PWM modulator circuit on a digital chip. PWM and PWM - what is it? Output signal from triangle pulse generator

Ideally, a method using pulse width modulation (PWM) is the answer to the quest for a nearly perfect regulated power supply. We have already said that in a pulsed source the switch is either on or off and control is carried out with zero power dissipation, in contrast to a linear stabilizer, where stabilization occurs due to power dissipation in the pass element. In real-world applications, pulse width modulation provides a reasonable approach to lossless switching due to lower switching frequency, for example in the range of 20 – 40 kHz. Looking at the situation from the other side can tell why this frequency range has been popular for so long.

Since the early days of PWM stabilization, designers have tried to move toward higher frequencies because they can reduce the size, weight, and cost of the magnetic core and filter capacitors. High switching frequencies also provide other benefits. By using higher frequencies, a reduction in radio interference and electromagnetic noise can be expected; you can expect fewer problems with shielding, decoupling, insulation and limiting

Research institute in the scheme. You can also expect faster response, as well as lower output impedance and ripple.

The main obstacle to the use of higher frequencies was the practical difficulty of creating fast and sufficiently powerful switches. Due to the fact that it is impossible to achieve instantaneous switching on and off, there is voltage on it during switching and at the same time current flows through it. In other words, trapezoidal rather than square oscillations characterize the switching process. This in turn leads to switching losses, which cancel out the theoretically high efficiency. an ideal switch that turns on instantly, has zero resistance when on, and turns off instantly. In Fig. 18.2 compares PWM and switching mode in the resonant mode, which will be discussed in more detail.

Rice. 18.2. Oscillograms showing the difference between PWM and resonant mode. With PWM, switching losses occur due to the simultaneous flow of current through the switch and the presence of voltage across it. Note that this situation does not exist in the resonant mode of operation, which uses frequency modulation (FM) to stabilize the voltage.

From the above, it is obvious that an ideal switch should not have any voltage drop during the on state. All these arguments suggest that high efficiency. was difficult to achieve, especially at high switching frequencies, until progress was made in switching semiconductor devices. It should also be pointed out that at the same time progress was needed in the creation of other devices such as diodes, transformers and capacitors. We must pay tribute to workers in all areas of technology for the fact that the switching frequency when using pulse width modulation was increased to 500 kHz. However, at higher frequencies, say 150 kHz, it is better to consider a different method. So, we come to the resonant mode of operation of the power source.

The stabilized power supply using resonant mode truly represents a great leap forward in technology. Although it must be said that the use of resonant phenomena in inverters, converters and power supplies precedes the era of semiconductors. It turned out that when using resonance phenomena it was often possible to obtain good results. For example, in the first televisions, the necessary high voltages for the picture tube were obtained using a radio frequency power source. It was a vacuum tube sine wave generator operating at a frequency of 150 to 300 kHz, in which an increase in alternating voltage was achieved in a resonant radio frequency transformer. Essentially similar circuits are still used to generate voltages of at least several hundred thousand volts for a variety of industrial and research purposes. Higher voltages are often achieved through the combined use of resonant operation and a diode voltage multiplier.

It has also long been known that resonant inverter output circuits stabilize the operation of electric motors and welding equipment. Usually, a coil with high inductance was connected to the break in the wire leading from the DC voltage source to the inverter. In this case, the inverter behaves in relation to the load as a current source, which makes it easier to satisfy the condition for the existence of resonant phenomena. In this case, it is more correct to call existing thyristor inverters quasi-resonant - the oscillatory circuit is periodically subjected to shock excitation, but there are no continuous oscillations. Between excitation pulses, the oscillatory circuit releases the stored energy to the load. Examples of the mentioned circuits are shown in Fig. 18.3, 18.4 and 18.5.

From the above it should be clear that the widespread use of the resonant mode of operation began after the creation of specialized control ICs. These ICs freed designers from the problems with failures that inevitably accompany the desire to use resonant mode at frequencies of several hundred kilohertz or several MHz, where small component sizes can provide significant reductions in size, weight and cost.

Rice. 18.3. An example of a resonant high-voltage source operating in the radio frequency range. This restored old circuit uses vacuum tubes in a Meissner oscillator. The operating frequency is determined by the step-up winding Z1 and its own distributed capacitance. No frequency stabilization is provided.

Rice. 18.4. An example of a current-triggered inverter with a resonant circuit at the output. Pay attention to the presence of a coil with high inductance L in the power circuit and a capacitor included in the resonant circuit at the output. A similar method is applicable to self-excited inverters. These circuits usually do not have stabilization.

Rice. 18.5. An example of a quasi-resonant inverter with one thyristor. By choosing the appropriate thyristor, it is possible to obtain an output power of several kilowatts and a switching frequency of about 30 kHz. If the ripple frequency is slightly lower than the resonant frequency of the series XC circuit, then the load will have a good sinusoidal voltage. There is no stabilization in the circuit. General Electric Semiconductor Products Dept.

Interestingly, the resonant voltage regulator has much in common with the long-popular pulse width modulation (PWM) circuit. Indeed, according to the block diagram, a source of pulses of constant duration and variable frequency, together with a resonant “circuit”, is used instead of a PWM circuit. During operation, due to the presence of a ZC circuit, either current flows through the switch or voltage is applied to it, in the form of sinusoid segments. The shape of the signals during switching, unlike high-frequency PWM circuits, is such that there is never a simultaneous presence of voltage on the switch and current flowing through it. Therefore, switching losses are negligible even at high frequencies.

Rice. Figure 18.6 illustrates the resonant mode of operation. The error signal is received in the same way as in PWM power supplies, that is, as the difference between the output and reference voltages. This error voltage is fed to a voltage controlled oscillator, the output of which drives the standby multivibrator. The modulation circuit is essentially a voltage-frequency converter. Pulses of the waiting multivibrator, having a fixed duration and variable repetition rate, are supplied to the input of the switch(es). Often a power amplifier is included at the output of the standby multivibrator to provide higher instantaneous current and lower resistance. One or two power MOSFETs are usually used as switches.

The output of the commutator(s) is connected to a resonant Z C-circuit and an output transformer. It can be seen that the amplitude of the almost sinusoidal voltage applied to the primary winding of the transformer depends on the proximity of the resonant frequency of the ZC circuit to the reciprocal of the fixed duration of the variable frequency pulses coming from the switch. Thus, stabilization of a constant output voltage can be achieved using frequency modulation. Too high a quality factor Z of the C-circuit will prevent power delivery, and a very low one will cause excessively large peak current values ​​​​in the switch.

Rice. 18.6. Simplified circuit of a resonant stabilized power supply. To a first approximation, we can assume that instead of a pulse-width modulator, the popular PWM stabilizer uses a voltage-frequency converter.

The resonant mode can be obtained in different ways: you can use either a series or parallel L C circuit. And the nominal operating frequency can be either lower or higher than the natural resonant frequency Z of the C-circuit. In any case, stabilization requires working on the falling portion of the resonance curve. In Fig. 18.6, the inductance of the primary winding of the output transformer is quite high, so that it practically does not affect the resonant frequency Z of the C-circuit.

In order to avoid misunderstandings due to inaccurate statements in the technical literature, it would be good to remember the following facts related to resonant stabilizers:

In a resonant Z C-circuit, oscillations always occur at its resonant frequency, regardless of the frequency of the pulses with which shock excitation is carried out. However, in most cases there are no conditions for the existence of free oscillations. The rectifier circuit receives half-cycles of a sinusoidal oscillation.

One of the most popular designs uses a series resonant circuit in which the output power is drawn from a capacitor through the high-resistance primary winding of an output transformer. Such a source is respectively called a converter or stabilizer with series resonance and parallel load. Unfortunately, these devices are sometimes referred to as parallel-resonant circuits (Fig. 18.7B).

Ideally, there are two ways to achieve near-zero switching losses. One is zero current switching, which is the most popular and allows operation at frequencies around 2 MHz, and the other is zero voltage switching, which allows operation up to 10 MHz. Zero current switching uses pulses of constant duration and variable repetition rate to shock excite the circuit. A fixed time interval between pulses is used in zero voltage switching mode.

Most often (especially when switching with zero current), the frequency range extends from low frequencies to 80% of the resonant frequency of the circuit. This provides enough time for the inductor current to decrease to zero or become negative. A pulse that determines the on time.

ends when the current becomes negative; the moment it ends is not very critical. Negative inductor current implies that current is no longer flowing through the power MOSFET, but rather through the clamping diode. The pulse duration is determined by the RC circuit connected to the control IC. The values ​​of R and C can be conveniently determined from graphs provided by the IC manufacturer. Typical data illustrating the choice of the RC value for determining the pulse duration, as well as the generator frequency, are shown in Fig. 18.8.

Rice. 18.8. Examples of graphs for determining the parameters of a resonant stabilized source. These curves correspond to the GP605 IC, but are typical of circuits from other manufacturers. (A) Allowable combinations of capacitance and resistance depending on the maximum frequency of the generator. (B) Allowable capacitance depending on the minimum frequency of the generator. (C) Resistor and capacitance combination for the selected pulse duration. Depending on whether we are dealing with circuit A or B, the LAN circuits will be different. Gennum Sof.

You must be sure that the “switching frequency” corresponds to the frequency at which the pulses enter the resonant circuit. This is not necessarily the frequency of the oscillator in the control IC. In a push-pull switching power supply, the generator frequency will be twice the switching frequency. For single-ended SMPS these frequencies usually coincide.

A source operating in intermittent mode approaches lossless switching. This simply means that for each pulse there should be only one period of oscillation in the Z C-loop. In practice, this requires the presence of “dead time” between the completion of one oscillation cycle and the appearance of the next impulse. This is why the pulse repetition rate should not approach the resonant frequency

LC circuit. Satisfaction of this requirement leads to a slight decrease in output power.

Stabilization is based on the fact that the energy stored in? The C-circuit is maximum when the repetition frequency of the pulses that carry out shock excitation of the ZC-circuit is close to its resonant frequency. Deviation of the pulse frequency from this optimal condition results in less power being received. Since the resonant frequency remains constant, the above-mentioned “dead time” changes to achieve stabilization.

Resonant power supplies are often equipped with current protection, making them similar to PWM sources that have such protection. Indeed, one can find a reference to the operation of a resonant source S in current limiting mode. However, there is a significant difference. In a PWM system, the current rise is taken into account, and the maximum source current is limited at any time throughout the cycle. In a resonant source, part of the sinusoidal oscillation is taken into account; this allows the maximum SMPS current to be limited, but not instantly. In both cases, protection is achieved, but in resonant sources it is not as fast or accurate as in PWM sources that have current protection. In PWM sources, monitoring the current value implements feed-forward stabilization; In resonant sources, reading the current value leads to the use of the shutdown method.

Last but most importantly, switches in resonant SMPS do not experience simultaneous voltage and current during the switching process. This results in high efficiency. with a significant reduction in the dissipated power in the switches, which in turn reduces temperature problems, creating a high density of elements.

The pulse width modulation (PWM) method is one of the most effective in terms of improving the quality of the output voltage of the AU. The main idea of ​​the method is that the output voltage curve is formed in the form of a series of high-frequency pulses, the duration of which varies (modulates) according to a certain law, in most cases sinusoidal. The pulse repetition rate is called the carrier (or clock) frequency, and the frequency with which the pulse duration changes is called the modulation frequency. Since the carrier frequency is usually significantly higher than the modulation frequency, harmonics that are multiples of the carrier frequency and are present in the output voltage spectrum are relatively easily suppressed using an appropriate filter.

Currently, quite a few types of PWM are known, classified according to various criteria. For example, based on the type of output voltage pulses, modulation is distinguished between unipolar and bipolar. The simplest example of bipolar modulation is the processes implemented in a single-phase half-bridge inverter circuit (Fig. 4.9). The control pulses supplied to the bases of the power transistors, as shown in Figure 4.9(b), are formed by comparing the modulating, low-frequency voltage with a sawtooth reference voltage, the frequency of which is the carrier frequency.

Let us assume that the control system is organized in such a way that if the instantaneous value of the reference voltage is greater than the value of the modulating voltage, then transistor VT2 is turned on and a pulse of positive polarity is formed at the load, as shown in Figure 4.9(c). Accordingly, if the reference voltage becomes less than the modulating voltage, then transistor VT2 turns off and transistor VT1 turns on, which leads to a change in the polarity of the voltage across the load. With the active-inductive nature of the load, the polarity of the output voltage changes due to the inclusion of a reverse diode VD1, through which the load current is closed, supported by the inductive emf L.

When the modulating voltage changes, the duration of the positive and negative output voltage pulses changes; accordingly, the average voltage value over the period of the carrier frequency changes.

The combination of these average values ​​of the output voltage forms a smooth component, the shape of which is determined by the modulating signal. The main disadvantage of bipolar modulation is the large amplitude of the first harmonic of the carrier frequency.

With unipolar modulation, as shown in Figure 4.10, in the output voltage curve during one half-wave of the modulating signal, pulses of only one polarity are formed, and instead of voltage pulses of the opposite polarity, an interval with zero voltage (zero shelf) is formed. In this case, when the duration of the voltage pulses changes, the duration of the zero shelf changes accordingly so that the period of the carrier frequency remains constant.

Unipolar modulation can be implemented in a single-phase bridge circuit AIN, provided that one pair of power transistors, for example, VT1 and VT4, switches with the frequency of the modulation signal, at moments, etc., and the second pair of transistors switches with the carrier frequency. The duration of the control pulses is formed in the same way as in the previous case, as a result of comparing the reference voltage and the modulating signal. The formation of a pulse at the output of the inverter, for example, of positive polarity, is ensured by simultaneously turning on transistors VT1 and VT2. Since transistor VT2 switches at a high frequency, when it is turned off, transistor VT1 remains on, which leads to the closure of the load current stored in the inductance through transistor VT1 and diode VD3. In this case, the voltage at the inverter output is equal to the sum of the voltage drops across the transistor and diode, i.e. close to zero. Similarly, a zero shelf is created when a negative half-wave of a smooth component is formed: when transistor VT3 is turned off, the load current is closed through transistor VT4 and diode VD2. Thus, the polarity of the smooth component of the output voltage is determined by switching on transistors VT1 or VT4, and the high-frequency filling and, accordingly, the shape of the smooth component is determined by switching transistors VT2 or VT3.

The main advantage of unipolar modulation, compared to bipolar modulation, is the reduction in the amplitudes of high-frequency harmonics.

It should be noted that unipolar modulation is not possible in some circuits, such as single-phase half-bridge. In this case, to implement unipolar modulation it is necessary to use more complex circuits, for example, the circuit shown in Figure 4.7.

Based on the method of forming the duration of high-frequency pulses, several types of pulse-width modulation are distinguished, the most common of which are PWM of the first and second types. With pulse-width modulation of the first kind (PWM-1), the duration of the generated pulse is proportional to the values ​​of the modulating signal, selected at certain, predetermined moments in time. The principle of forming pulse duration with PWM-1 is illustrated in Fig. 4.11(a).

The principle of forming pulse duration with PWM-2 is shown in Fig. 4.11(b). In this case, the pulse duration is determined by the value of the modulating signal at the end of the pulse.

Based on the method of changing the duration, one-way and two-way modulation are distinguished. For example, in Fig. 4.9 shows one-

third-party modulation, since when the modulating signal changes, the moment at which only the trailing edge of the pulse is generated changes. Accordingly, in Fig. Figure 4.10 shows an example of two-way modulation.

The ratio of the carrier frequency to the frequency of the modulating signal is called the carrier frequency multiple. The multiplicity can be either an integer or a fraction, and in the general case the multiplicity can also be an irrational fraction. The multiplicity significantly affects the spectral composition of the output voltage, and with fractional-rational multiplicities, harmonics with a frequency lower than the frequency of the modulating signal appear in the spectrum of the output voltage. Such harmonics are called subharmonics, and their amplitudes increase as the carrier frequency factor decreases, which can lead to disruption of the normal operation of the inverter. To suppress subharmonics, the carrier frequency multiplicity should be increased, but this inevitably increases switching losses in the inverter's power devices.

The useful component of the output voltage is determined by the shape of the smooth component, which in turn depends on the shape of the modulating signal or, as it is commonly called, on the modulation law. Currently, modulation according to the sinusoidal, trapezoidal or rectangular law is most often used. In particular, the method of pulse-width control at the carrier frequency discussed above is nothing more than the use of PWM according to the rectangular law.

• Back
• Forward

Random news

3.2. Algebraic stability criteria

One of the first criteria for durability was identified by Professor J. A. Vishnegradsky and given by him in his works “On Direct-Acting Regulators” and “On Indirect-Acting Regulators.” The criterion is formulated for processes described by third-order differential equations, the characteristic equation of which is reduced to the form: .

Figure 3.4 - Diagram that defines the stability area of ​​systems described by 3rd order equations. (Vishnegradsky diagram)

If we introduce the notation and, then according to Vishnegradsky, in order for the system to be stable it is necessary that, or. In Figure 3.4, the hyperbola ΧΥ =1 is plotted in the coordinates X and Υ, which gives the stability limit of the system. The line between the resistance areas is usually hatched, so that the resistance areas can be seen from the hatching without further explanation.

On the diagram in Figure 3.4 there is a plotted line of the aperiodicity boundary, determined by the condition with a face point at the values ​​​​of X = Υ = 3.

The Vishnegradsky stability criterion outlined above is a separate case of the Routh-Hurwitz stability criterion. This criterion can be formulated as follows, in the form proposed by Hurwitz: if the system is described by a linear differential equation, the characteristic equation of which is:

then in order for it to be stable, that is, for all the real roots and real parts of the complex roots of the characteristic equation to be negative, it is necessary and sufficient that all the coefficients of the equation have the same sign, and the diagonal determinant is of order n-1, composed of the coefficients of the equation, and all of its diagonal minors would be positive:

The diagonal determinant is composed as follows:

Thus, in order for the system to be stable, it is necessary that all coefficients have the same sign and all determinants be greater than 0.

The order of compiling diagonal minors can be analyzed using the example of a fifth-degree equation:

Then we get:

For a third order equation:

And also.

Note that for and we have the Vyshegradsky stability conditions

Both the Vishnegradsky criterion and the Routh-Hurwitz criterion determine the stability of the system based on the coefficients of the characteristic equation and are called algebraic stability criteria. Let's look at some examples of resistance research using the Routh-Hurwitz criterion.

Example 1. Characteristic equation of the system

For this:

Just as all the coefficients of this equation are greater than zero, so the determinants are also greater than zero - the system is stable.

A good definition of pulse width modulation (PWM) is in its name itself. This means modulating (changing) the pulse width (not the frequency). To better understand what is PWM, let's look at some highlights first.

Microcontrollers are intelligent digital components that operate on the basis of binary signals. The best representation of a binary signal is a square wave (a signal having a rectangular shape). The following diagram explains the basic terms associated with square wave.

In a PWM signal, time (period), and therefore frequency, is always a constant value. Only the on-time and off-time of the pulse (duty factor) change. Using this modulation method, we can obtain the voltage we need.

The only difference between a square wave and a PWM signal is that a square wave has equal and constant on and off times (50% duty cycle), while a PWM signal has a variable duty cycle.

A square wave can be considered a special case of a PWM signal that has a 50% duty cycle (on period = off period).

Let's look at the example of using PWM

Let's say we have a supply voltage of 50 volts and we need to power some load that operates at 40 volts. In this case, a good way to get 40V from 50V is to use what is called a step-down chopper.

The PWM signal generated by the chopper is supplied to the power unit of the circuit (thyristor, field-effect transistor), which in turn controls the load. This PWM signal can be easily generated by a microcontroller having a timer.

Requirements for a PWM signal to obtain 40V from 50V using a thyristor: power supply for a time = 400 ms and turn off for a time = 100 ms (taking into account the PWM signal period equal to 500 ms).

In general terms, this can be easily explained as follows: basically, a thyristor acts as a switch. The load receives supply voltage from the source through a thyristor. When the thyristor is in the off state, the load is not connected to the source, and when the thyristor is in the on state, the load is connected to the source.

This process of turning the thyristor on and off is carried out using a PWM signal.

The ratio of the period of a PWM signal to its duration is called the duty cycle of the signal, and the inverse of the duty cycle is called the duty cycle.

If the duty cycle is 100, then in this case we have a constant signal.

Thus, the duty cycle (duty cycle) can be calculated using the following formula:

Using the above formulas, we can calculate the turn-on time of the thyristor to obtain the voltage we need.

By multiplying the duty cycle of the pulses by 100, we can represent this as a percentage. Thus, the percentage of pulse duty cycle is directly proportional to the voltage value from the original one. In the above example, if we want to get 40 volts from a 50 volt power supply, then this can be achieved by generating a signal with a duty cycle of 80%. Because 80% of 50 instead of 40.

To consolidate the material, let's solve the following problem:

• Let's calculate the duration of switching on and off of a signal having a frequency of 50 Hz and a duty cycle of 60%.

The resulting PWM wave will look like this:

One of the best examples of using pulse width modulation is using PWM to adjust the speed of a motor or the brightness of an LED.

This technique of changing the pulse width to obtain the required duty cycle is called “pulse width modulation.”

Quite often, to build a welding inverter, the main three types of high-frequency converters are used, namely converters connected according to the following circuits: asymmetric or oblique bridge, half-bridge, and full bridge. In this case, resonant converters are subtypes of half-bridge and full-bridge circuits. According to the control system, these devices can be divided into: PWM (pulse width modulation), PFM (frequency control), phase control, and there may also be combinations of all three systems.

All of the above converters have their pros and cons. Let's deal with each one separately.

Half bridge system with PWM

The block diagram is shown below:

This is perhaps one of the simplest, but no less reliable push-pull converters. The “surge” of the voltage of the primary winding of the power transformer will be equal to half the supply voltage - this is a disadvantage of this circuit. But if you look from the other side, you can use a transformer with a smaller core without fear of entering the saturation zone, which is also a plus. For welding inverters with a power of about 2-3 kW, such a power module is quite promising.

Since power transistors operate in hard switching mode, drivers must be installed for their normal operation. This is due to the fact that when operating in this mode, transistors require a high-quality control signal. It is also necessary to have a no-current pause in order to prevent the simultaneous opening of transistors, which will result in the failure of the latter.

A rather promising view of a half-bridge converter, its circuit is shown below:

A resonant half bridge will be a little simpler than a PWM half bridge. This is due to the presence of resonant inductance, which limits the maximum current of transistors, and switching of transistors occurs at zero current or voltage. The current flowing through the power circuit will be in the form of a sinusoid, which will remove the load from the capacitor filters. With this design of the circuit, drivers are not necessarily needed; switching can be carried out by a conventional pulse transformer. The quality of control pulses in this circuit is not as significant as in the previous one, but there should still be a no-current pause.

In this case, you can do without current protection, and the shape of the current-voltage characteristic is , which does not require its parametric formation.

The output current will be limited only by the magnetizing inductance of the transformer and, accordingly, can reach quite significant values ​​in the event that a short circuit occurs. This property has a positive effect on the ignition and burning of the arc, but it also must be taken into account when selecting output diodes.

Typically, the output parameters are adjusted by changing the frequency. But phase regulation also provides some advantages and is more promising for welding inverters. It allows you to bypass such an unpleasant phenomenon as the coincidence of a short circuit with resonance, and also increases the range of regulation of output parameters. The use of phase control can allow the output current to be varied in the range from 0 to I max.

Asymmetrical or oblique bridge

This is a single-ended, forward-flow converter, the block diagram of which is given below:

This type of converter is quite popular both among ordinary radio amateurs and among manufacturers of welding inverters. The very first welding inverters were built precisely according to such schemes - an asymmetric or “oblique” bridge. Noise immunity, a fairly wide range of output current regulation, reliability and simplicity - all these qualities still attract manufacturers to this day.

Quite high currents passing through transistors, an increased requirement for the quality of the control pulse, which leads to the need to use powerful drivers to control transistors, and high requirements for installation work in these devices and the presence of large pulse currents, which in turn increase the requirements for - These are significant disadvantages of this type of converter. Also, to maintain normal operation of the transistors, it is necessary to add RCD chains - snubbers.

But despite the above disadvantages and the low efficiency of the device, an asymmetric or “oblique” bridge is still used in welding inverters. In this case, transistors T1 and T2 will operate in phase, that is, they will close and open simultaneously. In this case, energy accumulation will occur not in the transformer, but in the inductor coil Dr1. That is why, in order to obtain the same power with a bridge converter, double the current through the transistors is required, since the duty cycle will not exceed 50%. We will consider this system in more detail in the following articles.

It is a classic push-pull converter, the block diagram of which is shown below:

This circuit allows you to receive power 2 times more than when turning on the half-bridge type and 2 times more than when turning on the “oblique” bridge type, while the magnitudes of the currents and, accordingly, losses in all three cases will be equal. This can be explained by the fact that the supply voltage will be equal to the “drive” voltage of the primary winding of the power transformer.

In order to obtain the same power with a half-bridge (drive voltage 0.5U supply), the current required is 2 times! less than for the half-bridge case. In a full bridge circuit with PWM, the transistors will operate alternately - T1, T3 are on, and T2, T4 are off and, accordingly, vice versa when the polarity changes. The values ​​of the amplitude current flowing through this diagonal are monitored and controlled. To regulate it, there are two most commonly used methods:

• Leave the cut-off voltage unchanged, and change only the length of the control pulse;
• Carry out changes in the cut-off voltage level according to data from the current transformer while leaving the duration of the control pulse unchanged;

Both methods can allow changes in the output current within fairly large limits. A full bridge with PWM has the same disadvantages and requirements as a half bridge with PWM. (See above).

It is the most promising high-frequency converter circuit for a welding inverter, the block diagram of which is shown below:

A resonant bridge is not much different from a full PWM bridge. The difference is that with a resonant connection, a resonant LC circuit is connected in series with the transformer winding. However, its appearance radically changes the process of power transfer. Losses will decrease, efficiency will increase, the load on input electrolytes will decrease and electromagnetic interference will decrease. In this case, drivers for power transistors should be used only if MOSFET transistors are used that have a gate capacitance of more than 5000 pF. IGBTs can only get by with a pulse transformer. More detailed descriptions of the schemes will be given in subsequent articles.

The output current can be controlled in two ways - frequency and phase. Both of these methods were described in a resonant half-bridge (see above).

Full bridge with dissipation choke

Its circuit is practically no different from the circuit of a resonant bridge or half-bridge, only instead of a resonant LC circuit, a non-resonant LC circuit is connected in series with the transformer. Capacitance C, approximately C≈22μF x 63V, works as a balancing capacitor, and the inductive reactance of the inductor L as a reactance, the value of which will change linearly depending on the change in frequency. The converter is controlled by frequency. , As the voltage frequency increases, the inductance resistance will increase, which will reduce the current in the power transformer. Quite a simple and reliable method. Therefore, a fairly large number of industrial inverters are built according to this principle of limiting output parameters.

When working with many different technologies, the question is often: how to manage the power that is available? What to do if it needs to be lowered or raised? The answer to these questions is a PWM regulator. What is he? Where is it used? And how to assemble such a device yourself?

What is pulse width modulation?

Without clarifying the meaning of this term, it makes no sense to continue. So, pulse-width modulation is the process of controlling the power that is supplied to the load, carried out by modifying the duty cycle of the pulses, which is done at a constant frequency. There are several types of pulse width modulation:

1. Analog.

2. Digital.

3. Binary (two-level).

4. Trinity (three-level).

What is a PWM regulator?

Now that we know what pulse width modulation is, we can talk about the main topic of the article. A PWM regulator is used to regulate the supply voltage and to prevent powerful inertial loads in automobiles and motorcycles. This may sound complicated and is best explained with an example. Let’s say you need to make the interior lighting lamps change their brightness not immediately, but gradually. The same applies to side lights, car headlights or fans. This desire can be realized by installing a transistor voltage regulator (parametric or compensation). But with a large current, it will generate extremely high power and will require the installation of additional large radiators or an addition in the form of a forced cooling system using a small fan removed from the computer device. As you can see, this path entails many consequences that will need to be overcome.

The real salvation from this situation was the PWM regulator, which operates on powerful field-effect power transistors. They can switch high currents (up to 160 Amps) with only 12-15V gate voltage. It should be noted that the resistance of an open transistor is quite low, and thanks to this, the level of power dissipation can be significantly reduced. To create your own PWM regulator, you will need a control circuit that can provide a voltage difference between the source and gate within the range of 12-15V. If this cannot be achieved, the channel resistance will greatly increase and the power dissipation will increase significantly. And this, in turn, can cause the transistor to overheat and fail.

A whole range of microcircuits for PWM regulators are produced that can withstand an increase in input voltage to a level of 25-30V, despite the fact that the power supply will be only 7-14V. This will allow the output transistor to be turned on in the circuit along with the common drain. This, in turn, is necessary to connect a load with a common minus. Examples include the following samples: L9610, L9611, U6080B ... U6084B. Most loads do not draw more than 10 amps of current, so they cannot cause voltage sags. And as a result, you can use simple circuits without modification in the form of an additional unit that will increase the voltage. And it is precisely these samples of PWM regulators that will be discussed in the article. They can be built on the basis of an asymmetrical or standby multivibrator. It’s worth talking about the PWM engine speed controller. More on this later.

Scheme No. 1

This PWM controller circuit was assembled using CMOS chip inverters. It is a rectangular pulse generator that operates on 2 logic elements. Thanks to the diodes, the time constant of discharge and charge of the frequency-setting capacitor changes separately here. This allows you to change the duty cycle of the output pulses, and as a result, the value of the effective voltage that is present at the load. In this circuit, it is possible to use any inverting CMOS elements, as well as NOR and AND. Examples include K176PU2, K561LN1, K561LA7, K561LE5. You can use other types, but before that you will have to think carefully about how to correctly group their inputs so that they can perform the assigned functionality. The advantages of the scheme are the accessibility and simplicity of the elements. Disadvantages are the difficulty (almost impossibility) of modification and imperfection regarding changing the output voltage range.

Scheme No. 2

It has better characteristics than the first sample, but is more difficult to implement. Can regulate the effective load voltage in the range of 0-12V, to which it changes from an initial value of 8-12V. The maximum current depends on the type of field-effect transistor and can reach significant values. Given that the output voltage is proportional to the control input, this circuit can be used as part of a control system (to maintain the temperature level).

Reasons for the spread

What attracts car enthusiasts to a PWM controller? It should be noted that there is a desire to increase efficiency when constructing secondary ones for electronic equipment. Thanks to this property, this technology can also be found in the manufacture of computer monitors, displays in phones, laptops, tablets and similar equipment, and not just in cars. It should also be noted that this technology is significantly inexpensive when used. Also, if you decide not to buy, but to assemble a PWM controller yourself, you can save money when improving your own car.

Conclusion

Well, you now know what a PWM power regulator is, how it works, and you can even assemble similar devices yourself. Therefore, if you want to experiment with the capabilities of your car, there is only one thing to say about this - do it. Moreover, you can not only use the diagrams presented here, but also significantly modify them if you have the appropriate knowledge and experience. But even if everything doesn’t work out the first time, you can gain a very valuable thing - experience. Who knows where it might come in handy next and how important its presence will be.