Current waveform

The centrifugal pump directly driven by a variable-speed electric motor is the most commonly used hardware comoination for adjustable speed pumping. The motor is operated by an electronic-motor speed controller whose function is to generate the voltage or current waveform required by the motor to make the speed of the motor track the input command input signal from the process controller. 

A low di/dr will also help to smooth the d.c. link current waveform. 

Figure 19.27 Approximate representation of assumed voltage and current waveforms illustrating a current-chopping effect and its attenuation while interrupting a circuit having 0,3 p.f.

As one can notice, the boost-mode converter has the same parts as the forward-mode converter, but they have been rearranged. This new arrangement causes the converter to operate in a completely different fashion than the forward-mode converter. This time, when the power switch is turned on, a current loop is created that only includes the inductor, the power switch, and the input voltage source. The diode is reverse-biased during this period. The inductor s current waveform (Figure 3-4) is also a positive linear ramp and is described by... 

The power switch and rectifier ac-current loops contain very high trapezoidal current waveforms typical in PWM switching power supplies. These waveforms are rich in harmonics which extend far above the basic switching frequency. These ac currents can have peak amplitudes two to five times that of the... 

This capacitor experiences the same current waveform at the power switch, which is a trapezoid with an initial current of about 1A rising to 2.8 A with very sharp edges. This capacitor has much more rigorous operating conditions than the output filter capacitor. I will estimate the RMS value of the trapezoidal current waveform as a piecewise superposition of two waveforms, a rectangular 1A peak waveform and a triangular waveform with a 1.8 A peak. This yields an estimated RMS value of 1.1 A. The value of the capacitor is then calculated as ... 

I have decided to use a current transformer to sense the primary current waveform, since resistive methods are impractical in the half-bridge topology. Several transformer manufacturers make current transformers for such a purpose wound on a toroidal core. Coilcraft makes current transformers with 50, 100, and 200 turns on their secondaries. The secondary voltage must be determined in order to have representative current waveforms of the level to work with the control IC. The voltage needed on the output of the current transformer is... 

Every current-mode control application that exceeds 50 percent duty cycle must have slope compensation on the current ramp waveform. Otherwise an instability will occur whenever the duty cycle exceeds 50 percent. This is typically done by summing into the current waveform some of the oscillator ramp waveform. This will increase the slope of the current waveform and therefore trip the current sense comparator earlier. A common problem is the inadvertent loading of the oscillator, so I will use a PNP emitter-follower to buffer the oscillator. The circuit configuration can be seen in Figure 3-74. 

The conduction loss ( 2) is measured as the product of the switch terminal voltage and current waveforms. These waveforms are typically quite linear and the power loss during this period is given in Equation 4.1. 

Quasi-resonant converters are a separate class of switching power supplies that tune the ac power waveforms to reduce or eliminate the switching loss within the supply. This is done by placing resonant tank circuits within the ac current paths to create pseudo-sinusoidal voltage or current waveforms. Because the tank circuits have one resonant frequency, the method of control needs to be modified to a variable frequency control where the resonant period is fixed and the control varies the period of the non-resonant period. The quasi-resonant converters usually operate in the 300 kHz to 2 MHz frequency range. 

Quasi-resonant converters force the voltage or current waveform into a haver-sine waveshape. If the power switch(es) are switched at the right moments, then there are no switching losses experienced. Also because of the controlled rates of change for the voltage or current waveforms, much better RFI/EMI performance is realized. Most of the basic topologies that exist within the PWM family are also in the quasi-resonant family. 

A second type of quasi-resonant converter is the zero voltage switching (ZVS) quasi-resonant family. A ZVS QR buck converter and its waveforms are shown in Figure 4-11. Here the power switch remains on most of the time and performs resonant off periods to decrease the output power. Actually, the ZCS and the ZVS families mirror one another. If you were to compare the switch voltage and current waveforms between the two families, and if one inverts both the voltage and current waveforms in order to reference them to the power switch, the waveforms would have a striking resemblance to one another. 

Power factor correction circuits are intended to increase the conduction angle of the rectifiers and to make the ac input current waveform sinusoidal and in phase with the voltage waveform. The input waveforms can be seen in Figure C-2. This means that all the power drawn from the power line is real power and not reactive. The net result is that the peak and RMS current drawn from the line is much lower than that drawn by the capacitive input Alter circuit traditionally used. 

Another major source of noise is the loop consisting of the output rectifiers, the output filter capacitor, and the transformer secondary windings. Once again, high-peak valued trapezoidal current waveforms flow between these components. The output Alter capacitor and rectifier also want to be located as physically close to the transformer as possible to minimize the radiated noise. This source also generates common-mode conducted noise mainly on the output lines of the power supply. 

One subtle, but major noise source is the output rectifier. The shape of the reverse recovery characteristic of the rectifiers has a direct affect on the noise generated within the supply. The abruptness or sharpness of the reverse recovery current waveform is often a major source of high-frequency noise. An abrupt recovery diode may need a snubber placed in parallel with it in order to lower its high-frequency spectral characteristics. A snubber will cost the designer in efficiency. Finding a soft recovery rectifier will definitely be an advantage in the design. 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

In that spirit, we now detail a quick procedure for estimating harmonic amplitudes of the square current waveform (assuming fast transitions). We will apply it to our specific lOOkHz/lA example as we go along ... 

One of the things the preceding impedance calculations tell us is that if we are trying to draw a sharp current waveform from a capacitor, that capacitor had better be good. 

A little later, after we have better understood the current waveforms in the input capacitors, we will do a calculation to correctly compute the amount of bulk capacitance based on the 1% input ripple criterion mentioned above. 

The average of the switch current waveform, evaluated over the full cycle, is /, x I) = 0.67A (Block 1). 

Therefore, if we plot the current waveform we realize that during the OFF time, the capacitor current must be sitting at a steady -0.67A (Block 3). This is the capacitor charging (refresh) current. 

However, the entire capacitor current waveform is simply equivalent to taking the switch waveform and translating it down vertically, by an amount exactly equal to the DC value of the switch waveform. Doing so effectively subtracts the DC component from the switch waveform and provides the required AC component to the capacitors. 

Therefore, the total height (peak to peak) of the capacitor current waveform (measured to the center of the ramp) is still 1A, which is also the case for the switch waveform from which it is derived. 

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