1. Field of the Invention
The present invention relates to a power conversion controller, especially to a fixed-on-time controller for discontinuous mode PFC (Power Factor Correction) power conversion.
2. Description of the Related Art
Fixed-on-time power conversion is capable of achieving good PFC for discontinuous mode power conversion when the line input is a full-wave rectified voltage. The reason is as follows:
Let the line input be VIN(t)=|VA sin(ωt)|, the fixed on time=tON, the inductance=L, and the input current at the end of an on period=IINPEAK(tX), then IINPEAK(tx)=(VIN(tX)/L)tON=(|VA sin(ωtX)|/L)tON, wherein tON is much smaller than 2π/ω. As can be seen in the above equation, the input current will follow the line input at every on period and a good power factor correction is therefore achieved.
To get insight into the principle of the fixed-on-time discontinuous mode PFC power conversion, please refer to FIG. 1, which illustrates a block diagram of a discontinuous mode PFC power converter using a prior art fixed-on-time controller. As illustrated in FIG. 1, the discontinuous mode PFC power converter includes an input rectifier circuit 101, a transformer circuit 102, an output rectifier circuit 103, a load 104, a resistor 105, and a fixed-on-time controller 110.
The input rectifier circuit 101 is used to generate a full wave rectified voltage VFULL—WAVE according to an AC power VAC.
The transformer circuit 102 is used to transfer the power from the full wave rectified voltage VFULL—WAVE to the output rectifier circuit 103, under the control of a driving signal VDRV of the fixed-on-time controller 110.
The output rectifier circuit 103 is used to provide a DC voltage VO to the load 104, and the resistor 105 is used to provide a feedback signal VFB for the fixed-on-time controller 110.
The fixed-on-time controller 110 includes an error amplifier 111, a constant current source 112, a capacitor 113, a switch 114, a comparator 115, and a fixed-on-time driver circuit 116.
The error amplifier 111 has a negative input end coupled to the feedback signal VFB, a positive input end coupled to a reference voltage VREF, and an output end for providing a threshold signal VCOMP.
The constant current source 112, the capacitor 113, and the switch 114 are used to generate a saw signal VSW.
The comparator 115 has a negative input end coupled to the threshold signal VCOMP, a positive input end coupled to the saw signal VSW, and an output end for providing a turn-off signal VOFF.
The fixed-on-time driver circuit 116 is used to provide the driving signal VDRV for the transformer circuit 102 and a reset signal RESET for the switch 114, according to the turn-off signal VOFF from the comparator 115 and a sensing signal VAUX from the transformer circuit 102, wherein the sensing signal VAUX is used to indicate the end of an inductor current discharging period of the transformer circuit 102, and the active time point of the reset signal RESET follows that of the turn-off signal VOFF.
When in operation, the voltage of the driving signal VDRV will arise from a low level to a high level after the sensing signal VAUX becomes active, and fall from a high level to a low level after the turn-off signal VOFF becomes active. Besides, the period the driving signal VDRV remains at a high level—the on time of the transformer circuit 102—will be fixed to a value by the feedback control mechanism of the power conversion to transfer a specific amount of energy per cycle from the AC power VAC to the load 104, to regulate VFB at VREF.
However, as the period the driving signal VDRV remains at a high level—a fixed on time of the transformer circuit 102 corresponding to a load value of the load 104 and a line voltage of the AC power VAC—is ended when the saw signal VSW reaches the threshold signal VCOMP, the threshold signal VCOMP will exhibit a large level shift to change the period of the fixed on time from a short/long value to a long/short value when the load value of the load 104 or the line voltage of the AC power VAC changes drastically.
Please refer to FIG. 2, which illustrates the waveforms of major signals in the prior art fixed-on-time controller 110 of FIG. 1 corresponding to a low line and a high line of the AC power VAC respectively. As illustrated in FIG. 2, VDRV, LOW—LINE (the driving signal VDRV generated corresponding to a low line of the AC power VAC) has a fixed on time tON1, and VDRV, HIGH—LINE (the driving signal VDRV generated corresponding to a high line of the AC power VAC) has a fixed on time tON2, wherein tON1 is longer than tON2 such that same power is delivered to the load 104. As the fixed on time is ended when the saw signal VSW—of which the ramping up slope is fixed by the constant current source 112—reaches the threshold signal VCOMP, the fixed on time is then controlled by the level of the threshold signal VCOMP. As such, a higher VCOMP, LOW—LINE (the threshold signal VCOMP corresponding to the low line of the AC power VAC) is generated to allow a higher VSW, LOW—LINE (the saw signal VSAW corresponding to the low line of the AC power VAC) and therefore the longer tON1; and a lower VCOMP, HIGH—LINE (the threshold signal VCOMP corresponding to the high line of the AC power VAC) is generated to allow a lower VSW, HIGH—LINE (the saw signal VSAW corresponding to the high line of the AC power VAC) and therefore the shorter tON2, wherein the VCOMP, LOW—LINE is higher than the VCOMP, HIGH—LINE by ΔV1.
That is, ΔV1 of the prior art fixed-on-time controller 110 can be a large value when the load value of the load 104 or the line voltage of the AC power VAC changes drastically. However, large ΔV1 is adverse to the design of the error amplifier 111. To minimize ΔV1, one solution is to change the capacitance of the capacitor 113 to cope with the amplitude variation of the AC power VAC or the load value variation of the load 104. However, it is bothersome to change the capacitor 113 whenever the AC power VAC or the load 104 is changed, and besides, the solution is adverse to the integration of the capacitor 113 into the fixed-on-time controller 110.
In view of these problems, the present invention proposes a novel fixed-on-time controller for discontinuous mode PFC power conversion.