Self-driven synchronous rectification scheme for wide output range

[0002] The present invention relates to a logic integrated circuit. In particular, the present invention relates to a self-driven, synchronous rectification scheme for a power converter which is easily adapted to various circuit topologies.

[0003] There is an ever-increasing demand in the power electronics market for low voltage and high current DC—DC converters. As output voltage is desired to be 3.3V or lower, even a state-of-the-art schottky diode with a forward voltage drop of 0.3V has an unacceptable amount of power loss.

[0004] Because of this, synchronous rectifiers are often used to improve the efficiency of DC—DC converters. Generally, there are two types of synchronous rectifiers, self-driven and externally driven. Since the self-driven mode is usually less complex, less costly and more reliable, it is preferred for use with most low voltage DC—DC converter applications.

[0005] FIG. 1A illustrates a conventional self-driven synchronous rectification, asymmetrical, zero voltage switching (ZVS) half-bridge (HB) topology. Although this circuit is very simple, it is only suitable for applications where the output voltage is in the range of from about 3.3V to 6V. Referring to FIG. 1B, the gate-drive voltages V_gs3and V_gs4of synchronous rectifiers S₃and S₄, respectively, are as follows: $\begin{array}{cc}{V}_{\mathrm{gs3}}=\frac{2N_s}{N_p}{\mathrm{DV}}_{\mathrm{in}}=\frac{2}{N}{\mathrm{DV}}_{\mathrm{in}}=\frac{V_0}{1-D}\left(t_0\le t\le t_1\right)& \left(1\right)\\ V_{\mathrm{gs4}}=\frac{2N_s}{N_p}\left(1-D\right)V_{\mathrm{in}}=\frac{2\left(1-D\right)}{N}{V}_{\mathrm{in}}=\frac{V_0}{D}\left(t_1\le t\le t_2\right)& \left(2\right)\end{array}$

[0006] wherein, V_inis the input voltage; V_ois the output voltage; D is the steady-state duty cycle; N_pis the number of primary winding turns of the transformer; N_sis the number of secondary turns of the transformer; and N is the turns ratio of the transformer. The turns ratio of the transformer TR is calculated by dividing the number of primary windings by the number of secondary windings (i.e. N=N_p/N_s).

[0007] FIG. 1B illustrates the switching waveform occurring in the converter illustrated in FIG. 1A. As shown in FIG. 1B, the gate-drive voltage V_gs4of S₄is always higher than the gate-drive voltage V_gs3of S₃if D is less than 50%. If we assume that the minimum steady-state duty cycle D at heavy load is 30%, then V_gs3is about 1.4V, and V_gs4is about 3.3V. Since most synchronous rectifiers (including logic level devices) only work well with the gate-drive voltage between about 4V and 20V, the circuit shown in FIG. 1A only works well when the output voltage V_Ois between 2.9V to 6V. If the output voltage is below 2.9V, S₃would be under driven. If the output voltage were about 6V, then S₄would be over driven. In either case the synchronous rectifiers are easily rendered inoperative.

[0008] Accordingly, there remains a need for a self-driven synchronous rectifier which operates normally at various output voltages, such as low output voltages of 2.9V or lower and/or high output voltages above 6V.

[0009] The self-driven synchronous rectification circuit of the present invention includes two power switches S₁and S₂; a transformer Tr having a primary winding with N_pnumber of turns, a secondary winding with N_snumber of turns and an auxiliary winding with N_anumber of turns; two secondary synchronous rectifiers S₃and S₄; two diodes D₁and D₂; and two zener diodes ZD₁and ZD₂.

[0010] The number of auxiliary winding turns N_aof the transformer Tr ensure that the synchronous rectifiers S₃and S₄are supplied with an adequate gate-drive voltage. The selection of the number of auxiliary winding turns for use is determined according to the output voltage required. In the circuit of the present invention, when S₃conducts, the gate-drive voltage of S₄is clamped by D₁. Also, when S₄conducts, the gate-drive voltage of S₃is clamped by D₂. In other words, D₁and D₂prevent S₃and S₄from conducting at the same time. ZD₁and ZD₂restrain the gate overvoltage of S₃and S₄, respectively.

[0011] With the above circuit configuration, a self-driven synchronous rectification scheme can be implemented which operates normally at various output voltages, such as low output voltages of 2.9V or lower and/or high output voltages above 6V.

[0012] Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings, wherein:

[0013] FIG. 1A is a circuit diagram of conventional self-driven synchronous rectification, asymmetrical ZVS HB converter;

[0014] FIG. 1B illustrates the switching waveform occurring in the converter illustrated in FIG. 1A;

[0015] FIG. 2A is a circuit diagram of an asymmetrical ZVS HB converter which incorporates a self-driven synchronous rectifier circuit in accordance with the present invention;

[0016] FIG. 2B illustrates the switching waveform occurring in the converter of FIG. 2A;

[0017] FIG. 3 is a circuit diagram showing the self-driven synchronous rectifier circuit of the present invention applied to a forward converter; and

[0018] FIG. 4 is a circuit diagram showing the self-driven synchronous rectifier circuit of the present invention applied to a full-bridge converter.

[0019] Referring now to the drawings, FIG. 2A shows a circuit diagram of an asymmetrical ZVS HB converter incorporating the self-driven synchronous rectifier circuit of the present invention. The self-driven synchronous rectifier circuit of the present invention includes two power switches S₁and S₂; a transformer Tr having a primary winding with N_pnumber of turns, a secondary winding with N_snumber of turns and an auxiliary winding with N_anumber of turns; two secondary synchronous rectifiers S₃and S₄; two diodes D₁and D₂; and two zener diodes ZD₁and ZD₂.

[0020] The number of auxiliary winding turns N_aof the transformer Tr ensure that the synchronous rectifiers S₃and S₄are supplied with an adequate gate-drive voltage. The selection of the number of auxiliary winding turns for use is determined according to the output voltage required. Such a determination will be evident to one of ordinary skill in the art, and will depend upon, for example, the wire size chosen and the inductance required of the winding. In the circuit of the present invention, when S₃conducts, the gate-drive voltage of S₄is clamped by D₁. Also, when S₄conducts, the gate-drive voltage of S₃is clamped by D₂. In other words, D₁and D₂prevent S₃and S₄from conducting at the same time. ZD₁and ZD₂operate to restrain the gate overvoltage of S₃and S₄, respectively. Because of this circuit configuration, the self-driven synchronous rectifier circuit operates normally at various output voltages, such as low output voltages of 2.9V or lower and/or high output voltages above 6V.

[0021] FIG. 2B illustrates the switching waveform of the converter shown in FIG. 2A. V_gs1, and V_gs2represent the gate voltage waveforms of the two power switches S₁and S₂. VP is the primary voltage waveform of transformer Tr. V_Nais the voltage waveform of the auxiliary winding. V_gs3and V_gs4represent the gate voltage waveforms of the two synchronous rectifiers S₃and S₄. V_gs3and V_gs4are calculated as follows: $\begin{array}{cc}{V}_{\mathrm{gs3}}=\frac{N_a}{N_p}{\mathrm{DV}}_{\mathrm{in}}\left(t_0\le t\le t_1\right)& \left(3\right)\\ V_{\mathrm{gs4}}=\frac{N_a}{N_p}\left(1-D\right)V_{\mathrm{in}}\left(t_1\le t\le t_2\right)& \left(4\right)\end{array}$

[0022] wherein, D is the on-time of switch S₁in percent duty cycle; 1-D is the on-time of switch S₂in percent duty cycle; N_pis the number of primary winding turns of transformer Tr; N_ais the number of auxiliary winding turns of the transformer Tr; and V_inis the input voltage.

[0023] Comparing equations (3) and (4) above with equations (1) and (2), it can be seen that, while the input voltage V_in, the duty cycle D and the primary and secondary turns of the transformer (N_p, N_s) are determined, the gate voltage of self-driven synchronous rectifiers S₃and S₄is adjusted by selecting the number of auxiliary winding turns N_aof transformer Tr. This selection of the number of auxiliary winding turns N_aensures a reasonable gate-drive voltage for the synchronous rectifiers S₃and S₄even when the output voltage is lower than 2.9V or higher than 6V.

[0024] The principle of operation is as follows. Referring to FIG. 2B, during time t₀-t₁, S₂and S₃are on, and S₁and S₄are off. The primary voltage of transformer Tr is DV_in. Accordingly, the voltage of the auxiliary winding V_Na, namely the gate-drive voltage V_gs3of S₃is as follows: $\frac{N_a}{N_p}{\mathrm{DV}}_{\mathrm{in}}$

[0025] When S₃is on, the gate-drive voltage V_gs4of S₄is clamped to zero through the diode D₁so as to prevent S₃and S₄from conducting at the same time. At time t₁, S₂is turned off and S₁is turned on. When S₁is turned on, the voltage of the primary winding N_pof transformer T_rreverses and the voltages of the secondary winding N_sand auxiliary winding N_aalso reverse. When the voltages of the windings reverse, S₄is turned on and S₃is turned off. During this interval, the gate-drive voltage V_gs4of S₄is as follows: $\frac{N_a}{N_p}\left(1-D\right)V_{\mathrm{in}}$

[0026] When S₄is on, the gate-drive voltage of S₃is clamped to zero through the diode D₂so as to prevent S₃and S₄from conducting at the same time. The function of ZD₁and ZD₂is to restrain the gate overvoltage of each of the synchronous rectifiers S₃and S₄, respectively.

[0027] While the embodiment of the invention described above is presently preferred, many variations and modifications are possible depending on practical needs.

[0028] For example, the self-driven synchronous rectifier circuit of the present invention can be applied to a symmetrical HB converter. With the proper adjustments, which are well within the level of one of ordinary skill in the art given the present disclosure, the self-driven synchronous rectifier circuit can also be applied to a ZVS HB converter whose duty cycle is close to 50%.

[0029] In addition to an asymmetrical and a symmetrical HB converter, the self-driven synchronous rectifier circuit of the present invention can be applied to a forward converter, such as that shown in FIG. 3, and a full-bridge converter, such as that shown in FIG. 4.

[0030] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.