Voltage error amplifiers

The threshold for the current comparator is set by the output of the voltage error amplifier. If the voltage error amplifier indicates that the output voltage is too low, then the current threshold is raised to allow more energy to reach the load. The converse is true too. 

There are many buek eontroller ICs on the market, but the one that was ehosen is the UC3873. The internal referenee presented to the voltage error amplifier is 1.50 V-t/- 2 pereent. 

The internal reference presented to the voltage error amplifier is 2.5 V +1-2.5 percent. 

The eompensation of the voltage error amplifier should be a single-pole rolloff with a unity gain frequeney of 38 Hz. This is required to rejeet the fundamental line frequeneies of 50 and 60 Hz. The feedbaek eapaeitor around the voltage error amplifier beeomes... 

Fig. 11.6. Simple feedback electronics with integration compensation. The first op-amp amplifies the error signal with a variable gain. An RC network provides an integration compensation. A high-voltage op-amp provides an output of 100 V or more, to drive the z piezo.

An example of an elementary voltage feedbaek applieation is the nonisolated, single-output switehing power supply. If we negleet the error amplifier eompensation, then the design is quite simple. Tet us examine a situation where a 5 V output is regulated and a 2.5 V referenee is provided within the eontrol IC. This ean be seen in Figure 3-43. 

To begin the proeess, one deeides how mueh sense eurrent is to be drawn through the output voltage resistor divider. For the sake of reasonable values to be ealeulated for the error amplifier eompensation values, resistanee values in the range of 1.5 to 15 K should be used in the upper leg of the resistor divider. Tet us use a sense eurrent of 1 mA as the resistor divider sense eurrent. This makes the lower resistor in the divider (Ri)... 

The usual ehoiee for a seeondary error amplifier is the TL431 whieh has a temperature eompensated voltage referenee, and an amplifier within a three-leaded paekage. It does need a minimum of 1.0mA eontinuous eurrent flowing... 

The overvoltage override methods assume that the power supply is still operating and the voltage feedbaek has beeome open-eireuited, or that one of the outputs has beeome light loaded and its voltage rises above the maximum speeifieation. These methods have a separate eomparator or transistor and resistor dividers wired to eaeh output. The eomparator or transistor would then override the error amplifier. These are shown in Figure 3-53. 

The voltage feedbaek loop must be isolated from the input voltage line and the eontrol IC. An optoisolator must be used. To minimize the drift effeet of the optoisolator an error amplifier is desired on the seeondary side. A TT431CP does this job nieely. The topology of the feedbaek eireuitry is shown in Figure 3-70. 

The error amplifier within the MC34025 has a totem-pole output circuit, which means that its output is not easily overriden. It will be used as a simple voltage follower and the error amplifier function will take place completely within the TT431 on the secondary side of the power supply. 

I am designating the maximum voltage at the input to the error amplifier to be -1-4.5 V. The MOC8102 has a nominal C r of 100 percent and the TL431 requires a minimum of 1 mA passing through it to operate. This makes the value of the resistor Rl ... 

AVe is the peak-to-peak output voltage of the error amplifier. 

Then find the value of the feedback capacitor C. The designer knows the value of the input resistor (R). It is the upper resistor in the voltage divider responsible for the voltage feedback to the error amplifier. One then performs Equation B.15. 

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 ... 

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