Forward converter with improved reset circuitry

[0002] The present invention relates to DC-to-DC converters, AC-to-DC converters. The major characteristic of this converter is that transfer energy from input to output with high efficiency and low circulating energy.

[0003] In the field of power conversion technology, increasing the work duty and reducing the voltage stress are the most important demands. Basically, tertiary winding reset technology is desirable as it allows forward converter to transfer the power from input to output. However, the work duty of tertiary winding reset forward converter is limited at 50% maximum due to the requirement of voltage-second balance principle and the peak voltage of primary switching device is kept in two times of input voltage. In order to overcome limitations in work duty and voltage stress, the prior art has been devised the active clamp technology and passive clamped technology.

[0004] FIG. 1ashows the active clamp forward converter. The active clamp forward converter has been derived from conventional forward converter by adding the switch S_Ato select connecting the transformer to a reset capacitor C_Rwhen the primary power switch S_Pis turned OFF. The gate driving signals are separated by dead time with small period and supplied from a two channel complementary pulse-width-modulated controller. FIG. 1bshows the timing diagram of the circuit of FIG. 1a.

[0005] Active clamp reset technology reduces the voltage stress and allows the work duty is greater then 50% that the power handling of transformer is improved and reduce the power rating of primary switching device. However, the optimal gate drive waveform is generated from highly circuit complexity controller to prevent the damage from cross conduction between S_Aand S_P, the high side driver is required to control the auxiliary switch S_A. The voltage rating of auxiliary switch is equal to input voltage that the component selection is difficult to meet cost effect requirement in high input voltage applications.

[0006] FIG. 2ashows the passive clamp forward converter. The passive clamp forward converter has been derived from active clamp forward converter by relocating the clamp circuit from primary side to secondary side. Auxiliary switch S_Ais drives by secondary voltage to select connecting the transformer to a reset capacitor C_Rwhen the primary power switch S_Pis turned off. The gate drive signal is supplied from a single channel pulse-width-modulated controller. FIG. 2bshows the timing diagram of the circuit of FIG. 2a. This is a forward converter using the passive clamp technology and drive by a simple PWM controller. The current waveform of auxiliary switch shows low level magnetizing current with small period load current.

[0007] Passive clamp technology reduce the voltage rating of auxiliary switch and the circuit complexity of controller, it take advantages as active clamp technology but energy recover of leakage inductance. However, the maximum output current rated auxiliary switch is required to prevent the damage from simultaneous high current due to the transition of secondary voltage. The component selection is difficult to meet cost effect requirement in high output current applications.

[0008] What is the best reset technology? Passive drive, less circuit complexity of controller, low rating requirement in high frequency operation are the most important demands of transformer reset technology.

[0009] The invention utilizes a forward converter with new reset concept to enhance the performance of power converter. Briefly, this reset circuit consists of a clamp diode, a reset capacitor, an inductor, an auxiliary switch and an auxiliary diode. The clamp diode provides a single direction path to transfer the magnetizing energy from transformer to reset capacitor when primary power switch is turn OFF and secondary reverse voltage of transformer is turned positive. The auxiliary switch is turn ON to provide a path to discharge the energy from capacitor through the inductor to the source when primary power switch is turn OFF. The auxiliary diode provides the path to conducts the current of inductor when auxiliary switch is turn OFF.

[0010] The new converter provides optimal transformer reset: It eliminate the limitation of maximum work duty, as it form the voltage waveform keep in “quasi-square” automatically during OFF period; It simplifies converter construction, as it can be implemented without extra control circuits; It is worked well and do not require any large rating device, as it can be widely used with low cost.

[0011] FIG. 1ashows the active clamp forward converter.

[0012] FIG. 1bshows the timing diagram of the circuit of FIG. 1a.

[0013] FIG. 2ashows the passive clamp forward converter.

[0014] FIG. 2bshows the timing diagram of the circuit of FIG. 2a.

[0015] FIG. 3ashows the buck reset forward converter.

[0016] FIG. 3bshows the timing diagram of the circuit in FIG. 3a.

[0017] FIG. 4ashows the transformer magnetizing stage of circuit in FIG. 3a.

[0018] FIG. 4bshows the transformer de-magnetizing stage of circuit in FIG. 3a.

[0019] FIG. 5 shows the topological variations of buck reset forward converter.

[0020] FIG. 6 shows the buck reset forward converter with synchronous rectifier.

[0021] FIG. 7 shows buck reset forward converter with reorganized reset circuit.

[0022] FIG. 8 shows buck reset forward converter with primary side reset circuit.

[0023] Referring now to the drawings, and more particular to FIG. 3a, a buck reset forward converter in accordance with the present invention is illustrated. A buck reset forward converter comprises a conventional forward converter (input source V₁, primary switch S_P, controller PWM, transformer T₁, power rectifiers D₁and D₂, smooth choke L_O, smooth capacitor C_Oand resistive load R_Lare included, the smooth choke is large enough to used as an output DC current source and the auxiliary inductor is large enough to used as an resetting DC current source), a clamp diode D_C, an auxiliary diode D_A, a reset capacitor C_R, an auxiliary switch S_Aand an auxiliary inductor L_A. The clamp diode D_Cprovides a single direction path to transfer the magnetizing energy from transformer secondary to reset capacitor C_Rwhen primary power switch is turn OFF. The auxiliary switch S_Ais turn ON passively to provide a path to discharge the energy from reset capacitor C_Rthrough the auxiliary inductor L_Ato the output load R_Lwhen secondary voltage of transformer is negative. The auxiliary diode D_Aprovides the path to conducts the current of auxiliary inductor L_Awhen auxiliary switch S_Ais turn OFF.

[0024] A buck reset topology with forward converter show in FIG. 3awhose typical operation waveforms show in FIG. 3bis different from that of the prior art active clamp technology or passive clamp technology with the same converter. Under steady-state operation, two stages are equal to two equivalent circuits within one switching cycle shown in FIG. 4aand FIG. 4b.

[0025] Referring to FIG. 4a, during T0 to T1 interval, the primary power switch S_Pis turned ON at T0 and the inductor current of smooth choke as a current source I_Ois transferred from input source V_Ito the output load, the current of magnetizing inductance L_Mis charged by input source V_Iand increased linearly from zero. The current of auxiliary inductor is conducted through auxiliary diode D_Ato the smooth capacitor C_Oas a current source I_Rduring this interval.

[0026] Referring to FIG. 4b, from T1 to T0 interval, the primary power switch S_Pis turned OFF at T1 and the current of smooth choke L_Ois conducts through power rectifier D₂. The auxiliary switch S_Ais turned ON via reversed secondary voltage and the auxiliary diode D_Ais turned OFF. The magnetizing current of transformer is coupled to secondary side through the clamp diode D_Cand charge to reset capacitor C_R. The capacitance of reset capacitor C_Ris large enough to be used as a reset voltage source V_R. The current of auxiliary inductor is through the auxiliary switch S_Aand supplies from reset voltage source V_R. This interval ends at T0, when the magnetizing current of transformer goes zero, the primary power switch S_Pis turn ON again to starting the next switching cycle.

[0027] It should be noted that reorganize the power circuit and reset circuit by applying duality principle could derive the topological variations of buck reset forward converter show as FIG. 5.

[0028] Referring to FIG. 5, when the primary power switch S_Pis turned ON, the inductor current of smooth choke is transferred from input source V_Ito the output load, the current of magnetizing inductance L_Mis charged by input source V_Iand increased linearly from zero. The current of auxiliary inductor is conducted through auxiliary diode D_Ato the smooth capacitor C_O.

[0029] When the primary power switch S_Pis turned OFF, the current of smooth choke L_Ois conducts through power rectifier D₂. The auxiliary switch S_Ais turned ON via reversed secondary voltage and the auxiliary diode D_Ais turned OFF. The magnetizing current of transformer is coupled to secondary side through the clamp diode D_Cand charge to reset capacitor C_R. The current of auxiliary inductor is through the auxiliary switch S_Ato the smooth capacitor C_O. The current of magnetizing inductance L_Mis discharged to reset capacitor C_Rand decreased linearly to zero.

[0030] The power rectifiers D₁and D₂show as FIG. 3acould be replaced by MOSFET used as synchronous rectifier to improve the conversion efficiency. The buck reset forward converter with synchronous rectifier show as FIG. 6.

[0031] Referring to FIG. 6, when the primary power switch S_Pis turned ON, the inductor current of smooth choke is conducts from secondary winding through synchronous rectifier SR₁to the output load. The synchronous rectifier SR₁is turn ON via forward voltage of secondary winding and synchronous rectifier SR₂is turn OFF via reverse voltage. The current of magnetizing inductance L_Mis charged by input source V_Iand increased linearly from zero. The current of auxiliary inductor is conducted through auxiliary diode D_Ato the smooth capacitor C_O.

[0032] When the primary power switch S_Pis turned OFF, the current of smooth choke L_Ois conducts through synchronous rectifier SR₂. The synchronous rectifier S_R, is turned OFF by forward voltage of secondary winding and synchronous rectifier SR₂is turn ON via reverse voltage. The auxiliary switch S_Ais turned ON via reversed secondary voltage and the auxiliary diode D_Ais turned OFF. The magnetizing current of transformer is coupled to secondary side through the clamp diode D_Cand charge to reset capacitor C_R. The current of auxiliary inductor is through the auxiliary switch S_Ato the smooth capacitor C_O. The current of magnetizing inductance LM is discharged to reset capacitor C_Rand decreased linearly to zero.

[0033] Relocation the auxiliary switch and put a simple gat drive network to shift the average level of drive signal to return level may eliminate the auxiliary diode D_A. The buck reset forward converter with reorganized reset circuit is show in FIG. 7.

[0034] Referring to FIG. 7, when the primary power switch S_Pis turned ON, the inductor current of smooth choke is conducts from secondary winding through synchronous rectifier S_R, to the output load. The synchronous rectifier S_R, is turn ON via forward voltage of secondary winding and synchronous rectifier SR₂is turn OFF via reverse voltage. The gate voltage of auxiliary switch S_Ais turns to positive value to turn OFF the auxiliary switch S_A. The current of magnetizing inductance L_Mis charged by input source V_Iand increased linearly from zero. The current of auxiliary inductor is conducted through clamp diode D_Cand synchronous rectifier SR₁to the smooth capacitor C_O.

[0035] When the primary power switch S_Pis turned OFF, the current of smooth choke L_Ois conducts through synchronous rectifier SR₂. The synchronous rectifier SR₁is turned OFF via forward voltage of secondary winding and synchronous rectifier SR₂is turn ON via reverse voltage. The gate voltage of auxiliary switch S_Ais turn to negative value of output voltage to turn ON the auxiliary switch S_A. The magnetizing current of transformer is coupled to secondary side through the clamp diode D_Cand charge to reset capacitor C_R. The current of auxiliary inductor is through the auxiliary switch S_Aand reset capacitor C_Rto the smooth capacitor C_O. The current of magnetizing inductance L_Mis discharged to reset capacitor C_Rand decreased linearly to zero.

[0036] The reset circuit show as FIG. 7 can be apply in primary side and drive by main control signal to recover the stored energy of leakage inductance and reset the flux of transformer. The buck reset forward converter with primary side reset circuit is show in FIG. 8.

[0037] Referring to FIG. 8, when the voltage of PWM controller is turns to positive value, the primary power switch S_Pis turned ON and the auxiliary switch S_Ais turned OFF, the inductor current of smooth choke is conducts from secondary winding through synchronous rectifier SR₁to the output load. The synchronous rectifier SR₁is turn ON via forward voltage of secondary winding and synchronous rectifier SR₂is turn OFF via reverse voltage. The current of magnetizing inductance L_Mis charged by input source V_Iand increased linearly from zero. The current of auxiliary inductor is conducted through clamp diode D_Cand primary power switch S_Pto the input source V_I.

[0038] When the voltage of PWM controller is turns to negative value, the primary power switch S_Pis turned OFF and the auxiliary switch S_Ais turned ON, the current of smooth choke L_Ois conducts through synchronous rectifier SR₂. The synchronous rectifier S_R, is turned OFF via forward voltage of secondary winding and synchronous rectifier SR₂is turn ON via reverse voltage. The magnetizing current of transformer is through the clamp diode Dc and charge to reset capacitor C_R. The current of auxiliary inductor is through the auxiliary switch S_Aand reset capacitor C_Rto the input source V_I. The current of magnetizing inductance L_Mis discharged to reset capacitor C_Rand decreased linearly to zero.

[0039] A buck reset forward converter eliminate the limitation of maximum work duty, as it forms the voltage waveform keep in “quasisquare” automatically during OFF period; It simplifies converter construction, as it can be implemented without extra control circuits; It is worked well and do not require any large rating device, as it can be widely used with low cost.

[0040] While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.