Ways to improve the efficiency of isolated power supplies?

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Ways to improve the efficiency of isolated power supplies? [Copy link]

In most typical applications of buck regulators, it is standard practice to use active switches instead of Schottky diodes. This can greatly improve conversion efficiency, especially when generating low output voltages. In applications where galvanic isolation is required, synchronous rectification can also be used to improve conversion efficiency. Figure 1 shows a forward converter with synchronous rectification on the secondary side. Figure 1. Self-driven synchronous rectification of a forward converter Driving the switch for synchronous rectification can be achieved in different ways - 1.A simple method involves driving across the secondary winding of the transformer. This is shown in Figure 1. In this case, the input voltage range may not be very wide. When using the minimum input voltage, the gates of SR1 and SR2 need to have enough voltage so that the switches can turn on reliably. To ensure that the gate voltage of MOSFET SR1 and MOSFET SR2 does not exceed their maximum rated voltage, the maximum input voltage cannot be too high. In all power supplies with synchronous rectification, negative currents may be generated in the circuit. For example, if the capacitors at the output of the circuit are precharged before the circuit is powered on, current may flow from the output side to the input side. Negative currents can increase the voltage across MOSFET SR1 and MOSFET SR2, causing them to be damaged. Care must be taken to protect the switches from such events. 2.Figure 2 shows a method for implementing synchronous rectification using the LT3900. This controller drives the synchronous rectification switches SR1 and SR2 in a forward topology. Figure 2. Synchronous rectification of a forward converter with a dedicated driver IC. This concept works well. However, the LTC3900 needs to prevent negative currents from flowing through the external switches. First, the device needs to detect negative currents quickly; then, the SR1 and SR2 switches need to be turned off quickly. This is necessary to prevent circuit damage during startup or in possible burst modes. 3.Figure 3 shows a very elegant circuit design using the new ADP1074. The output voltage information is sensed via the feedback pin. To prevent the risk of negative currents flowing through the SR1 and SR2 switches under certain conditions (such as when the output voltage is precharged), synchronous rectification is not activated. The body diodes of the two switches perform the rectification. This prevents damage to the switches. The iCoupler technology built into the ADP1074 allows safe operation without negative current flow. Figure 3. Synchronous rectification in a forward topology is achieved through full integration with the ADP1074.