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Posted on 04 April 2019

Improving the Regulation of Multiple Output Flyback Converters

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The flyback topology is a good choice for multiple output applications, where several semi-stabilized outputs are required from a single supply. At low power levels the flyback is very attractive in that it is typically the least expensive isolated topology because it uses the fewest number of components.

By Florian Mueller, Texas Instruments

This article addresses some of the challenges of designing a multiple output flyback power supply. The main output is closed loop controlled and is thus fully regulated. The other outputs are only semi-regulated and could provide line and load regulation in the order of ±6%. Problems can appear if the power of the main output is low compared to the semi-regulated outputs. Low main output current could make it very difficult to maintain the other outputs in the ±6% tolerance.

Flyback Control Method

Figure 1 shows the simplified schematic of an isolated flyback converter. The shunt regulator TL431 from Texas Instruments is used in conjunction with an optocoupler to provide feedback loop isolation. A small variation of the output voltage due to line or load changes is sensed by the inverting input of the error amplifier and compared to the internal voltage reference of the TL431. Differences between the divided down output voltage and the voltage reference are gained up and converted into a proportional error current. This error signal is transferred to the primary controller through the optocoupler.

Secondary side control method

There are two methods for connecting the optocoupler

The Zener diode D2, the capacitor C2 and the resistor R1 provides the supply for the optocoupler (figure 1). Therefore the optocoupler supply voltage is fixed. This removes any optocoupler dependency on the output voltage and load changes in the ac gain characteristics. This independence on the output voltage is the reason that there is only one single feedback path in this circuit. The control loop does not contain an inner loop. Figure 2 shows the block diagram of the transfer functions.

Block diagram for optocoupler connection without inner loop

The total open loop is simply the product of all transfer functions.

Transfer functions

Supplying the optocoupler with the regulated output voltage is an alternative method. Therefore the zener diode D2 and the capacitor C2 must be removed. The optocoupler current depends now on the difference between the optocoupler supply voltage (Vout) and the error voltage and a second inner loop is introduced (see block diagram figure 3).

Block diagram for optocoupler connection inner loop included

The total open loop is now the product of the error amplifier transfer function and the closed loop of the Powerstage and optocoupler transfer function.

Optocoupler transfer function

The effect of closing the inner loop can be seen in the bode plot of the powerstage-optocoupler transfer function (figure 4). Because of equation 1 the gain is reduced to near 0dB at lower frequencies and it tracks the open loop gain at higher frequencies. Adding an inner loop by connecting the optocoupler to the regulated output voltage has an effect on the gain.

Bode plot

The advantage of the inner loop is that there is a reduction of the output impedance and an improvement of the transient response behavior. The system is faster because the information must not go through the error amplifier.

Compensating the closed inner loop is done by using a typ I or typ II network. For a more elaborate analysis, see Ref [2].

Multiple Output Flyback Design

Flyback converters are often used in power supplies requiring several output voltages. Each additional output only requires another transformer winding, rectifier and output capacitor.

Only one output is tightly regulated while the others are controlled via less accurate transformer actions. If perfect coupling between windings was possible, the output voltage would be defined by the winding ratio of the transformer. Unfortunately, perfect winding coupling is impossible, which often results in poor cross-regulation. There are two critical states.

First, the light load operation of the auxiliary windings while the main output is fully loaded. Due to transformer leakage inductance and parasitic capacitance, the secondary voltage tends to ring. If the auxiliary output is fully loaded, this ringing is clamped. If not, this ringing will charge up the output capacitor. This results in a much higher auxiliary output voltage. Solution of this problem can be the use of a minimum load, a post regulator or a Zener diode to clamp the voltage.

The other critical state occurs, if the load of the regulated output is very low. This condition can be the reason for fully unregulated auxiliary outputs.

Figure 5 shows an example of a multiple output Flyback Design.

Multiple output flyback converter

Problems can occur if the maximum output power of the regulated output (Vout1) is small compared to the total output power. The auxiliary output (Vout2) can be unregulated if the current from the regulated output is low. Decreasing the gain of the compensation network to achieve a very slow system sometimes doesn’t help to regulate Vout2. Disconnecting the optocoupler from Vout1 and supplying it with a constant voltage in order to remove the inner loop, usually does not lead to success also.

There is another method to get the design working. The inner loop must be taken from the semi-regulated output. Therefore the optocoupler is connected to the output Vout2. Vout1 is regulated from the outer voltage loop and Vout2 is regulated from the inner loop (see Figure 5). The combined signal contains the error information of the two outputs and the optocoupler transfers it to the primary. This signal sets the power switch duty cycle and therefore the primary side current.

TI offers a Quasi Resonant pulse width modulation controller which contains all of the features needed to implement a highly efficient off-line power supply (for example UCC28600). There are also current mode controllers with very low standby power available, for more details please visit www.ti.com.

Conclusion

Flyback switch mode converters are very popular due to their low cost and simplicity. Unfortunately the regulation of the auxiliary outputs can be bad in some applications. There are techniques to tighten the output regulation. The secondary windings or the output can be stacked or the feedbacks can be combined. Sometimes this results in better cross regulation. The above described way to connect the inner loop gives an additional possibility to improve the performance of the auxiliary outputs.

References
[1] “Fast Analytical Techniques for Electrical and Electronic Circuits”, Vatche Vorperian, Cambridge University Press 2002, ISBN 0 521 62442 8.
[2] “Closing the Loop with a Popular Shunt Regulator”, Robert Kollmann, John Betten, Texas Instruments, www.powerelectronics.com

For more information on the used parts and technologies described here please visit http://www.ti.com.

 

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