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The transfer function of the discontinuous flyback converter is such that the output current (I_{OUT}) is proportional to peak current (I_{PK}^{2}); I_{PK}^{2} is also proportional to on time (T_{ON}^{2}), which is also proportional to control voltage (V_{C}^{2}), as shown in Fig. 1.
The openloop gain, and therefore the bandwidth, varies directly with I_{PK}, T_{ON} and V_{C}. Using a logarithmic converter on V_{C} can stabilize the gain, resulting in improved overall performance, because the bandwidth at low current is increased while keeping the high current gain at the maximum value allowed by the circuit.
Fig. 2 shows a “statespace” SPICE simulation of a multiphase flyback converter.^{[1]} The flyback block, shown as X1 with inputs V_{IN} and V_{C} and outputs V_{OUT} and duty, linearizes the switching of the output power device (power MOSFET or BJT). This linearization enables SPICE simulations to run very quickly while still producing accurate line/load step, startup and other types of transient results, in addition to enabling the traditional smallsignal ac analysis (Bode plot).
Statespace techniques are a staple of powersupply designers who use SPICE.^{[2]} Statespace simulations allow SPICE to run as fast as or faster than many nonSPICEbased piecewiselinear simulators. The class of switching simulations where SPICE is at a disadvantage to periodic operating point/piecewiselinear simulators is truly very narrow. In addition, SPICE has the added advantage of much wider model support, and many average models for SPICE are readily available.
The flyback block and other statespace blocks for topologies, such as buck, boost and forward, can be found in the Power IC Model Library for PSPICE (www.emaeda.com) and in other literature.^{[3, 4]}
Logarithmic Compensation
Fig. 2 shows the circuit with linear control, which results in the output current being proportional to V_{C}^{2}. From Fig. 3, E1 and E2 are used to force the currentmode statespace flyback model to operate in voltage mode. R1 through R8, D1, D5, D6, D7 and X6 comprise the log converter, which performs the gain equalization. The components were selected to adjust the converter's response to approximate the squareroot function over the range of V_{C}. V_{RAMP} offset is the typical offset contained within most PWM integrated circuits. The transformers are ideal dcdc transformers with a secondarytoprimary turns ratio of 0.25.
Fig. 4 shows the output current versus controlvoltage response using normal linear gain control. The traces show the output current as a function of V_{C} (trace 2) and the derivative of I_{OUT} with respect to V_{C} (trace 3), which is the gain. This graph clearly shows that the gain is linearly proportional to V_{C}.
Fig. 5 shows the output current versus controlvoltage response using logarithmic gain control. The traces show the output current as a function of V_{C} (trace 1) and the derivative of I_{OUT} with respect to V_{C} (trace 2), which is the gain. This graph shows that the gain is relatively constant and nearly independent of V_{C} over a wide operating range. Using a more accurate logarithmic control would provide a flatter gain curve.
The openloop bandwidth of the control loop is directly proportional to the gain. This would mean that using the linear control results in a bandwidth tolerance of nearly 90% over the full operating range (~9 A/V average ±8 A/V) while the log control results in a gain tolerance of approximately 20% (~9 A/V average ±2 A/V) over the full operating range. The gain tolerance reduction offered by the log control makes it much easier to stabilize and also allows better closedloop performance. This gain equalization could be done accurately with a squareroot circuit; however, the approach presented here will work nearly as well.
Gain and Bandwidth
The main concern with the gain being proportional to V_{C} or I_{PK} is that the closedloop performance of a converter is a function of its bandwidth. The result is that if the gain is a function of V_{C}, then the closedloop performance will be a function of V_{C} (and, therefore, load dependent). The major performance characteristics affected are ripple rejection (and, therefore, the linestep response), output impedance (and, therefore, the stepload response), and the openloop bandwidth and phase margin. In the case of linepowered flyback converters (nonPFC), the ripple rejection may be important to remove the 120Hz ripple component from the output.
In order to tolerate a wide gain variation, it is generally required that the control loop have a very lowfrequency zero to ensure singlepole compensation at the zero crossing (the single, dominant pole is usually the output capacitor). The overall effect of the lowfrequency zero is a long response time. This requires, for example, that the softstart time be very long so as not to result in outputvoltage overshoot. The recovery time for load and line steps also will be long as a result of the lowfrequency zero. Some applications may not be sensitive to these performance characteristics, in which case a lowfrequency zero and a wide gain variation are not significant issues. In other cases, the openloop performance can be improved, such as with larger output capacitors.
The addition of gain equalization allows the optimization of the control loop for the best performance in those cases that do require or demand it. In select cases, it may be the difference between whether a linear postregulator is required, adding to the cost and reducing the efficiency of the powerconversion circuitry.
In specific applications, such as PFC converters, the bandwidth of the control loop must always be lower than the line frequency, or significant current distortion will occur. Further reduction of the bandwidth to accommodate the gain variation due to loading also will degrade the performance of the converter. For this reason, many of the PFCspecific control integrated circuits include a gain equalization network allowing the best performance possible, given the requirement for the low bandwidth necessary to achieve low linecurrent distortion.
References

Sandler, Steven M. Switchmode Power Supply Simulation with PSPICE and SPICE 3, Chapter 5, McGrawHill, 2006, ISBN 0071463267.

Basso, C. SwitchMode Power Supply SPICE Cookbook, McGrawHill, ISBN 0071376461.

PSPICE Power IC Model Library Documentation, AEi Systems, LLC, 2005.

Amran, Y., Huliehel, F., and BenYaakov, S. A Unified SPICE Compatible Average Model of PWM Converters. IEEE Trans. on Power Electronics, 6, 585594, 1991, www.ee.bgu.ac.il/~pel/public.htm.