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Jan 18 2018

Transistor Switching Circuit Design and Its Theory

Warm hints: The word in this article is about 3500 and reading time is about 18 minutes.

Summary

Transistor switching circuit (working in saturation state) is commonplace in modern circuit design applications. The classic 74LS, 74ALS and other integrated circuits use transistor switching circuits internally, but they have only common driving capability. Transistor switching circuit is divided into two categories, one is the classic TTL transistor switching circuit, the other is the MOS tube switching circuit. This paper will show you the knowledge about transistor switching circuit including TTL transistor switching circuit; Buzzer control circuit - passive buzzer ; and etc..

Article coreTransistor Switching Circuit Design and Its TheoryCategoryCircuit
English nameTransistor switching circuit



Catalogs

Catalogs

I.Transistor switching circuit

III.Buzzer control circuit - passive buzzer

4.3 A voltage regulator tube and MOS tube voltage regulator circuit

II.TTL transistor switching circuit

IV.IO control power switch is on - Use transistor and MOS tube

V.Signal level conversion

2.1 Emitter ground switch circuit

4.1 Via a IO pin to control the power

5.1 Basic Transistor switch to improve the circuit

2.2 Emitter follows the switch circuit

4.2 Two MOS tubes 3401

5.2 Acceleration capacitor




Introduction

II.Transistor switching circuit

Transistor switching circuit (working in saturation state) is commonplace in modern circuit design applications. The classic 74LS, 74ALS and other integrated circuits use transistor switching circuits internally, but they have only common driving capability. Transistor switching circuit is divided into two categories, one is the classic TTL transistor switching circuit, the other is the MOS tube switching circuit. 

II.TTL transistor switching circuit

TTL transistor switching circuit according to the driving ability is divided into small signal switching circuit and power switching circuit. According to the transistor connection style is divided into emitter ground (PNP transistor emitter connected to the power supply) and shooter follow the switch circuit.

2.1 Emitter ground switch circuit

NPN and PNP basic switch schematic--Transistor Switching Circuit Design and its theory

The basic circuit above is a bit further from the actual design circuit: there is a transition from on to off due to the accumulation of the base charge of the transistor (when the transistor is off, the base charge release is slowed due to the presence of R1, so Ic will not be immediately turns to zero). In other words emitter earthing switch circuit has turn-off time. It can not be directly applied to the high-frequency switch.

Practical NPN and PNP Switching Schematic (Add Acceleration Capacitance)--Transistor Switching Circuit Design and its theory

Explanation: When the transistor suddenly turns on (IN signal suddenly jumps), C1 momentarily presents short-circuits, which provides the transistor with the base current quickly, thus speeding up transistor conduction. When the transistor suddenly turns off (the IN signal suddenly trips), C1 turns on instantaneously, providing a low impedance path for discharging the base charge, thus speeding the transistor off. C  value is usually tens to hundreds of skin method. R2 in the circuit is to ensure that transistor remains off state when there is no IN high input.R4 is to ensure that transistor remains off when there is no IN low input . R1 and R3 are used in base current limiting.

Practical NPN Switching Schematic Schematic (Diode-based Diode Clamp)--Transistor Switching Circuit Design and its theory

Explanation: Since the TVS diode Vf is 0.2 to 0.4V smaller than Vbe, most of the base current flows from the diode and then the transistor, and last to ground when the transistor is turned on, so that the current flowing to the base of the transistor is small , Accumulating less charge. When the transistor is off (IN signal suddenly jumps), discharged charge become less, turn-off action naturally become faster.

The actual circuit design--Transistor Switching Circuit Design and its theory

In the actual circuit design, we need to consider the transistor Vceo, Vcbo to meet the pressure, and the transistor to meet the collector power consumption. Using load current and hfe (to take the minimum transistor hfe to calculate) to calculate the base resistance (base current to stay 0.5 to 1 Times the margin). Note that the special diode reverse withstand voltage.

2.2 Emitter follows the switch circuit

Explanation: The advantages of emitter follow are switching speed. Itcan be used in high-frequency signal switching. R2 can not be too large, the circuit is susceptible to interference. Of course, it can not be too small, or wasted the previous drive capability. Base does not need current limiting resistor, because the load current divided by hfe is the base current, the transistor will automatically take such a large current from the superiors.

Power switch circuit

The above-mentioned switch circuit is suitable for low-power switch. when the load current is larger, there are two ways to solve: First, select the high-hfe tube, so that is conducive to the previous stage to provide base current. The other is to use Darlington Connect the two transistors string together, so hfe = hfe1 * hfe2, but also conducive to provide the base current.

Note when using the Darlington connection: The potential difference between base and emitter is 1.2 to 1.4 volts. The tube voltage drop used to calculate the tube collector power dissipation is no longer its Vce (sat) but its Vbe, That is 0.6 to 0.7 volts.

Transistor switch circuit video site: 

This video is about a simple circuit of a transistor switch based on a bipolar transistor,Transistor switches are 

used in various radio-electronic appliances, analog and digital signal switches, automation and monitoring 

systems, automatic equipment control systems, home radio and TV appliances, as well as in many other devices.



Detail

III.Buzzer control circuit - passive buzzer

When BUZZ is on high voltage, the transistor T1 (N-type transistor) is turned on, the buzzer sounds. R5’s role is used for the current limit.

Buzzer control circuit - passive buzzer--Transistor Switching Circuit Design and its theory

The following circuit adds a capacitor C18 and a reverse diode D2 to filter and block the reverse. The reverse breakdown voltage of the diode is very high. General low-power triode trigger voltage is very low to 0.7V. Current is also very small, generally less than 1UA.

Buzzer control circuit - passive buzzer--Transistor Switching Circuit Design and its theory

IV.IO control power switch is on - Use transistor and MOS tube

MOS: One of the FET MOSFET tube, which can be made into enhanced or depleted, P-channel or N-channel altogether types. But the practical application is only enhanced N-channel MOS tube and enhanced P Channel MOS tube, that’s NMOS and PMOS.

For these two enhanced MOS tube, NMOS is commonly used, characterized by low on-resistance. It is usually applied to witching power supply and motor-driven.

Condution conditions:

NMOS turns on when Vgs is greater than a certain value. PMOS turns on when Vgs is less than a certain value.

Switching loss:

Whether it is NMOS or PMOS, there are on-resistance after conduction, resulting in inevitable losses. And now the MOS transistor on-resistance is generally tens of milliohms.

MOS tube AO3401: P-channel Enhancement Mode Field Effect Transistor

P-channel Enhancement Mode Field Effect Transistor--Transistor Switching Circuit Design and its theory

Conduction conditions: generally do not exceed -12V can be for the AO3401. The following is the impedance for different pressure drop:

Transistor Switching Circuit Design and its theory

The following is the switch control circuit in engineering applications.

4.1 Via a IO pin to control the power

Via a IO pin to control the power--Transistor Switching Circuit Design and its theoryVia a IO pin to control the power--Transistor Switching Circuit Design and its theory

4.2 Two MOS tubes 3401

The following are two MOS tubes 3401, without adding switch control. Just after power-on, VDD is equal to the input voltage. At this point you can power in two ways. If J5 doesn’t have input voltage, power by VBUS, output 5V voltage via the F1. The following circuit can replace R10 with a switch, Q201 is always on, the internal diode voltage drop is about 0.5V.

Note: the direction of the two transistors are different, Q200 left is S, the right is D. Q201 left is D, the right is s.

When J5 has voltage, Q200 turns on, Q201 also meets the conduction condition, the voltage is 0.1V.

Transistor Switching Circuit Design and its theory

Note: VBUS right side is disconnected.

Reference

J5voltage

R11

R9

VDD

VBUS

J5 and VBUS

5.09

0

0.46

5.07

4.6

Only VBUS

4.21

0

0.38

4.21

4.51

Only J5

5.09

0

0.46

5.09

4.75

4.3 A voltage regulator tube and MOS tube voltage regulator circuit

Description:

VCC can come from left side VDD5V_Control, also can come from PC PS2 mouth supply Vpc_IN. VCC adopts the one which voltage is high.

Regulator circuit measurement--Transistor Switching Circuit Design and its theory

The original circuit:

Transistor Switching Circuit Design and its theory

The left Vpc_IN is powered by PS2 power supply, the right is powered by VCC.

When the PS2 powers, and the left is 5V, the right is about 4.5V, It can meet the voltage requirements of the machine, when the PS2 port is turned off, the machine can work properly.

In order to reduce the voltage drop of PS2, I decide to adopt the following circuit:

Transistor Switching Circuit Design and its theory

When the PS2 port is powered, the three tubes Q412 is turned on, so Q411 is turned on and VCC is close to Vpc_IN. At this time the machine adopts PS2 port voltage (about 5V). when PS2 is not connected, The current can not flow from the machine to the PS2 port.

Using the above parameters test record:

Voltage at both ends of the voltage regulator

A(input)

B

C

D(output)

E

current

3.41

5.14

1.73

0.68

5.13

0

1.05MA

3.25

4.63

1.38

0.67

4.63

0

0.71MA

3.1

4.23

1.13

0.66

4.23

0

0.47 MA

2.9

3.8

0.9

0.64

3.8

0

0.26 MA

2.59

3.32

0.73

0.62

3.32

0

0.09 MA

2.34

2.9

0.56

0.5

2.35

2.25

0.05 MA

2.28

2.73

0.45

0.41

2.16

2.15

0.04 MA

The last two lines show:

MOS diode internal diode voltage drop is about 0.6V.

Zener leakage current can make the transistor conduction. PN junction can be turned on around 0.6V.

Conclusions:

The input voltage at 3.3V, the transistor is turned on, indicating that the resistance R436 is too large, you need to reduce.

Zener leakage current increases with increasing input voltage, but current should exceed 1 mA when the voltage at both ends reaches 3.9V.

In order to ensure that the input voltage around 5V can make it steady, you must increase the current, reduce the resistance, and when the input voltage is lower than 4.7V, you must turn off the transistor.

Voltage at both ends of the voltage regulator

A(input)

B

C

D(output)

E

current

3.94

5.15

1.21

0.69

5.15

0

5.2MA

3.85

4.9

1.05

0.65

4.9

0

4MA

3.8

4.76

0.96

0.63

4.76

0

3.3 MA

3.77

4.65

0.88

0.59

4.65

0

2.9 MA

3.76

4.62

0.86

0.58

4.62

0

2.8 MA

3.72

4.48

0.76

0.51

4.03

3.70

2.5MA

3.64

4.25

0.61

0.41

3.67

3.67

2 MA

The last two lines show:

To meet the PS2 input voltage in [4.6-5V] to meet the regulator's effect. And then further large keyboard connected to the machine, when the machine power off, the keyboard can work properly. When the power tool work can also work normally.

Problems detected:

The quality test shows that the terminal can not be shut down. It has been found that when the terminal is powered off, there is still voltage at Vpc_In. VCC (4.84V) passes Q411, resulting in a voltage of 4.8V at Vpc_In. And the voltage drop of D405 is 0.3V or so. When Vpc_IN suddenly become power-off, the power supply VCC is at the moment of power-off, the transistor is on, all VCC will pour into the terminal, the transistor is always on.

The range of the PS2 power supply voltage is not easy to determine. That is, when the terminal voltage is large, the circuit is forward conducting. At the same time, the Vpc_IN voltage must be less than a certain value to prevent the transistor Q412 from turning on.

For Example:

IRF530 features: ,General VGS take 12-15V, floating among plus or minus 20V

Transistor Switching Circuit Design and its theory

The circuit above is wrong. Vgs is too small.

For single-chip PWM drive high voltage MOS (VGS close to 10V at saturation turn-on state), we should consider the following issues:

Level conversion, high-level microcontroller output does not exceed 5V, general 12-15V, So the drive circuit must have level conversion capability.

Phase conversion, MOS said above is as an inverter, so according to the phase of the load and the microcontroller output phase conversion. Such as the request MOS output MOS turn-on, the drive circuit is required in phase.

Switching frequency, different drive circuit has a different frequency response, for up to 1.5M switching frequency, with a simple triode simple self-ride circuit is difficult to meet the requirements, the basic need to choose a dedicated driver IC. Also, the general optocoupler is not working at the frequency of dozens of K above the switch state, if you want to isolate, 6N137 is better, there are special with optical isolation and drive optocoupler, 1.5M still can not reach .

Drive current. Although MOS consumes no driving power when it is static, his input is capacitive. In order to turn on the switch as soon as possible and reduce the switching loss, it is necessary to charge the Cgs with the fastest speed, so the driving circuit has a very important parameter Peak drive current, such as 200MA, 600MA, 1A, 2A, 4A, 6A.

The operating voltage of the drive circuit, the general maximum VGS can not exceed 20V, so the operating voltage of the drive circuit should not exceed 18V as well, to the circuit above, you need to add a voltage of 15V, of course, can buck from 40V.

DV / DT problem, electromagnetic interference will increase because MOS is easily damaged under high DV / DT. In order to solve these problems, it is sometimes necessary to increase the rise / fall time of the output of the driver circuit. A simple method is to add a small resistance between the driver output and the G-pole.



Analysis

V.Signal level conversion

5.1 Basic Transistor switch to improve the circuit

Sometimes, the low voltage level we set may not be able to make the transistor switch off, especially when the input level close to 0.6 volts . To overcome this critical condition, we must take corrective steps to ensure that the transistor must be closed. Figure 1 shows the improved circuit designed for the two situation.

ensure the transistor switch action, the correct two modified circuit--Transistor Switching Circuit Design and its theory

Figure 1 ensure the transistor switch action, the correct two modified circuit

The circuit of Figure 1 (a) has a diode connected in series between the base and emitter so that the value of the input voltage that enables the base current to turn on is raised by 0.6 volts so that even if the value of Vin approaches the value due to a malfunction of the signal source 0.6 volts, the transistor will not lead to conduction, so the switch can still be in the off state.

The circuit of Figure 1 (b) incorporates a secondary hold-off resistor, R2, designed with appropriate R1, R2 and Vin values to ensure the switch is turned off at the critical input voltage. As shown in Fig. 1 (b), R1 and R2 form a series voltage divider circuit before the base emitter junction is not conducting (IB0), so R1 must pass a fixed (varying with Vin) voltage. And the base voltage must be lower than the Vin value. Even if Vin approaches the threshold (Vin = 0.6 volts), the base voltage will still be pulled down by the auxiliary-off resistance connected to the negative supply to below 0.6 volts. Due to the deliberate design of the R1, R2 and VBB values, as long as Vin is in the high range, the base will still have enough voltage to turn on the transistor without being affected by the auxiliary-off resistance.

5.2 Acceleration capacitor

In applications requiring fast switching actions, the switching speed of the triode switch must be increased. Figure 2 is a common method, this method only in parallel with an RB resistor on the acceleration capacitor, so when Vin rises from zero voltage and start sending current to the base, the capacitor can not be instantaneous charge, so the same short circuit However, at this moment, there is an instantaneous high current flowing from the capacitor to the base, thus speeding up the conduction of the switch. Later, until the charge is completed, the capacitance is the same as open circuit, without affecting the normal work of the transistor.

 circuit with the acceleration capacitor--Transistor Switching Circuit Design and its theory

Figure 2 circuit with the acceleration capacitor 

Once the input voltage drops from high level to zero voltage level, the capacitor turns the base emitter junction into a reverse bias in a very short period of time, causing the triode switch to switch off rapidly due to the left end of the capacitor Has been charged to a positive voltage, so the moment the input voltage drops, the voltage across the capacitor can not be instantaneously changed will remain at a fixed value, so the input voltage drops immediately to make the base voltage decreases, so that the base emitter junction Become reverse bias, and quickly turn off the transistor. Proper selection of the accelerating capacitor reduces the switching time of the triode switch below a few tenths of microseconds, and most of the accelerating capacitors are on the order of hundreds of pF.

Sometimes the load of the triode switch is not directly applied between the collector and the power supply, but then connected as shown in Figure 3. This connection and small signal amplifier circuit is very close, but lack of only one output coupling capacitor. This connection and the normal connection is exactly the opposite. When the transistor is off, the load is enabled. When the transistor is turned on, the load is cut off. The form of these two circuits are common, we must have a clear Resolution ability.

Improved circuit that connects the load to transistor circuit--Transistor Switching Circuit Design and its theory

Figure 3 Improved circuit that connects the load to transistor circuit

One of the most common applications of a transistor switch is to drive a pilot light, which can indicate the operating conditions of a particular point of the circuit, whether the motor's controller is energized, or whether a certain limit switch Pass or a digital circuit is in a high state.

For example, Figure 4 (a) shows the output state of a digital flip-flop using a transistor switch. If the flip-flop output is high (typically 5 volts), the transistor switch is turned on, leaving the indicator light, so the operator just a look at the light, you can know the flip-flop current working conditions , Without the need to use the meter to detect.

Sometimes the signal source (such as the flip-flop) output current capacity is too small, not enough to drive the transistor switch, this time to avoid signal source overload and malfunction, we must use the improved circuit shown in Figure 4(b) When the output is high, the first driver emitter current amplification with the transistor Q1 to do, and then turn on Q2 and drive the light, because the emitter with the input stage of the input impedance is quite high, so the flip-flop should be provided A small amount of input current, you can get a satisfactory job.

Digital display circuit of Figure 4(a) is often used on digital displays.

Figure 4 (a) Basic circuit diagram--Transistor Switching Circuit Design and its theoryFigure 4 (b) Improved circuit diagram--Transistor Switching Circuit Design and its theory

Figure 4 (a) Basic circuit diagram (b) Improved circuit diagram

Analysis: If FREOF is high 5V, the output FREOUT should be about 1.3K Hz square wave,

The waveforms are as follows: The left side of C39 and the right side of C41 are square wave of about 1.3K, and one high and one low.

Transistor Switching Circuit Design and its theory

About RC charge and discharge experiments:

In the following figure, when input 1Hz square wave signal, the interception of the left of the waveform C3 as follows. It takes about 4ms to be fully charged.

Theoretical calculations: charge and discharge shares the same principle. First calculate charge and discharge constant TC = RC, the unit is ohm and F.

The following circuit TC = 1K * 1uf = 1ms 3TC can usually reach 0.95E, and 4.75V, so 3ms can reach 4.75V, consistent with the waveform.

Transistor Switching Circuit Design and its theory

The figure is a simple control circuit:

When KSEL is high, KCLK1 and KCLK0 through, KDAT1 and KDAT0 through.

Whtn it's on Low level, the circuit blocks.

Transistor Switching Circuit Design and its theory



Book Recommendation

  • Design and Application of Transistor Switching Circuits Hardcover – March, 1968

--Louis A. Delhom (Author)

  • Transistor Circuit Design

--Texas Instruments Incorporated (Author)



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