different results to the corresponding control points through the firmware identification of playback, pause, start, and shut down, so as to achieve different working states!

The third: mos tube switch circuit diagram

The following is a typical application of the two MOS tubes: the first NMOS tube is a high level conduction, low level truncation, and the Drain end is connected to the ground end of the back circuit; The second type is a typical switching circuit of the PMOS tube, which is a high level disconnect, a low level conduction, and the Drain end is connected to the VCC end of the back circuit.

The fourth: mos tube switch circuit diagram

The drive circuit accelerates the MOSFet turn-off time

Figure 5 Isolation driver

In order to meet the high-end MOS tube drive as shown in Figure 5, transformer drive is often used, and sometimes transformer drive is also used to meet the safety isolation. Among them, R1 aims to inhibit the parasitic inductance on the PCB board and C1 form LC oscillation, and C1 aims to separate the DC through the AC, but also to prevent the core from saturating.

The fifth: mos tube switch circuit diagram

(a) For the commonly used low-power drive circuit, simple, reliable and low cost. Suitable for low-power switchgear that does not require isolation. (b) shown in the driver circuit switching speed is very fast, strong driving ability, in order to prevent two MOSFET tubes through, usually connected to a 0.5 ~ 1Ω small resistance for current limiting, the circuit is suitable for medium power switchgear that does not require isolation. These two circuits are characterized by simple structure.

Common unisolated complementary drive circuit

Power MOSFETs are voltage-type control devices that turn on as long as the voltage applied between the gate and the source exceeds its threshold voltage. Since the MOSFET has a junction capacitor, the sudden rise in the drain-source voltage when the MOSFET is turned off will generate interference voltage at both ends of the gate source through the junction capacitor. The turn-off circuit impedance of the commonly used complementary drive circuit is small and the turn-off speed is fast, but it can not provide negative pressure, so the anti-interference is poor. In order to improve the anti-interference of the circuit, a circuit composed of V1, V2 and R can be added on the basis of this drive circuit to generate a negative pressure. The circuit schematic diagram is shown in Figure 8.

Complementary circuits that provide negative pressure

When V1 is on, V2 is off, the gate and source of the upper tube of the two MOSFETs are discharged, and the gate and source of the lower tube are charged, that is, the upper tube is off and the lower tube is on, and the driven power tube is off. On the contrary, when V1 is off, V2 is on, the upper pipe is on, and the lower pipe is off, so that the driving pipe is on. Because the gate and source of the upper and lower two tubes are charged and discharged through different loops, including the loop of V2, because V2 will continue to exit saturation until it is turned off, so for S1, the turn-on is slower than the turn-off, and for S2, the turn-on is faster than the turn-off, so the heat degree of the two tubes is not exactly the same, S1 is more serious than S2.

The disadvantage of the drive circuit is that it needs two power supplies, and because the value of R can not be too large, otherwise it will saturate V1 depth and affect the turn-off speed, so R will have a certain loss.

Sixth: mos tube switch circuit diagram

Forward drive circuit

The circuit principle is shown in Figure (a), N3 is the demagnetization winding, S2 is the driven power tube. R2 is a damping resistance to prevent extreme voltage oscillation of the power tube grid and source. Because the leakage is not required to be small, and from the perspective of speed, the general R2 is small, so it is ignored in the analysis.

Forward drive circuit

The equivalent circuit diagram shown in the figure shows that the secondary side of the pulse is not required in parallel with a resistor R1, which is used as the false load of the forward converter to eliminate the output voltage oscillation and misdirection during the shutdown. At the same time, it can also be used as an energy leakage loop when the power MOSFET is turned off. The conduction speed of the drive circuit is mainly related to the size of the driven S2 gate, the equivalent input capacitance of the source, the speed of the drive signal of S1 and the current that S1 can provide. The simulation and analysis show that the smaller the duty cycle D, the larger R1 and L, the smaller the magnetization current, the smaller the U1 value, and the slower the turn-off speed. The circuit has the following advantages: ① The circuit structure is simple and reliable, and the isolation drive is realized. Only a single power supply can provide positive on, off negative pressure. ③ When the duty cycle is fixed, the drive circuit also has a fast switching speed through reasonable parameter design.

The disadvantages of the circuit are as follows: first, the loss of the circuit is large because the secondary side of the isolation transformer needs to choke and burp false load to prevent oscillation; Second, when the duty cycle changes, the switching speed changes greatly. When the pulse width is narrow, the MOSFET gate turn-off rate is slower due to the reduced stored energy.

The seventh: mos tube switch circuit diagram

Complementary drive circuit with isolation transformer

As shown in the figure, V1 and V2 are complementary, capacitor C plays a role in isolating DC, and T1 is a magnetic ring or magnetic tank with high frequency and high magnetic rate.

Complementary drive circuit with isolation transformer

The voltage on the isolation transformer is (1-D) Ui when it is on and DUi when it is off. If the reliable on-voltage of the main power tube S is 12V, the primary and secondary side-turn ratio of the isolation transformer N1/N2 is 12/ [(1-D) Ui]. In order to ensure the stability of GS voltage during switching, the C value can be slightly larger. The circuit has the following advantages:

① The circuit structure is simple and reliable, and has the effect of electrical isolation. When the pulse width changes, the turn-off ability of the drive does not change.

② The circuit needs only one power supply, that is, a single power supply. The function of the isolating capacitor C can provide a negative pressure when the driven tube is turned off, thus accelerating the power tube turn-off and has a high anti-interference ability.

However, a major disadvantage of this circuit is that the amplitude of the output voltage will change with the change of the duty cycle. When D is small, the negative voltage is small, the anti-interference performance of the circuit is poor, and the forward voltage is high, and attention should be paid to make its amplitude not exceed the allowable voltage of the MOSFET gate. When D is greater than 0.5, the forward voltage of the drive voltage is less than its negative voltage, and attention should be paid to the negative voltage value not exceeding the allowable voltage of the MOAFET gate. Therefore, the circuit is more suitable for the occasions where the duty cycle is fixed or the duty cycle has a small change range and the duty cycle is less than 0.5.

Eighth: mos tube switch circuit diagram

The drive circuit composed of integrated chip UC3724/3725

The circuit structure is shown in Figure 11. The UC3724 is used to generate high-frequency carrier signal, and the carrier frequency is determined by the capacitor CT and the resistor RT. Generally, the carrier frequency is less than 600kHz, and high-frequency modulated waves are generated at both ends of pin 4 and pin 6, which are isolated by a high-frequency small magnetic ring transformer and sent to the UC3725 chip after pin 7 and pin 8 are modulated by UC3725 to obtain the drive signal. The UC3725 has a Schottky rectifier bridge inside which the 7 - and 8-pin high-frequency modulated wave is rectified into a constant current voltage to supply the required drive power. Generally speaking, the higher the carrier frequency, the smaller the drive delay, but the higher the anti-interference becomes worse; The larger the magnetizing inductance of the isolation transformer, the smaller the magnetizing current, and the less the heat of UC3724, but the increase of the number of turns leads to the greater influence of parasitic parameters, which will also reduce the anti-interference ability.

According to the experimental data:

For the signal with a switching frequency less than 100kHz, the carrier frequency of (400 ~ 500) kHz is generally better, and the transformer uses a high frequency ring core such as 5K and 7K, and the magnetizing inductance on the primary side is less than about 1 millihen. This drive circuit is only suitable for signal frequency less than 100kHz occasions, because the signal frequency relative to the carrier frequency is too high, the relative delay is too much, and the required drive power increases, UC3724 and UC3725 chip heating temperature rise is higher, so the switching frequency above 100kHz only for the MOSFET with small pole capacitance can be. It is a good drive circuit for 1kVA or so switching frequency less than 100kHz occasions. The circuit has the following characteristics: single power supply operation, control signal and drive to achieve isolation, simple structure and small size, especially suitable for uncertain duty cycle changes or signal frequency changes occasions.

The drive circuit composed of integrated chip UC3724/3725

Ninth: mos tube switch circuit diagram

In the first application, the voltage is selected by PMOS. When V8V is present, the voltage is all provided by V8V, and the PMOS is turned off. VBAT does not provide voltage to VSIN, and when V8V is low, VSIN is supplied by 8V. Pay attention to the grounding of R120, which can stably pull down the gate voltage to ensure the normal opening of the PMOS, which is also the hidden danger caused by the high grid impedance described above. The function of D9 and D10 is to prevent voltage backflow. D9 can be omitted. It should be noted here that in fact the DS connection of the circuit is reversed, so that the function of the switching tube can not be achieved by the epiphytic diode conduction, and the practical application should pay attention to.

Look at this circuit, the control signal PGC controls whether V4.2 supplies power to P_GPRS. In this circuit, the source-drain ends are not reversed. The significance of the existence of R110 and R113 lies in that the R110 control grid current is not too large, and the normal state of R113 control grid will pull R113 up to the PMOS, which can also be regarded as the pull-up of the control signal. When the MCU internal pin does not pull up, that is, the output is open leak. It cannot drive the PMOS to shut down, at this time, it needs to be pulled up by the external voltage, so the resistor R113 plays two roles. The R110 can be smaller, down to 100 ohms.