CN102868303B - Synchronous rectifier - Google Patents

Abstract

The present invention relates to synchronous rectifiers, and the disclosed embodiments relate to an apparatus and method for reducing power consumption in a power supply. A device is provided, comprising: an element (214) for connecting a first signal (S2) to a reference level (ground potential) when the connecting element is conducting; elements (202, 204) for placing said connection element in a conducting state for a duration of a portion of a period of a second signal (S3); and an element (206) for varying the conduction of said connection element in response to the magnitude of said second signal duration.

Description

synchronous rectifier

This application is a divisional application of an invention patent application with an application date of September 28, 2005, an application number of 200580051717.8, and an invention title of "Synchronous Rectifier".

technical field

This invention relates generally to the field of power supplies, and more particularly to the use of synchronous rectification in switching power supplies.

Background technique

As shown in FIG. 1 , a typical switched mode power supply (SMPS) includes: a primary side component 150 and a secondary side component. Primary side (also referred to as "hot side") components include switch controller 106, switching metal oxide semiconductor (MOSFET) 108, MOSFET heat sink 110, current sense resistor 112, inrush current suppression capacitor 114, with primary winding 120 and Transformer 116 of secondary winding 118 , rectifier diode 102 , filter capacitor 104 and opto-isolator 126 . Secondary or “cold side” components include secondary transformer windings 122 and 124 , rectifier diodes 128 , 136 and respective heat sinks 130 , 138 , and filter capacitors 132 and 134 . The entire switching power supply is powered by an unregulated voltage source 100 . Controller 106 provides drive signal V _D to MOSFET 108 to generate current in primary winding 120 of transformer 116 . Secondary winding 118 of transformer 116 provides a voltage source that, when rectified and filtered by diode 102 and capacitor 104 , respectively, provides a supply voltage V _DD to controller 106 . The feedback signal V _FB is generated by the rectified and filtered secondary power supply +12V, and fed back to the controller 106 through the optical isolator 126 , thereby establishing a feedback loop to control the on and off of the MOSFET 108 . Adjustment of the operating level in the SMPS is achieved by comparing the feedback signal V _FB with a reference value in the controller 106 and varying the conduction period of the MOSFET 108 in response to the difference between the feedback signal and the reference level . Resistor 112 senses the primary current flowing in MOSFET 108 as a current feedback signal for current mode controller 106 . Using current mode control, excessive current is prevented from flowing from the switching power supply during overload conditions. The signals from secondary windings 122 and 124 of transformer 116 are rectified by diodes 128 and 136 respectively and filtered by capacitors 132 and 134 respectively to obtain regulated output voltages of +6.5V and +12V. Rectification of the resulting signal across windings 122 and 124 may be accomplished by diodes in series with the respective windings between ground and the power supply output. In the typical SMPS described, one of the diodes, 128, is arranged with its cathode connected to the positive output of its particular power supply, so that both the anode and cathode of diode 128 are remote from ground. In the exemplary +12V supply, diode 136 is configured with its anode connected to ground. In rectifiers of the type described in this exemplary switching power supply, typically the main source of inefficiency is the voltage drop across the rectifying diodes. In higher supplies, the inefficiency due to the voltage drop across the rectifier diodes can be severe, requiring a heat sink and possibly active measures such as forced air cooling.

To increase the efficiency of the rectifier, a transistor (usually a MOSFET) can be used as a low voltage drop switch instead of a diode. This technique is called synchronous rectification. Synchronous rectification requires controlling the drive to the synchronous rectifiers to turn the MOSFETs on or off at the appropriate portion of the signal being rectified. Typically an integrated circuit controller is used to control the conduction of the MOSFET. These ICs, such as the ST Microelectronics STS-R3 or the Anachip AP436, are somewhat expensive and require an additional 4 to 8 external components. These ICs often include clock generation circuits and other complex methods to determine the on/off control of the synchronous rectifier MOSFETs. The present invention includes a simpler control circuit, and can implement synchronous rectification in a switching power supply at low cost using discrete components.

Contents of the invention

Some aspects commensurate with the originally claimed scope of the present invention are set forth below; however, the present invention may encompass various aspects not set forth below.

The disclosed embodiments relate to an apparatus comprising: a first device, which may be a transistor, configured to connect a first signal to a reference level when the first device is turned on; a second device, which may be a differentiator or a high-pass filter and is responsive to a second signal which may be out of phase with the first signal, the second device being configured to control the conduction of the first device for a fraction of the period of the second signal and a detector, which may be a diode peak detector, configured to vary the on-duration of the first device in response to the magnitude of the second signal. In such an apparatus, the detector may reduce the conduction duration of the first device in response to an increase in the magnitude of the second signal.

Another embodiment comprises: means for connecting the first signal to a reference level when the connecting element is conducting; for placing said connecting element in a conducting state for a duration of a portion of a period of the second signal and an element for varying the conduction duration of the connecting element in response to the magnitude of the second signal.

Yet another embodiment is a method comprising the steps of: connecting a first signal to a reference level by turning on a device; differentiating a second signal; and, in response to said differentiated second signal, controlling turning on of the device; and varying the duration of conduction of the device in response to the magnitude of the second signal. A variation of this method may include reducing the conduction duration of the device in response to an increase in the magnitude of the second signal.

Description of drawings

Embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, and similar elements in each of the accompanying drawings use the same reference numerals:

Figure 1 is a block diagram of a typical switching power supply;

Fig. 2 is the schematic diagram of the embodiment of the present invention;

Figure 3 is a representative waveform of the drain voltage of the main switching MOSFET (108); and

FIG. 4 shows representative waveforms of drain voltage and gate voltage of a synchronous rectifier MOSFET in an embodiment of the present invention.

Detailed ways

The embodiment of the discrete control circuit shown in FIG. 2 satisfies the need for a low cost, synchronous rectifier controller. Figure 2 shows a representative switching power supply used in electronic equipment. The primary side circuit 150 is a typical switching power supply, which is well known to those skilled in the art, and is similar to the aforementioned power supplies. The switching transformer 116 has multiple secondary windings 122 and 124 to obtain different supply voltages. Diode 128 acts as a conventional rectifier in this system. The use of this high _- side rectifier for the +6.5V supply serves a dual purpose: to rectify the signal S3 from winding 122 to generate the +6.5V supply, and thus, the AC signal S3 at the anode _of diode 128 can be used to drive the synchronous rectifier MOSFET 214 switch control signal. MOSFET transistors can be used as rectifiers by controlling the conduction time of the MOSFET to coincide with the desired portion of the pulse waveform (synchronous rectification). Since the voltage drop of the MOSFET is much lower than even the Schottky diode, the efficiency of the power supply can be improved. In most cases, when synchronous rectification is used, the large heat sinks normally used to cool the diodes can be eliminated. In the exemplary embodiment shown in FIG. 2, MOSFET 214 is set to have its source connected to the ground potential of the secondary side. This configuration simplifies generating drive signals to MOSFET 214 . _A control voltage for gate driving of the synchronous rectifier 214 is obtained according to the signal S3. _The polarity of pulse signal S3 is opposite to the polarity of signal S2 _appearing at the drain of MOSFET 214 . This phase inversion is determined by phase adjustment of windings 122 and 124 such that the polarity _of signal S3 is the phase required to turn _on the gate of the MOSFET when signal S2 is at its most negative level. Drain _- to _- source conduction of MOSFET 214 when signal S2 is at its maximum negative level clamps signal S2 at ground potential, thereby rectifying signal S2 to an output of ₊ 12V. _The controller 106 is designed such that the signals S2 and S3 can have a variable duty cycle _; and the positive part _of the signal S3 increases for a duration with a higher line voltage. As a result, measures must be taken to shorten the duration of the pulses at the gate of MOSFET 214 to provide a suitable on-time for the MOSFET to ensure that MOSFET 214 is only turned _on when signal S2 is at a negative level. Capacitor 202 and resistor 204 form a high-pass filter that differentiates the waveform of signal S ₃ to produce a drive waveform for the gate of MOSFET 214 . Differentiating the waveform helps shorten the on-time of MOSFET 214 so that MOSFET 214 turns on when its drain voltage is negative or later, and turns off when its drain voltage rises or before. Low power (compared to conventional rectifier diodes such as diode 128) diode 136 conducts during negative excursions of signal S2, during which time intervals due to gate drive to MOSFET 214 may be shorter _than the negative _excursion of signal S2. duration of the offset, so the MOSFET may not turn on. The functions described above for diode 136 may also be performed by the internal parasitic diode of MOSFET 214 . Diode 206 and capacitor 208 rectify signal S3 to obtain a negative bias proportional to the value _of unregulated voltage source 100 (and proportional to the AC line input voltage). As the negative bias increases, the average voltage at the gate of MOSFET 214 is lowered, thereby reducing the MOSFET's on-time. A voltage divider formed by resistors 210 and 204 adjusts the negative bias obtained by diode 206 and capacitor 208 to establish the desired range of negative bias applied to the gate drive. This negative bias on the gate of MOSFET 214 prevents over-conduction (and increased losses) at high line voltages. Resistor 200 provides current limiting for the negative supply formed by diode 206 . Resistor 212 reduces the rise time of the drive voltage to the gate of MOSFET 214, thereby minimizing radiated noise due to fast switching transients.

The waveforms plotted in FIG. 3 show signal S ₁ switching the drain of MOSFET 110 , which is the primary voltage of winding 120 of transformer 116 . The upper trace in FIG. 4 shows the signal voltage S ₂ induced in the secondary winding 124 , which is also the voltage at the drain of the synchronous rectifier MOSFET 214 . The lower trace in FIG. 4 shows the gate drive to MOSFET 214 . The turn-on threshold of MOSFET 214 is approximately 2.5V to 3.0V, as shown by R ₁ and R ₂ in FIG. 4 . Projecting the point at which the gate drive signal crosses the turn-on threshold onto the drain voltage waveform in the upper region of FIG. ₄ shows that MOSFET 214 conducts well during the time interval during which signal S2 is negatively shifted. It can also be seen from the gate drive waveforms of FIG. 4 that increasing the negative bias applied to the gate of MOSFET 214 through detector 206 (as would occur at higher line voltages) will reduce the gate drive voltage above the conduction voltage. pass threshold _R1 or _R2 time period.

While the invention has been described with reference to preferred embodiments, it will be apparent that various modifications can be made to the various embodiments without departing from the spirit and scope of the invention.

Claims (29)

1. a synchronous rectificating device, this device comprises:

Transformer;

The source of input supply voltage;

First switching transistor, for described input supply voltage is periodically coupled to described transformer, to produce AC supply voltage in the first winding of described transformer;

Second switch transistor, this second switch transistor forms a synchronous rectifier, this synchronous rectifier is coupled to described first winding and is coupled to exporting through rectification of load, the conducting during the part in the cycle of described AC supply voltage of described second switch transistor for carrying out rectification to described AC supply voltage to produce; And

Detector, described detector is during the part in described cycle, when described input supply voltage is coupled to described transformer by described first switching transistor, described input supply voltage is responded, for producing the first control signal being coupled to described second switch transistor, described detector is configured to the size according to described input supply voltage, the moment terminated to the conducting of described second switch transistor during changing the described cycle in the mode changing the conduction duration of described second switch transistor.

2. synchronous rectificating device as claimed in claim 1, wherein, when described input supply voltage size increases, described detector reduces the described conduction duration of described second switch transistor.

3. synchronous rectificating device as claimed in claim 1, wherein, described second switch transistors couple is between described first winding and reference level.

4. synchronous rectificating device as claimed in claim 1, wherein, described second switch transistor comprises metal-oxide semiconductor (MOS).

5. synchronous rectificating device as claimed in claim 1, wherein, described transformer produces the second control signal, described second control signal is coupled to described second switch transistor from described transformer via a differentiating device, and wherein, described second control signal changes the ON time of described second switch transistor according to described input supply voltage size.

6. synchronous rectificating device as claimed in claim 5, wherein, described differentiating device comprises high pass filter.

7. synchronous rectificating device as claimed in claim 6, wherein, described high pass filter comprises capacitor and resistor.

8. synchronous rectificating device as claimed in claim 1, wherein, when the signal that described second switch transient response produces in the winding at described transformer starts conducting to described second switch transistor during controlling the described cycle.

9. synchronous rectificating device as claimed in claim 1, wherein, the phase place that the voltage produced at described second switch transistor two ends has is contrary with the phase place of described AC supply voltage.

10. synchronous rectificating device as claimed in claim 1, wherein, the negative peak of described detector to the signal produced in the winding of described transformer responds.

11. synchronous rectificating devices as claimed in claim 10, wherein, described detector comprises diode.

12. 1 kinds of synchronous rectificating devices, comprising:

Transformer;

The source of input supply voltage;

First switching transistor, for described transformer is periodically coupled in the source of described input supply voltage, to produce seasonal power voltage in the first winding of described transformer;

Second switch transistor, this second switch transistors couple is to described first winding to form a synchronous rectifier, and this synchronous rectifier is used for carrying out rectification to described seasonal power voltage and is coupled to exporting through rectification of load to produce;

Element, this element responds the signal produced in the winding of described transformer, for generation first switch controlling signal and second switch control signal, described first switch controlling signal and second switch control signal are all coupled to described second switch transistor, periodically to control moment that in described second switch transistor, conducting starts and the moment that conducting terminates respectively for providing synchronous rectification; And

Detector, being coupled to for detecting the signal produced in the winding of the described transformer of described second switch transistor, changing the ON time of described second switch transistor for the signal detected described in basis.

13. synchronous rectificating devices as claimed in claim 12, wherein, described detector is configured to change accordingly according in input supply voltage size, changes the conduction duration of described second switch transistor.

14. synchronous rectificating devices as claimed in claim 13, wherein, described detector is coupled to be received in the described signal produced in the described winding of described transformer.

15. synchronous rectificating devices as claimed in claim 12, wherein, described first switch controlling signal producing element comprises differentiator.

16. 1 kinds of synchronous rectified power devices, comprising:

Transformer;

The source of input supply voltage;

First switching transistor, for described transformer is periodically coupled in the source of described input supply voltage, to produce seasonal power voltage in the first winding of described transformer;

Second switch transistor, this second switch transistors couple to form a synchronous rectifier, is coupled to exporting through rectification of load for carrying out rectification to described seasonal power voltage to produce to described first winding;

Differentiator, this differentiator responds the signal produced in the winding of described transformer, for generation one switch controlling signal, this switch controlling signal is coupled to the control end of described second switch transistor to control the switching manipulation in described second switch transistor, for providing synchronous rectification; And

Peak rectifier, for carrying out peak value rectification to the signal produced in the winding of described transformer, to generate the second control signal, this second control signal is coupled to the described control end of described second switch transistor, for the ON time changing described second switch transistor according to this second control signal.

17. 1 kinds of switch mode power supplies, this device comprises:

Transformer;

The source of input supply voltage;

First switching transistor, for described input supply voltage is periodically coupled to described transformer, to produce seasonal power voltage in the first secondary winding of described transformer;

Second switch transistor (214), this second switch transistor forms a synchronous rectifier, this synchronous rectifier is coupled to described first secondary winding and is coupled to exporting through rectification of load for carrying out rectification to described seasonal power voltage to produce, when described second switch transistor (214) conducting, by the first signal (S that described first secondary winding by described transformer provides ₂) be connected to reference level;

Secondary signal (the S provided by the second subprime winding of described transformer is provided ₃) differentiating device (202,204), described differentiating device (202,204) for generating a switch controlling signal, this switch controlling signal is coupled to the control end of described second switch transistor to control the switching manipulation in described second switch transistor, for providing synchronous rectification; And

Detector (206), described detector configurations is the increase of the size in response to described input supply voltage, the moment terminated to the conducting of described second switch transistor during changing the described cycle in the mode reducing the conduction duration of described second switch transistor.

18. switch mode power supplies as claimed in claim 17, wherein, described second switch transistor (214) is metal-oxide semiconductor (MOS).

19. switch mode power supplies as claimed in claim 17, wherein, described differentiating device comprises high pass filter (202,204).

20. switch mode power supplies as claimed in claim 19, wherein, described high pass filter comprises capacitor and resistor (202,204).

21. switch mode power supplies as claimed in claim 17, wherein, described first signal (S ₂) have and described secondary signal (S ₃) contrary phase place.

22. switch mode power supplies as claimed in claim 17, wherein, described detector (206,208) responds described secondary signal (S ₃) negative peak.

23. switch mode power supplies as claimed in claim 22, wherein, described detector comprises diode.

24. 1 kinds of devices for the conducting of control synchronization rectifier, this device comprises:

First signal (S is provided ₂) the first source (124) and secondary signal (S is provided ₃) the second source (122), described first signal (S ₂) and described secondary signal (S ₃) phase place contrary;

Transistor (214), has the first main current conducting terminals and the second main current conducting terminals and conducting control terminal, and described first main current conducting terminals connects described first signal (S ₂), and described second main current conducting terminals connects reference level;

Capacitor (202), the first terminal of described capacitor (202) connects described secondary signal, and the second terminal of described capacitor (202) connects described conducting control terminal;

Resistor (204), the first terminal of described resistor (204) connects described conducting control terminal, and the second terminal of described resistor (204) connects described reference level; And

Diode (206), the negative electrode of described diode (206) connects described secondary signal (S ₃), and the anode of described diode (206) connects described conducting control terminal,

Wherein, described first source (124) and the second source (122) are the secondary winding of switch transformer (116).

25. devices as claimed in claim 24, wherein, described transistor is metal-oxide semiconductor (MOS).

26. 1 kinds of devices for synchronous rectification, this device comprises:

Transistor (214), this transistor forms a synchronous rectifier, this synchronous rectifier is coupled to the first secondary winding of a switch transformer, is coupled to exporting through rectification of load for carrying out rectification to the seasonal power voltage produced in described first secondary winding to produce;

Element (202,204), this element responds the signal produced in the second subprime winding of described switch transformer, for generation first switch controlling signal and second switch control signal, these two signals are all coupled to described transistor, periodically to control the moment that conducting in described transistor starts to terminate with conducting respectively, for providing synchronous rectification; And

Detector (206), for the increase of the amplitude peak in response to the signal produced in the second subprime winding of described switch transformer, reduces the conduction duration of described transistor.

27. devices as claimed in claim 26, wherein, described detector (206) comprises diode.

28. devices as claimed in claim 26, wherein, described first switch controlling signal producing element (202,204) is differentiator.

29. devices as claimed in claim 28, wherein, described differentiator comprises high pass filter.