Here are 3 "secrets" to help you improve the efficiency of isolated power supplies

Latest update time：2020-02-24

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How to improve the efficiency of isolated power supplies?

In most typical applications of buck regulators, it is standard 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 in a forward converter.

Driving the switch for synchronous rectification can be achieved in different ways -

0 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 can occur in the circuit. For example, if the capacitors at the output of the circuit are precharged before the circuit is powered on, current can flow from the output side to the input side. Negative currents can increase the voltage across MOSFET SR1 and MOSFET SR2, causing damage to them. Care must be taken to protect the switches from such events.

0 2

Figure 2 shows one way to implement 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 current from flowing through the external switches. First, the device needs to quickly detect the negative current; then, the SR1 and SR2 switches need to turn off quickly. This is necessary to prevent circuit damage during startup or possible burst mode operation.

0 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 current flowing through the SR1 and SR2 switches in certain situations (such as when the output voltage is precharged), the synchronous rectification is not activated. The body diodes of both switches perform the rectification. This prevents damage to the switches. Safe operation without negative current flow is achieved using the iCoupler® technology built into the ADP1074.

Figure 3. Synchronous rectification in a forward topology is achieved through full integration with the ADP1074.

Have these three "secrets" helped you? Please leave a message below and give your opinion.

ADP1074

Current Mode Controller for Active Clamp Forward Topology
Integrated 5 kV (wide-body SOIC package) or 3.0 kV (LGA package) dielectric insulation voltage rating,
using ADI’s patented iCoupler technology
Wide supply voltage range

Primary V IN : Up to 60 V
Secondary side V DD2 : up to 36 V

Integrated 1A primary MOSFET driver for power switch and active clamp reset switch
Integrated 1A Secondary MOSFET Driver for Synchronous Rectification
Integrated Error Amplifier and < 1% Precision Voltage Reference
Programmable slope compensation
Programmable frequency range: 50 kHz to 600 kHz (typical)
Frequency synchronization
Programmable Maximum Duty Cycle Limit
Programmable soft start
Smooth start from pre-charged load
Programmable dead band
Power saving light load mode using MODE pin
Protection functions such as short circuit, output overvoltage and over temperature protection

Cycle-by-cycle input overcurrent protection

Precision Enable UVLO with Hysteresis
PGOOD pin for system flag
Secondary side tracking function
Remote (secondary side) shutdown/reset function
Safety and regulatory certification (pending)

DIN V VDE V 0884-10 (VDE V 0884-10):2006-12
V IORM = 849 V peak
V IORM = 560 V peak (for LGA package)
5000 V rms for 1 minute, in accordance with UL 1577

UL Certification:
CSA Component Acceptance Notice 5A
VDE certificate of conformity

CQC certification complies with GB4943.1-2011
Available in 24-pin SOIC_W and 24-pin LGA packages

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