Error Amplifier with Forced Equilibrium Adaptor

by Liviu Pascu Each power supply, whether linear or switcher, uses at least two operating modes that characterize its operation: constant voltage (CV) and constant current (CC). The power supply can work in either one, while the other is usually a protective mode, both for the load and for the power supply itself. For example, a constant voltage supply will have a protective CC mode and a constant current supply will have a protective CV mode. The protective or limit mode can be designed to: Function as precisely as the main loop Be a rough limitation only Be designed to limit both output current and voltage, e.g., as in a power supply with a foldback-type limit feature, maintaining a quasi-constant dissipated power on the power control element. The transitions between these two modes, main operating mode and the protective mode, must be automatic and as smooth as possible. Unfortunately, that is not always the case because of the performance of the supply's error amplifier. However, this can be corrected by added a Forced Equilibrium Adapter that enables the error amplifier to cope with these operating mode changes.

Articles TOUR

• The particular structure of the error amplifiers used to control the power supply
• The difference between the new programming values and final output values

Figure 1. Rectangular type output characteristics for a unipolar power supply: (a) Ideal form, (b) Real - dynamic form

Figure 2. Functional schematic of a linear unipolar power supply with rectangular output characteristics.

• The intrinsic delay of the operational amplifier working at its limit (close to +E_aux rail)
• The effective slew rate of the operational amplifiers used for both error amplifiers
• The time necessary to charge or to discharge the local feedback capacitors, C1 and C2, to the new output voltage of the corresponding error amplifiers.

The transients described above are significant for power supplies with a small value output capacitor, that produces a short equivalent time constant, compared with the other time constants in the supply. This kind of power supply could be a power amplifier or a regular power supply designed to work in two modes: "fast mode," with a small value output filter capacitor and "slow mode," with a large value output filter capacitor. When working in "slow mode" output transients are "absorbed" by the output filter capacitor, if it is big enough, which spares the load. However, the stress remains on the power supply, because, during transient, the current through the power element reaches an uncontrolled very high value when the output voltage of the power supply goes from a low to high value.

Figure 3 illustrates the error amplifier with Forced Equilibrium Adaptor, that significantly improves the power supply behavior by minimizing transients appearing at the output when the control of the power supply changes from one error amplifier to the other.

The circuit functions as follows: when the error amplifier is in control, IC1 - the Basic Error Amplifier (BEA) regulates the potential at the pass element (PE) input and eventually the output signal (voltage or current). Current through R11 is sunk partially, depending of the structure of the pass element, by IC1. The voltage drop on R5, VR5 >> V_OS (V_OS = offset input voltage of IC101), is positive with respect to the input of IC101 of the Forced Equilibrium Adaptor (FEA) It is sufficient to force its output to the positive rail of the auxiliary supply (+E_aux). At this time diode D101 is off, so the Forced Equilibrium Adaptor will not affect the equilibrium at the input of the error amplifier when the main loop is closed: V_a = 0V and then, V_fbk / R1 = V_prog / R2.

Thus with D101 off, the Forced Equilibrium Adaptor will not interfere with the normal operation of the regulating loop, performed by the controlling error amplifier. The solid line above R5 in Figure 3 represents the current flowing through R5 in this situation.

Because the other error amplifier has a similar structure, we can apply similar reasoning while referring to Figure 3 and the schematic of a generic power supply shown in Figure 2. In this case, because V_fbk / R1 < V_prog / R2, the potential V_a will be positive, forcing the output of IC1 toward the positive rail of the auxiliary supply. However, then the current through R5 sourced by IC1 through R5 and R104 toward -E_aux, (broken line below R5 in Figure 3), tries to change direction, IC101, upon receiving a negative input, it changes the polarity of its output from positive to negative, thus forward biasing diode D101. With diode D101 ON, IC101 will close a local loop around the basic error amplifier IC1 due to the huge open-loop amplification factor, keeping VR5 = V_OS.

Figure 3. Schematic of the Error Amplifier with Forced Equilibrium Adaptor.

The values of R104 and R5 are chosen such that E_aux / R104 > V_OS / R5.

So, even if the error amplifier does not control the overall loop through its corresponding diode (D1 or D2), a small current will flow through D1 or D2, from +E_aux through R11 toward -E_aux The result is the diode is forward biased so that it is ready to work when the error amplifier takes control. Furthermore, the voltage drops across the local feedback capacitors C1 and C2 in Figure 2, placed around the error amplifiers, are almost equal; the difference is merely 0.1 to 0.2V, given by the difference between the voltage drop across the diodes D1 and D2, where the "controlling" diode conducts more current than the "non-controlling" diode. This is very important, because it minimizes the time required to update the charges across the feedback capacitors when the control is switched between error amplifiers.

To obtain maximum performance from the circuit, IC101 should be a high slew rate type op amp, with very small bias currents. The amplifier IC1 can remain the one initially chosen for the application. Generally, it must be an op amp with very small bias currents, input offset voltage and input offset current. No special restrictions are imposed on the slew rate. Zener diode D102 limits the excursion at the output of the error amplifier upon power-on of the circuit and when the output is programmed from a high value to a low value.

Figures 4 and 5 compare the output current and voltage from a power supply of 2000VDC / 0.1ADC (adjustable from zero to the nominal values), working in "fast mode," when the power supply is driven from CV to CC mode. The waveforms are recorded for the power supply using regular error amplifiers (Figures 4(a) and 5(a)) and using Error Amplifiers with Forced Equilibrium Adaptor (Figures 4b and 5b).

Figure 4. Output current (CH1 = 20mA/cm) and output voltage (CH2 = 50V/cm) for a power supply driven from CV to CC. (a) Regular error amplifiers, (b) Error amplifiers with forced equilibrium adaptors.

From the above description, the advantages of the error amplifier with Forced Equilibrium Adaptor could be summarized as:

• Reduces the overall output transition time and implicitly minimize output transition amplitude during time that the error amplifier either is going out of control or is coming under control. This is accomplished by making the swing at the output of the error amplifiers almost zero and keeping the "non-controlling" error amplifier and the corresponding OR diode in a ready state.
• Allows a much smaller local feedback capacitor to be used to make the loop stable, thereby again reducing the transition time and amplitude at the overall output
• The circuit is very simple and can easily be adapted to the existing error amplifiers
• The circuit can also be applied to other kind of instruments, devices or installations configured to use two or more error amplifiers "fighting" to control the same process.

Figure 5. Output current (CH1 = 20mA/cm) and output voltage (CH2 = 50V/cm) for a power supply driven from CV to CC: (a) Regular error amplifiers, (b) Error amplifiers with forced equilibrium adaptors.