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Power tip: Selecting rectifiers for flybacks

Posted: 01 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:power supply? output voltage regulation? flyback? resistors? control?

In Power tip: Weighted feedback in multiple-output supplies, we tackled the use of 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.

Figure 1: Cross regulating a two-output flyback saves money at the expense of regulation.(Click on image to enlarge)

Next, pick the rectifier style from an ultra-fast recovery diode, Schottky diode, or MOSFET. Figure 2 shows I-V characteristics of each at C40 and +125C 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 minimumother 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 0.6V-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.

Figure 2: MOSFETs can mitigate large rectifier voltage variations which ruin cross regulation. (Click on image to enlarge)

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