WO2005015723A1

WO2005015723A1 - Switched mode power supply

Info

Publication number: WO2005015723A1
Application number: PCT/EP2004/008095
Authority: WO
Inventors: Reinhard Kögel; Jean-Paul Louvel
Original assignee: Thomson Licensing SAS
Current assignee: Thomson Licensing SAS
Priority date: 2003-07-31
Filing date: 2004-07-20
Publication date: 2005-02-17
Anticipated expiration: 2006-01-31
Also published as: DE10334958A1

Abstract

The switched mode power supply contains an inductance (T1), a switching transistor (Q1) which is coupled to the inductance (T1), a sensing resistor (R1) which is connected in series with the switching transistor (Q1) and a control circuit (IC1) for controlling an output voltage (U3). In this case, the control circuit (IC1) is coupled to the sensing resistor (R1), and a node (a) is arranged between the control circuit and the sensing resistor (R1) and is used to add an auxiliary signal. The auxiliary signal has a characteristic which is at least sometimes rising, in particular once the switching transistor (Q1) has been switched on. In preferred exemplary embodiments, the node (a) is connected to an output (6) of a driver stage of an integrated circuit (IC1) via a resistor (R4) or a highpass filter.

Description

Switched mode power supply

Background

The invention is based on a switched mode power supply (SMPS) comprising an inductance, a switching transistor coupled to the inductance, a sensing resistor connected in series with the switching transistor and a control circuit for controlling an output voltage the control circuit being coupled to the sensing resistor. Switched mode power supplies of this kind are used, for example, in television sets, video recorders and settop boxes.

Appliances of this type usually use switched mode power supplies based on the flyback converter principle, which provide a large number of stabilized supply voltages on the output side. The control circuit is used to keep to one of the output voltages during operation by means of a control loop. The control circuit in this case controls the switching transistor by means of a driver voltage such that the output voltage connected to the control loop is kept constant by means of, for example, pulse-width modulation (PWM) or by varying the frequency of the driver voltage. The other output voltages of the switched mode power supply are also stabilized in this way.

Integrated circuits (ICs) are often used as the control circuit and considerably simplify the design of a switched mode power supply. Integrated circuits of this type usually contain circuits for control purposes, an oscillator, a driver stage for directly driving a switching transistor, circuits for generating internal operating voltages, and protection circuits.

A known switched mode power supply, which has an integrated circuit ICl, is illustrated in Fig. 1. The switched mode power supply uses, on the input side, a bridge rectifier BR, by means of which an AC voltage Un applied to a system terminal is rectified. The rectified voltage Ul is smoothed by means of a storage capacitor Cl and is applied to a primary winding Wl of a transformer Tl . The transformer Tl brings about system isolation between the primary side and the secondary side and has, on the primary side, an auxiliary winding W2 for generating an operating voltage VCC for the integrated circuit ICl and, on the secondary side, windings W3 - W5 for generating stabilized output voltages U3 - U5. By means of diodes Dl - D3 , rectified voltages are tapped off at the windings W3 - W5 , which are then smoothed by means of bandpass filters LCI - LC3.

Connected in series with the primary winding Wl is a switching transistor Ql, in this exemplary embodiment a OSFET, which is connected on the output side to ground via a sensing resistor Rl . The control input of the switching transistor Ql is connected to a driver stage DR of the integrated circuit ICl which is used to control the switching transistor Ql by means of a driver voltage Ud. The switched mode power supply is in the form of a flyback converter, with the result that, during operation when the switching transistor Ql is on, energy is stored in the transformer Tl and is transferred to the windings W2 - W5 in the subsequent Off phase of the switching transistor Ql .

The known switched mode power supply has primary-side control which functions by means of the supply voltage

VCC. The supply voltage VCC is generated by the auxiliary winding W2 , diodes D , D5 and capacitors C2 , C3. The supply voltage VCC is applied to a terminal 7 of the integrated circuit ICl, as a result of which the driver stage DR is supplied with voltage, and to a terminal 8, by means of which the integrated circuit ICl generates internal reference voltages and stabilized supply voltages for operating its circuits. A control voltage Ur is generated from the supply voltage VCC by means of a voltage divider and is applied, via an input 2, to a fault amplifier EA in the integrated circuit ICl . The output of the fault amplifier EA is fed back, via a terminal 1 and an RC filter RC1, to the input 2 and generates a threshold voltage Uf for control purposes.

The integrated circuit ICl has an oscillator 0, whose frequency can be set by means of it being connected externally to the terminal 4 by means of a resistor Rt and a capacitor Ct . The oscillator O prescribes the switching frequency for the switching transistor Ql, and the pulse width of the driver voltage Ud generated in the driver stage DR is varied using the control circuit of the integrated circuit. The fault amplifier EA is used to keep the supply voltage VCC constant. As a result of this, the output voltages U3 - U5 are also stabilized, since the windings W2 - W5 are coupled to one another .

The integrated circuit ICl may also be used for switched mode power supplies which are controlled on the secondary side. A switched mode power supply based on the flyback converter principle which has secondary-side output voltage control is described, for example, in US 4,876,636, to which reference is made here. Improved voltage stabilization is achieved by means of secondary-side control. However, the control loop requires for this purpose a transformer, for example an optocoupler, by means of which the control signal is transmitted from the secondary side to the primary side.

Connected in series with the switching transistor Ql is a sensing resistor Rl, which produces a voltage drop when the switching transistor Ql is switched on. This voltage U2 is applied, via a bandpass filter, resistor R2 and capacitor C4, as a control signal Us to the output 3 of the integrated circuit ICl .

The integrated circuit ICl uses the control voltage Ur applied to the input 2 and the control signal Us applied to the input 3 for control purposes. The fault amplifier EA in this case operates as a first comparator, which prescribes the threshold voltage Uf for a second comparator RA. The control signal Us is applied to the second input of the second comparator RA, with the result that the comparator RA is used to monitor the current through the sensing resistor Rl . If the control signal Us exceeds the threshold voltage Uf, the second comparator RA passes an output signal on to the logic circuit LO, as a result of which the switching transistor Ql is turned off by the driver stage DR.

Since the switching frequency for the switching transistor Ql is prescribed by the oscillator 0, this control results in pulse-width modulation of the driver voltage Ud, by means of which the output voltages U3 - U5 are stabilized. This control method has the advantage that, by means of the control signal Us, fluctuations in the system voltage Un are also included directly in the control .

The integrated circuit ICl described with reference to Fig. 1 is a frequently used type UC3844 which can be procured, for example, from the company On Semiconductor (http : //onsemi . com) . Other SMPS controller ICs also use a control method of this type, which is known as "Current Mode Control" . The switched mode power supply illustrated in Fig. 1 is specified as an application at this Internet address. The switched mode power supply also has a startup circuit AS, by means of which the integrated circuit ICl is supplied with a current once the switched mode power supply has been connected. For the purpose of attenuating voltage peaks, connected to the switching transistor Ql on the input side is a first attenuating network SN1, by means of which voltage peaks are passed on to the storage capacitor Cl, and a second attenuating network SN2 , which is connected in parallel with the switching transistor Ql .

The interaction between the threshold voltage Uf and the control voltage U2 is illustrated in Fig. 2. The threshold voltage Uf is shown here as a threshold value for the control voltage U2. During the time interval, during which the switching transistor Ql is off, the control voltage U2 is equal to zero. If the switching transistor Ql is switched on, U2 has an increasing value, which largely depends on the inductance of the primary winding Wl . However, a voltage peak is produced at the start of the voltage rise, since, once the switching transistor Ql has been switched on, capacitances present at its output, in particular the capacitance of a snubber network SN2 which is connected in parallel with the switching transistor Ql, see

Fig. 1, are discharged.

If a switched mode power supply designed for a specific power is switched over to a mode of operation having a low output power, for example to a standby mode of operation, the integrated circuit ICl prescribes a driver voltage Ud having a low pulse width for the switching transistor Ql . The rise of the control voltage U2 is therefore likewise low. This case is illustrated in Fig. 3. The control voltage Ur then very rapidly reaches the threshold value applied to the fault amplifier EA, since the switched mode power supply is operating virtually off-load, as a result of which the RC element RC1 causes the threshold voltage Uf to drop. At very low output powers, the threshold voltage Uf may in this case be so low that interference voltages, which are produced when the switching transistor Ql is switched on, impair the control, since they cause the switching transistor Ql to be turned off too early.

Summary of the invention

The object of the present invention is to specify a switched mode power supply of the type mentioned initially which has reliable control at low powers.

This object is achieved by the features specified in claim 1. Advantageous developments of the invention are specified in the subclaims.

The switched mode power supply according to the invention contains an inductance, a switching transistor which is coupled to the inductance, a sensing resistor which is connected in series with the switching transistor and a control circuit for controlling an output voltage. The control circuit is coupled to the sensing resistor, and a node is arranged between the control circuit and the sensing resistor being used to add an auxiliary signal to the control voltage generated by the sensing resistor.

The auxiliary signal has, in particular, a time characteristic, by means of which interference signals, which are produced once the switching transistor has been switched on, are suppressed, resulting in the switching transistor being turned off at a correct point in time even at low power. The auxiliary signal is, for example, a voltage or a current and has, once the switching transistor has been turned off, a characteristic which is at least sometimes rising. By this means, the control signal is weighted, and the latter component of the control signal which is important for control purposes is highlighted.

The magnitude of this auxiliary signal is irrelevant for control purposes, since the level of the controlled output voltage depends on the control voltage applied to the fault amplifier of the control circuit. By virtue of the fact that a constant output voltage is kept to, this control voltage is matched accordingly.

In preferred embodiments, the node is connected, via a highpass filter, to an output of a driver stage of the control circuit or is coupled to a primary-side auxiliary winding of a transformer, which has a primary winding and further secondary windings .

Brief Description of the drawings

The invention is explained by way of example in more detail below with reference to schematic drawings, in which:

Fig. 1 shows a switched mode power supply having an integrated circuit according to the prior art,

Figs 2-4 show voltage diagrams for the purpose of explaining the control voltage, the control signal and the auxiliary signal,

Fig. 5 shows a switched mode power supply, in which an output of a driver stage of a control circuit is connected to the control signal,

Fig. 6 shows a switched mode power supply as shown in Fig. 5, which also has a highpass filter between the output of the driver stage and the node , and Fig. 7 shows a detail of a switched mode power supply having a control circuit, in which an auxiliary voltage from a secondary winding is added to the control voltage .

Description of preferred embodiments

According to the invention, an auxiliary signal Ux is added to the control voltage U2 via a node, which is arranged between the control circuit of the switched mode power supply and the sensing resistor, as is shown in Fig. 4. The auxiliary signal is used to weigh the control voltage U2 such that the component of the control voltage U2 which is essential for control purposes is highlighted. The auxiliary signal is in this case a saw-tooth voltage, for example, which rises once the switching transistor has been switched on. As can be seen from Fig. 4, this produces a control signal Us which rises monotonously. The auxiliary signal Ux is not used for control purposes, since a constant output voltage is kept to, with the result that a signal having a different amplitude is corrected at the input of the control circuit. The threshold voltage Uf is therefore matched accordingly.

Fig. 5 shows a switched mode power supply, in which an auxiliary signal, a voltage Ud, generated by the driver stage of a control circuit ICl, is added to the control voltage U2 via the node a. The integrated circuit ICl is an IC, for example, as is described with reference to Fig. 1.

The switched mode power supply also has a transformer Tl having a primary winding Wl and a secondary winding W3. Connected in series with the primary winding Wl is a switching transistor Ql, which is connected on the output side to ground via a sensing resistor Rl . Connected in parallel with the switching transistor Ql is a capacitor C5, by means of which voltage peaks, which are produced when the switching transistor Ql is turned off, are attenuated. Connected to the secondary winding W3 is a diode D5 and a capacitor C7 for the purpose of generating a stabilized output voltage U3. Connected in parallel with the diode D5 is a capacitor C6, which likewise serves the purpose of suppressing voltage peaks .

A control voltage U2 , which is used to control the current in the integrated circuit ICl and is connected to a corresponding "sense" input 3, is tapped off at the sensing resistor Rl by means of a resistor R2. Arranged between the input 3 and the resistor R2 is the node a, to which a capacitor C4 is connected. The resistor R2 , together with a resistor R3 , is connected as a voltage divider in order to match the maximum current through the switching transistor Ql and the resistor Rl for the input 3 of the integrated circuit

ICl . The capacitor C4 is thus in the form of a bandpass filter which is charged via the resistors R2 and R4.

The switching transistor Ql is controlled by the driver voltage Ud, which is generated by a driver stage of the integrated circuit ICl and is output at a corresponding output 6. The output 6 of the driver stage is in this case connected to the node a via a resistor R4 , in order to add an auxiliary signal to the control voltage U2, as is described with reference to Fig. 4.

The circuit operates as follows: The switching transistor Ql is switched on by a constant, positive voltage Ud. This produces a constant current through the resistor R4 , by means of which the capacitor C4 is charged linearly. This current is therefore seen by the input 3 as a saw-tooth voltage which is added to the voltage. The capacitor C4 is then discharged again in the Off phase of the switching transistor Ql via the resistors R2, Rl, R3 and R4. The control signal Us present at the input 3 thus has a waveform which corresponds to that in Figure 4.

A further exemplary embodiment is illustrated in Fig. 6, in which an auxiliary signal Ux is added to the node a and is essentially only effective in the standby mode of operation, but has no influence in the normal mode of operation, and, in particular, does not influence a so-called "foldback point" . For this purpose, arranged between the output 6 of the driver stage of the integrated circuit ICl and the node a is a

^■highpass filter, capacitor C4 and resistor R4 , which has a short time constant in comparison with a switching cycle of the switching transistor Ql , with the result that once Ql has been switched on, a current flows via the highpass filter to the capacitor C4 only for a short period of time.

In the normal mode of operation, in which a pulse width ratio of approximately 40% - 50% is usually used, this highpass filter has no effect. The control of the "foldback points", by means of which fluctuating system voltages are directly included in the control, is therefore not affected in the normal mode of operation. Further circuitry for the switched mode power supply illustrated in Fig. 6 corresponds to that in Fig. 5. Fig. 7 shows a further exemplary embodiment, in which the auxiliary signal Ux is generated by a primary-side auxiliary winding of the transformer Tl (not shown in the figure) . The node a is in this case connected via a resistor R5 to the auxiliary winding. The further circuitry for the switched mode power supply corresponds to that in Fig. 5 or Fig. 1. The resistor R5 is connected, in particular, to a winding end, to which a positive voltage (positive forward) is applied when the switching transistor is switched on. This voltage is used to likewise suppress interference voltage peaks, which are produced when the switching transistor Ql is switched on, in the standby mode of operation. At the same time, control of the "foldback points" is also taken into account here, since the forward voltage of the auxiliary winding is proportional to the system voltage.

Preferably used as an integrated circuit ICl for the switched mode power supplies described in Figures 5 - 7 is the integrated circuit UC3844. However, other SMPS controller ICs, such as, in particular, current mode controller ICs, may also be used. The invention may also be used in switched mode power supplies which use a discrete control circuit. Further modifications of the invention are obvious to those skilled in the art. The invention may be used, for example, even in so-called pre-converters, which are used for power factor correction. These do not use a transformer but only use a coil and an inductance. The invention can be used for switched mode power supplies which are controlled both on the secondary side and on the primary side. The invention may also be used for other converters, such as step-up converters or forward converters .

Claims

1. A switched mode power supply having an inductance (Tl) , a switching transistor (Ql) coupled to the inductance (Tl) , a sensing resistor (Rl) connected in series with the switching transistor (Ql) , and a control circuit (ICl) for controlling an output voltage (U3 - U5) , the control circuit (ICl) being coupled to the sensing resistor (Rl) , characterized in that a node (a) is arranged between the control circuit and the sensing resistor (Rl) for adding an auxiliary signal (Ux) .

2. The switched mode power supply as claimed in claim 1, characterized in that, when the switching transistor (Ql) is switched on, the auxiliary signal (Ux) has a characteristic which is at least partly rising.

3. The switched mode power supply as claimed in claim 1 or 2, characterized in that the sensing resistor (Rl) is coupled via an integrating means (C4) to an input (3) of the control circuit (ICl) .

4. The switched mode power supply as claimed in claim 3, characterized in that the integrating means (C4) has a time constant which is smaller than the period corresponding to the switching frequency of the switching transistor (Ql) .

5. The switched mode power supply as claimed in claim 3 or 4, characterized in that the integrating means (C4) has a capacitor, and in that a current, which is additional once the switching transistor (Ql) has been switched on and is at least sometimes constant, is supplied to the capacitor (C4) via the node (a) .

6. The switched mode power supply as claimed in claim 1, characterized in that the switched mode power supply has a capacitor (C4) , which is coupled to the node (a) , and in that the auxiliary signal (Ux) is a current which has a characteristic which is at least sometimes rising when the switching transistor (Ql) has been switched on.

7. The switched mode power supply as claimed in one of the preceding claims, characterized in that the node (a) is connected to the output (6) of the driver stage (DR) of the control circuit (ICl) .

8. The switched mode power supply as claimed in claim 7, characterized in that the output (6) of the driver stage (DR) is coupled, via a highpass filter (R4, C7) , to the node (a) .

9. The switched mode power supply as claimed in one of the preceding claims, characterized in that the inductance is a transformer (Tl) , which has a primary winding (Wl) and at least one secondary winding (W2 - W5) , and in that the secondary winding (W2) is coupled to the node (a) .

10. The switched mode power supply as claimed in claim 9 , characterized in that the secondary winding (W2) is coupled to the node (a) via a resistor (R5) .

11. The switched mode power supply as claimed in one of the preceding claims, characterized in that the switched mode power supply operates based on the flyback converter principle, and in that the control circuit is integrated in an integrated circuit (ICl) , which operates as a so-called "current mode controller" .