Current mode

As the last section discussed the voltage in a modal domain, this section discusses the current in a modal domain. We first need to find the eigenvalues of (= YZ as the second equation of Equation 1.115 tells us. Since P( P in general, we define Q as the eigenvalue matrix of Pj and B as the eigenvector matrix of P,  

Since a matrix returns to the original matrix when it is transposed twice. 

The earlier equation shows that the eigenvalues for the voltage are equal to those for the currents. Since y= Q, propagation constants for the voltage 

However, the current transformation matrix B is not equal to the voltage transformation matrix A. Taking transpose of the first equation of Equation 1.144, 

The aforementioned shows that the current transformation matrix can be found from the voltage transformation matrix. In general, D is assumed as an identity matrix. Under this assumption. 

---

Fig. 4. Operational modes for stm. (a) Constant height mode, (b) Constant current mode (18).

The constant height mode of operation results in a faster measurement. In this analysis, the tip height is maintained at a constant level above the surface and differences in tunneling current ate measured as the tip is scaimed across the surface. This approach is not as sensitive to surface irregularities as the constant current mode, but it does work well for relatively smooth surfaces. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Designing the output filter choke La) in a forward-mode converter is done first. This simple procedure can be seen in Section 3.5.5. A key design factor is to design the inductor to operate in the continuous current mode. The typical value of peak inductor current is 150 percent of the rated output current. The typical valley (minimum) current is about 50 percent of the rated output current. 

Current-mode, turn-on with eloek Boost Very good OC proteetion many ICs typieally GND-driven SW... 

Current-mode control is best used in topologies where the linear slopes within the eurrent waveforms are higher. This would be the boost-mode topologies sueh as boost, buek-boost, and flybaek. 

Current mode control has an inherent overcurrent protection. The highspeed current comparator provides pulse-to-pulse current limiting. This form of protection is a constant power form of overload protection (see Section 3.11). This form of protection folds back the current and voltage to maintain a constant power into the load. This may not be optimum for all products, especially where the typical failures slowly increase the failure current. Another form of overload protection can also be placed in the circuit. 

Another form current-mode control is called hysteretic current-mode control. Here both the peak and the valley currents are controlled. This is obviously better for continuous-mode forward for boost converters. It is somewhat complicated to set-up, but it does offer very fast response times. It is not a very common method of control and its frequency varies. 

Use the MTPIONIOM. I have selected the current sensing style of power MOSFET since I wish to implement a current-mode controller and this will reduce my sensing losses by three orders of magnitude. 

Figure 3-66 Schematic for design example 3.15.2. A 28W current-mode, flyback dc-to-dc converter.

The controller to be used is to be the UC3843P current-mode controller IC running at a frequency of 50 kHz. 

Selecting the SMPS controller IC. The important factors within this application that affect the choice of switching power supply controller IC are MOSFET driver needed (totem-pole driver), single-ended output, 50 percent duty cycle limit desired, and current-mode control desired. The popular industry choice that meets these needs is the UC3845B. 

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. 

Current-mode eoiitrolled, forward-mode eoiiverters have a one pole filter ehar-aeteristie. The optimum eompensation is the single-pole, single-zero method of eompensation. 

A15 Watt, ZVS Quasi-resonant, Current-mode Controlled Flyback Converter... 

The control-to-output characteristic curves for a current-mode controlled flyback-mode converter, even though it is operating in variable frequency, are of a single-pole nature. So a single pole-zero method of compensation should be used. The placement of the filter pole, ESR zero, and dc gain are... 

B.2.2 Voltage-mode Controlled Flyback Converter and Current-mode Controlled Forward-mode Converter Control-to-Output Characteristics... 

The operation of a discontinuous-mode, flyback converter is quite different from that of a forward-mode converter, and likewise their control-to-output characteristics are very different. The topologies that fall into this category of control-to-output characteristics are the boost, buck/boost, and the flyback. The forward and flyback-mode converters operating under current-mode control also fall into this category. Only their dc value is determined differently. Their representative circuit diagram is given in Figure B-12. 

AVe in this case can be the peak-to-peak voltage of the oscillator ramp if the method of control is voltage-mode, or the maximum peak voltage representing the primary current within the current-mode method of control. The gain can be converted into decibels, which is shown in Equation B.7. 

The current-mode controlled forward-mode converter exliibits the same dc gain as the voltage-mode controlled forward converter, as shown in Equation B.6. 

The output filter pole in both voltage-mode controlled flyback converter and the current-mode controlled forward and flyback is highly dependent on the equivalent resistance of the load. This means that when the load current increases or decreases, the location of the output filter pole moves. The filter pole can be found from... 

The current-mode controlled forward converter has one additional consideration there is a double pole at one-half the operating switching frequency. The compensation bandwidth normally does not go this high, but it may cause problems if the closed-loop gain is not sufficiently low enough to attenuate its effects. Its influence on the control-to-output characteristic can be seen in Figure B-14. 

Boost-mode with voltage-or current-mode control X X ... 

Figure B-19 An example of single-pole with in-band gain limiting compensation used with a voltage/current-mode controlled flyback converter.

The next task is to determine the plaeement of the eompensating zero and pole within the error amplifier. The zero is plaeed at the lowest frequency manifestation of the filter pole. Since for the voltage-mode controlled flyback converter, and the current-mode controlled flyback and forward converters, this pole s frequency changes in response to the equivalent load resistance. The lightest expected load results in the lowest output filter pole frequency. The error amplifier s high frequency compensating pole is placed at the lowest anticipated zero frequency in the control-to-output curve cause by the ESR of the capacitor. In short ... 

---