Power Tip 72: Select the right rectifiers for multiple output flybacks
In Power Tip 70, we discussed using weighted feedback to improve cross regulation in multiple output flybacks. Choosing the output rectifiers is another important design aspect as they influence cross regulation over load and temperature, efficiency, and transformer design. Today let’s look at these impacts in a flyback with a 3.3 V/0.5 A and a 5 V/2 A output.
In the circuit shown in Figure 1, we chose rectifiers D1 and D3 to determine the turns ratio between the 3.3 V and 5 V windings. First you need to decide on the rectifier’s required voltage rating. The voltage rating is determined by the difference between the output voltage and reflected voltage from the primary, plus an allowance for ringing and derating. In this case, 30 V diodes are suitable for the 3.3 V output, and 40 V diodes are needed for the 5.0 V output.
Cross regulating a two-output flyback saves money at the expense of regulation.
Next, pick the rectifier style from an ultra-fast recovery diode, Schottky diode, or MOSFET. Figure 2 shows I-V characteristics of each at -40 and +125°C temperatures over a wide current range. Usually, you are given a maximum current that your power supply will be asked to deliver, but there is little guidance on the minimum — other than that the supply sometimes will have no load. These large variations in voltage drop play havoc with regulation, as we will see, especially at low voltage.
Many times the designer puts preload resistors in the power supply to help constrain the load range. Also important is that, in the discontinuous flyback being considered, the diode peak current is four times the output current. So a 1 amp output means 4 amps peak in the rectifier. In Figure 2, the fast recovery diode has the largest voltage drop and a 0.6-volt-wide variation over the expected current range. The Schottky diode voltage drop is lower than the fast recovery diode, although it still varies more than 0.4 V over current. The MOSFETs have the lowest drop and the smallest variation. However, they cost the most and require drive circuitry, which further adds to the cost and increases the power supply size.
MOSFETs can mitigate large rectifier voltage variations which ruin cross regulation.
Once you understand the voltage drops associated with the rectifiers, next determine the number of turns for the transformer secondary. The available flux swing due to saturation or core loss sets the minimum number of turns. With this minimum number as a starting point, you can construct Table 1. In case 1, this table starts with a one-turn winding between the transformer 3.3 V and 5 V outputs, and it calculates the 5 V output based on a perfectly regulated 3.3 V.
Large rectifier voltage variations ruin cross regulation.
The 5 V output rises to 5.2 V (or 4% set-on error) with nominal voltage drops for the diodes and a two-turn 3.3 V winding. The error is worse due to circuit parasitics like transformer resistances and inductances.
Case 2 shows the voltage extremes due to a heavily loaded 3.3 V and a lightly loaded 5 V. Now the error is 13%. Cases 3 and 4 replace the Schottky on the 5 V output with a fast-recovery diode with less than stellar results. The nominal case is somewhat improved, but the worst case has dropped to 20%.
In cases 5-8, I adjusted the turns to try to find a better combination. Cases 7 and 8 are clearly better with about a 5% set-on error. The tradeoff in the number of turns is the secondary copper loss is much greater. Each turn occupies half the area, increasing the resistance per turn by two, and the winding length is twice, which doubles its resistance. For improvement in set-on error, the secondary loss increases by four. Case 9 shows the best results will be obtained with the MOSFETs, due to their low voltage drop. At nominal drops, there is no set-on error.
To summarize, rectifier drops can greatly degrade across regulation in multiple-output, low-voltage flybacks. The problem is further complicated if the load currents are wide ranging, which results in large rectifier forward voltage swings. Preloads may help to mitigate the load range at the expense of power loss and efficiency. The best regulation results are achieved using rectifiers with low-voltage drops.
MOSFETs used as synchronous rectifiers give the best results at the expense of cost and circuit size/complexity. Schottky diodes are the next best choice, followed by junction diodes. Additionally, the selection of transformer turns is an iterative problem, which becomes a tradeoff between set-on accuracy and losses in the transformer.
For more detail on this design, check out our PowerLab posting. While you are there, examine some of the other 1,400 reference designs that we have built, tested, and documented for your use. Check out TI Power Lab Notes for a designer’s perspective on his power supply designs.