Shim operation diagram. PWM controller. Pulse width modulation. Scheme. PWM – pulse width modulation

Adjusting the speed of electric motors in modern electronic technology is achieved not by changing the supply voltage, as was done before, but by supplying current pulses of different durations to the electric motor. PWM, which has recently become very popular, is used for these purposes ( pulse width modulated) regulators. The circuit is universal - it also controls the engine speed, the brightness of the lamps, and the current in the charger.

PWM regulator circuit

The above diagram works great, attached.

Without altering the circuit, the voltage can be raised to 16 volts. Place the transistor depending on the load power.

Can be assembled PWM regulator and according to this electrical circuit, with a conventional bipolar transistor:

And if necessary, instead of the composite transistor KT827, install a field-effect IRFZ44N, with resistor R1 - 47k. The polevik without a radiator does not heat up at a load of up to 7 amperes.

PWM controller operation

The timer on the NE555 chip monitors the voltage on capacitor C1, which is removed from the THR pin. As soon as it reaches the maximum, the internal transistor opens. Which shorts the DIS pin to ground. In this case, a logical zero appears at the OUT output. The capacitor begins to discharge through DIS and when the voltage on it becomes zero, the system will switch to the opposite state - at output 1, the transistor is closed. The capacitor begins to charge again and everything repeats again.

The charge of capacitor C1 follows the path: “R2->upper arm R1 ->D2”, and the discharge along the path: D1 -> lower arm R1 -> DIS. When we rotate the variable resistor R1, we change the ratio of the resistances of the upper and lower arms. Which, accordingly, changes the ratio of the pulse length to the pause. The frequency is set mainly by capacitor C1 and also depends slightly on the value of resistance R1. By changing the charge/discharge resistance ratio, we change the duty cycle. Resistor R3 ensures that the output is pulled to a high level - so there is an open-collector output. Which is not able to independently set a high level.

You can use any diodes, capacitors of approximately the same value as in the diagram. Deviations within one order of magnitude do not significantly affect the operation of the device. At 4.7 nanofarads set in C1, for example, the frequency drops to 18 kHz, but it is almost inaudible.

If after assembling the circuit the key control transistor gets hot, then most likely it does not open completely. That is, there is a large voltage drop across the transistor (it is partially open) and current flows through it. As a result, a lot of power is dissipated for heating. It is advisable to parallel the circuit at the output with large capacitors, otherwise it will sing and be poorly regulated. To avoid whistling, select C1, the whistling often comes from it. In general, the scope of application is very wide; its use as a brightness regulator for high-power LED lamps, LED strips and spotlights will be especially promising, but more on that next time. This article was written with the support of ear, ur5rnp, stalker68.

Any radio amateur, novice TV technician or electrician will sooner or later come across such a thing as a PWM controller. Abroad it is labeled as PWM. Therefore, today I want to dwell on the question of what a PWM controller is, how it works and what it is needed for. Even if you do not plan to repair electronic equipment, this article will still be interesting for general information.

Pulse width modulator - operating principle

The abbreviation PWM stands for pulse width modulator. In English it will be like this - pulse-width modulation or PWM. In television and radio technology, PWM controllers are used to convert voltage; they can even be used as components of a speed control system for electric drives in household appliances, changing the speed of the electric motor. There is a PWM controller even in conventional switching power supplies.

There, the constant voltage at the input is converted into rectangular pulses, which are formed at a certain frequency and with a certain duty cycle. At the output, using control signals, it is possible to regulate the operation of an entire high-power transistor module. Thus, the developers received an adjustable voltage control unit, which is much smaller and more convenient than the old ones, which use a step-down transformer, a diode bridge and a noise filter.

The main advantages of PWM:

Small dimensions; - excellent performance; - high reliability; - low cost.

On the Internet you can find a PWM controller based on Arduino or NE555. This is not exactly a controller, but rather a generator of PWM pulses, in which there is no possibility of connecting a feedback circuit. Such devices are more suitable for voltage regulators than for providing stable power to devices, because they can only be used to regulate output parameters, but not to stabilize them.

PWM controller outputs

The standard circuit of a PWM controller, which is used in television, radio and other electronic equipment, is characterized by the presence of several outputs.

Common pin (GND)— the contact is connected to the common wire of the controller’s power supply circuit. It is connected to a similar pin of the module’s power supply circuit and controls the voltage at the circuit’s output, turning it off when the value drops below a threshold.

Power pin (VC)— this pin of the PWM controller is responsible for power supply to the circuit and power connection. Typically, the power control pin and the power pin are located next to each other. Don't confuse this with the VCC pin.

Power Control Pin (VCC)— ensures that the supply voltage of the microcircuit is above a certain value. Typically this pin is connected to VC. If the voltage at this pin drops below the specified threshold for a given PWM controller, the controller turns off. If this is not done, then when the voltage at the output of the circuit decreases, the transistors will not open completely and will quickly heat up, which will lead to breakdown.

Controller output OUT– this is the output control voltage, in other words, the control PWM signal for the power switches is supplied from here. It should be noted here that microcircuits are different. For example, there are two outputs - push-pull, which are used to control two-arm cascades. And the output stage itself can be single- or two-cycle. The main thing here is not to get confused!

VREF output— Reference voltage. Provides operation of the function of generating a stable reference voltage. As a rule, it is recommended to connect it to the common wire with a 1 µF capacitor to improve the quality and stability of the reference voltage.

ILIM output— Output current limiter. This is a signal from the current sensor. If the voltage at this pin exceeds a specified threshold (usually 1 Volt), then the PWM controller closes the power switches. If an even higher threshold is exceeded (usually 1.5 Volts), then the PWM controller resets the voltage on the soft start leg and the output pulses stop.

ILIMREF output— sets the output current limit value at the ILIM pin.

SS output- the so-called “soft start”. The voltage at this pin limits the maximum possible pulse width. The PWM controller supplies a fixed current here.

RtCt output– used to connect a timing RC circuit used to determine the frequency of the PWM signal.

RAMP output is the comparison input. It works like this. A sawtooth voltage is applied to the contact. As soon as it exceeds the voltage value at the error amplification output, a trip signal appears at the OUT pin. This is the basis of PWM regulation.

CLOCK output– clock pulses. Used to synchronize several PWM controllers with each other. In this case, the RC circuit is connected only to the master controller, the RT of the slaves is connected to Vref, and the CT of the slaves is connected to the common.

INV output- this is the inverting input of the comparator. An error amplifier is built on it. The higher the voltage across INV, the longer the output pulses.

Conclusion NONINV is the non-inverting input of the comparator. It is usually connected to the common wire - GND.

EAOUT pin— error amplifier output — Error Amplifier Output. From this pin, frequency correction of the error amplifier is carried out by applying signals to the INV through frequency-dependent circuits. The fact is that the PWM controller reacts quite slowly to influence through the input of the error amplifier and therefore the circuit can burn out due to excitation. That's why the EAOUT pin is used.

How to test a PWM controller

There are several ways to test a PWM controller. You can, of course, do this without a multimeter, but why bother if you can use a normal device.

Before checking the operation of the PWM controller, it is necessary to perform basic diagnostics of the power supply itself. It works like this:

Step 1. Carefully inspect the power supply itself, in which the PWM is installed, when it is turned off. In particular, electrolytic capacitors should be carefully inspected for swelling.

Step 2. Check the fuse and input filter elements of the power supply for serviceability.

Step 3. Check for short circuit or open circuit of the rectifier bridge diodes. You can ring them without desoldering them from the board. In this case, you must be sure that the circuit being tested is not shunted by the transformer windings or resistor. If you suspect this, you will still have to unsolder the elements and check them separately.

Step 4. Check the serviceability of the output circuits, namely electrolytic capacitors of low-frequency filters, rectifier diodes, diode assemblies, etc.

Step 5. Check the power transistors of the high-frequency converter and the transistors of the control cascade. In this case, be sure to check the return diodes that are connected in parallel to the collector-emitter electrodes of the power transistors.

Checking the PWM controller - video instructions:

Previously, to power devices, they used a circuit with a step-down (or step-up, or multi-winding) transformer, a diode bridge, and a filter to smooth out ripples. For stabilization, linear circuits using parametric or integrated stabilizers were used. The main disadvantage was the low efficiency and large weight and dimensions of powerful power supplies.

All modern household electrical appliances use switching power supplies (UPS, IPS - the same thing). Most of these power supplies use a PWM controller as the main control element. In this article we will look at its structure and purpose.

Definition and Main Benefits

A PWM controller is a device that contains a number of circuit solutions for controlling power switches. In this case, control occurs on the basis of information received through feedback circuits for current or voltage - this is necessary to stabilize the output parameters.

Sometimes PWM pulse generators are called PWM controllers, but they do not have the ability to connect feedback circuits, and they are more suitable for voltage regulators than for providing stable power to devices. However, in the literature and Internet portals you can often find names like “PWM controller, on NE555” or “... on Arduino” - this is not entirely true for the above reasons, they can only be used to regulate output parameters, but not to stabilize them.

The abbreviation “PWM” stands for pulse-width modulation - this is one of the methods of modulating a signal not due to the output voltage, but precisely by changing the pulse width. As a result, a simulated signal is formed by integrating pulses using C- or LC-circuits, in other words, by smoothing.

Conclusion: A PWM controller is a device that controls a PWM signal.

Main characteristics

For a PWM signal, two main characteristics can be distinguished:

1. Pulse frequency - the operating frequency of the converter depends on this. Typical frequencies are above 20 kHz, in fact 40-100 kHz.

2. Duty factor and duty cycle. These are two adjacent quantities characterizing the same thing. The duty cycle can be denoted by the letter S, and the duty cycle by D.

where T is the signal period,

The part of the time from the period when a control signal is generated at the controller output is always less than 1. The duty cycle is always greater than 1. At a frequency of 100 kHz, the signal period is 10 μs, and the switch is open for 2.5 μs, then the duty cycle is 0.25, as a percentage - 25 %, and the duty cycle is 4.

It is also important to consider the internal design and purpose of the number of keys managed.

Differences from linear loss schemes

As already mentioned, the advantage over linear circuits is the high efficiency (more than 80, and currently 90%). This is due to the following:

Let's say the smoothed voltage after the diode bridge is 15V, the load current is 1A. You need to get a stabilized 12V power supply. In fact, a linear stabilizer is a resistance that changes its value depending on the value of the input voltage to obtain a nominal output - with small deviations (fractions of volts) when the input changes (units and tens of volts).

As is known, resistors release thermal energy when electric current flows through them. The same process occurs on linear stabilizers. The allocated power will be equal to:

Ploss=(Uin-Uout)*I

Since in the considered example the load current is 1A, the input voltage is 15V, and the output voltage is 12V, we will calculate the losses and efficiency of the linear stabilizer (KRENK or type L7812):

Ploss=(15V-12V)*1A = 3V*1A = 3W

Then the efficiency is:

n=Puseful/Pconsumed

n=((12V*1A)/(15V*1A))*100%=(12W/15W)*100%=80%

The main feature of PWM is that the power element, let it be a MOSFET, is either completely open or completely closed and no current flows through it. Therefore, efficiency losses are due only to conductivity losses

And switching losses. This is a topic for a separate article, so we will not dwell on this issue. Also, power supply losses occur (input and output, if the power supply is network-powered), as well as on conductors, passive filter elements, etc.

General structure

Let's consider the general structure of an abstract PWM controller. I used the word “abstract” because, in general, they are all similar, but their functionality may still differ within certain limits, and the structure and conclusions will differ accordingly.

Inside the PWM controller, like any other IC, there is a semiconductor crystal on which a complex circuit is located. The controller includes the following functional units:

1. Pulse generator.

2. Reference voltage source. (AND HE)

3. Circuits for processing the feedback signal (OS): error amplifier, comparator.

4. Pulse generator controls built-in transistors, which are designed to control a power key or keys.

The number of power switches that a PWM controller can control depends on its purpose. The simplest flyback converters in their circuit contain 1 power switch, half-bridge circuits (push-pull) - 2 switches, bridge circuits - 4.

The choice of PWM controller also depends on the type of key. To control a bipolar transistor, the main requirement is that the output control current of the PWM controller is not lower than the transistor current divided by H21e, so that it can be turned on and off simply by sending pulses to the base. In this case, most controllers will do.

In the case of management, there are certain nuances. To quickly turn off, you need to discharge the gate capacitance. To do this, the gate output circuit is made of two keys - one of them is connected to the power supply with the IC pin and controls the gate (turns on the transistor), and the second is installed between the output and ground, when you need to turn off the power transistor - the first key closes, the second opens, closing shutter to the ground and discharges it.

Interesting:

Some PWM controllers for low-power power supplies (up to 50 W) do not use built-in or external power switches. Example - 5l0830R

Generally speaking, a PWM controller can be represented as a comparator, one input of which is supplied with a signal from the feedback circuit (FC), and a sawtooth changing signal is supplied to the second input. When the sawtooth signal reaches and exceeds the OS signal in magnitude, a pulse appears at the output of the comparator.

When the signals at the inputs change, the pulse width changes. Let's say that you connected a powerful consumer to the power supply, and the voltage at its output drops, then the OS voltage will also drop. Then, in most of the period, the sawtooth signal will exceed the feedback signal, and the pulse width will increase. All of the above is reflected to a certain extent in the graphs.

Functional diagram of a PWM controller using the TL494 as an example; we will look at it in more detail later. The purpose of the pins and individual nodes is described in the following subheading.

Pin assignment

PWM controllers are available in various packages. They can have from three to 16 or more conclusions. Accordingly, the flexibility of using the controller depends on the number of pins, or rather their purpose. For example, a popular microcircuit most often has 8 pins, and an even more iconic one has TL494- 16 or 24.

Therefore, let’s look at typical pin names and their purpose:

GND- the common terminal is connected to the minus of the circuit or to ground.

Uc(Vc)- power supply of the microcircuit.

Ucc (Vss, Vcc)- Output for power control. If the power sags, then there is a possibility that the power switches will not open completely, and because of this they will begin to heat up and burn out. The output is needed to disable the controller in such a situation.

OUT- as the name suggests, this is the output of the controller. The control PWM signal for power switches is output here. We mentioned above that converters of different topologies have different numbers of keys. The name of the pin may differ depending on this. For example, in half-bridge controllers it may be called HO and LO for the high and low switches, respectively. In this case, the output can be single-ended or push-pull (with one switch and two) - to control field-effect transistors (see explanation above). But the controller itself can be for single-cycle and push-pull circuits - with one and two output pins, respectively. It is important.

Vref- reference voltage, usually connected to ground through a small capacitor (units of microfarads).

ILIM- signal from the current sensor. Needed to limit the output current. Connects to feedback circuits.

ILIMREF- the activation voltage of the ILIM leg is set on it

SS- a signal is generated for a soft start of the controller. Designed for smooth transition to nominal mode. A capacitor is installed between it and the common wire to ensure a smooth start.

RtCt- terminals for connecting a timing RC circuit, which determines the frequency of the PWM signal.

CLOCK- clock pulses to synchronize several PWM controllers with each other, then the RC circuit is connected only to the master controller, and the RT slaves with Vref, the CT slaves are connected to the common one.

RAMP is the comparison input. A sawtooth voltage is applied to it, for example from the Ct pin. When it exceeds the voltage value at the error amplification output, a shutdown pulse appears at OUT - the basis for PWM regulation.

INV and NONINV- these are the inverting and non-inverting inputs of the comparator on which the error amplifier is built. In simple words: the higher the voltage on INV, the longer the output pulses and vice versa. The signal from the voltage divider in the feedback circuit from the output is connected to it. Then the non-inverting input NONINV is connected to the common wire - GND.

EAOUT or Error Amplifier Output rus. Error amplifier output. Despite the fact that there are error amplifier inputs and with their help, in principle, you can adjust the output parameters, but the controller reacts to this rather slowly. As a result of a slow response, the circuit may become excited and fail. Therefore, signals are supplied from this pin through frequency-dependent circuits to the INV. This is also called error amplifier frequency correction.

Examples of real devices

To consolidate the information, let's look at a few examples of typical PWM controllers and their connection circuits. We will do this using the example of two microcircuits:

TL494 (its analogues: KA7500B, KR1114EU4, Sharp IR3M02, UA494, Fujitsu MB3759);

They are actively used. By the way, these power supplies have considerable power (100 W or more on the 12V bus). Often used as a donor for conversion into a laboratory power supply or a universal powerful charger, for example for car batteries.

TL494 - review

Let's start with the 494th chip. Its technical characteristics:

In this particular example, you can see most of the findings described above:

1. Non-inverting input of the first error comparator

2. Inverting input of the first error comparator

3. Feedback input

4. Dead time adjustment input

5. Terminal for connecting an external timing capacitor

6. Output for connecting a timing resistor

7. Common pin of the microcircuit, minus power supply

8. Collector terminal of the first output transistor

9. Emitter terminal of the first output transistor

10. Emitter terminal of the second output transistor

11. Collector terminal of the second output transistor

12. Supply voltage input

13. Input for selecting single-cycle or push-pull mode of operation of the microcircuit

14. Built-in 5 volt reference output

15. Inverting input of the second error comparator

16. Non-inverting input of the second error comparator

The figure below shows an example of a computer power supply based on this chip.

UC3843 - review

Another popular PWM is the 3843 chip - computer and other power supplies are also built on it. Its pinout is located lower, as you can see, it has only 8 pins, but it performs the same functions as the previous IC.

Interesting:

There are UC3843 in a 14-leg case, but they are much less common. Pay attention to the markings - additional pins are either duplicated or not used (NC).

Let's decipher the purpose of the conclusions:

1. Comparator (error amplifier) input.

2. Feedback voltage input. This voltage is compared with the reference voltage inside the IC.

3. Current sensor. It is connected to a resistor located between the power transistor and the common wire. Needed for overload protection.

4. Timing RC circuit. With its help, the operating frequency of the IC is set.

6. Exit. Control voltage. Connected to the gate of the transistor, here is a push-pull output stage to control a single-ended converter (one transistor), which can be seen in the figure below.

Buck, Boost and Buck-Boost types.

Perhaps one of the most successful examples will be the widespread LM2596 microcircuit, on the basis of which you can find a lot of converters on the market, as shown below.

Such a microcircuit contains all the technical solutions described above, and also, instead of an output stage on low-power switches, it has a built-in power switch capable of withstanding a current of up to 3A. The internal structure of such a converter is shown below.

You can be sure that in essence there are no special differences from those discussed in it.

But here is an example on such a controller, as you can see, there is no power switch, but only a 5L0380R microcircuit with four pins. It follows that in certain tasks the complex circuitry and flexibility of the TL494 are simply not needed. This is true for low-power power supplies, where there are no special requirements for noise and interference, and the output ripple can be suppressed with an LC filter. This is a power supply for LED strips, laptops, DVD players, etc.

Conclusion

At the beginning of the article, it was said that a PWM controller is a device that simulates the average voltage value by changing the pulse width based on the signal from the feedback circuit. I note that the names and classifications of each author are often different; sometimes a PWM controller is called a simple PWM voltage regulator, and the family of electronic microcircuits described in this article is called “Integrated subsystem for pulse-stabilized converters.” The name does not change the essence, but disputes and misunderstandings arise.

Every radio amateur is familiar with the NE555 microcircuit (analogous to KR1006). Its versatility allows you to design a wide variety of homemade products: from a simple single-vibrator pulse with two elements in the harness to a multi-component modulator. This article will discuss the circuit for switching on a timer in the mode of a rectangular pulse generator with pulse-width adjustment.

Scheme and principle of its operation

With the development of high-power LEDs, NE555 again entered the arena as a dimmer, recalling its undeniable advantages. Devices based on it do not require deep knowledge of electronics, are assembled quickly and work reliably.

It is known that the brightness of an LED can be controlled in two ways: analog and pulse. The first method involves changing the amplitude value of the direct current through the LED. This method has one significant drawback - low efficiency. The second method involves changing the pulse width (duty factor) of the current with a frequency from 200 Hz to several kilohertz. At such frequencies, the flickering of LEDs is invisible to the human eye. The circuit of a PWM regulator with a powerful output transistor is shown in the figure. It is capable of operating from 4.5 to 18 V, which indicates the ability to control the brightness of both one powerful LED and an entire LED strip. The brightness adjustment range ranges from 5 to 95%. The device is a modified version of a rectangular pulse generator. The frequency of these pulses depends on the capacitance C1 and resistances R1, R2 and is determined by the formula: f=1/(ln2*(R1+2*R2)*C1), Hz

The operating principle of the electronic brightness control is as follows. At the moment the supply voltage is applied, the capacitor begins to charge through the circuit: +Usupply – R2 – VD1 –R1 –C1 – -Usupply. As soon as the voltage on it reaches the level of 2/3U, the internal timer transistor will open and the discharge process will begin. The discharge begins from the top plate C1 and further along the circuit: R1 – VD2 –7 IC pin – -U supply. Having reached the 1/3U mark, the timer power transistor will close and C1 will again begin to gain capacity. Subsequently, the process is repeated cyclically, forming rectangular pulses at pin 3.

Changing the resistance of the trimming resistor leads to a decrease (increase) in the pulse time at the timer output (pin 3), and as a result, the average value of the output signal decreases (increases). The generated sequence of pulses is supplied through the current-limiting resistor R3 to the gate VT1, which is connected according to a circuit with a common source. The load in the form of an LED strip or sequentially connected high-power LEDs is connected to the open drain circuit VT1.

In this case, a powerful MOSFET transistor with a maximum drain current of 13A is installed. This allows you to control the glow of an LED strip several meters long. But the transistor may require a heat sink.

Blocking capacitor C2 eliminates the influence of interference that may occur along the power circuit when the timer is switched. The value of its capacitance can be any within the range of 0.01-0.1 µF.

Board and assembly parts of the brightness control

The single-sided printed circuit board has dimensions of 22x24 mm. As you can see from the picture, there is nothing superfluous on it that could raise questions.

After assembly, the PWM dimmer circuit does not require adjustment, and the printed circuit board is easy to make with your own hands. The board, in addition to the tuning resistor, uses SMD elements.

DA1 – IC NE555;
VT1 – field effect transistor IRF7413;
VD1,VD2 – 1N4007;
R1 – 50 kOhm, trim;
R2, R3 – 1 kOhm;
C1 – 0.1 µF;
C2 – 0.01 µF.

Transistor VT1 must be selected depending on the load power. For example, to change the brightness of a one-watt LED, a bipolar transistor with a maximum permissible collector current of 500 mA will be sufficient.

The brightness of the LED strip must be controlled from a +12 V voltage source and match its supply voltage. Ideally, the regulator should be powered by a stabilized power supply specifically designed for tape.

The load in the form of individual high-power LEDs is powered differently. In this case, the dimmer's power source is a current stabilizer (also called an LED driver). Its rated output current must match the current of the LEDs connected in series.