Synchronous rectification technology DC-DC module power supply

1. Overview

2. Basic synchronous rectification circuit

As shown in Figure 1, the secondary side of the circuit is a basic synchronous rectification circuit, and the key waveforms are shown in Figure 2. When the primary main switch Q1 is turned on, energy is transmitted to the secondary side through the transformer T1, and the secondary side works in the rectification state. At this time, the Vgs voltage of SR1 is the voltage of the secondary winding of the transformer, and the polarity is positive. The Vgs voltage of SR2 is zero, so SR1 is turned on and SR2 is turned off; when the primary main switch Q1 is turned off, the excitation current and load current of the primary winding of the transformer T1 flow through C1, and the voltage on C1 begins to rise. When the voltage of C1 rises to Vin, the load current in the primary winding drops to 0. Under the action of the excitation current, the primary excitation inductance Lm resonates with the capacitor C1, and the resonant voltage Vr is a sine wave. The resonance period Tr=2π√LmC2. The resonant voltage Vr is added to the primary winding of the transformer T1 to reset the magnetic field of T1. At the same time, the secondary side also enters the freewheeling state. At this time, the Vgs voltage of SR1 is 0, and the Vgs voltage of SR2 is the voltage of the secondary winding of the transformer. The voltage waveform is a sine wave with a positive polarity, so SR1 is turned off and SR2 is turned on; this working state will be repeated periodically.

3. Problems with basic synchronous rectification circuits

3.1. Driving the freewheeling tube

As shown in Figure 2, the Vgs waveform of SR2, since SR2 is driven by a sinusoidal resonant voltage, affected by the duty cycle and resonant parameters of the main switch, the voltage waveform changes greatly, the driving effect is not ideal, and the module efficiency is low.

3.2 Output parallel connection

Connecting two DC-DC power modules with basic synchronous rectification circuits in parallel will cause many problems, one of which is "current backflow". The following is a simple example to illustrate the "current backflow" phenomenon. As shown in Figure 3, when module 2 is working normally and module 1 is turned off, the output voltage VOUT of module 2 will be added to the G and S of SR1 and SR2 respectively through the secondary windings of L and T1 inside module 1. SR1 and SR2 will be turned on and a large current will flow through them. At the same time, the output voltage VOUT of module 2 will be pulled down. For module 1, the current at this time flows into the module in the reverse direction, which is called the "current backflow" phenomenon. In a system with N modules connected in parallel, assuming that the maximum output current of each module is Io, when one of the modules is turned off, the backflow current flowing into this module will reach (N-1)×IO, which will have serious consequences.

4. Improved synchronous rectification circuit

4.1 Circuit Description

The improved synchronous rectification circuit is shown in Figure 4. The secondary synchronous rectifier SR1 is moved to the upper end. SR1 and SR2 are connected in a common drain manner. The N1 and N2 windings are extracted from the transformer. The N1 winding is used to drive SR1. The N2 winding is used to drive SR2 after half-wave rectification. The primary synchronous signal SYNC is isolated and drives the low-power MOSFET S1 to turn off SR2. The isolation drive circuit can use a typical circuit similar to Figure 5. The timing relationship of the key signals is shown in Figure 6.

4.2. Driving the freewheeling tube

The improved synchronous rectification circuit drives SR2 by half-wave rectification. The driving signal charges the equivalent capacitor Ci between G and S of SR2 through diode D1. Since the input impedance of the MOSFET gate is large, Vgs will keep the peak value of the driving signal unchanged until the SYNC signal turns on S1 to discharge the charge between G and S of SR2. Therefore, the Vgs waveform of SR2 is close to a square wave and can be maintained until the end of the freewheeling process (see the Vgs waveform of SR2 in Figure 6). The improved efficiency will be higher.

4.3 Output parallel connection

The improved synchronous rectification circuit can support multiple modules in parallel. As shown in Figure 7, since separate windings N1 and N2 are used to drive synchronous rectifiers SR1 and SR2, the gate of the synchronous rectifier has no direct connection with the output terminal VOUT. When module 1 is turned off, the driving voltages of SR1 and SR2 are both 0, which is equivalent to the diode characteristics. In other working states, such as startup, standby, dynamic load, etc., the parallel modules can also work normally.

5. Application results

The improved synchronous rectification technology is applied to the DC-DC module power supply with 48V input and 5V@20A output, and the efficiency can reach more than 90%. Figure 8 shows the driving waveform of the synchronous rectifier tube during normal operation, where channel 1 is the driving waveform of the freewheeling tube and channel 2 is the driving waveform of the rectifier tube. It can be seen that the driving waveforms of the two tubes not only ensure the appropriate dead zone to avoid direct conduction, but also minimize the conduction time through the diode, so the efficiency of synchronous rectification is very high. Figure 9 shows the voltage waveform on the output parallel bus when two modules are connected in parallel and one of the modules is shut down, where channel 1 is the shutdown signal of module 1 and channel 2 is the voltage waveform on the output parallel bus. It can be seen that when one of the modules is shut down, the voltage on the output parallel bus is not affected. Figure 10 shows the output voltage waveform of a single module when it is shut down under light load and no load. It can be seen that the output voltage of the module drops smoothly after shutdown, and there will be no oscillation. Its characteristics are basically consistent with the module power supply of Schottky rectification.

6. Summary

This article analyzes some problems existing in the application of basic synchronous rectification technology, and proposes an improved synchronous rectification technology and specific circuit. This technology has been applied in the brick series DC-DC module power supply with industrial standards, and has shown excellent performance and compatibility in practical applications.