Active Shunt Regulator and Pre-load
In the world of line-voltage-AC to low-voltage-DC switching power supplies, the flyback is by far the most popular topology. One main reason for this is the cost effectiveness with which multiple output voltages can be provided by simply adding additional windings to the transformer secondary.
Typically, feedback is taken from the output requiring the tightest output tolerance. That output then defines the turns-per-volt for all other secondary windings. Due to the effect of leakage inductance, it is not always possible to achieve the required output voltage cross regulation, especially when a given output may be unloaded or only lightly loaded when other outputs are fully loaded.
Post regulators or pre-loads may be used to prevent an output's voltage from rising under those conditions. However, the higher cost and the reduction in efficiency that post regulators and pre-loads cause have made them less attractive, especially with respect to the recent regulatory requirements for low no-load and/or standby input power consumption in many consumer applications. The active shunt regulator shown in figure 1 solves the regulation problem while it minimizes the cost and efficiency impacts.
Figure 1. Active shunt regulator for multiple output flyback converters
The circuit operates as follows. While the outputs are both within regulation, resistor divider R14 and R13 bias transistor Q5 on, which keeps Q4 and Q1 turned off. When in this mode of operation, the current through Q5 acts as a small pre-load for the 5 V output.
The nominal difference between the 5 V output and the 3.3 V output is 1.7 V. When the load demands additional current from the 3.3 V output without the same increase in load current being drawn from the 5 V output, its output voltage will increase with respect to that of the 3.3 V output. As that difference exceeds about 100 mV, Q5 becomes biased off, which turns on Q4 and Q1 and allows current to flow from the 5 V output to 3.3 V output. This current lowers the voltage of the 5 V output, which reduces the difference between the two outputs.
The amount of current that flows in Q1 is determined by the difference in the two voltages. Therefore, this circuit helps to keep both outputs in regulation, regardless of their loading; even in the worst case condition of the 3.3 V output being fully loaded while the 5 V output is unloaded. The arrangement of Q5 and Q4 provides temperature compensation, because the VBE temperature variations of each transistor cancel the other out. Diodes D8 and D9 are not required but reduce the dissipation in Q1, which removes the need for it to have a heatsink.
As the circuit only reacts to relative differences between the two voltages, it is largely inactive at both full load and light load. Because the shunt is connected from the 5 V output to 3.3 V output, the active dissipation of the circuit is reduced by 66% when compared to a shunt regulator that is connected to ground. The result is that efficiency remains high at full load and light-to-no-load power consumption is kept low.