Overview of PWM Pulse Width Modulation)

What are the best PWM techniques and strategies for different types of power devices and loads?

Best PWM techniques and strategies explained...

How PWM works

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PWM works by switching the power device (such as a transistor, a diode, or an IGBT) on and off at a high frequency, while varying the duty cycle (the ratio of on-time to off-time) of the switch. The duty cycle determines the average output voltage and current of the converter. By changing the duty cycle, the converter can adjust the output to match the desired load demand. For example, in an inverter, PWM can create a sinusoidal output voltage from a DC input source by switching the power device at a frequency much higher than the output frequency, and varying the duty cycle according to a reference waveform.

Benefits of PWM

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Pulse-width modulation (PWM) offers several advantages for power electronics systems, such as reducing harmonic distortion and noise in the output waveform, thus improving power quality and reducing electromagnetic interference (EMI). It also increases efficiency and reliability of the converter, as the power device is either fully on or fully off, minimizing switching losses and thermal stress. Furthermore, PWM enables a wide range of output voltage and current regulation due to its adjustable duty cycle from zero to one, allowing the converter to operate in various modes and applications. In addition, PWM simplifies the design and implementation of the converter since it does not require complex filters, transformers, or feedback circuits.

Drawbacks of PWM (Well, I am a hard nosed realist);

PWM has some drawbacks when used in power electronics systems. It increases the switching frequency and stress of the power device, reducing its lifetime and increasing its switching losses and EMI emissions. Additionally, a high-performance controller and driver circuit are needed to generate and apply the PWM signals to the power device, which can increase costs and complexity of the converter. Lastly, it can cause adverse effects on the load, such as voltage spikes, current ripples, acoustic noise, and torque pulsations, particularly for sensitive or nonlinear loads like motors, batteries or LEDs.

Best PWM techniques and strategies?

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Different PWM techniques and strategies can be used for various power devices and loads, depending on the system requirements and constraints. Sinusoidal PWM uses a sinusoidal reference waveform to modulate the duty cycle of the switch, resulting in a low harmonic distortion and a smooth output voltage. Space vector PWM employs a vector representation of the output voltage to modulate the duty cycle of the switch, creating an optimal output voltage. Hysteresis PWM uses a hysteresis band around the reference waveform to modulate the duty cycle, generating a variable switching frequency and a constant output current. Lastly, Random PWM utilizes a random or pseudo-random variation of the switching frequency or the duty cycle of the switch, leading to a spread spectrum output voltage with low EMI emission and high noise immunity.

What is a PWM Inverter? Types and Their Applications

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Pulse Width Modulated inverters (PWM inverters) replaced the older versions of inverters and has a wide range of applications. Practically these are used in the power electronics circuits. The inverters based on the PWM technology possess MOSFETs in the switching stage of the output. Most of the inverters available nowadays possess this PWM technology and are capable of producing ac voltage for varying magnitudes and frequencies. There are multiple protection and control circuits in these types of inverters. The implementation of PWM technology in the inverters makes it suitable and ideal for the distinct loads connected.

In depth: What is a PWM Inverter?

An inverter whose functionality depends upon the pulse width modulation technology is referred to as PWM inverters. These are capable of maintaining the output voltages as the rated voltages depending on the country irrespective of the type of load connected. This can be achieved by changing the switching frequency width at the oscilator.

There are various circuits used in the PWM inverters. Some of them are listed below.

Battery Charging Current Sensor Circuit:

The purpose of this circuit is to sense the current utilized in charging the battery and maintain it at the rated value. It is important to avoid the fluctuations to protect the batteries’ shelf life.

Battery Voltage Sensing Circuit:

This circuit is used to sense the voltage required to charge the battery when it is exhausted and begin trickle charging of the battery once it gets fully charged.

AC Mains Sensing Circuit:

This circuit is to sense the availability of AC mains. If it is available then the inverter will be in a state of charging and in the absence of mains the inverter will be in battery mode.

Soft Start Circuit:

It is used to delay the charging for 8 to 10 seconds after resuming the power. It is to protect the MOSFETs from the high currents. This is also referred to as Mains delay.

Change Over Circuit:

Based on the mains availability this circuit switches the operation of the inverter between the battery and the charging modes.

Shut Down Circuit:

This circuit is to monitor the inverter closely and shut it down whenever any abnormality incurred.

PWM Controller Circuit:

To regulate the voltage at the output this controller is used. The circuit needs to perform PWM operations are incorporated in the IC’s and these are present in this circuit.

Battery Charging Circuit:

The process of charging a battery in the inverter is controlled by this circuit. The output generated by the sensing circuit of the mains and the sensor circuits of the battery is the inputs for this circuit.

Oscillator Circuit:

This circuit is incorporated with the IC of PWM. It is used to generate the switching frequencies.

Driver Circuit:

The output of the inverter gets driven by this circuit based on the switching signal of frequency generated. It is similar to that of a preamplifier circuit.

Output Section:

This output section comprises a step-up transformer and it is used to drive the load.

Working Principle:

An inverter designing involves various topologies of power circuits and the methods to control the voltage. The most concentrated part of the inverter is its waveform generated at the output. For the purpose of filtering the waveform inductors and the capacitors are used. In order to reduce the harmonics from the output low pass filters are used.

If the inverter possesses a fixed value of output frequencies resonant filters are used. For the adjustable frequencies at the output, filters are tuned above the maximum value of fundamental frequency. PWM technology changes the square wave characteristics. The pulses used for switching are modulated and regulated before it supplied to the connected load. When there is no requirement for voltage control fixed width of the pulse is used.

PWM Inverter Types & Waveforms

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The technique of PWM in an inverter comprises of two signals. One signal is for the reference and the other will be the carrier. The pulse required for switching the mode of the inverter can be generated by the comparison among those two signals. There are various PWM techniques.

Single Pulse Width Modulation (SPWM):

For every half cycle, there is only one pulse available to control the technique. The square wave signal will be for reference and a triangular wave will be the carrier. The gate pulse generated will be the result of the comparison of the carrier and the reference signals. Higher harmonics is the major drawback of this technique.

Single Pulse Width Modulation:

Multiple Pulse Width Modulation (MPWM)

MPWM technique is used to overcome the drawback of SPWM. Instead of a single pulse, multiple pulses are used for every half cycle of the voltage at the output. The frequency at the output is controlled by controlling the frequency of the carrier.

Sinusoidal Pulse Width Modulation:

In this type of PWM technique, instead of a square wave, a sine wave is used as a reference and the carrier will be a triangular wave. The sine wave will be the output and its RMS value of voltage is controlled by the modulation index.

Modified Sinusoidal Pulse Width Modulation:

The carrier wave is applied for the first and the last sixty-degree interval per every half cycle. This modification is introduced to improve the harmonic characteristics. It decreases the loss due to switching and increases the fundamental component.

Finally, the Applications of a PWM inverter:

Most commonly PWM inverters are utilized in the speed AC drives where the speed of the drive is dependent on the variation in the frequency of the applied voltage. Majorly the circuits in power electronics can be controlled by using PWM signals. To generate the signals in analog form from digital devices like microcontrollers, the PWM technique is beneficial. Further, there are various applications where PWM technology is used in different circuits.