Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
  • H02M3/00—Conversion of dc power input into dc power output
  • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
  • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
  • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
  • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
  • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
  • H02M3/33561—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having more than one ouput with independent control

Definitions

• the invention relates to a switched-mode power supply comprising a transformer which is provided with primary and secondary windings and through which power is transferred from the primary to the secondary, the transformer comprising at least two secondary windings for forming at least two separate outputs for the power supply; a first switch provided in the primary circuit for chopping a primary current flowing through the primary winding of the transformer; and an adjust ⁇ ment circuit controlling said first switch, said cir ⁇ cuit adjusting the output voltage of the power supply by pulse width modulation by varying the ratio between the durations of the ON and OFF periods of the switch.
• the role of switched-mode power supplies in the power supply design is becoming more and more import ⁇ ant.
• Switches-mode power supplies employ flyback topology to ar ever increasing degree (topology refers to the circuit configuration de ⁇ termining how the power is transferred in a power supply).
• topology refers to the circuit configuration de ⁇ termining how the power is transferred in a power supply.
• the biggest advantage of a flyback-type power supply is its simple and inexpensive structure, which is suitable for use even in multiple output power supplies.
• the switch-mode power supply according to the present invention is preferably of the flyback type, even though the solution according to the inven ⁇ tion is applicable in any type of switched-mode power supply comprising several (at least two) outputs.
• the ratios of the numbers of turns of the secondary windings of the transformer are selected so that the other output voltages will have a desired value. For instance, if an output of 5 V is adjusted accurately, the number of turns of the secondary winding supplying a voltage of 12 V is selected relative to the number of turns of the secondary winding supplying a voltage of 5 V in the ratio of 12/5. In practice, the ratio is slightly different from this, as the voltage losses of the secondary windings also have to be compensated for.
• a problem with the above-described method is that it can provide only voltages at which both of the numbers of turns are integers, as the magnetic circuit of the transformer will otherwise be magnetized asym ⁇ metrically and may be saturated.
• the number of turns of the secondary winding has to be relatively small at low voltages in order for the losses created in the transformer coils to be low. For instance, the number of turns of 5 V windings typically varies be ⁇ tween 4 and 8 in a switched-mode power supply of about 10 W. Each turn thus corresponds to a voltage ranging between 1.25 V and 0.625 V.
• the fre ⁇ quently needed generation of a voltage of -4.5 V cannot thereby be performed directly by the number of turns of the transformer, but the closest voltages are -5 V (the same number of turns as in the secondary winding of 5 V) or between -3.75 V and -4.37 V (one turn less than in the secondary winding of 5 V).
• the voltage of -4.5 V is often generated by forming a voltage of -5 V and diminishing the voltage by a separate series regulator by the required amount, 0.5 V.
• the resulting power losses having a magnitude of 0.5 V x the magnitude of the load current may be significant.
• the object of the present invention is thus to avoid the above-described disadvantages and to provide a switched-mode power supply which allows desired out ⁇ put voltages to be produced with a very high accuracy without any significant power losses.
• This object is achieved by a ywitched-mode power supply according to the invention, which is characterized in that an autotransformer known per se is connected after at least one of the secondary windings.
• the idea of the invention is to connect a separ ⁇ ate autotransformer after the main transformer of the switched-mode power supply, which allows the output voltage to be decreased to a desired level.
• the additional transformer according to the in ⁇ vention is very simple in structure and it allows several accurate output voltages to be produced.
• Figure 1 shows a flyback-type switched-mode power supply known per se with three different output voltages
• Figure 2 shows a part of a secondary circuit in a switched-mode power supply when the solution accord ⁇ ing to the invention is applied to the circuit, an autotransformer being used as a voltage reducing means; and
• Figure 3 illustrates the dimensions of the secondary circuit according to the invention.
• FIG. 1 shows a general switching diagram il ⁇ lustrating a flyback switched-mode power supply known per se when a PWM adjustment circuit 13 is positioned on the side of the secondary voltages.
• the power supply comprises a main transformer Tl, through which power is transferred from primary to secondary, and a switch SW1, such as a power mosfet (shown in the figures) or a bipolar transistor, provided in the primary circuit.
• the switch SW1 having its drain electrode connected to one terminal of the primary winding and its source electrode connected directly to the minus terminal of an input voltage Uin, chops a primary current flowing through the primary winding N of the main transformer.
• the power supply further comprises an adjustment circuit 13 controlling the switch.
• the adjustment cir ⁇ cuit controls output voltages, +12 V, +5 V (main out- put), and -4.5 V in this example, by adjusting the duty cycle of the switch SW1. This adjustment takes place by a pulse width modulation (PWM), i.e. by adjusting the ratio between the durations of the ON and OFF periods of the switch.
• PWM pulse width modulation
• the adjustment circuit 13 controlling the width of the switch pulse may oper ⁇ ate either in a voltage mode based on the output volt ⁇ age or in a current mode based on the primary current and output voltage.
• the adjustment circuit shown in the example of Figure 1 is therefore the current- mode adjustment circuit 13, which operates on the basis of voltage information from the output of the switched-mode power supply and primary current information.
• Voltage information is derived to a voltage feedback input Vfb of the adjustment circuit by sampling the output voltage of the main output by voltage division resistors R4 and R5.
• Information about the primary current is derived to a current measuring input Is of the adjustment circuit through a current measuring transformer 21 provided in the primary circuit.
• the adjustment circuit 13 As the adjustment circuit 13 is positioned on the secondary side, and the switch SW1 on the primary side, a separate amplifier circuit 22 with galvanic separation is needed for controlling the switch. Operating voltage to the adjustment circuit is obtained from the +12 V output. The input for the operating voltage is indicated by Vc.
• the adjustment circuit 13 may be e.g. of the type UC3843 (or any other circuit of the same family), manufacturer e.g. Unitrode Corporation, USA.
• the secondary circuit of the power supply comprises three secondary windings, which are indicated by the references Nl, N2 and N3.
• One terminal of the secondary winding Nl is connected to the earth, and a series connection of a rectifying diode Dl and an output capacitor Coutl is connected between this terminal and the other terminal of the winding.
• one terminal of the second ⁇ ary winding N2 is connected to the earth, and a series connection of a rectifying diode D2 and an output capacitor Cout2 is connected between this terminal and the other terminal of the winding.
• the secondary winding N3 and the rectifying diode D3 are connected in a corresponding way, except that the direction of the winding and the diode is reversed, as the output voltage is negative.
• a series regulator 23 diminishes the output voltage to a desired level.
• the power supply described above has the above- described problems if the -4.5 V output, for instance, is to be implemented without the power losses caused by the series regulator.
• Figure 2 shows a portion of the secondary cir ⁇ cuit of a switched-mode power supply, such as the flyback-type switched-mode power supply shown in Figure 1, when the solution according to the invention is applied to the circuit.
• a switched-mode power supply such as the flyback-type switched-mode power supply shown in Figure 1, when the solution according to the invention is applied to the circuit.
• an autotransformer T2 is connected after that secondary winding of the main transformer Tl which forms the output to be adjusted (in this example, winding N3).
• the autotransformer diminishes the output voltage U22 to a desired level.
• it comprises a single winding, which is used both as a primary and secondary winding. In other words, it comprises two windings, one winding forming part of the other (the windings are not separated from each other).
• NAB turns in the winding between the terminals A and B of the autotransformer T2 and NBC turns between the terminals B and C (which is the common section of the winding).
• the first terminal of the secondary winding N3 is connected to the terminal A of the autotransformer, and the rectifying diode D3 is connected to the terminal B of the autotransformer.
• the voltage Ul is selected so that a voltage Ull rectified by the diode Dl is of a desired level, the voltage Ull being kept constant by varying the pulse ratio of the switched-mode power source (the duty cycle of the switch SW1).
• the number of turns ⁇ BC can be made very high as compared with the number of turns ⁇ AB, whereby the ratio ⁇ BC/( ⁇ BC+ ⁇ AB) in the above equation of the volt- age U2 can be selected very accurately.
• the voltage U21 (and the voltage U22) can be determined with a very high accuracy.
• the proportion of the current II in the load current IL will be very small, so that the winding ⁇ BC can be made of a very thin conductor wire.
• the invention also includes a resistance Rl, the significance of which will be described below.
• Im(t) the magnetizing current of the autotran- sformer, flowing in the closed secondary current circuit
• IL(t) current to the load, distributed in the autotransformer, the corresponding circuit being closed partly through the autotransformer and partly through the secondary winding ⁇ 3 of the main trans ⁇ former;
• the resistance R2 represents the common resistance of the windings N3 and NAB, which is of the order of 10 to 50 milliohms in a 10 W power supply.
• the resistance R2 causes power losses in the secondary, wherefore one attempts to keep it low.
• Limiting factors include the transformer construction and losses due to a high-frequency current acting on the winding.
• the resistance Rl represents the sum of the resistance of the winding NBC and the resistance of a possible resistance component to be added. If the resistance component is added, it is connected between the terminal C of the auto ⁇ transformer and the earthed terminal. A voltage appearing in the secondary is indicat ⁇ ed with the reference U2(t). This voltage has to be symmetrical relative to the zero level, i.e. its time integral has to be zero in magnitude as calculated over the period of the duty cycle T. Otherwise the main transformer of the power supply would be satur ⁇ ated.
• the integral of the voltage acting across the autotransformer should also be zero in magnitude across the recurrent duty cycle T. Otherwise the magnetizing current Im flowing through the auto ⁇ transformer starts to increase, and the magnetizing current Im(t) contains a dc component.
• a voltage acts across the auto- transformer when UA is the voltage acting in the ter ⁇ minal A and UC is the voltage acting in the terminal C.
• UA the voltage acting in the ter ⁇ minal A
• UC the voltage acting in the terminal C.
• UA(t)-UC(t) U2(t)-ILa(t)xR2+ILc(t)xRl-Im(t)x(Rl+R2).
• the integral of the voltage U2(t) calculated over the duty cycle T also has to be zero in magnitude in order that the main transformer would not be saturated. This results in that the time integral of the middle term in the equation of the voltage acting across the autotransformer has to be zero, i.e.
• the ratio R1/R2 need not be quite accurate, as the dc current component ImO need not be exactly zero. ImO thereby depends on the value of the load current IL(t) so that the above integral of Equation (2) is still zero.
• the dc current component ImO will have the general expression (5) :
• the resistance Rl can be accomplished solely by the resistance of the winding NBC, without any separ ⁇ ate resistance component.
• the above discussion is general and applies equally well to a positive and negative output voltage when the direction of the voltages, winding, and cur ⁇ rents is selected correspondingly.
• the autotransformer also refers more generally to windings connected in a similar way, i.e. windings interconnected both magnetically and galvanically.
• the solution according to the invention can be applied to any type of switched-mode power supply.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Dc-Dc Converters (AREA)
• Control Of Electrical Variables (AREA)
• Ac-Ac Conversion (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

Family

ID=8538748

Family Applications (1)

Country Status (5)

Cited By (2)

Citations (1)

• 1993
  • 1993-10-11 FI FI934477A patent/FI94198C/fi active
• 1994
  • 1994-10-10 GB GB9607227A patent/GB2298500B/en not_active Expired - Fee Related
  • 1994-10-10 AU AU78145/94A patent/AU7814594A/en not_active Abandoned
  • 1994-10-10 DE DE4497667T patent/DE4497667T1/de not_active Withdrawn
  • 1994-10-10 WO PCT/FI1994/000454 patent/WO1995010877A1/en active Application Filing

Patent Citations (1)

Non-Patent Citations (1)

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