Soft switching DC/DC converters and methods

A soft switching apparatus comprises an energy recovery channel formed by two diodes in series connection and a resonant tank formed by an inductor and a capacitor. The soft switching apparatus is coupled to the primary side of a bridge converter. An energy transfer process during L-C resonance helps to reduce the amplitude of the current flowing through the inductor in a freewheeling period. Furthermore, the soft switching apparatus can help to reduce the voltage stress across the secondary switching devices as well as the shoot-through currents flowing through the secondary switching devices, and thus enabling the reduction or elimination of dead time in a secondary synchronous rectifier control scheme.

TECHNICAL FIELD

The present invention relates to isolated DC-DC converters and methods, and more particularly, to isolated bridge DC-DC converters and methods employing a soft switching apparatus at a primary side of a transformer.

BACKGROUND

A telecommunication network power system usually includes an AC-DC stage converting the power from the AC utility line to a 48V DC distribution bus and a DC-DC stage converting the 48V DC distribution bus to a plurality of voltage levels for all types of telecommunication loads. Both stages may comprise isolated DC-DC converters. Isolated DC-DC converters can be implemented by using different power topologies, such as flyback converters, forward converters, half bridge converters, full bridge converters and the like. As known in the art, bridge converters generally are employed when the power of a DC-DC converter is more than 100 watts. As shown inFIG. 1, a full bridge converter100is a conventional full bridge converter having a full wave rectifier coupled to a center-tapped secondary winding. The full bridge converter100includes four switches Q1, Q2, Q3and Q4at a primary side of a transformer Tx. The four switches Q1, Q2, Q3and Q4form a bridge having two legs. Q1and Q3in series connection have a junction point, referred to as A. Q2and Q4in series connection have a junction point, referred to as B. The primary winding of the transformer Tx is connected to A and B. A DC supply Vin is connected to the two legs to provide power to the full bridge converter100. According to the operating principle of a hard switching full bridge converter, the switches Q1and Q4are turned on simultaneously for an adjustable time during a first half cycle. After a period of dead time, the switches Q2and Q3are turned on simultaneously for an equal time during the second half cycle. As a result, Vin and −Vin are applied to the primary side of the transformer Tx in alternate half periods.

In a fixed duty cycle control scheme, the turn-on time of the switches Q1and Q4is equal to the turn-on time of the switches Q2and Q3. When all four switches are turned off, both S1and S2are turned on. The load current flows through S1and S2. This interval is referred to as a freewheeling period. The output voltage of the bridge converter100is proportional to the turn-on time of the switches. A controller (not shown) may detect the output voltage Vo and adjust the turn-on time via a negative feedback control loop (not shown). The secondary side of the transformer Tx is center-tapped. Such a center-tapped secondary and two switches S1and S2can form a full wave rectifier, which can convert the primary voltage having double polarities (Vin and −Vin) of the transformer Tx to a secondary voltage having a single polarity. Then, the secondary voltage having a single polarity is fed to an output filter including an inductor Lo and an output capacitor Co. The output filter averages the square voltage pulses at the output of the full wave rectifier and generates a DC voltage at Vo, which is then supplied to a load represented by a resistor RL.

The conventional full bridge converter described above may have large magnetic components. In order to reduce the size of the magnetic components such as the transformer Tx and the output inductor Lo, the switching frequency may be increased to a higher level so as to reduce the transformer and the inductor in size. Consequently, the power density of a full bridge converter can be increased substantially. However, as the switching frequency of full bridge converters increases, the total efficiency is reduced due to extra switching losses in response to a higher switching frequency. Therefore, there is a need to have a soft switching full bridge converter to reduce switching losses.

A phase shift full bridge converter is capable of reducing switching losses by means of the zero voltage switching control technique. As shown in a dashed rectangle120ofFIG. 1, instead of turning on two primary switches (e.g., Q1and Q4) simultaneously, the turn-on time of these two switches are shifted by a period of time. More particularly, as depicted in the dashed rectangle110, a waveform106and a waveform110show Q1is on for a period of time before Q4is turned on. There is an overlap between Q1's turn-on time and Q4's turn-on time. After Q1is turned off, Q4stays on for a period of time. Likewise, a waveform108and a waveform112show there is a phase shift between Q2and Q3's turn-on time. The phase shift full bridge can achieve a zero voltage switching by utilizing the L-C resonance between transformer leakage inductance and MOSFET (e.g., Q1) output capacitance. For example, Q3has a parasitic capacitor (not shown) across its drain and source. During the period when both Q1and Q4are on, the voltage across Q3's parasitic capacitor is charged to a voltage approximately equal to Vin. According to the basic principle of the phase shift control technique, Q1is off prior to Q4. After Q1is off, the primary side current cannot change instantaneously. As a result, the primary side current will flow through the parasitic capacitors of Q1and Q3. The flow of the primary side current through both parasitic capacitors may cause the voltage at the junction between Q1and Q3to be discharged to zero, enabling a zero voltage switching when Q3is turned on without substantial power loss. Similarly, the phase shift operation may enable lossless turn-on of other switches, namely Q1, Q2and Q4.

As described above with respect toFIG. 1, a phase-shift full bridge converter may reduce the switching losses by employing a zero voltage switching technique. However, the primary side may have a large amount of current stress due to the primary side switches' longer conduction time of the primary current having an amplitude close to the peak current during a freewheeling period, and the reverse recovery of second switches' body diodes may cause further power losses.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provide a system, apparatus and method for reducing the current and voltage stress of a bridge converter.

In accordance with an embodiment, a system for reducing the current and voltage stress of a bridge converter comprises a transformer having a primary and secondary winding, a bridge circuit having at least one leg coupled to the primary winding, a rectifier coupled to the secondary winding, and a soft switching apparatus. The soft switching apparatus comprises a first diode having an anode and a cathode coupled to a positive terminal of a DC source and a second diode having an anode coupled to a negative terminal of the DC source and a cathode connected to the anode of the first diode. The soft switching apparatus further comprises an inductor and a capacitor, which are connected in series and coupled to the two diodes and the primary winding. In the system, the inductor and the capacitor form a resonant tank. Moreover, the capacitor helps to reduce a current in the inductor during a freewheeling period.

In accordance with another embodiment, a method of reducing the current and voltage stress of a bridge converter is disclosed. The method includes providing a bridge having at least one leg and a transformer having a primary winding and a second winding. The primary winding is serially connected to an inductor and a capacitor. The method further includes providing an energy recovery channel formed by two diodes in series connection and forming a resonant tank by placing the inductor, the capacitor and the primary winding in series connection. Furthermore, the method includes coupling a rectifier to the secondary winding, coupling the energy recovery channel to the resonant tank at a junction point of the resonant tank, coupling a first terminal of the resonant tank to a junction point of a first leg of the bridge and coupling a second terminal of the resonant tank to a junction point of a second leg of the bridge. The method can reduce a current flowing through the inductor by at least 20% during a freewheeling period through an L-C resonant process between the inductor and the capacitor. The method also limits shoot-through currents flowing through the rectifier by using the inductor as a current limiting element. Furthermore, the method reduces the voltage stress across the rectifier by means of the resonant tank and the energy recovery channel.

An advantage of a preferred embodiment of the present invention is reducing the voltage and current stress of a bridge converter so as to improve the bridge converter's reliability and efficiency.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to preferred embodiments in a specific context, namely a full bridge converter having a soft switching apparatus. The invention may also be applied, however, to a variety of bridge converters including half bridge and full bridge converters.

Referring initially toFIG. 2A, a full bridge converter including a soft switching apparatus is illustrated. The basic operation principle of a full bridge converter has been described above with respect toFIG. 1. A soft switching apparatus200includes four terminals. As shown inFIG. 1, two terminals of the soft switching apparatus200are connected to the positive terminal and the negative terminal of Vin respectively. The other two terminals are connected to the junction of Q1and Q3, referred to as A, and one end of the primary side of the transformer Tx, respectively. It should be noted that the soft switching apparatus200and the primary side of the transformer Tx are in series connection. A person having ordinary skill in the art will recognize that a position exchange between the soft switching apparatus200and the primary side of the transformer Tx is still within the scope of the present invention.

As shown inFIG. 2B, the soft switching apparatus200includes two diodes D5and D6, an inductor Lr and a capacitor Cr. D5and D6are in series connection. More particularly, the anode of D5is connected to the cathode of D6. The junction between D5and D6is connected to one end of Lr. The other end of Lr is connected to Cr. It should be noted that because Lr and Cr are in series connection, the location exchange between Lr and Cr has no impact on the electrical characteristics of the soft switching apparatus200. As a result, an apparatus merely swapping the locations of Lr and Cr is within the scope of the present invention. Furthermore, after the soft switching apparatus200is added into a full bridge converter, as shown inFIG. 1, Lr, Cr and the primary side winding of the transformer Tx are in series connection. For the reason provided above with respect to the location exchange between Lr and Cr, in a circuit having three components in series connection, the connection sequence of Lr, Cr and the primary side winding of the transformer Tx has no impact on the electrical characteristics of the full bridge converter. Therefore, the connection sequence among these three components can be Lr, Cr, the primary side winding of the transformer Tx, or any combination thereof.

FIG. 3illustrates a plurality of variations of the soft switching apparatus200. In the soft switching apparatus200, Lr and Cr form a resonant tank in which the energy oscillates back and forth between Cr and Lr. D5and D6form an energy recovery channel. The resonant energy may be recovered back to the DC supply coupled to the bridge converter through the energy recovery channel.FIG. 3includes three circuit configurations. A dashed rectangle302illustrates an exemplified circuit configuration in which the resonant tank formed by Lr and Cr is placed at the left side of the energy recovery channel formed by D5and D6. Likewise, in a dashed rectangle304, Cr and Lr may be placed in a symmetrical pattern. That is, the two components of the resonant tank are symmetrical relative to the energy recovery channel. Similarly, by exchanging Lr and Cr's positions shown in the dashed rectangle304, a circuit configuration shown in a dashed rectangle306can be formed.

It should be noted that whileFIG. 3shows several possible circuit configurations based upon the soft switching apparatus200, one having ordinary skill in the art will recognize the circuit configurations described above are merely exemplary circuit configurations and are not meant to limit the current embodiments. For example, when the soft switching apparatus200is serially connected to the primary side winding of the transformer Tx, the primary side winding may be placed in series with the resonant tank or be placed in between Lr and Cr. All such variations are fully intended to be included within the scope of the embodiments discussed herein.

FIG. 4Aillustrates a circuit diagram of a full bridge converter400having a soft switching apparatus in accordance with an embodiment. The full bridge converter400shown inFIG. 4Acan be considered having two portions. Along the isolation barrier provided by the transformer Tx, the left side of the transformer Tx into which power flows is called the primary side and the circuit connected to the primary side winding is called the primary circuit of the full bridge converter400. On the other hand, the right side of the transformer Tx from which power flows is called the secondary side, and the circuit connected to the secondary side winding is called the secondary circuit of the full bridge converter400. The primary side's circuit diagram has been described in detail with respect toFIG. 1, and therefore not discussed herein. The soft switching apparatus200is placed in series with the primary side winding of the transformer Tx. The energy recovery channel of the soft switching apparatus200has two terminals. As shown inFIG. 4A, one terminal (the cathode of D5) is connected to the positive terminal of the DC supply Vin and the other terminal (the anode of D6) is connected to the negative terminal of the DC supply Vin.

InFIG. 4A, the secondary circuit of the full bridge converter400employs a full wave rectifier formed by a secondary winding and four switches S1, S2, S3and S4(these switches are controlled to work as diodes, and such switches are usually called synchronous rectifiers). The operation of the full wave rectifier shown inFIG. 4Ais known in the art, and hence is not discussed herein. In comparison to the full bridge converter100shown inFIG. 2, the full bridge rectifier shown inFIG. 4Aemploys a different type of full wave rectifier. Nevertheless,FIG. 4Afurther illustrates that the soft switching apparatus200may be not only applied to a full bridge converter having a center-tapped secondary winding as shown inFIG. 2, but also applied to a full bridge converter having a non center-tapped secondary winding as shown inFIG. 4. Furthermore, the soft switching apparatus200is applicable to bridge converters having other secondary configurations, such as voltage doubler rectifiers and current doubler rectifiers. In sum, the soft switching apparatus200can be applied to all types of bridge converters, including full bridge converters having different types of secondary configurations, and half bridge converters having different types of secondary configurations. Furthermore, different control techniques of bridge converters have no impact on the application of the soft switching apparatus200. For example, the soft switching apparatus200can be applied to a hard switching full bridge converter as well as a phase shift full bridge converter. A plurality of applicable control techniques will be described in detail later with respect toFIGS. 5A-5E.

FIG. 4Billustrates another circuit diagram of a full bridge converter400having a soft switching apparatus in accordance with an embodiment. The full bridge converter400has been described in detail with respect toFIG. 4A.FIG. 4Billustrates the full bridge converter400with the soft switching apparatus304. As described with respect toFIG. 3, the soft switching apparatus304is a derivative of the soft switching apparatus200. The circuit shown inFIG. 4Bmay have electrical characteristics similar to those of the circuit shown inFIG. 4A.FIGS. 4C and 4Dillustrate another two variations, which have electrical characteristics similar to those of the circuit diagram shown inFIG. 4A. More particularly,FIG. 4Cincludes the full bridge converter400with the soft switching apparatus302andFIG. 4Dincludes the full bridge converter400with the soft switching apparatus306. In summary, the derivatives of the soft switching apparatus200are within the scope of the present invention.

As described above, the soft switching apparatus200can be applied to bridge converters having different control schemes.FIGS. 5A-5Eillustrates a plurality of control schemes applicable to the bridge converters having the soft switching apparatus200. InFIG. 5A, the primary switches Q1and Q3form a switch pair. Likewise, the primary switches Q2and Q4form the other switch pair. Both switches in each switch pair operate in complementary mode, that is, a high side switch (e.g., Q1shown in a waveform506) is driven on for a duty cycle D and the corresponding low side switch (e.g., Q3shown in the waveform510) is driven on for a duty cycle approximately 1-D in consideration of a short dead time between the conduction of the high side switch and the conduction of the low side switch. A waveform508and a waveform512show Q2and Q4are driven also in complementary mode. According to another embodiment,FIG. 5Bshows a conventional phase shift control scheme, which has been described in detail with respect toFIG. 1.FIG. 5Cshows a conventional hard switching control scheme, that is, in a waveform528, Q1and Q4are turned on simultaneously for a duty cycle D in a first half period; likewise, in a waveform526, Q2and Q3are turned on simultaneously for a duty cycle D in a second half period.

FIG. 5Dillustrates a hybrid control scheme, in which two switches (e.g., Q1shown in a waveform536and Q3shown in a waveform538) of one leg of the full bridge converter are turned on for a duty cycle D in the first half and the second half of one period. The other two switches (e.g., Q2shown in a waveform540and Q4shown in a waveform542) of the other leg operate in complementary mode, which has been described with respect toFIG. 5A.FIG. 5Eillustrates a control scheme derived fromFIG. 5D. Similar toFIG. 5D, in a waveform548and a waveform550, Q1and Q3are turned on for duty cycle D except that the duty cycle shown inFIG. 5Eis trailing edge triggered rather than leading edge triggered as shown inFIG. 5D. InFIG. 5E, a waveform552and a waveform546shows the operation of Q2and Q4is similar to that ofFIG. 5D. One advantageous feature of the soft switching apparatus200is that it can be applied to bridge converters having a variety of control schemes. It should be noted that the control schemes shown above are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives and modifications.

FIGS. 6A-6Fshow a number of advantageous features by using the soft switching apparatus200.FIG. 6Aillustrates a primary side current waveform showing the operation of the full bridge converter100inFIG. 1. The horizontal axis ofFIG. 6Ais time. The vertical axis ofFIG. 6Arepresents the current flowing through the primary side of the transformer Tx.FIG. 6Eillustrates a voltage waveform across V_AB of the full bridge converter100inFIG. 1. The vertical axis ofFIG. 6Erepresents the voltage across two junction points (e.g., junction points A and B shown inFIG. 1). The operation of full bridge converters and corresponding waveforms as shown inFIG. 6AandFIG. 6Eare known in the art, and hence are not discussed herein. In order to illustrate the difference between a conventional full bridge converter and a full bridge converter having the soft switching apparatus200, a dashed circle602highlights the amplitude of the primary current inFIG. 6Awhen the voltage across two junction points A and B inFIG. 6Eis approximately zero and the full bridge converter enters a freewheeling period. As shown inFIG. 6A, the current remains approximately 10 A.

FIGS. 6B and 6Fillustrate a primary side current waveform and a voltage waveform across V_AB showing the operation of the full bridge converter400shown inFIG. 4A. As described above with respect toFIG. 4A, the full bridge converter400includes the soft switching apparatus200. The resonant tank formed by Lr and Cr in the soft switching apparatus200is capable of reducing the amplitude of primary current by transferring the energy of Lr into Cr through L-C resonance. As shown in a dashed circle606, by employing Cr capable of transferring some energy from Lr to Cr, the amplitude of the primary current in a freewheeling period is reduced by at least 20% at full load. Furthermore, the energy stored in Cr will be recovered and transferred to the load. This will be discussed in detail with respect toFIG. 7B.

The soft switching apparatus200not only reduces the amplitude of the primary current of a full bridge converter, but also alleviates the voltage stress across the secondary switches. In accordance with an embodiment,FIG. 6Cillustrates the voltage stress of a secondary switch (e.g., S3) if the full bridge converter does not include the soft switching apparatus200. The vertical axis ofFIG. 6Crepresents the voltage across the drain and source of a secondary switch. According to an embodiment, the ringing across Vds_S3is up to 35V as shown in a dashed circle604. In contrast,FIG. 6Dshows with similar operation conditions the full bridge converter400is capable of limiting the ringing across Vds_S3below 30V. An advantageous feature of the soft switching apparatus200is that the energy transfer between Lr and Cr when V_AB is approximately zero can reduce the amplitude of the primary current as well as the voltage stress across the secondary switches.

FIGS. 7A and 7Bfurther illustrate the difference between the full bridge converter400shown inFIG. 4Aand the full bridge converter100shown inFIG. 1. In a conventional full bridge converter, such as the full bridge converter100, it is not uncommon to add a blocking capacitor in series with the primary side of the transformer Tx. However, the blocking capacitor normally has a large amount of capacitance because the function of the blocking capacitor is to generate a DC voltage to cancel the volt-second imbalance across the transformer Tx. The large amount of capacitance of the blocking capacitor causes the voltage across the blocking capacitor to remain almost constant in a switching cycle. As shown inFIG. 7A, the voltage across a blocking capacitor (e.g., V_Cr inFIG. 7A) is approximately zero because the controller maintains duty cycle symmetry between half-cycles according to an embodiment. In contrast, as shown inFIG. 7B, the voltage across Cr (referred to as V_Cr) in the soft switching apparatus200varies in a range from −4V to 4V according to an embodiment. The swing of the voltage across Cr represents the energy transfer during L-C resonance, which in a freewheeling period reduces the primary current of the transformer by at least 20%.

In accordance with another embodiment, the soft switching apparatus200may alleviate shoot-through and reverse recovery issues existed in a bridge converter having a secondary synchronous rectifier. As known in the art, in order to avoid shoot-through current, an adequate dead time is required to prevent a direct path formed by two fully or partially on switches.FIG. 8Aillustrates a conventional way to solve this issue by using dead times. As shown inFIG. 8A, a waveform802and a waveform806show the conduction of S2and S3is terminated before Q1and Q4are turned on. The gap between the S2and S3's turn-off and the Q1and Q4's turn-on is the dead time1, during which the current previously flowing through S2and S3is transferred to the diodes (not shown), which are usually the parasitic body diodes of the switches S2and S3. Similarly, the gap between the Q1and Q4's turn-off and the S1and S3's turn-on is the dead time2. In order to prevent shoot-through in all operating conditions, a long dead time may be employed. However, too long a dead time means high conduction losses caused by the higher voltage drop of the diodes and high reverse recovery losses caused by the storage charge of the diodes. On the other hand, too short a dead time may cause shoot-through. Especially, when noise in a converter causes variation in timing, a predetermined short dead time may not be enough to prevent shoot-through of the secondary switches. Such a shoot-through current not only causes high power losses, but also generates high voltage and current stresses, which may detrimentally damage the components of the converter. InFIG. 8A, waveforms802,804,806,808,810and812show long dead time1and dead time2are added between the turn-on pulses of S2, S3and the turn-on pulses of Q1and Q4. The typical consideration in the dead time control is to make sure the outgoing secondary switches are turned off before V_AB changes its state.

FIG. 8Billustrates a dead time control scheme based upon the full bridge converter400. As shown by waveforms816,818,820,822,824and826, in comparison toFIG. 8A, the dead time1and dead time2between two pulses are reduced to a level vulnerable to shoot-through. However, Lr in the primary side, as an inductive element, can limit a flow of shoot-through currents. In addition, D5and D6(not shown but illustrated inFIG. 2), as clamping elements, can help to reduce the oscillation due to shoot-through. One additional advantageous feature of the soft switching apparatus200is that it not only reduces unnecessary power losses resulted from a long dead time, but also improves the reliability of bridge converters by alleviating the shoot-through issues. Because of the soft switching apparatus200, it is possible to eliminate the dead time so that the current in the secondary side may not flow through any diode during normal operation. Furthermore, the conduction of the switches being turned off (e.g., S2and S3inFIG. 8B) can be extended to a point, which is slightly after the point at which V_A changes from low to high, so that the dead time becomes negative. In sum, the soft switching apparatus200can improve the operation of a converter by reducing the dead time of the converter.

Referring now toFIGS. 9-12, the concept of adding the soft switching apparatus200into a full bridge converter can be extended to a family of bridge converters including full bridge converters having a secondary diode rectifier, and a half bridge converter having either a synchronous rectifier or a diode rectifier. The diode rectifier shown inFIG. 9is known in the art. Replacing a synchronous rectifier shown inFIG. 4with a diode rectifier generally has no impact on the operation of the soft switching apparatus200. In fact, the soft switching apparatus200works in a similar way to that inFIG. 4.

InFIG. 10, the soft switching apparatus200is applied to a half bridge converter. Similar to that in the full bridge converter400, the soft switching apparatus200can reduce the amplitude of the primary side current in a freewheeling period as well as the voltage stress across the secondary rectifier. The operation of the circuits shown inFIG. 10andFIG. 11is similar except that the secondary ofFIG. 11employs a synchronous rectifier rather than a diode rectifier.

Furthermore, the soft switching apparatus200is applicable to half bridge converters having different control schemes. Even if a half bridge converter may have a different name by employing a different control scheme, the derivative from the basic half bridge converter is still within the scope of the present invention. For example, inFIG. 12A, waveforms1202,1204,1206and1208represent a fix duty cycle control of a half bridge converter. In contrast, FIG.12B includes a set of waveforms1212,1214,1216and1218representing an asymmetrical duty cycle control scheme. Under such a control scheme, a half bridge converter is also called an asymmetrical half bridge converter. Nevertheless, the soft switching apparatus200is applicable to the asymmetrical half bridge converter too. A person having ordinary skill in the art will recognize a variety of alternatives are within the scope of the present invention. In summary, the soft switching apparatus200can be applied to a variety of bridge converters including half bridge and full bridge converters.