Single-pole compensation

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... 

The schematic and Bode plot for the single-pole method of compensation are given in Figure B-16. At dc it exhibits the full open-loop gain of the op amp, and its gain drops at -20dB/decade from dc. It also has a constant -270 degree phase shift. Any phase shift contributed by the control-to-output characteristic... 

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

B.4.2 Single-pole Compensation with In-band Gain Limiting... 

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

Note that the integrator has a single-pole — at zero frequency . Therefore, we will often refer to it as the pole-at-zero stage or section of the compensation network. This pole is more commonly called the pole at the origin or the dominant pole. 

We can guess that the phase shift introduced by a single pole at its resonant frequency is 45°, so the new phase margin should be around 79° — 45° = 34°. We plot the gain-phase plots with this new high-frequency pole criterion (and freshly calculated compensation component values) and we get the curve shown in Figure 7-17. 

Current mode control In this method, as shown in Fig. 10.79, a second inner control loop compares the peak inductor current with the control voltage, which provides improved open-loop line regulation. All of the problems of the direct duty cycle control method (1) and (2) are corrected with this method. An additional advantage of this method is that the two pole second-order filter is reduced to a single pole (the filter capacitor) first-order filter resulting in simpler compensation networks. There are several current mode control ICs available in the market. 

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