WO2001011762A1

WO2001011762A1 - Transformer resetting

Info

Publication number: WO2001011762A1
Application number: PCT/FI2000/000667
Authority: WO
Inventors: Gösta BAARMAN; Juha RIIHIMÄKI
Original assignee: Nokia Corporation
Priority date: 1999-08-06
Filing date: 2000-08-04
Publication date: 2001-02-15
Also published as: AU6445400A; FI19991677A

Abstract

This invention relates to resetting the transformer in a DC-to-DC converter. Especially this invention relates to resetting the transformer with an arrangement in an auxiliary output circuit. According to the invention an auxiliary output is used for resetting. At least one capacitor (C1) in a reset circuit is used for resetting the transformer (T1), which capacitor (C1) is charged essentially during the ON period of the switching element and discharged essentially during the OFF period of the switching element (Q1) producing a demagnetizing current and at least one output of the forward DC-to-DC converter is taken from said reset circuit.

Description

Transformer resetting

Field of Invention

This invention relates to resetting the transformer in a DC-to-DC converter. Especially this invention relates to resetting the transformer with an arrangement in an auxiliary output circuit.

Background of the Invention

DC-to-DC converters are widely used in many applications. The idea of a DC-to- DC converter is to change a first DC voltage level to another DC voltage level. The second DC voltage is fed to a load. Typically the DC-to-DC converter comprises primary and secondary sides. These sides are separated from each other by a transformer. The primary side of the DC-to-DC converter comprises at least a switch by means of which the feeding of voltage and possibly current to the transformer is controlled. The secondary side of the DC-to-DC converter comprises several components by means of which the output voltage is at least rectified and filtered when needed.

One special type of DC-to-DC converter is a so called forward DC-to-DC converter. In Figure 1 it is shown a typical arrangement for a forward DC-to-DC converter. Typically the converter comprises a transformer T which divides the converter to two sides; primary side and secondary side. In the primary side there is at least one switching element means for switching the voltage into the primary side of the transformer T. Typically the switching element is some kind of transistor Ql, which is controlled by a control element 101. Advantageously, the control element 101 gets feedback from the secondary side of the converter. In the secondary side of the converter there are two diodes Dl; D2 coupled so that the current from the transformer can in all situations be directed to the output. First of all, when the switching element is ON (the transistor Ql conducts), the energy transfers to the secondary side. The secondary side of the transformer transfers the energy as a current through the diode Dl and the inductance LI into the load. When the switching element is OFF (the transistor Ql does not conduct), diode Dl is reverse biased. Inductor LI tends to maintain the current flow in the secondary side. The diode D2 forms a path for the current. As a result, the current in the secondary side is fed to the output in all phases of the switching cycle. Capacitor Cl in the secondary side of the converter is for stabilizing the output voltage Vout. In this example voltages Nin and Vout are DC voltages.

Especially, in forward type DC-to-DC converters there is a need to reset the transformer. The resetting of the transformer means that the stored energy is removed from the transformer. The need for resetting the transformer arises when there is a possibility that the transformer can saturate. If the transformer is allowed to saturate, the transformer behaves like a short circuit seen by the input voltage source. As a result, the forward DC-to-DC output circuit does not work as it should.

There are several known methods to reset the transformer. A first method is shown in Figure 2 and described in patent publication US-5521807 (Chen et al.) in which a small circuit arrangement is used for resetting the transformer. This converter topology has a multiple output configuration. Each of the secondary circuits has an own reset circuit arrangement. For resetting it is used an extra parasitic reset capacitance Cshl, Csh2, Cshn with the secondary forward rectifier diode Dl. The reset circuit arrangement works in the following way. When the switch Ql in the primary side is turned OFF, the magnetizing current is immediately reflected to the secondary winding of the transformer. The reset circuit initiates a half resonant cycle which first discharges the magnetizing current to zero and then further charge it to negative with the final value approximately equal to its starting value. This process forces the transformer to reset completely. As seen in Figure 2 Cd, Cm, Cp, Csn are the equivalent parasitic capacitances of series diode D3 and Zener diode Zl, MOSFET switch Ql, transformer primary winding, transformer secondary windings and output Schottky rectifier diode Dl. Another similar solution is described in patent document US 4688160 where the resetting is achieved with a capacitor that is connected across the output diode.

These methods have the disadvantage that the resetting of the transformer is dependent on the components. Different component manufacturers produce components, whose characteristics differ from each others. In addition, the characteristics of components are temperature dependent, which in the worst case means that the resetting cannot be carried out totally.

A further method for resetting the transformer in a forward DC-to-DC converter is described in patent publication US-4441146. The method is shown in Figure 3. For resetting the transformer a storage capacitor 20 is arranged to the secondary side of the converter. In addition, the arrangement comprises an auxiliary solid state switch 21 and a switch control circuit 22. The switch control circuit 22 operates the auxiliary switch in its open state during the converter's ON period, when the primary switch is closed, and in its closed state during the converter's OFF period, when the primary switch is open. The resetting of the transformer's core is achieved by implementing the conceptual function of a "magnetizing current mirror". The magnetizing current mirror takes the magnetization at the end of the ON period and creates a mirror image of it prior to the initiation of the following conversion cycle. The image is created via charging and discharging of the storage capacitor which forms a resonant circuit with the transformer's magnetizing inductance. The resonant circuit recycles the transformer's magnetizing energy by creating a mirror image of the magnetic flux between ON periods. This maximizes the flux swing available within a given core.

The problem of this method to reset the transformer is that the controlling of the auxiliary switch needs electronics. This makes the converter more expensive to manufacture. Also the need for extra electronics makes the electronics design difficult to carry out. In addition, with this method it is not possible to get an auxiliary output.

There are also several other methods to carry out the resetting of the transformer. One very common way to this is to use an extra reset winding into the transformer. This makes the structure of the transformer more complicated as well as needs quite much extra electronics.

Summary of the Invention

This invention describes a method by means of which the resetting of the transformer can be carried out in a simple and cheap way. According to the invention an auxiliary output is used for resetting. Advantageously, a small circuit arrangement is implemented, which circuit arrangement comprises means for resetting the transformer of the forward DC-to-DC converter. The reset circuit comprises at least capacitors and diodes. Advantageously, these elements are arranged so that the current path is dependent on the switching cycle in the primaiy side. When the switching element in the primary side is ON, at least one capacitor is charged. When the switching element in the primaiy side is OFF, the charged capacitor/capacitors is/are discharged resulting in a demagnetizing current, which resets the transformer of the forward DC-to-DC converter. An auxiliary output can also be taken from the reset circuit. Brief Description of the Drawings

Figure 1 discloses a typical circuit arrangement for forward DC-to-DC converter,

Figure 2 discloses a first method to reset the transformer according to prior art,

Figure 3 discloses a second method to reset the transformer according to prior art,

Figure 4 discloses a block diagram of the invention,

Figure 5 discloses another block diagram of the invention,

Figure 6 discloses a third block diagram of the invention,

Figure 7 discloses a first embodiment of the invention,

Figure 8 discloses periodically the voltage over the switching element,

Figure 9 discloses a second embodiment of the invention,

Figure 10 discloses a third embodiment of the invention,

Figure 11 discloses a fourth embodiment of the invention,

Figure 12 discloses a fifth embodiment of the invention and

Figure 13 discloses a sixth embodiment of the invention.

Detailed Description of the Invention

In Figure 4 it is disclosed a block diagram of the invention. The forward DC-to-DC converter in Figure 4 comprises a transformer 403, which divides the converter to a primary side and a secondary side. Typically, in the primary side there is an arrangement 402 by means of which the input voltage NIN can be filtered. This kind of arrangement can be implemented by using for example a capacitor. In the primary side there is advantageously a switching element 404. Controlling the switching element 404 the current is arranged to flow through the transformer 403. The switching element 404 is controlled by a control circuit 101. Typically the control circuit 101 comprises some kind of pulse-width modulator by means of which the duty cycle can be defined. Advantageously, the control circuit 101 gets feedback from the secondary side of the converter. In the secondary side there is a rectifier circuit 405. The rectifier circuit 405 normally comprises several diodes, which controls the direction of the current so that the result is rectified signal. The current is stored into the energy storing element 406, which typically is an inductor. The output signal VOUT1 is filtered before the load by a filter arrangement 407. The filter arrangement 407 can be a capacitor. The reset circuit 408 is arranged to the auxiliary output VOUT2 of the forward DC-to-DC converter. The first input of the reset circuit 408 is taken from the first side of the transformer 403 and the second input of the reset circuit 408 is taken from the second side of the transformer 403. The reset circuit 408 comprises means, which produce an effect by means of which the transformer 403 can be reset.

Figure 5 discloses the same block diagram of the invention as the previous one. Here the reset circuit 408 is disclosed more accurately. The reset circuit comprises a first capacitor Cl, which is coupled from its first side to the first side of the transformer 403. The other side of the capacitor Cl is coupled to the anode of the diode Dl. The cathode of the diode Dl is an auxiliary output VOUT2. Capacitor C3 is connected between the cathode of the diode Dl and the anode of diode D2. The cathode of diode D2 is coupled to the anode of diode Dl.

In Figure 6 is shown a third block diagram of the invention. In this preferred embodiment an auxiliary output is not taken from the reset circuit 408. Here parallel to the capacitor C3 is arranged a resistor Rp. By means of this resistor Rp the capacitor C3 can be discharged so that the reset circuit 408 works in all cases. In the following descriptions it is shown only topologies which have the auxiliary output. To a man skilled in the art it is obvious that in all cases it is possible to add a resistor Rp parallel to the capacitor C3. In these cases the auxiliary output VOUT3 can be removed. It is also possible to have an auxiliary output VOUT3 and the parallel resistor at the same time so that the operation of the reset circuit 408 does not suffer.

In Figure 7 it is shown one preferred embodiment of the invention. In this preferred embodiment the converter comprises multiple output circuits and thus multiple outputs VOUT1; VOUT2. In the secondary side of the transformer there is a winding for every output circuit. In addition, from the second output circuit in the secondary side it is taken an auxiliary output VOUT3 into which in this preferred embodiment it is arranged a reset circuit for the transformer Tl. In the primary side of the forward DC-to-DC converter it is arranged a MOSFET transistor Ql to operate as a switching element. The transistor Ql is controlled by a control circuit 101. Capacitor C2 filters the input voltage VIN, which advantageously is a DC voltage. The secondary side of the forward DC-to-DC converter shown in Figure 7 is typical to this kind of converters. First of all, there is a rectifier circuit, which comprises a first diode D5; D6 and a second diode D4; D3. Secondly, there are inductors in the output circuits of the secondary sides. Strictly, in this preferred embodiment the inductors in the first and in the second output circuits are coupled together. This kind of inductor arrangement is called coupled inductor LK1. Finally, the output voltages VOUT1; VOUT2 are filtered by capacitors C5; C4. The input to reset circuit in this preferred embodiment is taken from the anode of diode D6. The current from the transformer Tl flows through the capacitor Cl and diode Dl to the output. Capacitor C3 charge during the operation and feeds the current to the output when the switching element is OFF. To a man skilled in the art it is obvious that the capacitors Cl and C3 and the diodes Dl and D2 forms a voltage doubler circuit arrangement. Using this arrangement the output voltage of the auxiliary circuit can be adjusted to the correct level. For this purpose any kind of voltage multiplier circuit can advantageously be used. It is also obvious to a man skilled in the art that the reset circuit described here can be arranged to any output circuit in the converter topology.

Next we consider the operation of the reset circuit in the embodiment shown in Figure 8. To make this easier in Figure 8 it is shown the voltage Vce between the drain and the source of the switching transistor Ql. To a man skilled in the art it is obvious that the same applies to any other kind of switching element like for example to FET transistor. The first time period tl is when the switching transistor Ql conducts. During the time period t2 the transistor Ql is switched OFF and because of this the voltage Vce rises quite high, even higher than double input voltage Vin. The shape of this voltage curve depends on the LC-circuit, which is formed by the primary winding of the transformer Tl and the intrinsic capacitance of the transistor Ql and Cl. In some cases there is arranged an extra capacitor over the transistor Q 1 to protect the component and/or a clamp parallel to the transformer Tl. During the third time period t3 the switching effect is removed and the voltage over the transistor is essentially the same as the input voltage Vin. Time t3 may be almost zero when nήnimum operating input voltage is applied.

More closely the operation of the reset circuit is the following. During the time period tl, when the switching transistor Ql is conducting and the voltage Vce is zero the converter circuit operates normally. The current is also flowing to the reset circuit through the capacitor Cl and the diode Dl to the output VOUT3. The capacitor C3 is charged. During the second time period t2 when the switch is not conducting the voltage Vce rises rapidly. At the same time the polarity of the transformer changes. Capacitors Cl discharges so that the current flows from the capacitor Cl through the transformer Tl and diode D2. This demagnetizing current Idm is marked to the Figure 8 with dotted line. As a result the current resets the capacitor Cl and the transformer Tl. The flow of demagnetizing current ends latest, when the polarity of the capacitor Cl changes again. The capacitor C3 is discharged to the load at the same time. During the third time period t3 the demagnetizing current does not exist and the transformer as well as the capacitor Cl are reset. The capacitor C3 still feeds current to the load.

In Figure 9 it is shown a second preferred embodiment of the invention. The circuit topology of the converter is the same as described previously. The reset circuit in this embodiment is arranged in a different way. The input to reset circuit is taken from the anode of diode D6. Capacitor Cl is connected between the anode of the diode D6 and the anode of the diode Dl . The cathode of diode D2 is connected to the anode of the diode Dl. Capacitor C3 is arranged between the cathode of the diode Dl and the anode of the diode D2. The anode of the diode D2 and thus the capacitor C3 are also connected into the positive signal of the output VOUT2. When the transistor Ql conducts (time period tl), the current flows to the reset circuit through the capacitor Cl and the diode Dl. Capacitors Cl and C3 charge until the transistor Ql changes its state. After the state has changed the capacitors Cl and C3 start discharging. The capacitor Cl discharges to the transformer Tl. The current produced by the capacitor Cl flows through the transformer Tl, capacitor C4 and diode D2 resetting the transformer Tl. The capacitor Cl changes its polarity and the current flows as long as the capacitor Cl has approximately reached the voltage of the transformer.

In Figure 10 it is shown a third preferred embodiment of the invention. In this preferred embodiment the output circuits of the forward DC-to-DC converters are the same as in the previously described embodiments. Here the capacitor C3 is connected between the cathode of the diode D 1 and the ground signal of the output VOUT2. Again, the capacitors Cl and C3 charge, when the switching element is conducting. After that the capacitor Cl starts discharging by producing a current, which flows in a similar way as in the previous embodiment. To a man skilled in the art it is obvious that the circuits shown in Figures 9 and 10 operate in the same way. The only difference is the operating point of capacitor C3, which is defined by the connection point of the capacitor.

In Figure 11 it is shown a fourth preferred embodiment of the invention. In this preferred embodiment the forward DC-to-DC converter has two output circuits as in the previous examples. The reset circuit is coupled to the first output circuit in the secondary side of the converter. The input to the reset circuit is taken from the anode of the diode D5. The reset circuit comprises capacitor Cl and diode Dl through which the current is fed into a load during the first time period i.e. when the switch Ql is ON. When the switch Ql is OFF (time period t2), the secondary side of the transformer and the capacitor Cl change polarities and the current starts flowing to the opposite direction through the diodes D6 and D2. The path of the demagnetizing current Idm is marked in the Figure 11 with dotted line. The flow of the current continues as long as the capacitor Cl is discharged as long as it approximately reaches the voltage of the transformer. The transformer Tl and advantageously the capacitor Cl are reset. During the third time period t3 the capacitors C3 and C5 still discharge. Capacitor C3 is coupled between the auxiliary output and the ground.

In this preferred embodiment the inductors or more precise the coupled inductor LK1 is connected so that the inductor is in the ground signal between the anode of the diode D4 and the capacitor C5 which belongs to the basic circuit arrangement of the forward DC-to-DC converter. The reason for this kind of arrangement is that sometimes it is used a common cathode double diodes in converters. These diodes have a common cathode which forms a rather big layer which tends to behave like a capacitor with another layer or even point in a circuit especially in high frequency applications. This increases the EMC problems. Another problem can arise especially in SMD (Surface-Mounted Device) technology in which the common cathode layer can capacitively be coupled to the ground or even to the output voltage resulting the bypassing of the inductor. To avoid this kind of problems the inductors are coupled to the ground signal. To a man skilled in the art it is obvious that the circuit arrangement described here does not produce any kind of problems in using the reset circuit.

In figure 12 it is shown a fifth preferred embodiment of the invention. In this embodiment the reset circuit is arranged to the first output circuit. In the embodiment shown in Figure 12 it is arranged two voltage doublers to the auxiliary output circuit by means of which the output voltage VOUT3 can be multiplied and adjusted appropriate for the output VOUT3. The first voltage doubler comprises diodes Dl and D2 and capacitors Cl and C3. The second voltage doubler comprises diodes D7 and D8 and capacitors C6 and C3. The capacitor C3 is common to both voltage doublers. The components are connected in the following way. The first side of the capacitor Cl is connected to the first side of the transformer Tl. The anode of the diode Dl is connected to the second side of the capacitor Cl. The first side of the capacitor C3 is connected to the cathode of the diode Dl and the second side of the capacitor C3 is connected to the ground signal of the output VOUT1. The cathode of the diode D2 is connected to the second side of the capacitor Cl and the anode of the diode Dl and the anode of the diode D2 is connected to the voltage signal of a output VOUT1. The first side of the capacitor C6 is connected to the second side of the transformer Tl. The anode of diode D7 is connected to the second side of the capacitor C6 and the cathode of the diode D7 is connected to the cathode of diode Dl and the first side of capacitor C3. The anode of the diode D8 is connected to the voltage signal of the output VOUT1 and the cathode of the diode D8 is connected to the second side of the capacitor C6 and the anode of the diode D7. The auxiliary output is taken from the cathode of the diode Dl, the cathode of the diode D7 and the first side of capacitor C3. In this preferred embodiment the demagnetizing current Idm flows through the capacitor Cl, the transformer Tl, the diode D4 and the diode D2 resetting the transformer Tl.

The preferred embodiment shown in Figure 13 comprises the same DC-to-DC converter topology as the previous examples. The voltage doubler circuit are coupled the same way as in Figure 12, besides the anodes of diodes D2 and D8 are connected to the ground signal of the output VOUT1. The resetting in this preferred embodiment works in the following way. During the first time period tl the current flows to the output of the auxiliary output circuit through the capacitor C 1 and the diode Dl. During the time period t2 the polarities of the transformer Tl and the capacitor Cl change and the demagnetizing current resets the transformer Tl. The demagnetizing current flows through the transformer Tl, the capacitor C6, the diode D7, the capacitor C3, the diode D2 and the capacitor C2. The current path is marked with dotted line in Figure 13.

To a man skilled in the ait it is obvious that the reset circuit can be implemented with other components than described here. The idea of the invention is to use an auxiliary output circuit in which there is an arrangement by means of which a demagnetizing current can be fed into the transformer, when the switching element is not conducting in the primary side of the forward DC-to-DC converter. As a result the transformer is reset. For clarity, the operations of the other output circuits, which belong to a typical DC-to-DC converter circuit are not described in detail. To a man skilled in the art it is obvious that the basic topology of the converter circuit can vary with the limits of the forward DC-to-DC converters.

In view of the foregoing description it will be evident to a man skilled in the art that various modifications may be made within the scope of the invention. While a several preferred embodiments of the invention have been described in detail, it should be apparent that many modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention.

Claims

1. A method for resetting the transformer (Tl) in a forward DC-to-DC converter comprising a switching element (Ql), characterized in that at least one capacitor (Cl) in a reset circuit is used for resetting the transformer (Tl), which capacitor (Cl) is charged essentially during the ON period of the switching element and discharged essentially during the OFF period of the switching element (Ql) producing a demagnetizing current (Idm) and at least one output of the forward DC- to-DC converter is taken from said reset circuit.

2. A forward DC-to-DC converter comprising a switching element (Ql), characterized in that the converter comprises a reset circuit in which at least one capacitor (Cl) is arranged to charge essentially during the ON period of the switching element (Ql) and to discharge essentially during the OFF period of the switching element (.Ql) producing a demagnetizing current for resetting the transformer (Tl) and at least one output of the forward DC-to-DC converter is arranged to be taken from said reset circuit.

3. A forward DC-to-DC converter according to Claim 2, characterized in that the reset circuit comprises at least a first diode (Dl) and a second diode (D2) and a first capacitor (Cl) and a second capacitor (C3) and in the reset circuit

- the first side of the first capacitor (Cl) is connected to the first side of the trans- former (Tl),

- the anode of the first diode (Dl) is connected to the second side of the first capacitor (Cl),

- the first side of the second capacitor (C3) is connected to the cathode of the first diode (Dl),

- the cathode of the second diode (D2) is connected to the second side of the first capacitor (Cl) and the anode of the first diode (Dl),

- the anode of the second diode (D2) is connected to the second side of the second capacitor (C3) and the second side of the transformer (Tl) and

- a second output is taken from the cathode of the first diode (Dl) and the first side of the second capacitor (C3).

4. A forward DC-to-DC converter according to Claim 2, characterized in that the reset circuit comprises a first diode (Dl) and a second diode (D2) and a first capacitor (Cl) and a second capacitor (C3) and in the reset circuit

- the anode of the second diode (D2) is connected to the second side of the second capacitor (C3) and the voltage signal of the first output and

- a second output is taken from the cathode of the first diode (Dl) and the first side of second capacitor (C3).

5. A forward DC-to-DC converter according to Claim 2, characterized in that the reset circuit comprises a first diode (Dl) and a second diode (D2) and a first capacitor (Cl) and a second capacitor (C3) and in the reset circuit

- the second side of the second capacitor (C3) is connected to the second side of the transformer (Tl),

- the cathode of the second diode (D2) is connected to the second side of the first capacitor (Cl), - the anode of the first diode (Dl) and the anode of the second diode (D2) is connected to the voltage signal of a first output and

6. A forward DC-to-DC converter according to Claim 2, characterized in that the reset circuit comprises a first diode (Dl) and a second diode (D2) and a first capacitor (Cl) and a second capacitor (C3) and in the reset circuit

- the first side of the first capacitor (Cl) is connected to the first side of the transformer (Tl),

- the second side of the second capacitor (C3) is connected to the ground signal of the first output,

- the anode of the second diode (D2) is connected to the voltage signal of a first output and

7. A forward DC-to-DC converter according to Claim 2, characterized in that the reset circuit comprises a first diode (Dl), a second diode (D2), a third diode (D7), a fourth diode (D8) and a first capacitor (Cl), a second capacitor (C3), a third capacitor (C6) and in the reset circuit

- the anode of the first diode (Dl) is connected to the second side of the first capacitor (Cl), - the first side of the second capacitor (C3) is connected to the cathode of the first diode (Dl),

- the anode of the second diode (D2) is connected to the voltage signal of a first output,

- the first side of the third capacitor (C6) is connected to the second side of the transformer (Tl),

- the anode of third diode (D7) is connected to the second side of the third capacitor (C6),

- the cathode of the third diode (D7) is connected to the cathode of first diode (Dl) and the first side of the second capacitor (C3),

- the anode of the fourth diode (D8) is connected to the voltage signal of the first output,

- the cathode of the fourth diode (D8) is connected to the second side of the third capacitor (C6) and the anode of the third diode (D7) and

- a second output is taken from the cathode of the first diode (Dl), the cathode of the third diode (D7) and the first side of the second capacitor (C3).

8. A forward DC-to-DC converter according to Claim 2, characterized in that the reset circuit comprises a first diode (Dl), a second diode (D2), a third diode (D7), a fourth diode (D8) and a first capacitor (Cl), a second capacitor (C3), a third capacitor (C6) and in the reset circuit

- the anode of the second diode (D2) is connected to the ground signal of a first output,

- the cathode of the third diode (D7) is connected to the cathode of first diode (Dl) and the first side of second capacitor (C3),

- the anode of the fourth diode (D8) is connected to the ground signal of the first output,

- a second output is taken from the cathode of the first diode (Dl), the cathode of the third diode (D7) and the first side of second capacitor (C3).

9. A forward DC-to-DC converter according to Claim 2, characterized in that the reset circuit is a voltage multiplier circuit.

10. The use of a circuit arrangement comprising capacitors and diodes for resetting the transformer and for forming an output in a forward DC-to-DC converter.