Simple control circuit for synchronous rectifiers used in ZVS phase shifted full bridge converter

[0002] The present invention relates generally to switching power supplies and, more particularly, to a simplified control circuit for synchronous rectifiers used in ZVS phase shifted full bridge converters.

[0003] Recent advancements in MOSFET switch technology makes synchronous rectifiers a feasible and competitive rectification technique for low voltage, high current DC/DC converters. Synchronous MOSFET switches are used to increase efficiency, reduce power losses, reduce electromagnetic interference and facilitate zero voltage switching (ZVS) in such converters. However, proper timing of the synchronous rectifiers is critical to the efficient operation of the converter.

[0004] In one known technique, the synchronous rectifiers may be driven by the control circuit used to drive the primary side of the transformer in the converter. This technique is commonly referred to as control driven synchronous rectification. One proposed implementation of this technique is presented by Laszlo Balogh in “Design Review: 100W, 400 kHz, DC/DC Converter with Current Doubler Synchronous Rectification Achieves 92% Efficiency”, SEM-1100 edition of the Unitrode Switching Regulated Power Supply Design Seminar Manual, by Unitrode corporation, 1996.

[0005] In this proposed prior art technique, the DC/DC converter is implemented using a phase shifted full bridge configuration on the primary side and a current doubler output with synchronous rectification on the secondary side. Of particular interest, a secondary control circuit 10 as shown in FIG. 1 is used to drive the synchronous rectifiers (not shown). The secondary control circuit 10 receives as input the primary drive signals 12 for each of the switching devices on the primary side of the converter. The drive signals 12 are in turn transmitted via a signal transformer 14 to the secondary side of the converter. The secondary control circuit 10 then outputs a secondary drive signal 16 for each of the synchronous rectifiers.

[0006] Referring to FIG. 2, a timing diagram further illustrates the operation of the prior art secondary control circuit 10. In this proposed technique, both of the synchronous rectifiers are turned on during a dead time period of operation as shown at 22, where a dead period is defined as the time at which no current is flowing from the input voltage source through the primary winding of the transformer of the converter. When diagonal switching devices on the primary side of the transformer are conducting, one of the two synchronous rectifiers is generally in an on state. The onset of a dead time period occurs when one of the two diagonal conducting switching devices is driven to an off state. It should be noted that the second of the two synchronous rectifiers is driven to an on state at the same time as the diagonal switching device is driven to an off state. However, due to parasitic delays and other variations inherent in the switching devices, it is conceivable that the second synchronous rectifier may be driven to an on state before the diagonal conducting switching device is driven to an off state. In this scenario, an opportunity for cross conduction between the synchronous rectifiers is introduced into the converter.

[0007] Therefore, it is desirable to provide an improved control circuit for controlling the synchronous rectifiers employed in switching power converters. It is envisioned that the improved control circuit is operable to drive both synchronous rectifiers to an on state during a freewheeling period, and yet eliminate the risk of cross conduction between the synchronous rectifiers.

[0008] In accordance with the present invention, an improved secondary control circuit is provided for controlling synchronous rectifiers in a switching power converter. The secondary control circuit employs control signals available for driving primary full bridge switches to drive two synchronous rectifiers. In particular, the secondary control circuit is operable to drive both synchronous rectifiers to an on state during a dead time period of operation. The dead time period is defined as the time at which no current is flowing from the input voltage source through a primary winding of a primary transformer of the converter. The onset of the dead time period occurs when the diagonal conducting switching devices is driven to an off state. In other words, the secondary control circuit is further operable to drive the second of the two synchronous rectifiers to an on state only after one of the diagonal switching devices is driven to an off state by the primary control circuit. In this way, the improved secondary control circuit eliminates the risk of cross conduction between the synchronous rectifiers.

[0009] For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.

[0010] FIG. 1 is a logical circuit diagram for a conventional secondary control circuit for controlling synchronous rectifiers in a phase shifted full bridge converter circuit;

[0011] FIG. 2 is a timing diagram for the conventional secondary control circuit;

[0012] FIG. 3 is a schematic of a full bridge converter having current doubler synchronous rectifiers in accordance with the present invention;

[0013] FIG. 4 is a logical circuit diagram for an improved secondary control circuit for controlling synchronous rectifiers in accordance with the present invention;

[0014] FIG. 5 is a timing diagram for the improved secondary control circuit in accordance with the present invention;

[0015] FIG. 6 is a schematic of a full bridge converter having an alternative synchronous rectifier configuration in accordance with the present invention.

[0016] A well known full bridge converter 30 having current doubler synchronous rectifiers is depicted in FIG. 3. The converter 30 generally includes an input voltage source 32, a primary switching circuit 34, a transformer 36, a rectifying circuit 38, a primary control circuit 40 and a secondary control circuit 42. While the following description is provided with reference to a full bridge converter, it is readily understood that the broader aspects of the present invention are applicable to other types of converter topologies, such as push pull or half bridge topologies, which may employ synchronous rectification.

[0017] The switching circuit 34 includes a first switching device Q_Aand a second switching device Q_Bconnected in series to form a first switching leg, and a third switching device Q_Cand a fourth switching device Q_Dconnected in series to form a second switching leg. Each of the switching legs is connected in parallel to the input voltage source 32. In a preferred embodiment, the primary switching device are further defined as MOSFET switches.

[0018] The switching circuit 34 is in turn connected to a primary winding of the transformer 36. In particular, a first junction point between the first switching device Q_Aand the second switching device Q_Bis connected to one end of the primary winding, and a second junction point between the third switching device Q_Cand the fourth switching device Q_Dis connected to the other end of the primary winding. An inductor Lr is commonly inserted in series with the primary winding of the transformer as shown. The input voltage source 32, the switching circuit 34, and the primary winding of the transformer are collectively referred to as the primary side of the converter.

[0019] A primary control circuit 40 operable to generate drive signals for each of the switching devices is connected to the switching circuit 34. In a preferred embodiment, the primary control circuit 34 is operable to supply substantially complementary drive signals to the switching devices in one of the switching legs and relatively phase shifted substantially complementary control signals to the switching devices in the other switching legs, so that diagonal switching devices conduct alternately with a dead time period therebetween. As is well known, the dead time period is defined as the time at which no current is flowing from the input voltage source through a primary winding of a transformer. In other words, the primary control circuit 34 provides zero voltage switching (ZVS) phase shifted control over the switching circuit 34. Although soft switching control is presently preferred, this is not intended as a limitation on the broader aspects of the present invention. On the contrary, hard switching control may be suitably employed by the primary control circuit 34. In this case, external delays may need to be introduced to ensure proper timing of the drive signals as is well known in the art.

[0020] The rectifying circuit 38 provides an output voltage V_Ofor the converter. The rectifying circuit 38 generally includes two synchronous rectifiers which are connected to a secondary winding of the transformer 36. Although the present invention is not limited thereto, MOSFET switches are preferably used to implement the synchronous rectifiers.

[0021] More specifically, the rectifying circuit 30 includes a first rectifying switch Q₁connected in series with a second rectifying switch Q₂. In this configuration, the drain for the first rectifying switch Q₁is connected to one end of the secondary winding of the transformer, the drain for the second rectifying switch Q₂is connected to the other end of the secondary winding of the transformer, and a junction point between the first rectifying switch Q₁and the second rectifying switch Q₂is connected to ground. The rectifying circuit 30 further includes a first inductor L₁connected between the drain of the first rectifying switch Q₁and an output terminal for the output voltage V₀of the converter, and a second inductor L₂connected between the drain of the second rectifying switch Q₂and the output terminal for the output voltage V_O. The output voltage V₀is measured across a capacitor C connected between the output terminal and ground. In other words, the rectifying circuit 38 is preferably implemented in a current doubler synchronous rectifier configuration. While the following description is provided with reference to a current doubler synchronous rectifier configuration, it is readily understood that the broader aspects of the present invention are applicable to other types of synchronous rectification circuits. For example, a full bridge converter having an alternative synchronous rectifying circuit is shown in FIG. 6.

[0022] In accordance with the present invention, an improved secondary control circuit 42 is connected to the rectifying circuit 38. The secondary control circuit 42 employs control signals from primary control circuit 40 to drive the two synchronous rectifiers. The secondary control circuit is operable to drive both synchronous rectifiers to an on state during a dead time period of operation. When diagonal switching devices on the primary side of the transformer are conducting, one of the two synchronous rectifiers is generally in an on state. The onset of the dead time period occurs when one of the diagonal conducting switching devices is driven to an off state. The secondary control circuit 42 is operable to drive the second of the two synchronous rectifiers to an on state only after one of the diagonal switching devices is driven to an off state by the primary control circuit 34. In particular, the secondary control circuit 42 drives the second of the two synchronous rectifiers to an on state after either delay period, Delay A-B or Delay C-D, as shown in FIG. 5. In this way, the improved secondary control circuit 42 eliminates the risk of cross conduction between the synchronous rectifiers.

[0023] FIG. 4 illustrates a preferred embodiment for the improved secondary control circuit 42. The secondary control circuit 42 is implemented using two logic OR gates, such that each OR gate receives the control signals used to drive diagonal switching devices and outputs a drive signal to one of the two synchronous rectifiers. In the context of the above-described full bridge converter, a first OR gate 52 receives as input the control signals used to drive the first switching device Q_Aand the fourth switching device Q_D; whereas a second OR gate 54 receives as input the control signals used to drive the third switching device Q_Band the third switching device Q_C. The first OR gate 52 and the second OR gate 54 in turn output drive signals to the first rectifying switch Q₁and the second rectifying switch Q₂, respectively. As will be apparent to one skilled in the art, various hardware configurations may be used to implement the secondary control circuit 42. It is also envisioned that a totem pole or similar buffer stage may be inserted between the output of the secondary control circuit 42 and the synchronous rectifiers to facilitate faster switching.

[0024] The operation of the secondary control circuit 42 is better understood from the timing diagram depicted in FIG. 5. The drive signals for each of the primary switching devices is shown in relation to the drive signals for the synchronous rectifiers as generated by the secondary control circuit 42. As noted above, substantially complementary drive signals serve as input to the first switching device Q_Aand the second switching device Q_B, and relatively phase shifted substantially complementary control signals serve as input to the third switching device Q_Cand the fourth switching device Q_D. As a result, diagonal switching devices conduct alternately with a dead time period therebetween.

[0025] For instance, when diagonal switching devices Q_Aand Q_Dare driven to an on state, current flows up through the primary winding of the transformer. On the secondary side, the first rectifying switch Q₁is in an on state and the second rectifying switch Q₂is in an off state, such that the first inductor L₁freewheels and the second inductor L₂stores energy.

[0026] A dead time period occurs when the fourth switching device Q_Dis driven to an off state. As a result, no current is flowing from the input primary source through the primary winding of a transformer. In a conventional approach, the second rectifying switch Q₂is simultaneously driven to an on state. However, the improved secondary control circuit 42 of the present invention provides a delay before the second rectifying switch Q₂is driven to an on state as shown at 60. In other words, the secondary control circuit is operable to drive the second of the two synchronous rectifiers to an on state only after one of the diagonal switching devices is driven to an off state by the primary control circuit.

[0027] Next, the third switching device Q_Cis driven to an on state. This delay between the fourth switching device Q_Dand the third switching device Q_Cis set in primary control circuit. When the third switching device Q_Cis driven to an on state, the second rectifying switch Q₂is also driven to an on state. As a result, both of the rectifying switches are turned on during the dead time period and both the inductors on the secondary side are in freewheeling mode. To terminate the freewheeling period, the first switching device Q_Ais driven to an off state. At the same time, the first rectifying switch Q₁is also driven to an off state.

[0028] To complete the cycle, the second switching device Q_Bis driven to an on state. As a result, diagonal switching device Q_Band Q_Care in an on state and current flows down through the primary winding of the transformer.

[0029] Thus, the dead time period is terminated. The cycle is then repeated.

[0030] It should be appreciated that the improved secondary control circuit of the present invention eliminates the risk of cross conduction between the synchronous rectifiers. In addition, the improved secondary control circuit may be constructed using a minimal part count as well as takes advantage of the standard delays set in the primary control circuit. Lastly, the improved secondary control circuit allows output choke current to flow in the secondary side of the converter during a portion of the delay period, thereby assisting the soft switching control.

[0031] While the invention has been described in its presently preferred form, it will be understood that the invention is capable of modification without departing from the spirit of the invention as set forth in the appended claims.