WO2010110802A1

WO2010110802A1 - Rectifier circuit with reduced power dissipation

Info

Publication number: WO2010110802A1
Application number: PCT/US2009/038611
Authority: WO
Inventors: Prassad R. Atluri; William H. Webb
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2009-03-27
Filing date: 2009-03-27
Publication date: 2010-09-30
Also published as: TW201041291A

Abstract

A rectifier circuit for use in a power supply to convert ac input power to dc output power to supply a dc load provides for reduced rectifier circuit power dissipation. The power supply includes a first ac source input line, a second ac source input line, a DC power supply line, and a DC power return line. The rectifier circuit includes a first field effect transistor (FET) having its drain coupled to the first ac source input line and its source coupled to the DC power return line, and a second FET having its drain coupled to the second ac source input line and its source coupled to the DC power return line. During an ac cycle, the first FET closes during the positive half cycle and the second FET closes during the negative half cycle.

Description

RECTIFIER CIRCUIT WITH REDUCED POWER DISSIPATION

Diode bridge rectifiers are widely used in power supplies where input ac power is converted to dc output power. Such applications include computer power supplies. For power supplies operating in the 1000 watt to 2000 watt range (e.g., mid to high end servers), the power dissipated (lost) in a diode bridge rectifier is in the range of 20 to 25 watts for a low line (100 volt) ac input source. Higher values of power dissipation reduce power supply efficiency, and increase heat generation, with a corresponding requirement to increase cooling capacity through larger heat sinks or increased cooling air flow.

Description of the Drawings

The Detailed Description will refer to the following drawings in which like numerals refer to tike objects, and in which;

Figure 1 illustrates an exemplary rectifier circuit, having reduced power

Figure 2 illustrates an alternate, exemplary rectifier circuit with reduced power dissipation; and Figure 3 illustrates another alternate, exemplary rectifier circuit with reduced power dissipation.

Detailed Description

Diode bridge rectifiers are widely used in power supplies where input ac power is converted to dc output power. Such applications include computer power supplies. In a computer power supply, a forward diode bridge rectifier circuit may include four diodes. Assuming a near unity power factor and that the current input to the bridge rectifier circuit is in phase with the voltage input to the bridge rectifier circuit, during the positive half cycle, two of the four diodes are forward biased and the input current flows through these two diodes. During the negative half cycle, the other two diodes are forward biased, and current flows through these two diodes to cany the load current from source to load. In tins circuit, the forward voltage drop is 1.0 volts or more. Consequently, the power dissipation in this diode bridge rectifier will be in the range of 20 to 25 watts at 100 amps load. These conditions may be expected for power supplies operating in the 1000 watt to 2000 watt range (e.g., mid to high end servers). Thus, the power dissipated (lost) in a diode bridge rectifier is a significant portion of the available power, and results in lower efficiency of the power supply. In addition to lowering efficiency, increasing power dissipation increase heat generation, with a corresponding requirement to increase cooling capacity through larger heat sinks or increased cooling air flow.

Figure 1 illustrates a switching power supply 100 having an alternating current source 110, a dc output load 120, and an exemplary rectifier circuit 200, among other components- Because of differences in ac input sources, especially between the U.S. and certain foreign countries, the ac source 110 may provide voltages that range from 90 volts rms to 264 volts rms. In Figure 1, the ac source is supplied by one phase of a three phase supply and has a nominal voltage of 208 volts. The load 120 may be a computer system such as a mid to high end server. The switching power supply 100 converts the 208 volt rms ac input to a 12 volt dc output for supply to the load 120. The power supply 1.00 also includes first and second ac source input lines 1.12 and 113, respectively, DC power supply line 222, and DC power return line 223.

The rectifier circuit 200 includes diode D2 (202) D3 (204), Do (206) and D7 (208X arranged as shown. The rectifier circuit 200 also includes MOSFETs Q4 (212) and Q5 (214). Associated with MOSFET 212 is control circuit 213, including diode D4, and associated with MOSFET 2.14 is control circuit 215, including diode D5. The immediate input to the rectifier circuit 200 is through the first and second ac source lines 112 and 113. The immediate output of the rectifier circuit 200 is fed to power factor correction circuit 250. Between the power factor correction circuit 250 and the load 120 are various other power supply components including an isolation transformer. Current flow in the power supply 100 is, in the positive half cycle of the alternating current supply, from ac source 110 through ac input line ϊ 12 to point A at the start of the rectifier circuit 200. Diode 202 has its anode coupled to the ac source input line 112 (at point A) and its cathode coupled to the DC supply line 222. Diode 204 has its anode coupled to the ac source input line 313 and its cathode coupled to the DC supply line 222. Diode 206 has Hs anode coupled to the DC return line 223, and its cathode coupled to the anode of the diode 202. Diode 20S has its anode coupled to the DC return line 223 and its cathode coupled to the ac source input line 113. Diode 206 is shunted by, or placed in parallel with, field effect transistor (in the embodiment shown, a MOSFET) 212. Diode 208 is shunted by, or placed in parallel with MOSFET 21.4. MOSFET 212 has its drain coupled to the cathode of diode 206, and its source coupled to the DC return line 223. MOSFET 214 has its drain coupled to the cathode of diode 208 and its source coupled to the DC return line 223. MOSFET 212 turns on during the negative half cycle when diode D4 forward biases. MOSFET 214 turns on during the positive half cycle when diode D5 forward biases.

As one skilled in the art will appreciate, the rectifier circuit 200 operates to provide power from the ac source 110 to the dc toad 120. However, as noted above, operation of diodes, such as the diodes of the rectifier circuit 200, comes with a penalty - reduced power supply efficiency owing to conduction losses (power dissipation) in the diodes. More specifically, the average current in the output diodes is equal to the dc output current, and the peak current can be several times higher, depending on the duty cycle. To reduce this power dissipation, the MOSFETs 212 and 214 operate to shunt current around their corresponding diodes 206 and 208. Consider the instant at which the input voltage to the rectifier circuit 200 (at point A) is positive. The input current passes through diode 202 and returns through diode 208. Turning MOSFET 214 on during the positive half cycle of the ac input causes current to flow (return) through the MOSFET 214 instead of through the diode 208. Similarly, during the negative half cycle of the ac input, turning on MOSFET 212 causes current How through MOSFET 212 instead of diode 206. Using these MOSFEss 212 and 214 in the DC return phase significantly reduces conduction losses as compared to use of the corresponding diodes 206 and 208.

Table 1 below shows the results of incorporating the MOSFETs 212 and 214 into the power supply 100. Note that for two MOSFETs (Figure 1) at low power levels, the power savings are on the order of 1 to 2 watts. However, at higher power levels (where, ideally, the power supply 100 will operate), the power savings are on the order of 7 watts. Saving 7 watts of power provides about a 0.5 percent gain in efficiency.

To further improve power supply efficiency by reducing power dissipation in the bridge rectifier circuit, four MOSFETs may be used and all the diodes replaced with MOSFETs. Figure 2 illustrates a power supply 300 in which the normal rectifier circuit is replaced with an exemplary rectifier circuit 400 having all MOSFETs. The rectifier circuit 400 includes MOSFETs Q26 and Q27 on the DC supply line 322 and MOSFETs Q29 and Q30 on the DC return line 323. Using the circuit 400 in the power supply 300 reduces power dissipation losses by as much as 14 watts (at high power levels), this reduction in power dissipation provides an efficiency gain to about 1. percent

Yet another exemplary rectifier circuit includes MOSFET drivers to the MOSFETs 212 and 214 of circuit 200. The schematic is shown in Figure 3 in which power supply 500 includes exemplary rectifier circuit 600 with MOSFETs 612 and 614, and corresponding MOSFET drivers 613 and 614. The efficiency gain of the rectifier circuit 600 is on par with that of the circuit 200 of Figure 1.

Claims

We claim:

1. A rectifier circuit (200, 400, 600) for use in a power supply (100, 300, 500) to convert ac input power to dc output power to supply a dc load, the power supply comprising a first ac source input line, a second ac source input line, a DC power supply line, and a DC power return line, the rectifier circuit, comprising: a first field effect transistor (FET) (212) having its drain coupled to the first ac source input tine (112) and its source coupled to the DC power return line (223); and a second FET (214) having its drain coupled to the second ac source input line (113) and its source coupled to the DC power return line (223), wherein during an ac cycle, the second FET (214) closes during a positive half cycle and the second FET (212) closes during the negative half cycle.

2. The rectifier circuit of claim 1, further comprising: a First control circuit coupled to the first FET; and a second control circuit coupled to the second FET, wherein the first and second control circuits provide a control signal to close the first and second FETs, respectively.

3. The rectifier circuit of claim 2, wherein the first and second control circuits each comprise a diode (D4, D5) that forward biases to provide the control signal.

4. The rectifier circuit of claim 1 , further comprising: a first diode (202) coupled on its anode side to the first ac source input line and on its cathode side to the DC power supply line (222); and a second diode (204) coupled on its anode side to the second ac source input line and on its cathode side to the DC power supply line (222), wherein during the positive half cycle, the first diode forward biases, and during the second half cycle, the second diode forward biases.

5. The rectifier circuit of claim 1. further comprising: a third FET (Q26) having its source coupled to the first ac source input line and its drain coupled to the DC power supply fine; and a fourth FET (Q27) having its source coupled to the second ac source input line and its drain coupled to the DC power supply line, wherein during the positive half cycle the third FET closes and during the negative half cycle, the fourth FET closes.

6. The rectifier circuit of claims 1. or 5, wherein the FETs are MOSFETs.

7. lite rectifier circuit of claims 1 or 5, further comprising driver circuits coupled to the gates of each of Ae FETs, wherein the driver circuits operate to close their respective FETs.

8. A method for reducing power dissipation in an ac power supply, the power supply comprising a first ac source input fine, a second ac source input line, a DC power supply line, a DC power return line, and a rectifier circuit, the method, comprising: during a negative half cycle, closing a first field effect transistor (FET) (212) whose drain is coupled to the first ac source input line and whose source coupled to the DC power return fine; and during a positive half cycle, dosing a second FET (214) whose drain is coupled to the second ac source input line and whose source coupled to the DC power return line.

9. The method of claim 8, further comprising: during the negative half cycle, closing a third FBT (Q26) whose drain is coupled to the first ac supply input line and whose drain is coupled to the DC power supply line; and during the positive naif cycle, closing a fourth FET (Q27) whose source is coupled to the second ac source input line and whose drain is coupled to the DC power supply line.

10. A rectifier circuit for reducing power dissipation in an ac power supply, the ac power supply comprising a first ac source input line, a second ac source input line, a DC power supply line, and a DC power return line, the rectifier circuit, comprising: a first diode (202) coupled on its anode side to the first ac source input line and on its cathode side to the DC power supply tine; a first field effect transistor (FET) (212) having its drain coupled to the anode of the first diode, and its source coupled to the DC power return line; a second diode (204) coupled on its anode side to the second ac source input line, and coupled on its cathode side to the DC power supply line; a second FET (214) having its drain coupled to the anode of the second diode, and its source coupled to the DC power return line, wherein during a positive half cycle of the ac source voltage, current flows in the first ac source input line, through the first diode, through the second FET, and into the second ac source input line.

11. The rectifier circuit of claim 10, wherein during a negative half cycle of the source voltage, current flows in the second ac source input line, through the second diode, and then through the first FET.

12. The rectifier circuit of claim 10, further comprising: a third diode (206) having its cathode coupled to the drain of the first FET, and having its anode coupled to the source of the first FET; and a fourth diode (20S) having its cathode coupled to the drain of the second FET, and having its anode coupled to the source of the second FBT.

13. The rectifier circuit of claim 10,, further comprising: a first driver circuit coupled to the first FET; and a second driver circuit coupled to the second FHT, wherein the driver circuits operate to close and opentheir respective FETs.

14. The rectifier circuit of claims 12 or 13, wherein during a positive half cycle of the source voltage, current flows through the first ac source line, then through the first diode to the DC supply line, and returns to the source by flowing through the DC return line.

15. The rectifier circuit of claims 12 or 13, wherein during a negative half cycle of the source voltage, current flows through the second ac source input line, then through the second diode to the DC supply line, and returns to the source by flowing through the DC return line.