Voltage mode control

B.2.1 The Voltage-mode Controlled Forward-mode Converter... 

The resulting control-to-output Bode plots for the voltage-mode controlled forward converter are given in Figure B-11. 

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

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... 

Figure B-11 The control-to-output curve for a forward-mode converter with voltage-mode control.

Figure B-12 The control-to-output model for a voltage-mode controlled flyback converter.

Figure B-17 Single-pole compensation used with a voltage-mode controlled forward regulator.

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 ... 

This eompensation is intended for voltage-mode eontrolled forward eonverters whieh exliibit a seeond order output filter pole eharaeteristie. This would also inelude a quasi-resonant forward-mode eonverter that uses variable frequency, voltage-mode control. The T-C filter has a severe 180 degree phase lag and a -40dB/decade gain rolloff. To get any sort of wide bandwidth from the supply at all, this type of compensation must be used. 

In the Boost and the Buck-Boost, we see that the output capacitor is in the critical path. So this capacitor should be close to the control IC, along with the diode. A paralleled ceramic capacitor can also help, provided it does not cause loop instability issues (especially in voltage mode control). 

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)... 

Figure 8-4 Signs of Core Saturation in the Push-Pull, with Voltage-Mode Control...

The SG1524A advanced regulating pulse width modulator can be configured to create a voltage mode controlled buck regulator. This type... 

As mentioned briefly, a recent contender to the hall of fame is the 5033, officially labeled a lOOV Push-Pull Voltage-Mode PWM controller, and released in 2003. Luckily, this analog vendor s datasheet only shows a Half-Bridge at work, and that we are aware is a good match for voltage-mode control. So I personally tend to think that marketing (alone) was responsible for this misleading push (or pull). 

We will also learn that the so-called voltage-mode control switching regulators actually rely on the ESR of the output capacitor for ensuring loop stability — even under normal operation. As indicated previously, loop stability refers to the ability of... 

In voltage mode control, the ramp applied to the PWM comparator is derived from an internal (fixed) clock. However in current mode control, it is derived from the inductor current (or switch current). And the latter leads to a rather odd situation where even a slight disturbance in the inductor current waveform can become worse in the next cycle (see upper half of Figure 2-10). 

To get the disturbance to decrease every cycle and eventually die out, it can be shown that we need to do one of two things. Actually, both methods effectively amount to mixing a little voltage-mode control into current-mode control. So... 

A paralleled ceramic capacitor can also help, provided it does not cause loop instability issues (especially in voltage mode control - see Chapter 7). 

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