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Modulation Systems and Characteristics
Published in Jerry C. Whitaker, Power Vacuum Tubes, 2017
A number of modulating schemes have been developed to take advantage of the noise immunity afforded by a constant amplitude modulating system. Pulse time modulation (PTM) is one of those systems. In a PTM system, instantaneous samples of the intelligence are used to vary the time of occurrence of some parameter of the pulsed carrier. Subsets of the PTM process include the following: Pulse duration modulation (PDM), where the time of occurrence of either the leading or trailing edge of each pulse (or both pulses) is varied from its unmodulated position by samples of the input modulating waveform. PDM also may be described as pulse length or pulse width modulation (PWM).Pulse position modulation (PPM), where samples of the modulating input signal are used to vary the position in time of pulses, relative to the unmodulated waveform. Several types of PTM waveforms are shown in Figure 2.28.Pulse frequency modulation (PFM), where samples of the input signal are used to modulate the frequency of a series of carrier pulses. The PFM process is illustrated in Figure 2.29.
Energy Harvesting for Sensors: DC Harvesters
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
Oscar Lopez-Lapeña, Maria Teresa Penella, Manel Gasulla
Switching converters are chosen because of their high efficiencies. Figure 10.7 shows a boost converter that can be used whenever the storage unit voltage is higher than the voltage of the PV panel. The state of the transistor can be controlled by a pulse width modulator (PWM) or by a pulse frequency modulator (PFM). The traditional PWM used in switching converters is not the most suitable solution in low-power applications. A periodic switching sequence holds the transistor in on state during a portion of the sequence period, called duty cycle. The voltage of the PV panel is adjusted by means of the duty cycle. As the transistor is continuously switching, the resulting power consumption is too high for low-power PV panels (<1 W). Instead, PFM can be used to further reduce power consumption. The converter remains inactive, with low-power consumption, during a long period of time while the capacitor Cin is charged by the PV panel. When the voltage V reaches a higher threshold value, the converter is activated to discharge the capacitor to a lower threshold value. In this way, V is held inside a small hysteresis cycle that is centered in VMPP. In [5,6], the authors propose implementations of the PFM switching converter using commercial voltage regulators. There are also commercial MPPT ICs, such as the BQ25504 from Texas Instruments, that uses a PFM switching converter and achieves a current consumption as low as 330 nA [7].
Signal Conversion Methods
Published in Clarence W. de Silva, Sensor Systems, 2016
In PFM, as well, the carrier signal is a pulse sequence of constant amplitude. In this method, it is the frequency (or period) of the pulses that is changed in proportion to the value of the data signal, while keeping the pulse-width constant. PFM has the same advantages as those of ordinary FM. Further advantages exist due to the fact that electronic circuits (digital circuits in particular) can handle pulses very efficiently. Furthermore, pulse detection is not susceptible to noise because it involves distinguishing between the presence and the absence of a pulse, rather than accurate determination of the pulse amplitude (or width). PFM may be used in place of PWM in most applications, with better results.
A Highly Efficient Coupled-Inductor SEPIC Topology Based PFC DC–DC Converter for Low Power LED Lighting Systems
Published in IETE Technical Review, 2019
Somnath Pal, Bhim Singh, Ashish Shrivastava
The proposed converter operates in CC and constant voltage (CV) modes. For the operation of CC at the output, current sensing from the load side is always required. Sometimes a current sense resistor is used, which results in additional sensing loss. Primary-side regulation (PSR) for power supplies could be the optimal solution for output current regulation as well as CV operation. The PSR controls the output voltage and current precisely by eliminating current sense resistor as well as the requirement of opto-couplers for an isolated solution. The efficiency of the controller then improves without incurring additional cost. The proposed converter also operates at fixed “Ton” time for a given supply voltage and load combinations. As a consequence, the operation of the converter is described with PFM technique, where the controller of the proposed converter generates pulses of fixed duration but with variable repetition rate as per the required output voltage. PFM has several advantages over commonly used pulse width modulation (PWM), like it has improved feedback stability and noise immunity. It also provides higher efficiency in light load conditions [17]. Under light load, there is period when the MOSFET switches slowly or not at all. The controller may skip pulses so as the main pulse frequency to compensate the leakages at the output, reducing the switching losses. During PFM operation, the output power is having proportionality with average frequency of the pulse train. The converter injects more power when the value of output voltage drops below the desired output voltage as measured by the feedback control loop as shown in Figure 1. Then the switching frequency of converter increases till the output voltage approaches a typical value between the desired output voltage and 0.8%–1.5% above the desired output voltage. The controller for PFM does not require an error amplifier like in PWM, but it has some limitations of higher peak current through the switch and may also increase EMI noises.
A High Efficiency Modified Forward Converter for Solar Photovoltaic Applications
Published in Electric Power Components and Systems, 2023
Praveen V. Pol, Sanjaykumar L. Patil
The output voltages and efficiencies are measured at various loading conditions and at different input voltages and the peak efficiency power point is noted. After the evaluation of the converter performance at the constant operating frequency and duty cycle, in the next stage of experimentation, a pulse frequency modulation (PFM) technique is applied at low output power levels that are below the peak efficiency power point value. The pulse width is kept constant at 4 s and the switching frequency or the pulse rate is decreased to reduce switching losses at low output power and the maximum efficiency points were tracked manually by using a trial and error method. The output voltages at the maximum efficiency points are also measured. From the peak efficiency power point onwards, the operating frequency is kept constant at 100 kHz with a duty cycle of 40 % (4 s pulse width) which is the normal switching waveform between the peak efficiency power point and the maximum rated output power of converter prototype. The efficiencies at various power levels and the corresponding output voltages are measured. Precision digital multi-meters are used to measure the input, output voltages, and currents at various operating conditions. The graphs of the percentage power conversion efficiency of the converter versus the converter output power without using a PFM at low power are shown in Figure 17. It is observed that, at a low power level of approximately 30 W, the minimum efficiency is below 89 %. However, due to the use of the PFM technique at low power levels, the switching losses are reduced and the efficiency is improved. The minimum efficiency goes above 94 % and almost, flat efficiency curves are observed in Figure 18. The peak efficiency of 97.6 % is obtained at 40 V input voltage and 166 W output power. The output voltages across the output filter capacitor are also measured simultaneously. Figure 19 shows the output voltage of the converter with respect to the output power at a constant switching frequency of 100 kHz and 40 % duty cycle. It is observed that there are considerable variations in the output voltages with respect to the load power. However, when PFM is used at low power, the output voltage variations are less, as observed in Figure 20, almost flat output voltage curves with respect to the output power are obtained.