19-09-2017, 04:59 PM
Pulse width modulation (PWM), or pulse duration modulation (PDM), is a modulation technique used to encode a message in a pulsed signal. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to the electrical devices, especially inertial charges such as motors. In addition, PWM is one of the two main algorithms used in photovoltaic solar battery chargers, the other is tracking the maximum power point.
The average value of the voltage (and current) supplied to the load is controlled by turning the switch between the power supply and the on / off load at a rapid speed. The longer the switch compared to the off periods, the greater the total power supplied to the load.
The PWM switching frequency must be much greater than that which would affect the load (the device using the power), ie the resulting waveform perceived by the load must be as smooth as possible. The speed (or frequency) at which the power supply should change can vary considerably depending on the load and the application, for example
Switching must be done several times a minute on an electric stove; 120 Hz in a lamp dimmer; between a few kilohertz (kHz), and tens of kHz for an engine drive; and well in the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of time 'on' to the regular interval or 'period' of time; a low duty cycle corresponds to low power because the power is off most of the time. The duty cycle is expressed as a percentage, 100% is fully activated.
The main advantage of PWM is that the loss of power in the switching devices is very low. When a switch is off there is virtually no current, and when it is turned on and the power is being transferred to the load, there is almost no voltage drop across the switch. The loss of power, the product of voltage and current, is therefore close to zero in both cases. PWM also works well with digital controls, which, due to their on / off nature, can easily set the required duty cycle. PWM has also been used in certain communication systems where its duty cycle has been used to transmit information through a communications channel.
The average value of the voltage (and current) supplied to the load is controlled by turning the switch between the power supply and the on / off load at a rapid speed. The longer the switch compared to the off periods, the greater the total power supplied to the load.
The PWM switching frequency must be much greater than that which would affect the load (the device using the power), ie the resulting waveform perceived by the load must be as smooth as possible. The speed (or frequency) at which the power supply should change can vary considerably depending on the load and the application, for example
Switching must be done several times a minute on an electric stove; 120 Hz in a lamp dimmer; between a few kilohertz (kHz), and tens of kHz for an engine drive; and well in the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of time 'on' to the regular interval or 'period' of time; a low duty cycle corresponds to low power because the power is off most of the time. The duty cycle is expressed as a percentage, 100% is fully activated.
The main advantage of PWM is that the loss of power in the switching devices is very low. When a switch is off there is virtually no current, and when it is turned on and the power is being transferred to the load, there is almost no voltage drop across the switch. The loss of power, the product of voltage and current, is therefore close to zero in both cases. PWM also works well with digital controls, which, due to their on / off nature, can easily set the required duty cycle. PWM has also been used in certain communication systems where its duty cycle has been used to transmit information through a communications channel.