KR20080094565A - Switched mode power supply comprising a rectifier circuit - Google Patents

Abstract

A switching mode power supply having a rectifier circuit is provided to be used in a fly-back converter which operates with a DC/DC up-converter, down-converter, or a pulse width modulation. A switching mode power supply having a rectifier circuit includes an inductor(TR), a switching transistor(T1), a switch(T3), and a control circuit(MC1). The inductor provides an output voltage. The switching transistor is coupled to the inductor in series. The switch is coupled to the inductor to rectify the output voltage. The rectifier circuit has the inductor, the switching transistor and the switch. The control circuit operates in a monostable mode for the operation of the switch. The rectifier circuit has a diode(Ds), especially, schottky diode which is coupled to the switch in parallel. The switch operates in a synchronization mode of a switching frequency equal to the switching transistor.

Description

SWITCHED MODE POWER SUPPLY COMPRISING A RECTIFIER CIRCUIT

The present invention relates to a switching mode power supply comprising a transformer, a switching transistor, a control circuit for providing rectification of the output voltage in synchronous current mode and a rectifier circuit with a switch.

Diodes used for the rectification of AC voltages are known to consume considerable energy due to the forward voltage drop, especially when used for rectification of small AC voltages. Therefore, to improve the efficiency of the rectifier circuit, a controllable switch such as a MOSFET switch with a small drain-source resistance is often used. A disadvantage of the MOSFET rectifier circuit is that a control circuit is required for the operation of the MOSFET.

Power supplies with synchronous rectifier circuits comprising current transformers for the operation of MOSFET switches are known from DE-A-19704604. Switching of the switch is provided by the positive voltage provided by the first secondary winding of the transformer of the power supply, and the switching of the switch when the polarity of the current passing through the second secondary winding reverses ( A current transformer is used to sense the current flow of the second secondary winding for blocking. Rectifier circuits of this kind can be used, in particular, for flyback converters.

From US 2006 / 0187692A1 a control circuit having a driver circuit with a first comparator and a second comparator for the operation of the switch and a rectifier circuit with a MOSFET switch are known. The first comparator is used to control the voltage polarity across the switch, and the second comparator is coupled with an adjustable reference voltage of the switch to provide a stabilized output voltage.

Also known from EP-A-1717938 is a switched mode power supply having a rectifier circuit comprising a comparator connected to a reference voltage to provide a stabilized DC output voltage. DC rectifier circuits of this kind can be used, for example, to provide regulated output voltages independent of the regulation of the additionally regulated output voltages of switched mode power supplies. Switched mode power supplies include a soft-switching half-bridge device operating in a quasi-resonant mode with feedback control from the secondary side. Half-bridge type DC / DC converters with soft switching are also known from EP-A-1717940 and FR 2738417.

Also known are integrated driver circuits for operating as synchronous rectifier circuits, for example MOSFET switches such as the integrated circuits STSR3 and STSR2, in which case STSR2 is designed for full-wave rectifier circuits and STSR3 is designed for half-wave rectifier circuits. The STSR3 operates over a wide operating frequency range and includes anticipation circuitry to prevent secondary shoot-through situations at turn-on of the switch. Several expected times can be set to provide switch-off of the switch prior to switching on the primary side switching transistor.

A switching mode power supply according to the invention comprises a rectifier circuit having an inductor, a switching transistor coupled in series with the inductor, and a switch coupled with the inductor for rectifying the output voltage, the rectifier circuit operating the switch. The control circuit further comprises a control circuit operating in a monostable (monostable) mode to control. This switch operates in synchronous mode, especially at the same switching frequency as the switching transistor, which operates essentially at a constant switching frequency. The control gate comprises, for example, a control circuit which is constant and especially independent of any load state and has a gate-on time well below the switching time of the switching transistor. Thus, the switch always operates in a safe operating state and any unwanted reverse current through the switch is avoided.

The rectifier circuit advantageously comprises a diode, in particular a Schottky diode, which takes the rest of the current under high load when the switch is already closed, and therefore in the case of such a high load Can take part of the consumption losses.

In a further aspect of the invention, a switched mode power supply comprises a feedback loop for coupling the output voltage of the switched mode power supply to the driver circuit of the switching transistor. The switch-off time of the switch is determined only by the time constant of the control circuit of the switch, and thus is independent of the switching signal of the switched mode power supply.

Monostable operation of the control circuit can be provided, for example using a digital mono-flap circuit or using a comparator, to which the ramp generator and the threshold circuit are coupled. Thus, the control circuit for operating the switch can be designed with cheap circuit components and does not particularly require a current measurement of the current flowing through the inductor.

Rectifier circuits include a switch-mode power supply having a push-pull half-bridge configuration and operating as a resonant converter or soft-switching quasi-resonant converter for applications in a plasma television set. It can be used advantageously. The switch is a MOSFET in one preferred embodiment.

Referring now to the schematic drawings, by way of example a preferred embodiment of the invention is described in more detail below.

The rectifier circuit of the present invention can be used, for example, in a flyback converter that operates with pulse width modulation or any DC / DC up-converter or down-converter. In the case of switch T3, in particular a wide variety of suitable semiconductor switches can be used, as will be appreciated by those skilled in the art.

1 shows a switched mode power supply comprising a rectifier circuit according to the invention. The switched mode power supply comprises a transformer TR having a primary winding Lp and a secondary winding Ls1, which transformer is designed to provide isolation from the mains. The first switching transistor T1 is coupled in series between the primary winding Lp and the DC input voltage Ve provided by the input capacitor Ce, and the second switching transistor T2 is connected to the primary winding Lp. ) And are arranged in parallel and connected to ground. The switching transistors T1 and T2 operate in a push-pull mode by the driver circuit IC1 and are arranged in this embodiment as a half bridge configuration.

The winding sense of the primary winding Lp and the secondary winding Ls1 is indicated by a dot, so that the terminals labeled 1 and 2 are labeled with a dot of the switching mode power supply. Have the same voltage polarity during operation. In this embodiment, in series with the primary winding Lp between Lp and ground, a resonant capacitor Cr is coupled to provide quasi-resonant operation of the switched mode power supply. Switching mode power supplies of this kind are described, for example, in FR 27387417 and EP1717940.

In this embodiment, the switch T3, which is a MOSFET, is arranged on the secondary side in series with the secondary winding Ls1. The output capacitor Cs is coupled in parallel with the secondary winding Ls1 and the switch T3 to provide a smooth DC output voltage Vs. The switch T3 is coupled between the secondary winding Ls1 and ground in this embodiment. The diode Ds is arranged in parallel with the switch T3, allowing the current i (Ds) to flow in the direction of the secondary winding Ls1 from ground even when the switch T3 is closed.

For regulation of the output voltage Vs, a feedback loop FB is provided, which, via a voltage divider with resistors R1 and R2, transfers part of the output voltage Vs to 1 of the driver circuit IC1. Refit to the vehicle side. Driver circuit IC1 operates essentially at a fixed switching frequency and provides a pulse-width modulated push-pull drive signal for changing the duty cycle of switching transistors T1 and T2 to compensate for output load variations. .

The switch T3 is controlled by the control circuit MC1, which operates in accordance with the present invention in a monostable mode. This control circuit MC1 comprises an inverter I, which is a junction disposed between the switch T3 and the secondary winding Ls1 for sensing the voltage polarity at the secondary winding Ls1. (junction) is coupled to the input to (4). The inverter I provides a control signal for the first input of the AND gate A and the control gate G.

The control gate G operates in the monostable mode and includes, for example, a timer circuit. Control gate G is switched from low to high at output Q when a high pulse is received at input A and to low after a finite period of T = τ = constant. The control gate G comprises, for example, a ramp generator and a threshold detector, the ramp generator comprising a timer capacitor that is charged when an input signal is received, and when reaching a defined threshold, the threshold detector Discharges the timer capacitor and provides an off signal for providing a low signal after T = τ. Monostable circuits of this kind are known in the art.

The output signal of the control gate G is coupled to the second input of the AND gate A, the output of which is connected to the control input of the switch T3 to control the operation of the switch T3. The output is coupled to the input of the switch driver D to which it is coupled. The control circuit MC1 acts as a synchronous rectifier circuit while the switching mode power supply with the switch T3 is in operation.

The operation of the rectifier circuit is as follows: If the voltage at the terminal 2 of the secondary winding Ls is positive and the switch T3 is switched, the current i (Ls) will charge the capacitor Cs. Is provided for. When the polarity across the winding Ls1 changes, the voltage at the terminal 2 goes low, goes high at the junction 4, and the output of the inverter I becomes "low" accordingly. This "low" signal provides a "low" signal at the output of AND gate A, which keeps switch T3 closed.

When the polarity across the winding Ls1 is changed again, the voltage at the terminal 2 becomes positive, at the junction 4 the voltage becomes low, and then the output at the inverter I is switched to "high". . The "high" signal at the output of the inverter I triggers the control gate G to provide a "one shot" signal at the output Q, so the AND gate switches via the driver D. It is switched to "high" to switch T3 because both input signals are "high" at the AND gate.

The control gate G is switched after a time T = τ defined by the "high" to "low" at the output Q. The output of AND gate A then switches from "high" to "low", so driver D switches off switch T3 to block the current flowing through switch T3. The current flowing through the secondary winding Ls still flows through the diode Ds until the polarity of the secondary winding Ls is inverted again according to the switching operation of the switching transistors T1 and T2. .

When the polarity in the secondary winding Ls is inverted again with the next switching cycle, the voltage at the junction 4 becomes high, so that the output of the inverter I becomes "low". Thereafter, the AND gate A is cut off and thus the switching of the switch T3 is not possible. The synchronous rectifier circuit therefore acts as a half-wave rectifier circuit.

One preferred embodiment of the rectifier circuit of FIG. 1 is shown in FIG. 2, in which separate analog components, in particular cheap circuit components, are used to realize the control circuit MC1 of FIG. 1. The primary side of the switching mode power supply with switching transistors T1 and T2 and driver circuit IC1 essentially corresponds to the circuit shown in FIG. 1, and the same reference symbols for the same circuit elements in FIGS. 1 and 2. Is used.

On the secondary side, the secondary winding Ls1, the diode Ds, the switch T3, and the capacitor Cs are arranged to provide the output voltage Vs, as described above in connection with FIG. In the operation of switch T3 as a synchronous rectifier, a comparator CO is provided whose output controls a push-pull switching stage with transistors T4 and T5 and transistors T4 and T5. Provides the necessary current for the switching of the switch T3, which is the MOSFET in the embodiment of FIG. The transformer TR comprises another secondary winding Ls2 for providing a negative operating voltage of -2.5 V for the comparator CO and transistor T5, which transistor T5 is a comparator CO. And extends the output voltage range of transistors T4 and T5 to the negative voltage for fast switching off of the MOSFET.

A capacitor Cp is coupled between the positive input V + of the comparator CO and ground, which is charged to a positive voltage via the diode Dp and the resistor Rp. Rp is connected to the output voltage Vs. A second capacitor Cm is coupled between the negative input V- of the comparator CO and ground, which is also connected to the resistor Rp by the junction 5 between the diode Dp and the resistor Rp. Combined. Since capacitor Cp is separated from junction 5 via diode Dp, the voltage across capacitor Cp remains essentially constant and unaffected by the voltage drop across capacitor Cm. Therefore, elements Rp, Dp, and Cp provide threshold voltages for the operation of comparator CO.

Capacitor Cm is also charged via resistor Rp, but is periodically discharged by transistor Qc and diode Dc1. At the current input of transistor Qc, the collector is coupled to capacitor Cm via diode Dc1 and further coupled to junction 6 via diode Dc2, which junction 6 is a transistor. When Qc is switched, the output of the comparator CO and the base terminals of the transistors T4 and T5 to keep the transistor T4 in the switched-off state and thus also the MOSFET switch T3 in the switched off state. Is connected to.

The base of the transistor Qc is coupled with the junction 4 via the diode Dc3 and the diode Dc4, and when the voltage at the junction 4 is high, the transistor Qc is switched by Dc4, When the voltage at junction 4 is low, transistor Qc is blocked by Dc3.

Therefore, the operation of the comparator CO is as follows: When the voltage at the junction 4 is high, the diode Dc4 conducts and thus the transistor Qc is switched. The diode Dc1 also conducts and discharges the capacitor Cm, and the diode Dc2 conducts to block the transistor T4, thereby closing the switch T3.

When the voltage across the secondary winding Ls1 reverses its polarity according to the operation of the switching transistors T1 and T2, the voltage at the junction 4 goes low and accordingly the transistor Qc via the diode DC3. ). The capacitor Cm is then charged via the resistor Rp. As long as the voltage across capacitor Cp is higher than the voltage across capacitor Cm, the output of comparator CO goes high, so transistor T4 is switched and so is switch T3.

When the voltage across capacitor Cm reaches the threshold voltage of capacitor Cp, comparator CO switches low and thus switch T3 is interrupted via transistors T4 and T5. Therefore, the switch-on time of the switch T3 becomes independent of the load of the switching mode power supply and the switching frequency of the driver circuit IC1, the value of the resistor Rp and the capacitor Cm, and the threshold voltage of the capacitor Cp. , And only by the stabilized output voltage (Vs). The switch-on time τ is specially arranged such that the switch T3 closes well before the polarity at the terminal 2 of the winding Ls1 changes from high to low, and thus is reversed through the switch T3. Current flow in the direction is avoided under all operating state conditions of the switched mode power supply.

When the voltage at the terminal 2 is switched low, the voltage at the junction 4 is switched high, and then again, the transistor Qc discharges the capacitor Cm via the diode Dc1, and the diode ( It is switched to shut off transistor T4 via Dc2). Thus, the switch T3 operates in response to the switching transistors T1 and T2. Switch on of switch T3 is initiated via transistor Qc, which causes the voltage at terminal 2 to be switched high and switched low at junction 4. When it is blocked. Transistor T4 is switched by comparator CO. The switch T3 is switched off after t = t3 when the capacitor Cm is charged to a threshold voltage as defined by the voltage across the capacitor Cp.

Thus, a rectifier circuit as described with respect to FIG. 2 provides a very efficient and reliable solution in terms of operation of the switch T3. By using a negative operating voltage as provided by the secondary winding Ls2, in particular a very low supply voltage Vs can be produced by the switched mode power supply. The start of the rectifier circuit is made by a resistor R3, which switches on the switching mode power supply via this resistor R3, and when the voltage at the terminal 2 is high, the transistor T4 and the switch are switched on. T3 is switched. The feedback loop FB in this embodiment comprises an optocoupler Opt for conveying the regulation signal to the driver circuit IC1.

The operation of the switched mode power supply shown in FIG. 2 is now further described with respect to the voltage and current diagrams shown in FIG. 3, which corresponds to the full load operation of the switched mode power supply. The voltage and current plots shown at 4 correspond to the operation of the switched mode power supply in standby mode. In the voltage diagram of FIG. 3, a) shows the voltage at the gate of MOSFET T1, and b) shows the voltage at the gate of MOSFET T2, respectively. The driver circuit IC1 provides delays tm2 and tm1, respectively, between switching on MOSFET T2 and vice versa after MOSFET T1 is switched off, thereby avoiding a short circuit situation for the supply voltage Ve.

The voltage Ve at the junction 3 between the MOSFET T1 and the MOSFET T2 changes according to the switching operation of the T1 and T2, and in Fig. 3c, the voltage across the primary winding Lp and the capacitor Cr relative to ground is shown. Indicates. Therefore, the current flowing through the primary winding Lp rises when the voltage at the junction 3 is high, and as the voltage at the junction 3 is low, as shown in Fig. 3D. The voltage applied to the capacitor Cr shown in (e) of FIG. 3 represents a resonance voltage in phase with the switching operations of the switching transistors T1 and T2.

The voltage of the junction 4 at the drain of the MOSFET switch T3 shown in FIG. 3 (f) is essentially in phase with the voltage across the primary winding Lp, and the input voltage Ve, the transformer ( It has a voltage value that depends on the turns ratio m of TR and the voltage across the capacitor Cr when the switch T3 is closed. The gate voltage for MOSFET T3 in this embodiment, which is the voltage that controls the operation of switch T3, is shown in FIG. 3 (h). The corresponding current flowing through the secondary winding Ls1 under the full load condition consists of a current flowing through the switch T3 and a current flowing through the diode Ds as shown in FIG.

As can be seen, the switch T3 is switched immediately after the time t1 at a time t2 at which the switching transistor T1 is cut off. The current flowing through the switch T3 continues to flow until the time t3 at which the switch T3 is interrupted in response to the operation of the comparator CO. The remaining energy stored in the transformer TR is then until the polarity across the secondary winding Ls1 is inverted at time t4 as shown in FIG. 3 (c) in response to the voltage at the junction 3. Generates a current flowing through the diode Ds.

3 (i) shows the voltages at the inputs V + and V- of the comparator CO. As can be seen, the voltage at the input V-, the voltage across the capacitor Cm, is shown in FIG. 3 (h) when the switch T3 is switched in response to the blocking of the transistor Qc. It starts to rise at time t2. At time t3, the voltage at input V- reaches a threshold voltage at input V +, causing the comparator CO to shut off switch T3. Time t3 is well ahead of time t4, and at time t4 the next switching cycle of the switched mode power supply is started to provide a save margin for the switching off of switch T3.

4A-4E show the same voltage and current diagrams for the situation where the switched mode power supply operates under low load conditions, such as in standby mode. As can be seen in FIG. 4 (d), the current flowing through the primary winding Lp is considerably lower than in FIG. 3 (d), and the switching on time of the switching transistor T1 is also shorter. The switch-off time of the switch T3 at time t3 is still the same as compared to FIG. 3G, which means that the voltage Vs charging the capacitor Cm is under the feedback loop FB under all load conditions. Is controlled by and therefore constant. The current flowing through the switch T3 to charge the capacitor Cs is now very small because the power consumption of the load in this operating state is very small. The current flowing through the diode Ds is essentially zero. Thus, only the switch-on time of the switch T3 is controlled in response to the operation of the switching transistors T1 and T2, while the switch-off time of the switch T3 is not and the control circuit MC1 operates in the monostable mode. ) And comparator (CO) respectively.

The present invention is not limited to the embodiments as described above with respect to the drawings, and various modifications are possible by those skilled in the art without departing from the scope of the invention. Therefore, the invention described above is present in the claims.

1 shows a switched mode power supply comprising a rectifier circuit according to the invention.

FIG. 2 shows one preferred embodiment of a switched mode power supply and rectifier circuit as shown in FIG. 1. FIG.

3 is a voltage and current diagram of the circuit of FIG. 2 operating at high load;

4 shows the circuit of FIG. 2 operating in a standby state.

※ Explanation of code about main part of drawing ※

T1: first switching transistor T2: second switching transistor

T3: switch Lp: primary winding

Ls1: secondary winding IC1: driver circuit

Ds: Diode MC1: Control Circuit

I: Inverter 3, 4, 5: Junction

G: control gate CO: comparator

Claims (10)

Switching mode power supply, An inductor TR for providing an output voltage Vs, a switching transistor T1 coupled in series with the inductor TR, and a switch coupled with the inductor TR for rectification of the output voltage Vs. A switching mode power supply comprising a rectifier circuit comprising (T3), Rectifier circuit (T3, MC1, Ds) comprises a control circuit (MC1) operating in a monostable (monostable) mode for the operation of the switch (T3). Switched power supply according to claim 1, wherein the rectifier circuit (T3, MC1, Ds) comprises a diode (Ds), in particular a Schottky diode coupled in parallel with the switch (T3). Switched mode power supply according to claim 1 or 2, wherein the switch (T3) operates in a synchronous mode at the same switching frequency as the switching transistors (T1, T2). The driver circuit IC1 for controlling the operation of the switching transistor T1 and the output voltage Vs for stabilizing the output voltage Vs. And a feedback loop (FB) coupled to the driver circuit (IC1). 5. The switching mode according to claim 1, wherein the driver circuit IC1 provides a pulse-width modulated driver signal with an essentially constant switching frequency for the operation of the switching transistor T1. 6. Power supply. The inductor TR is a transformer having a primary winding W1 and secondary windings W2 and W3, wherein the primary winding W1 is A switch mode power supply, coupled in series with a switching transistor (T1), the switch (T3) coupled in series with a secondary winding (W2). The switching transistor T1 is coupled in series with the second switching transistor T2, and the switching transistor T1 and the second switching transistor T2 are all push-through via the junction 3. Switched mode power supply coupled with a primary winding (W1) to operate in a push-pull half-bridge configuration. 8. In series with the primary winding Lp according to claim 6 or 7, for operating as a resonant converter or a quasi-resonant converter with soft switching. Switching mode power supply comprising a resonant capacitor (Cr). 9. The control circuit (MC1) according to any one of the preceding claims, wherein the control circuit (MC1), for example after a finite time (τ), to switch off the switch (T3), a comparator CO, a ramp generator Switching mode power supply comprising a monostable gate circuit (G) having Rp, Cm, Qc, and threshold circuits Rp, Dp, Cp. 10. The method of claim 9, wherein to control the operation of the switch T3, the ramp generators Rp, Cm, Qc and threshold circuits Rp, Dp, Cp are connected to the first input of the comparator CO. A switch mode power supply, each coupled to two inputs, wherein the switch T3 is a MOSFET.

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