Synchronous rectification DC-DC module power
1 OverviewDC-DC power module to meet the miniaturization requirements of the general will choose a simple and reliable power stage circuit, in which the resonant reset forward + synchronous rectification DC-DC power module in the application of more extensive, following the example of this circuit analysis.2, the basic synchronous rectifier circuitThe secondary side synchronous rectification circuit for the basic, key waveforms shown in Figure 2. When the primary master switch Q1 open, through the transformer T1 to the secondary transmission of energy of both sides of the state work in the rectifier, then the Vgs voltage of SR1 transformer secondary voltage, polarity is positive, SR2 the Vgs voltage to zero , and thus turn SR1, SR2 off; when the primary master switch Q1 is off, the transformer T1 primary winding excitation current and load current flows through C1, C1 on the voltage began to rise when the C1 voltage rises when Vin , the primary winding of the load current drops to zero, under the effect of the excitation current primary magnetizing inductance Lm and the resonant capacitor C1, the resonant voltage Vr is the sine wave, the resonant period Tr = 2π √ LmC2, resonant transformer voltage Vr applied on the primary winding of T1 to T1 magnetic reset, while the secondary also entered into the freewheeling state, then the Vgs voltage of SR1 0, SR2 the Vgs voltage of the transformer secondary voltage, the voltage sine wave, polarity is positive, and therefore off SR1, SR2 conduction; this kind of work status periodically repeated3, the basic problem of synchronous rectifier circuit3.1, continued flow tube driveFigure 2, the Vgs waveform SR2, SR2 is due to drive voltage sine wave resonance, by the main switch duty cycle and resonance parameters, large changes in voltage waveform to drive results are not satisfactory, the module efficiency is low.3.2, the output in parallelThe two basic synchronous rectification circuit using a DC-DC module power output will have a lot of problems in parallel, one serious problem is the "current anti-irrigation." The following example shows a simple "current anti-irrigation" phenomenon. Shown in Figure 3, when the normal work Module 2 Module 1 is turned off, the module's output voltage VOUT 2 module 1 through the inside of the L, T1's secondary windings were added to the SR1, SR2 of G, S between , SR1, SR2 and will therefore turn a larger current flow through, while module 2 of the output voltage VOUT will be pulled down. For module 1, the time of the reverse current is flowing into the module, called "current anti-irrigation" phenomenon. In the N-module parallel system, set the maximum output current of each module is Io, when one module is shut down, the flow of this module will sink to the anti-(N-1) × IO, this will serious consequences.4, improved synchronous rectification4.1 Circuit DescriptionImproved synchronous rectifier circuit shown in Figure 4, the secondary-side synchronous rectifier SR1 moved to the top, SR1, SR2 use of drain connection, taken from the transformer N1, N2 windings, N1 winding for driving SR1, N2 winding through the half-wave rectifier used to drive SR2, the primary synchronization signal SYNC by isolation, drive low-power MOSFET S1, for off SR2. Isolation driving circuit which can be used similar to Figure 5 a typical circuit. Critical signal timing relationships shown in Figure 6.4.2, continued flow tube driveImproved synchronous rectifier circuit by way of half-wave rectifier driver SR2, the drive signal through diode D1 to SR2 of G, S Ci equivalent capacitance between the charge, the MOSFET gate input impedance of a large, Vgs will remain the driving signal peak unchanged until the SYNC signal on-S1, the SR2 of G, S let go between charges. Close to the Vgs waveform thus SR2 square wave, and can maintain the freewheeling end of the process. Improved efficiency will be higher.4.3, the output in parallelImproved synchronous rectification circuit output can support multiple modules in parallel. Shown in Figure 7, the use of separate windings N1, N2-driven synchronous rectifier SR1, SR2, synchronous rectifier gate and the output VOUT is no direct contact, when the module 1 shutdown, SR1, SR2 driving voltage are 0, the equivalent of diode characteristics. Work in other states, such as starting, standby, the dynamic load such circumstances, the module can work in parallel.5, the application of the resultsImproved synchronous rectification applications in 48V input, 5V @ 20A output DC-DC power modules supply, efficiency can reach 90%. Shows the synchronous rectifier during normal operation of the drive waveform, in which Channel 1 is the continued flow tube drive waveform, channel 2 is the rectifier drive waveform. Shows two drive waveforms both to ensure the appropriate dead in order to avoid direct, but also through the diode can minimize the time, and thus the high efficiency synchronous rectification. Shows the two modules in parallel, when one module is shut down, the output voltage waveform on the parallel bus, where channel 1 is the module 1 shutdown signal, channel 2 is the output voltage waveform on the parallel bus. Can be seen when one module shutdown, the output voltage on the parallel bus is not affected. Figure 10 shows the output of a single module in the case of light load and no-load output voltage waveform shutdown, shutdown can be seen in the flat after the module output voltage drop, will not oscillate, its features and power Schottky rectifier modules is consistent .6 SummaryIn this paper, the basic application of synchronous rectification in a number of problems were analyzed, and proposed an improved synchronous rectification technology and specific circuit, the technology has been applied in a series of industry-standard brick DC-DC module power supplies, and demonstrated the practical application of excellent performance and compatibility.