Current mode control

The plant transfer functions presented earlier were only for voltage mode control. In current mode control, the ramp to the pulse width (duty cycle) modulator is derived from the 

However everyone does seem to agree that current mode control alters the poles of the system (as compared to voltage mode control), but the zeros are unchanged. So the boost and the buck-boost still have the same RHP zero, as we discussed earlier. And care is still needed to ensure that the RHP zero is at a much higher frequency than the chosen crossover frequency. 

For subharmonic instability to occur, two conditions have to be met simultaneously — the duty cycle should be close to or exceed 50%, and simultaneously, we should be in CCM 

If we take the Bode plot of any current mode controlled converter (one that has not yet entered this wide-narrow-wide-narrow state), we will discover an unexplained peaking in the gain plot, at exactly half the switching frequency. This is the source of subharmonic instability. Because, though this point is much past the crossover frequency, it is potentially dangerous because of the fact that if it peaks too much, it can end up intersecting the 0 dB axis again — which we know is one of the prescriptions for full instability. 

Subharmonic instability is nowadays being modeled as a pole at half the switching frequency. Note that in any case we never consider setting the crossover frequency higher than half the switching frequency. So in effect, this subharmonic pole will always occur at a 

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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. 

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. 

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. 

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 ... 

Boss Ng You must have been dreaming all right. Because I think you have gone and ruined the slope compensation You should never, never, never fool around with that capacitor in this current mode control IC. If you were just a little smarter, you would have tried paralleling a larger resistor across the sense resistor to adjust its value, but you had to go and do just that ... 

Voltage Mode Control converters shown above (with Current Mode Control, it may sometimes bo possible to omit the droop resistors if trensconductance error amps are used)... 

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