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High-Efficiency Switching Power Supply Unit with Synchronous Rectifier Rectification method Input voltage (V dc) Output voltage (V dc) Output current (A) Output power (W) Efficiencv (%) Y. Nakayash i k i , H. Sh imamor i , T. Satoh. T. Ohno Fujitsu Limited 4-1-1 Kami kodanaka, Nakahara-ku Kawasak i -c i ty, 21 1-8588, Japan Schottky Synchronous (SBD) (Sync Rect) 3.6 2.5 / 3.3 switchable 300 360 1080 1188 75 81 barrier diode rectifier 200-373 240-400 1. Introduction Power loss (W) Size W / H I D (mm) Power output per unit volume (W I Liter) Air flow (msl min) Weight (kg) Operating temperature This report describes an insulation type switching power supply unit (DC/DC converter) for mainframe computers that uses a newly developed synchronous rectifying circuit for the secondary rectification component. Compared with conventional switching power supply units, this unit offers 6% higher processing efficiency, and power output per unit volume that has been boosted by 1.75 times. As a result, this unit provides high efficiency, low voltage, and high power output. Table 1 shows the summary of specifications. The unit is illustrated in Figure 1. 360 278 9.1 (Liter) 5.7 (Liter) 120 1381 1200 60 I381 1250 119 208 8.5 5.3 2.4 1.5 0 - 45 0 - 45 Table 1 Specifications 2. Background In recent years, in LSI technology, there has been a shift from the use of transistortransistor logic (TTL) and emittercoupled logic (ECL) to the use of highly integrated complementary metal-oxide semiconductors S. Yamash ita, K. Fuoh i gam i , T. Yamamoto. Fujitsu Denso Limited 1-1 7-3 Sakado, Takatsu-ku. Kawasak i-c i ty, 21 3-8586, Japan (CMOS). At the same time, the logic component of CPUs, memories, and the like have been undergoing miniaturization at a relatively rapid pace. However, since it the power supply component is difficult to integrate, it is beginning to occupy most of the space inside the mainframe. Until now, the water cooling method has been used to cool the logic component. Because the water cooling method is highly effective, it has been used in power supply unit too. However, using CMOS reduces the total amount of electric power used by the logic component, making the water cooling unit (an expensive component) miniaturizing the power supply component unnecessary. Therefore, it has become all the more prohibitively expensive to use a water cooling unit just for the power supply component alone. It has become necessary to miniaturize the power supply unit by reducing the internal loss of the power supply and Figure 1 Previous (left) and new unit (right) 0-7803-5069-3 198/$10.00 01998 IEEE 398 improving the processing efficiency of the unit. In addition, the powersupply voltage has been decreasing over time, from +5.0V to +3.3V, and then to +2.5V. Moreover, the loss of the secondary rectification component made up of the Schottky barrier diode(SBD) has come to be relatively important. We therefore concluded that reducing the loss of the secondary rectification component would be the most important factor in the miniaturization process, and developed a switching power supply unit that uses the synchronous rectifying circuit. 3. Conventional circuit and loss analysis Figure 2 shows an outline of the switching supply circuit, and Figure 3 shows the result of the loss analysis. These figures indicate that the loss of the secondary rectification component occupies more than 40% of the total loss. From the point of view of academic analysis, the results of this analysis suggested that using the synchronous rectifying circuit would lead to the improved processing efficiency shown in Figure 4. (7-F SBD Figure 2 Circuit of conventional unit 90 I I Q 80 W ic ! . i . .y . . i . . . i . . i . . . j 0 , : :a . r)v 0 1 2 3 45 6 Output voltage (VI Synchronous rectifier (Sync Rect) - SBD rectifier - Figure 4 Efficiency comparisons 4. Current circumstances of Sync Rect The fundamental concept of synchronous rectification has long existed. However, its practical application as a product has taken place only recently. Synchronous rectification in mobile equipment, beginning with laptop computers, is an important technology useful in extending the service life of the batteries that power such devices. Since it is relatively easy to control the circuit for the synchronous rectification operation when the non- insulation mode is used, thereby facilitating the switch to the use of ICs, synchronous rectification is used for the majority of DC/DC converters in CPUs. However, it is difficult to control the circuit for the synchronous rectification operation when the insulation method (required for the transformer) is used. Consequently, the problems given below were encountered. Accordingly, the insulation method is only used with specific products. 5. Problems encountered with Sync Rect Figure 5 shows the synchronous rectifying circuit generally used. There are a number of problems associated with this circuit. Moreover, the processing efficiency was not as good as we had anticipated. Furthermore, another serious problem involving parallel operation was encountered. Figure 3 Power loss comparisons 399 INPUT - OUTPUT Figure 5 Circuit of previous synchronous rectifier 1) Insufficient driving of low side switch As Figure 6 illustrates, when the main switches(Q01,2 in Figure 5) are turned off, the transformer is reset, resulting in the elimination of the generated voltage. Then, the low side switch Q2 is turned off, the current flows into the BD2 (which is a body diode of Q2), leading to an increase in the loss. The Schottky barrier diode D2 therefore needs to be mounted parallel with Q2 to increase the physical quantity. shortened and the amount of time during which the current flows into the D2 is extended. Since the processing efficiency achieved is almost the. same as that obtained with the SBD rectifying circuit, the positive effects of using the synchronous rectifying circuit are not very significant. 2) Current detour at-parallel operation Figure 7 shows the units operating in parallel without using the blocking diode. When the main switches Q01,2 of one unit are turned off, the other unit outputs and applies the voltage to the Q2 gate. As a result, one unit (Q2) is turned on. Then high current flows through the choke coil into the drain. This results in the Q2 being damaged and the output voltage being reduced. Also, when the time of the generation of the poweron and poweroff signal differ between the units, it causes the same symptom to be generated, resulting in damage to the unit. Power supply f Other unit outputs I A 4 I Y output I :- I I: D2 -f on Figure 7 Parallel operation F - 7 - : I I Vt 0 plb-r . L. v g s q Low I + magnifL vgs ov : Power dissipation Figure 6 Waveforms of previous circuit When the range of the input voltage of the power supply TofF-B Toff-A is broad and such input voltage is high, the reset time is Figure 8 2-types of gate waveforms 400 3) Delay of turning off time In most of the currently developed MOS-FET, the threshold voltage Vth of Vgs is reduced from 4 V to 2 V. As Figure 8 illustrates, when turning off Q1 and Q2 by OV gate voltage (slope A), the moderate part of the slope of the transient phenomena indicates the threshold. The loss is increased by the pass current that flows from the transformers due to the delay in the turning off time. 4) Surge voltage Since the operation of synchronous rectifying circuit is bi-directional, the current of the choke coil L1 will not be discontinuous. When the load current is small, the L1 current flows in the reverse direction. Even the slightest delay in turning on the high side switch Q1 causes the current route of L1 to be cut, and the surge voltage to be generated, as Figure 9 illustrates. As a result, Q1 and Q2 incur damage. I 3odv diode l - I + T Figure 9 Mechanism of producing a surge voltage 5) Circulating current in parallel operation When the units with the synchronous rectifying circuit are operated in parallel, if there is even the slightest difference in the output voltages, the large current flows from the higher output voltage unit into the lower output voltage unit. (This is illustrated in Figure 10.) The current is fed back to the primary side by the U2 inverter, after which it is once again returned to the secondary side by the U1 inverter. As a result, a large circulating current flows between U1 and U2. Therefore, even with a light-load current, the loss generated in the unit is as extensive as that occurring with heavy loads. I12 Iout I1 , 1 I I IOUt = I2 - I3 Figure 10 Root of circulating current 6. Solving problems and the new circuit The following describes a solution to the problems described in Section 5. Figure 11 shows the new synchronous rectifying circuit. Figure 13 is a diagram illustrating the configuration at parallel operation. 1) Insufficient driving of low side switch Figure 11 shows Q2 and its driving switch Q3. As Figure 12 illustrates, when Q01,2 are turned off and it generates the transformer reset voltage, Q3 is turned off and the positive voltage is applied to the Q2 gate through D3. When the reset is completed and the voltage is removed, D3 is turned off and the Q2 gate is disconnected. At this point, Q2 holds the electric charge in Ciss between th gate and the source and stays on. As a result, the Schottky barrier diode connected to Q2 in parallel becomes unnecessary. The synchronous rectifying circuit is able to minimize the loss on its own. t 2) Current detour at parallel operation Since both Q1 and Q2 use separate windings (N2 and N3) for the drive as shown in Figure 1 1 , a sneak current from other unit can be avoided, and they can operate in parallel. 3) Delay of turning off time As shown in Figure 11, winding of transformer Q1 is directly connected between the gate and the source. When Q1 is turned off, a negative voltage can be applied to the gate. In the transformer of Q2, positive voltage is -. I *Q4 is controlled by the current of L1 Figure 11 New circuit 4) Surge voltage When a current equivalent to the current of L1 is detected (the transformer current is detected in Figure ll), Q4 is turned off before the current of L1 or the equivalent flows in the negative direction. By turning off Q4, the reverse voltage is applied to the Q2 gate and Q2 is turned off. BD2 then operates the rectification instead of Q2. Turning off Q2 before the choke coil current flows in the negative direction can stop the negative current of L1 and avoid the surge voltage. At this time, the forward drop of the rectifier increases from 4 3 on 0.2 V to 1 V. However, because the load current is small, no thermal failure occurs. 0 . . . . . . Vds QO tLLr-Lr 0 Off Off Gs; A 0 vgs2 5) Circulating current in parallel operation The circulating current is eliminated by using the separate winding for the gate driving and stopping the synchronous rectifier in the cut-off area of the choke coil. 7. Characteristics Figure 12 Waveform of new circuit Figure 14 shows the measured results of the internal loss and the efficiency of this new unit. As expected for the results in Figure 4, the loss is reduced by 23% and the efficiency is improved by 6%. generated when Q01,2 are turned on. Thereby, the gate charge is discharged through Q3. Then the reverse voltage is applied to the Q2 gate and Q1 is promptly turned off. (slope B in Figure 8) 402 17- I I 31 I I I I I i I 90