Switched-mode supply circuit and control circuit for use in such a power supply circuit

Abstract: In switched-mode power supply circuits the switch is rendered conducting for a longer period (or more frequently), dependent on the load connected to the power supply circuit, so as to ensure that the transformer can transfer enough energy to the secondary side for charging the output capacitor to a sufficient extent. To ensure that the transformer can always supply all energy stored in the transformer to the secondary side, a demagnetizing detection circuit detects the demagnetization of the transformer. If the load decreases (for example, in standby operation) the on-time of the switch is reduced. However, this on-time may achieve a minimum value so that the power supply circuit no longer functions properly. To obtain an efficiently operating power supply circuit, both in normal operation and in a low-load state, the control circuit of the power supply circuit has at least two modes, i . e . a first mode (standby operation) in which the control pulses are applied to the switch at a fixed frequency and the demagnetizing signal has no influence, and a second mode (normal operation) at a variable frequency, in which second mode the frequency of the control pulses is variable and, if necessary, the demagnetizing signal can delay the control pulses applied to the switch when the transformer is not fully demagnetized yet.

Description: The invention relates to a switched-mode power supply circuit as defined in the preamble of claim 1. European Patent Application EP-A 380033 discloses a power supply circuit which has a pulse width modulation control in which the control circuit receives two input signals, i.e. a first input signal for determining the starting moment of a new pulse and a second input signal for receiving a signal related to the output voltage. This known power supply circuit has the drawback that it does not operate efficiently, while the transformer may become saturated. It is an object of the invention to eliminate the above-mentioned drawbacks. To this end, a first aspect of the invention provides a power supply circuit as defined in claim 1. A second aspect of the invention provides a control circuit for use in such a power supply circuit. It is thereby achieved that the power supply circuit optimally functions in normal operation as well as in standby operation. For normal operation it is of great importance that the transformer is fully demagnetized before the switch is rendered conducting again, because otherwise the transformer becomes saturated so that it no longer operates correctly and the energy can no longer be transported to the secondary side. In standby operation the demagnetizing signal is not important, because the transformer will not become saturated. However, in standby operation it should be ensured that the switch is conducting during the minimum on-time. Since the load in standby operation is much smaller than in normal operation, the switch should be conducting for a much shorter time. To prevent the minimum on-time from being reached, the frequency of the control pulses of the switch in standby operation is set at a fixed value (generally a lower frequency). It is to be noted that European Patent Application EP-A 420997 discloses a power supply circuit in which the control circuit receives a demagnetizing signal at an input. However, in this power supply circuit there is no switching in any way between normal operation and standby operation. Advantageous embodiments are defined in the subclaims. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings, Fig. 1 shows a first embodiment of a switched-mode power supply circuit according to the invention, Fig. 2 shows voltage-time diagrams for the different signals, Fig. 3 shows an embodiment of a demagnetizing detection circuit for a power supply circuit according to the invention, and Fig. 4 shows an embodiment of a part of an oscillator circuit. Fig. 1 shows an embodiment of a switched-mode power supply circuit VS with input terminals 1 and 2 at which an input voltage Vi is supplied (for example, from a rectifier connected to the mains). A series arrangement of a primary winding Lp of a transformer T and a switch S is connected across the input terminals 1, 2. The switch S may be, for example a bipolar high-voltage transistor or a MOS field effect transistor. A series arrangement of a diode Do and a storage capacitor Co is connected across a secondary winding Ls of the transformer. Output terminals 3 and 4 of the power supply circuit are connected across the storage capacitor for supplying an output voltage Vo. A load Rb may be connected across the output terminals 3, 4. The switch S is rendered conducting and non-conducting by means of control pulses P from an output 9 of a control circuit 6, the on-time (period of conductance) of the switch being determined , inter alia by the load connected to the power supply circuit. An input 8 of the control circuit receives a feedback signal Vb which is a measure of the power to be supplied by the power supply circuit to the load. A second input 7 of the control circuit 6 receives a signal Id indicating whether the transformer T is fully demagnetized. The second input 7 is connected to an extra winding Ld on the transformer for detecting whether the transformer is demagnetized or not demagnetized. The control circuit 6 comprises a load detection circuit 63 which is coupled to the input 8 for receiving the feedback signal Vb. The control circuit further comprises a demagnetizing detection circuit 61 coupled to the input 7 for receiving the signal Id. With reference to this signal, the demagnetizing detection circuit determines whether the transformer is fully or not fully demagnetized. It is of great importance that the transformer is fully demagnetized before the switch S becomes conducting again. If the switch is rendered conducting before the transformer is fully demagnetized, the transformer will be fully saturated after some time and no longer operate as a transformer so that the power supply circuit can no longer supply the required output voltage. Moreover, the power supply circuit will use (dissipate) much more energy when the transformer becomes saturated or operates proximate to its range of saturation, as compared with the situation in which the transformer does not operate in its range of saturation. The control circuit further comprises an oscillator circuit 65 for generating and supplying a control signal to the switch. This oscillator circuit comprises switching means which switch the oscillator operation between two different modes. In a first mode the oscillator frequency is variable and the frequency is controlled in dependence upon the load Rb. However, to prevent the transformer from becoming saturated, the oscillator circuit 65 receives a demag-signal Dd from the demagnetizing detection circuit 61, which demag-signal in this mode prevents the oscillator circuit from supplying a subsequent control pulse to the switch before the transformer T is fully demagnetized. In this first mode the oscillator circuit 65 operates as long as the load of the power supply circuit is high enough, i . e . during normal operation. If the load falls below a predetermined value, the oscillator circuit switches over to a second mode, in which second mode the oscillator supplies control pulses at a fixed frequency and at which the demag-signal Dd is no longer considered. At a low load the transformer can never become saturated and the demag-signal thus need not be considered. By causing the control circuit to operate in two different modes, dependent on the load, a power supply circuit is obtained which operates efficiently in both modes and thus has a minimum dissipation. Fig. 2 shows voltage-time diagrams for the two modes. Fig. 2A shows the voltages for a low load and Fig. 2B shows the same voltages for a high load. Fig. 2A shows, one below the other, the voltage Vs across the switch S, the demagnetizing signal Id, the oscillator sawtooth Os from which the control pulses P are derived and the demag-signal Dd. As described above, the oscillator circuit 65 does not consider the demag-signal Dd at a low load (for example, during standby), and in Fig. 2A the demag-signal is already low before the oscillator circuit sends the next control pulse to the switch S. The demag-signal has no influence on the starting moment of the rising edge of the oscillator sawtooth Os. Fig. 2B also shows, one below the other, the voltage Vs across the switch S, the demagnetizing signal Id, the oscillator sawtooth Os from which the control pulses P are derived and the demag-signal Dd. Here the starting point is a high load Rb and the Figure shows that the rising edge of the oscillator sawtooth (hence the next control pulse to the switch) does not start until the demag-signal Dd has become low and, consequently, the transformer is first fully demagnetized before the switch S is rendered conducting again. Fig. 3 shows the demagnetizing detection circuit 61 in greater detail. As described hereinbefore, the input of the demagnetizing detection circuit is connected to the input 7 of the control circuit, at which input the demagnetizing detection circuit receives the demagnetizing signal Id. The input 7 is connected to a clamping circuit 611 for clamping the demagnetizing signal. An output of the clamping circuit is connected to a non-inverting input of a comparator 613 which receives a reference voltage Vref of, for example 65 mV at the inverting input. The output of the comparator 613 is connected to a reset input R of a flipflop 615. The set input S of the flipflop receives a signal from the oscillator circuit (a high signal at a rising edge of the oscillator sawtooth Os (see Fig. 2) and a low signal at a falling edge of the oscillator sawtooth). As long as the demagnetizing signal Id is high, a digital high signal is present at the input R of the flipflop so that the output Q of the flipflop will be low. The output of the comparator is also connected to a first input of an AND circuit 617, and a second input of the AND circuit is connected to the output Q of the flipflop. The AND circuit supplies the demag-signal Dd (to the oscillator circuit) at the output. In this way it is ensured that the oscillator circuit cannot apply control pulses to the switch in the first mode (hence at a high load) as long as the demagnetizing signal Id is high and the transformer is consequently not fully demagnetized. Fig. 4A shows an embodiment of a part of the oscillator circuit 65 in greater detail. A current source 651 is arranged in series with a switch S2 and a capacitor Ct, the current source charging the capacitor when the switch is closed. A series circuit of a switch S3 and a current source 652 for discharging the capacitor is connected to the junction point of the switch S2 and the capacitor Ct. The junction point of the switch S2 and the capacitor Ct is also connected to an input of an operational amplifier operating as a Schmitt trigger whose output is connected to a control input of the switch S3 for opening the switch again. The output of the amplifier is also connected to a set input of a flipflop 655 via an inverter 654. A reset input of the flipflop receives the inverted demag-signal. An inverting output of the flipflop is connected to an input of a NAND circuit 656. A second input of the NAND circuit 656 receives the inverted demag-signal. The output of the NAND circuit is connected to an input of an AND circuit 657. A second input of the AND circuit is connected to the inverter 654. The output of the AND circuit 657 supplies a signal Vosc which also serves as a switching signal for the switch S2. In this way the rising edge of the oscillator sawtooth (see Fig. 4B) is delayed when the demag-signal Dd is high (and hence the inverted demag-signal is low). As described above, the demag-signal is used only during the mode in which the load is high so as to delay, if necessary, the rising edge of the oscillator sawtooth. In order to operate at a variable frequency in the second mode, without the demag-signal having any influence, the reset input of the flipflop 655 should be digitally low in this situation. This is not shown in the Figure but will be evident to those skilled in the art. Fig. 4B shows the demag-signal Dd and the oscillator sawtooth plotted against time. It will be evident that the different elements of this power supply circuit can be implemented in all kinds of alternative ways. For example, the demagnetizing detection circuit can be adapted in all kinds of analog or digital ways without its operation essentially differing from the operation described. Moreover, the oscillator circuit can be adapted in all kinds of ways, for example a fixed frequency instead of a variable frequency in the first mode.

Claims: A DC-to-DC switched-mode power supply circuit (VS) comprising: input terminals (1,2) for receiving an input voltage (Vin); output terminals (3,4) for supplying an output voltage (Vout); a transformer (T) comprising a primary and a secondary winding (Lp,Ls), the secondary winding (Ls) being coupled to said output terminals (3,4); a switch (S), whereby a series arrangement of the primary winding (Lp) and the switch (S) is coupled to said input terminals (1,2); and a control circuit (6) comprising an input (8) for receiving a feedback signal (Vb), and an output (9) for applying a control signal (P) to the switch (S), the control circuit (6) having a first and a second mode of operation, wherein the first mode of operation, the control signal (P) comprises pulses having a frequency which is dependent on the feedback signal (Vb), characterized in that, the input (8) of the control circuit (6) is coupled to the output terminals (3,4) for receiving the feedback signal (Vb) in dependence upon the power supplied by the power supply circuit (VS) at the output terminals (3,4); said control circuit (6) has a further input (7) for receiving a demagnetizing signal (Id) from an extra winding (Ld); in the first mode of operation, the control signal (P) is delayed by means of a delay circuit (61) under the influence of the demagnetizing signal (Id); and in the second mode of operation, the pulses occur at a predetermined fixed frequency, the feedback signal (Vb) determining the duration of the pulses, and the demagnetizing signal (Id) being not considered. A power supply circuit (VS) as claimed in Claim 1, characterized in that the control circuit (6) operates in the second mode of operation at a feedback signal (Vb) being smaller than a predetermined value. A power supply circuit (VS) as claimed in Claim 1, characterized in that the control circuit (6) operates in the first mode of operation at a feedback signal (Vb) being larger than a predetermined value.

Inventors: MARINUS ANTONIUS ADRIANUS M, MARINUS, ANTONIUS ADRIANUS MARIA, MARINUS, ANTONIUS ADRIANUS MARIA, C/O INTERN.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: H02M3
IPC codes: H02M1, H02M3