ON Semi PWM controller chips for network power supplies. What is a PWM controller and how does it work?

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Pulse width modulation(PWM, English) Pulse-width modulation (PWM)) - approximation of the desired signal (multi-level or continuous) to actual binary signals (with two levels - on/off), so that, on average, over a certain period of time, their values ​​are equal. Formally, it can be written like this:

Where x(t) - the desired input signal in the limit from t1 before t2, and ∆ T i- duration i th PWM pulse, each with amplitude A. ∆T i is selected in such a way that the total areas (energies) of both quantities are approximately equal over a sufficiently long period of time, and the average values ​​of the quantities over the period are also equal:

The controlled “levels” are usually the power plant power parameters, for example, the voltage of pulse converters/DC voltage regulators/or the speed of an electric motor. For pulsed sources x(t) = U const stabilization.

THORN- a pulse-width converter that generates a PWM signal based on a given control voltage value. The main advantage of PWM is the high efficiency of its power amplifiers, which is achieved by using them exclusively in switching mode. This significantly reduces the power output at the power converter (PC).

Application

In pulse-width modulation, a periodic sequence of rectangular pulses is used as a carrier oscillation, and the information parameter associated with the discrete modulating signal is the duration of these pulses. A periodic sequence of rectangular pulses of the same duration has a constant component, inversely proportional to the duty cycle of the pulses, that is, directly proportional to their duration. By passing pulses through a low-pass filter with a cutoff frequency significantly lower than the pulse repetition rate, this constant component can be easily isolated, obtaining a constant voltage. If the duration of the pulses is different, the low-pass filter will release a slowly varying voltage that tracks the law of change in the duration of the pulses. Thus, using PWM you can create a simple DAC: the signal sample values ​​are encoded by the duration of the pulses, and the low-pass filter converts the pulse sequence into a smoothly varying signal.

Wikimedia Foundation. 2010.

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 equal to:

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 for 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 below, 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.

For an ordinary person who does not delve into electronics, the transition of all power supply devices from linear to pulsed was invisible. It is switching power supplies (SMPS) that are installed in all modern equipment. The main reason for switching to this type of voltage converter is the reduction in size. Since all the time, from the beginning of their appearance and invention, electronic devices require a constant reduction in their size. The figure shows, for comparison, the dimensions of a conventional and pulsed direct current source. Differences in size are visible to the naked eye.

The principle of operation of the SMPS and its design

A switching power supply is a device that operates on the principle of an inverter, that is, it first converts alternating voltage into direct voltage, and then again converts it into an alternating voltage of the desired frequency. Ultimately, the last stage of the converter is still based on voltage rectification, since most devices still operate at a reduced DC voltage. The essence of reducing the size of these power supply and converting devices is based on the operation of the transformer. The fact is that the transformer cannot operate with constant voltage. Simply, an EMF (electromotive force) will not be induced at the output of the secondary winding when direct current is supplied to the primary. In order for voltage to appear on the secondary winding, it must change in direction or magnitude. Alternating voltage has this property; the current in it changes its direction and magnitude with a frequency of 50 Hz. However, in order to reduce the size of the power supply itself and, accordingly, the transformer, which is the basis of galvanic isolation, it is necessary to increase the frequency of the input voltage.

At the same time, pulse transformers, unlike conventional linear ones, have a ferrite magnetic core, rather than a steel plate core. And also modern power supplies working on this principle consist of:

1. mains voltage rectifier;
2. a pulse generator operating on the basis of PWM (pulse width modulation) or a Schmitt trigger;
3. DC stabilized voltage converter.

After the mains voltage rectifier, a pulse generator using PWM generates it into alternating voltage with a frequency of about 20–80 kHz. It is this increase from 50 Hz to tens of kHz that makes it possible to significantly reduce both the dimensions and weight of the power source. The upper range could be larger, however, then the device will create high-frequency interference, which will affect the operation of radio frequency equipment. When choosing PWM stabilization, it is imperative to also take into account the higher harmonics of the currents.

Even when operating at these frequencies, these pulsed devices produce high frequency noise. And the more of them in one room or in one enclosed space, the more of them there are in radio frequencies. To absorb these negative influences and interference, special noise suppression filters are installed at the input of the device and at its output.

This is a clear example of a modern switching power supply used in personal computers.

A - input rectifier. Half-bridge and full-bridge circuits can be used. Below is an input filter having inductance;
B - input smoothing capacitors with a fairly large capacity. To the right is a radiator for high-voltage transistors;
C - pulse transformer. A radiator for low-voltage diodes is mounted to the right;
D - output filter coil, that is, group stabilization choke;
E - output filter capacitors.
The coil and large yellow capacitor below E are components of an additional input filter mounted directly on the power connector, and not part of the main circuit board.

If a radio amateur invents a circuit himself, he must look into the reference book on radio components. The reference book is the main source of information in this case.

Flyback switching power supply

This is one of the types of switching power supplies that have galvanic isolation of both primary and secondary circuits. This type of converter was immediately invented, which was patented back in 1851, and its improved version was used in ignition systems and in horizontal scanning of televisions and monitors, to supply high-voltage energy to the secondary anode of the kinescope.

The main part of this power supply is also a transformer or maybe a choke. There are two stages in its work:

1. Accumulation of electrical energy from the network or from another source;
2. Output of accumulated energy to the secondary circuits of the half-bridge.

When the primary circuit opens and closes, current appears in the secondary circuit. The role of the disconnecting key was most often performed by a transistor. To find out the parameters of which you must use the reference book. This transistor is most often controlled by a field-effect transistor using a PWM controller.

PWM controller control

The conversion of the mains voltage, which has already passed the rectification stage, into rectangular pulses is performed with some periodicity. The turn-off and turn-on period of this transistor is performed using microcircuits. The PWM controllers of these keys are the main active control element of the circuit. In this case, both forward and flyback power supplies have a transformer, after which re-rectification occurs.

In order to ensure that the output voltage in the SMPS does not drop with increasing load, a feedback loop was developed that was inserted directly into the PWM controllers. This connection makes it possible to completely stabilize the controlled output voltage by changing the duty cycle of the pulses. Controllers operating on PWM modulation provide a wide range of output voltage changes.

Microcircuits for switching power supplies can be of domestic or foreign production. For example, NCP 1252 are PWM controllers that have current control and are designed to create both types of pulse converters. Master pulse generators of this brand have proven themselves to be reliable devices. NCP 1252 controllers have all the quality features to create cost-effective and reliable power supplies. Switching power supplies based on this microcircuit are used in many brands of computers, televisions, amplifiers, stereo systems, etc. By looking in the reference book, you can find all the necessary and detailed information about all its operating parameters.

The advantage of switching power supplies over linear ones

Switching-based power supplies offer a number of advantages that distinguish them qualitatively from linear ones. Here are the main ones:

1. Significant reduction in dimensions and weight of devices;
2. Reducing the amount of expensive non-ferrous metals, such as copper, used in their manufacture;
3. No problems when a short circuit occurs, this applies to a greater extent to flyback devices;
4. Excellent smooth regulation of the output voltage, as well as its stabilization by introducing feedback into PWM controllers;
5. High efficiency indicators.

However, like everything in this world, pulse blocks have their drawbacks:

1. Emission of interference that can appear due to faulty noise suppression circuits, most often due to the drying out of electrolytic capacitors;
2. Undesirable operation without load;
3. A more complex circuit using a larger number of parts, to find analogues of which a reference book is needed.

The use of power supplies based on high-frequency modulation (pulse) in modern electronics, both in everyday life and in production, has significantly influenced the development of all electronic equipment. They have long displaced outdated sources built on a traditional linear circuit from the market, and will only improve in the future. PWM controllers are the heart of this device and the development of their functionality and technical characteristics is constantly improving.

Video about the operation of a switching power supply

To date, about 14 different topologies of switching power supplies have been developed (Table 1). Each has unique properties that allow it to be used to solve a specific range of problems.

Table 1. Basic circuit topologies used in the construction of switching power supplies

Topology	Scheme	Power, W	Application area	Peculiarities
Flyback (flyback)		up to 300	Power supplies for household equipment (TV, DVD, etc.), powerful chargers and external power supplies.	Simplicity of the circuit, low cost
Straight forward (feed forward)		up to 300	Power supplies for household equipment (TV, DVD, etc.), powerful chargers, external and built-in power supplies.	Reduced noise, increased efficiency at low output voltages
Resonant (resonance)		up to 300	Power supplies for household equipment (TV, DVD, etc.)	High operating frequency and, as a result, small dimensions, easy filtering of interference
Push-pull (push-pull)		100…5000	External and built-in power supplies for household, industrial and automotive equipment	Reduced interference level
Half bridge (half-bridge)		100…1000	External and built-in power supplies (for example, computers)	Small dimensions Reduced interference level
Mostovoy (full-bridge)		100…3000	Uninterruptible power supplies, chargers	Increased efficiency

Today, the “heart” of almost any modern medium- and high-power transformer switching power supply is a specialized IC that controls the operation of an external power transistor/transistors. The vast majority of such sources use several control modes for the operation of power transistors: pulse-width (PWM), pulse-frequency (FPM), quasi-resonant (QR). Also, in order to increase efficiency, a mixed mode is often used: PFM or quasi-resonant modes at low output power, and PWM at medium and high powers.

The tasks and functions of PWM controllers are reduced not only to controlling external power transistors and maintaining the output voltage at the required level with a given error. In fact, the list of these functions necessarily includes:

monitoring the state of key transistors (limiting current and duty cycle of control pulses);

smooth start after power supply (soft start);

control of the input voltage level and its “dips” and “surges”;

protection against breakdown of the power transformer and output circuits of the output rectifier;

temperature control of the controller itself (less often of power transistors).

Conventionally, all produced PWM controllers by STMicroelectronics (Table 2) can be divided into three groups: voltage control, current control and mixed control.

Table 2. Brief characteristics and parameters of STMicroelectronics PWM controllers

SG2525A/SG3524/SG3525A- a series of voltage-controlled PWM controllers (Fig. 1) with a fixed conversion frequency, specially designed for building any types of switching power supplies (according to the manufacturer’s statement) and allowing to reduce the number of required external components to a minimum.

Rice. 1.

This became possible thanks to the presence of a built-in reference power supply (+5.1 V ±1%), the ability to control the operating frequency of an external RC circuit, the duration of the “dead” time interval - with one external resistor, the duration of the soft start time - with one external capacitor (pin SOFT-START), built-in drivers (±200 mA) to control external power transistors or an external low-power transformer. In addition to all of the above, the IC provides the ability to synchronize multiple sources from a single external clock signal (SYNC pin) and current protection of external power transistors (SHUTDOWN pin). Scope of application - almost any DC/DC converter of low and medium power (Fig. 2 and Fig. 3).

Rice. 2.

Rice. 3.

UC2842B/3B/4B/5B and UC3842B/3B/4B/5B — a popular series of small-sized PWM controllers with a fixed conversion frequency and current control, housed in 8-pin SO and MiniDIP packages (Fig. 4).

Rice. 4.

Despite the fact that it has been in production for about 10 years, it still remains one of the most popular series, mainly due to its low cost and high reliability, partly due to its ease of implementation. Designed for building single-ended DC/DC converters with input voltages up to 8.2…30 V. The presence of an RC generator (operating frequency up to 500 kHz), a built-in powerful driver (±200 mA) for controlling an external field-effect or bipolar transistor, a built-in thermally stabilized reference source +5 V ± 1% make it possible to build flyback power supplies based on ICs of this series with the necessary set of protective functions - input overvoltage protection, current protection of an external power transistor, temperature protection of the IC. To eliminate false operation of the built-in current comparator (Current Sense) due to possible interference that occurs when switching an external power transistor, the so-called comparator blocking mode (Leading Edge Blanking) for a fixed time (about 100 ns) from the moment the transistor switches (Fig. 5).

Rice. 5.

Feature of the series — current control of an external power transistor, which makes it possible to exclude additional galvanically isolated feedback circuits (optocoupler) from the circuit, which can significantly reduce the size and cost of the final DC/DC converter. In addition, when building low-power converters (up to 3 W), it is possible to eliminate the external power transistor and use the built-in output driver instead.

L5991/L5991A - a series of PWM controllers with current control, high operating frequency (up to 1 MHz) and increased functionality (Fig. 6).

Rice. 6.

The distinctive features of the ICs of this series include: a powerful driver with an output current of up to 1 A for controlling a powerful field-effect transistor, programmable soft start, the ability to synchronize both input (Slave) and output (Master), shutdown input with reduced current consumption to 120 µA, the ability to limit the maximum duty cycle by external RC circuits, the presence of a Standby mode that increases efficiency (working with low or no load). The series was created for the construction of powerful flyback DC/DC converters.

To eliminate false operation of the built-in current comparator (Current Sense) due to possible interference that occurs when switching an external power transistor, the so-called comparator blocking mode (Leading Edge Blanking) for a fixed time (about 100 ns) from the moment the transistor switches (Fig. 7).

Rice. 7.

L6566A/L6566B/L6668 — a series of multifunctional PWM controllers, specially designed to work as part of flyback pulsed voltage converters of medium and high power (Fig. 7). Distinctive features of the IC: two selectable operating modes - fixed frequency mode (Fixed Frequency - FF) and quasi-resonant mode (Quasi-resonant - QR). The operating frequency is in fixed frequency mode, which is determined by the ratings of the external RC circuit. An additional FMOD input allows operation in frequency modulation mode, which reduces interference from the source. The IC has a built-in high-voltage input power supply for initial startup.

Separately, it is worth noting the features of the operation of the IC in the quasi-resonant mode, in which the source operates on the verge of continuous and intermittent current modes. For this purpose, an additional winding must be provided in the power transformer, designed to accurately determine the moment of opening of the power transistor. In this mode, maximum efficiency of the converter is achieved: at low loads the operating frequency is low, and losses on the power transistor are minimal. At medium and heavy loads, the operating frequency increases to the set frequency determined by the external RC circuit.

L6566A/L6566B/L6668 are primarily aimed at use as part of single- and multi-channel AC/DC converters of medium and high power (Fig. 8). The main applications are external power supplies for laptops, household appliances, built-in power supplies for industrial equipment, etc.

Rice. 8.

Conclusion

Today, the family of PWM controllers from STMicroelectronics has confidently and firmly occupied a niche among inexpensive, reliable, multifunctional, and at the same time easy-to-use switching power supplies of low, medium and high power. For the most part, they can be found both in ordinary household appliances (computers, laptops, DVD players, LCD TVs and monitors, etc.) and in complex industrial and medical equipment. One of the reasons for this was the very low price with high functionality in small-sized 8- and 16-pin SO and DIP packages, high reliability with an extended life cycle (according to the experience of many developers). The great popularity of some series, which has persisted for more than ten years, provides a certain guarantee to power supply manufacturers that PWM controllers from STMicroelectronics will not be discontinued for many years to come.

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TI announced new DSPs

System modeling and initial implementation of the algorithm are in most cases based on floating point arithmetic. After which, the debugged algorithm is loaded onto a microcontroller or fixed-point digital signal processor. Floating-point processors are used only in applications that require high precision and performance, where the cost of the end device is not critical.

For such applications, Texas Instruments has released floating-point digital signal processors TMS320F28335, TMS320F28334, TMS320F28332. But, as before, she didn’t stop there. There are new TMS320F2823x fixed-point DSPs that are software and hardware compatible with TMS320F2833x floating-point processors.

Users can now simulate a system, debug it on a floating-point platform (TMS320F2833x), and then simply recompile the resulting program code under the TMS320F2823x, thereby reducing development time (the time it takes to load an application onto a fixed-point platform) and the cost of the end device.

Serial production of the TMS320F2823x and TMS320F2833x will begin in the second quarter of 2008.

TI reveals details of its 45nm process technology

Texas Instruments (TI) is ready for mass production of its first 45-nanometer chips. The transition to 45 nm standards is said to have reduced chip power consumption by 63% and increased performance by 55% compared to 65 nm products

TI is currently shipping evaluation samples of the first 45nm processor for 3.5G devices. In the production of the new product, strained silicon, immersion lithography and dielectrics with an ultra-low dielectric constant (ultra-low K) are used.

This processor will allow the production of more compact and lightweight devices for 3.5G networks.

About ST Microelectronics

Over the past decade, we have seen an accelerated pace of development in electronic devices. Along with it, the requirements for the power supply device are also growing. Linear voltage regulators have low efficiency and cannot always meet the requirements for the device. Circuits with synchronous rectifiers have become widespread today. The range of ICs produced by various manufacturers is very large. This article will discuss the features of using a synchronous switch in a synchronous rectifier and will consider several types of PWM controllers from International Rectifier.

The synchronous rectifier circuit was developed a very long time ago. To build it, ordinary n-channel field-effect transistors are used, only they work in power supplies with low output voltage and replace rectifier diodes. The drain-source voltage of such transistors is usually small, but the capacitances between the drain-source and gate-drain are very, very significant. A characteristic feature of the operation of field-effect transistors as synchronous rectifiers is their operation in the fourth quadrant of their current-voltage characteristic, that is, the current flows through them in the opposite direction - from source to drain. In Fig. Figure 1 shows a diagram of the construction of a synchronous rectifier.

Figure 1 Schematic diagram of a synchronous rectifier

Figure 2 Block diagram of types of devices for constructing synchronous regulators produced by International Rectifier

The requirements for the selection of circuit elements when constructing a synchronous rectifier are as follows:

To summarize the selection of elements, we note that when choosing transistors, the company recommends that developers choose synchronous switches with a minimum resistance value. For the switching switch, it is necessary to select a transistor with a minimum gate charge value.

International Rectifier offers a wide range of PWM controller ICs with different functionality (see Figure 2). The family of pulse synchronous regulators includes integrated assemblies in monolithic packages (SupIRBuck, IPower) and PWM controllers without internal switches. Dual-channel assemblies are represented, in the first case, by monolithic integrated circuits and PWM controllers with or without an internal linear reference converter. Multiphase systems are represented by ICs of the X-Phase and I-Power families.

The IR3651SPBF synchronous PWM controller integrated circuit is designed for high-efficiency synchronous step-down DC/DC converters with input voltages up to 150 V. Programmable operating frequencies in the range up to 400 kHz allow the chip to be used in power supplies for telecommunications equipment and base stations, network servers, automotive and industrial applications. control units. When using the chip in low-power devices, the output voltage level can be precisely adjusted thanks to the built-in reference voltage source (1.25 V). The IR3651S PWM controller IC coupled with a pair of DirectFET transistors provides conversion efficiency of over 88% at a 48V supply voltage and 3.3V output voltage at 6A current without the use of heatsinks or airflow. Another advantage of this IC over analogues on the market today is the increased maximum supply voltage. The IC is designed using 160V HVIC technology. This allows you to increase the reliability parameters of the development as a whole. The IR3651S PWM controller IC is designed to drive two external N-channel MOSFETs at drive currents up to 25 A and has several protection options: programmable soft start, current protection and low voltage lockout. The IC also has a synchronization function for its coordinated operation on a common phase. Thus, this chip can be used both for low-power (less than 60 W) non-isolated DC/DC converters of network equipment, and for high-power (more than 200 W) pre-regulation stages in controlled isolated converters. In Fig. Figure 3 shows the connection diagram for the IR3651S IC.

Figure 3 Connection diagram of the IR3651S controller

The 3-phase PWM controller circuit for the synchronous DC-DC converter IR3094MPbF, together with the use of MOSFET transistors in a DirectFET package, allows the board size to be reduced by 40% when compared with today's analogues. The small size of the IR3094 controller is ideal for building compact synchronous converters for high-density systems. Typically, solutions for synchronous converters with three output voltages require 14 elements: 3 controllers, 6 switches, 3 chokes, components that provide external activation, plus feedback components. Converters assembled using the IR3094 controller and MOSFET transistors in a DirectFET package, IRF6637 and IRF6678 reduce the number of converter elements to 7 units.

Three pairs of DirectFET transistors can be placed in close proximity to the IR3094, creating a solution that minimizes PCB and package size. The IR3094's built-in high-power drivers, combined with a pair of DirectFET transistors in each phase, create a high-current density power control solution for POL (point-to-load) converters. The IR3094M controller is designed for applications requiring supply voltages from 0.85 to 5.1 V. It is housed in a compact 7 mm MLPQ package? 7 mm and contains a built-in 3 A key control driver, 1% reference voltage source, output voltage setting for each phase, programmable switching frequency up to 540 kHz.

The controller provides the following types of protection:

• programmable soft start;
• short circuit protection in the form of hiccupping current at the output of each phase;
• overvoltage protection;
• output indicating the current state of the controller - “power good”.

In conjunction with this type of controller, it is recommended to use the IRF6678 transistor, which is an ideal synchronous MOSFET that shows a low resistance value of 1.7 mOhm -10 V. The IRF6637 transistor has a low gate charge value (4 nC) and is less susceptible to the Miller effect, the junction resistance is 5.7 mOhm at 10 V.

To achieve accurate output voltage within 1% deviation, International Rectifier produces the IR3637 IC. It is used where high quality supply voltage is required. This IC allows the user to operate in the input voltage range from 4.5 to 16 V. The main advantage of this PWM controller is the simplified design and increased compactness of the DC-DC converter. The IC is housed in a compact SO-8 package and has protections such as short circuit protection, low voltage lockout, and soft start function with external programming.

The controller provides a PWM signal duty cycle of up to 85% at 400 kHz, which reduces the inductor size and improves the dynamic characteristics of the converter. In Fig. Figure 4 shows the connection diagram of the IR3637 PWM controller IC.
Previously, in 12 V applications, designers had limited choice of options and focused primarily on the use of integrated, non-isolated DC-DC converters that occupy a significantly larger area. The use of an alternative solution on discrete components (new PWM controllers and MOS transistors) allows you to take advantage of the integration of the converter circuit into the board.

When developing a synchronous rectification circuit, the designer is recommended to pay attention to three main points in the wiring of the PWM controller ground circuit:

The range of PWM controllers and integrated assemblies based on them from International Rectifier includes more than 100 items. In table 1 shows the main parameters of some PWM controllers. To speed up the development of a synchronous voltage converter, International Rectifier presents a project for My-Power developers on the on-line website - /engine/api/go.php?go=https://www.irf. com/design-center/mypower/index.html. Here the developer can not only calculate the circuit parameters and see oscillograms of the device’s operation, but also get recommendations on the type of transistors and see their main parameters.