Input filtering

Sample times for the microprocessor-based SLCs vaiy from O.I to 0.4 seconds. Low-pass analog electronic filters are installed on the process inputs to stop abasing errors caused by fast changes in the process signal. Input filter time constants are typically in the range From O.I to I s. Microprocessor-based SLCs may be made part of a DCS by using the communication port (RS-488 is common) on the controller or may be operated in a standalone mode independent of the DCS. 

Its operation ean be seen as analogous to a meehanieal flywheel and a one-piston engine. The L-C Alter, like the flywheel, stores energy between the power pulses of the driver. The input to the L-C Alter choke input filter) is the ehopped input voltage. The L-C Alter volt-time averages this duty-cyele modulated input voltage waveform. The L-C Altering funetion ean be approximated by... 

The output filter converts the rectified rectangular ac waveform into the dc output. Forward-mode converters have a two-pole L-C filter which produces the dc average of the rectified rectangular waveform. Boost-mode converters have a single-pole, capacitive input filter which produces a dc voltage which is the peak voltage of the rectified waveform. Both are reactive impedance filters and exliibit very little loss. 

Figure 3-54 Typical ac and dc input filter circuits (a) ac input filter circuit for a single or universal input power supply (common-mode EMI filter shown) (b) a voltage doubling ac input circuit for 110V and 220V ac inputs (c) single dc bus input filter.

In ae off-line eonverters, aluminum eleetrolytie eapaeitors are used as the input filter eapaeitor. They have proven to be more rugged than any other type of eapaeitor in the hazardous environment existing on the ae line. Choosing the final eapaeitor depends mostly upon the expeeted operating temperature range, quality level, and ease dimensions. 

Layout the input source loop and input filter. 

The major concern of both output and input filter capacitors is the ripple current entering the capacitor. In this application, the ripple current is identical to the inductor ac current. The maximum limits of the inductor current is 2.8 A for I peak and about one-half the maximum output current or 1.0 A. So the ripple current is 1.8 A p-p or an estimated RMS value of 0.6 A (about one-third of the p-p value). 

Designing the reetifier and input filter eireuit (refer to Seetion 3.12)... 

The approximate value of the bulk input filter capacitor is found from... 

As one can see, there is the familiar choke input filter (T-C) on the output, which is characteristic of the buck and all forward-mode converters. The configuration shown in Figure 4—10 is called a parallel resonant topology because the load impedance (the T-C filter acting as a damping impedance) is placed in parallel to the resonant capacitor. The input to the T-C filter stage... 

In terms of strietly passive reaetive loads, the power faetor is the resulting phase between the voltage and the eurrent waveforms. In power supplies though, it is the distortion to the voltage waveform resulting from the time whieh input reetifiers eonduet. Power faetor is measured from 0 to I where I is where all the power is used by the load (purely resistive). The typieal eapaeitive input filter found in power supplies has an average power faetor of 0.5 to 0.7. 

Note that a quiet supply line almost always helps in debugging, whatever the problem being investigated. And luckily, enhancing the input filtering rarely causes any problems. So I... 

I usually prefer to remove any additional input filter components only after I have solved whatever problem I had been going after. In the case of the vertically stacked capacitors, for example, I would then start removing each additional capacitor one by one (with the help of two soldering irons applied simultaneously to both sides of the stack), confirming, at each step, that the problem had not come back, and that there was not a new one now ... 

A simple device for measuring and delivering gas can be made from a football bladder, tubing, and a three-way tap as shown in Fig. 7 Fill the bladder with C02, close the tap, and detach from the cylinder. Turn the tap so that the syringe can be filled with the gas. Attach the tubing to the input filter on the reservoir, and turn the tap so that the C02 can be injected into the reservoir. In this fashion, a predetermined volume can be delivered. 

To know what the measured voltages Vl and Vn are, we now look at Figure 9-3. The voltage due to the common mode component is 25 2 multiplied by the current flowing in the earth connection (i.e. 50 Q times the current in each leg). The voltage due to the differential mode component is 100 2 times the differential mode current. Therefore the LISN provides the following load impedances to the noise generators (in the absence of any input filter)... 

Figure 13-4 Input Filter Interaction and Stability Criteria...

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