RU2320066C1 - Method for dc-to-dc voltage conversion in electrically isolated power supplies - Google Patents

Abstract

FIELD: electrical engineering; electrically isolated pulsed power supplies.

SUBSTANCE: proposed method for DC-to-DC voltage conversion that can be used to build power supplies from AC mains with power factor close to unity and unit power capacity of 500 V and higher using single-stage circuit arrangement includes storage of energy picked off input power supply across transformer primary winding, charging of first capacitor with magnetizing current upon completion of energy storage, transfer of stored energy upon completion of charge at reverse polarity of voltage across transformer windings to load and part of energy to second capacitor which is connected to primary winding as soon as voltages across primary winding and across second capacitor are equalized; then energy obtained by second capacitor is transferred through transformer and rectifier to load. Reverse-polarity magnetizing current is set up by energy of second capacitor at reverse polarity of voltage across transformer windings, signal obtained as difference of signals proportional to primary and secondary currents is compared with signal difference proportional to input voltage and voltage across second capacitor provided this difference is positive or incorporates zero, if it is negative and voltages are equal, second capacitor is turned off and first one is discharged by means of reverse-polarity magnetizing current until voltage across primary winding reverses polarity and becomes equal to input voltage, remaining energy of reverse-polarity magnetizing current is returned to input supply, and voltage conversion cycle is repeated.

EFFECT: enhanced efficiency, mass, and size of converter, as well as maximal unit power output of power supply.

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Description

The invention relates to electrical engineering and can be used in switching power supplies with galvanic isolation for converting direct voltage to constant voltage, as well as for building AC power sources with a power factor close to unity, according to a single-stage scheme, with a unit power of 500 W and more.

A known method of energy conversion, implemented in the power source [1], which consists in the fact that using the first key with a parallel capacitance accumulate energy in the primary winding of the transformer and then, when the first key is open, transmit it using the secondary winding of the transformer, rectifier and output filter into the load, while on the interval of energy transfer to the load, the voltage on the primary winding is limited using the RCD circuit, while the dissipation energy of the transformer is released into the ballast this circuit.

The disadvantage of this method is the low efficiency caused by the loss of dissipation energy and dynamic losses in the key.

The closest analogue is the energy conversion method implemented in the power source [2], in which, using the first key with parallel capacity, the energy is accumulated in the primary winding of the transformer and then, when the first key is open, it is transmitted using the secondary winding of the transformer, a rectifier and an output filter to the load, while the dissipation energy of the primary winding of the transformer is transferred to a capacitor, which is connected to the primary winding using a second switch when the voltage on the primary the coil will become equal to the voltage across the capacitor, and then through the open second key is transferred back through the transformer to the load. The on time of the second switch is determined by differentiating the voltage of the additional winding of the transformer.

The disadvantage of this power source is that a high converter efficiency can be obtained with fixed input and output voltage and load current, which does not allow it to be used in the power factor corrector mode, where the input voltage varies from zero to the network amplitude and in sources with a wide range output voltage. It is not possible to increase the capacitance of a capacitor connected in parallel with the key to reduce dynamic losses when it is turned off and to make full use of its switching ability. The non-optimal determination of the moment of turning off the second key leads to an increase in the “transformer downtime”, which is not associated either with the process of energy storage or with its transfer to the load. This worsens the current shape of the transformer windings, increases losses and leads to underutilization of the overall power of the transformer.

The problem solved in the proposed method is to reduce losses on energy conversion, as well as improving the specific weight and size indicators of converters for similar purposes.

The technical result from the application of the proposed method consists in increasing the efficiency of the converter, reducing the mass and dimensions of the cooler for the converter keys, increasing the unit power of the converter due to the more efficient use of the capabilities of the keys and the transformer.

This object is achieved by the fact that a method is proposed for converting direct voltage to constant voltage with galvanic isolation, which consists in the fact that energy is stored in the primary winding of the transformer from the input power source, at the end of the energy storage process, the first capacitor is charged, at the end of the charge, at the reverse the polarity of the voltage across the transformer windings, the stored energy is transferred to the load and part of the energy to the second capacitor, which is connected to the primary winding at the moment of equal voltage on the primary winding and voltage on the second capacitor, and then transfer the energy received by the second capacitor through a transformer and rectifier to the load.

In the proposed method, when the voltage polarity is reversed on the transformer windings with the energy of the second capacitor, a magnetization current of reverse polarity is created, the signal obtained as the difference of the signals proportional to the primary and secondary currents is compared with the difference of the signals proportional to the input voltage and the voltage of the second capacitor, if the difference is positive, or with zero, if the difference is negative, if they are equal, turn off the second capacitor and use the magnetizing current of reverse polarity to cut he first capacitor to a voltage reversal of the primary winding and the equality of its input voltage, energy return residue magnetizing current of opposite polarity to the input voltage source and the conversion cycle is repeated.

The application of the proposed method allows using the energy of the transformer to discharge the parallel capacitance and hold zero voltage on it until the first key is opened, which makes it possible to significantly increase the parallel capacitance to reduce dynamic losses when the key is turned off without large losses when it is recharged. It provides the converter operating mode in a wide range of input and output voltage and load current, with a minimum “transformer downtime”, which is not associated with the main processes of energy storage in the transformer and with the transfer of this energy to the load, and, therefore, to use the overall power more fully transformer and thus increase the efficiency, increase the specific weight and dimensions of the power source and increase its unit power.

The essence of the proposed method is illustrated by the timing diagrams shown in figure 1, and the equivalent circuit of figure 2.

The time charts (Figure 1) show: Ug.cl1 - control pulses of the first key; Ikl1 - current of the first key; Iμ - magnetization current of the transformer (Iμ = I1-12), I1 and I2 - current of the primary and secondary windings of the transformer; Ikl2 - current of the second key; Ukl1 - voltage on the first key. In the diagrams: ΔIμ is the increment of the magnetization current of reverse polarity to the moment the second key is turned off; Uc and Uin - voltage across the capacitor and input voltage.

On the equivalent circuit (Figure 2) disclosed an equivalent circuit of the transformer, where:

Lμ is the magnetization inductance of the transformer;

Ls1 and Ls2 - scattering inductance of the primary and secondary windings;

1 - input voltage source; 2 - the first key; 3 - the first capacitor (parallel capacitance); 4 - second capacitor; 5 - the second key; 6 - transformer; 7 - output rectifier; 8 - output filter; 9 - load.

The possibility of implementing the proposed method is illustrated by the timing diagrams of Figure 1 and the equivalent circuit of Figure 2.

On the interval t1-t2, the necessary amount of energy is accumulated in the primary winding of the transformer 6 (Icl1, Iμ) and at the moment t2 the first key is locked. The voltage on the first key rises at a speed determined by the value of the first capacitor 3, which allows to reduce the dynamic loss on switching off the first key. When the voltage on the secondary winding of the load voltage is reached, the rectifier 7 starts to conduct and the accumulated energy enters the load. At time t3, when the voltage on the first key becomes equal to the voltage of the second capacitor 4, the second switch 5 is unlocked, the scattering energy of the primary winding and part of the energy of the main stream are transferred to the capacitor (Icl2) - interval t3-t4. In the interval t4-t6, the energy of the main stream and the energy stored in the second capacitor are transferred to the load. At time t5, the energy of the main stream decreases to zero and the accumulation of the magnetization current of another sign begins due to the energy of the second capacitor. At time t6, when the difference in the signals proportional to the input voltage and the voltage of the second capacitor becomes equal to the signal proportional to the magnetizing current of the transformer Iμ, the second switch is locked and the first capacitor 3 is discharged with negative magnetizing current to zero. Excessive negative magnetization current closes through the reverse diode of the first key (not shown in the diagram) and returns to the input source. Next, the conversion process is repeated.

The moment of locking the second key is determined based on the required energy for recharging the first capacitor 3 to zero (without taking into account losses). This moment is determined when the signal is proportional to the current of the primary winding and the signal is proportional to the difference between the reduced values of the input and output voltages, which follows from the expressions for the energy of the capacitor and inductance:

;

;

from where

;

;

where: E _μ is the magnetization inductance energy of the transformer;

I _μ - magnetization current reduced to the primary side;

U _c is the voltage across the capacitor;

L _μ - magnetization inductance of the transformer;

U _BX - input voltage of the converter;

E _C is the energy of the capacitance connected in parallel with the first key;

C is the capacitance of the capacitor.

The process of recharging the first capacitor 3 proceeds in the following sequence. Initially, the voltage on it is equal to Ucl1 = Ubx + Uc, where Uc is the voltage on the second capacitor, Ubx is the voltage of the input power source. Then the voltage across the capacitor drops, and as soon as it takes the value Uc = Ubx, the voltage on the primary winding becomes equal to zero, and the energy of the first capacitor corresponding to Uc is transferred to the transformer. Then this component of the energy of the transformer cuts the parallel capacitance to a voltage of Ucl1 = Ubx-Uc. In order for the capacitor to discharge to zero, it is necessary to first accumulate a current equal to in the transformer:

.

.

When implementing the method in a power source combined with a power factor corrector, where the input voltage pulsates with the mains frequency, there is a mode when the input voltage is less than the voltage of the second capacitor. In this mode, excess negative current is accumulated in the transformer during the discharge of the first capacitor, and it is necessary to lock the second key when the magnetization current reaches zero.

To implement the method, it is necessary to determine the value of the capacitance of the second capacitor. The optimal value is determined from the condition that in the maximum power mode of the converter, the half-cycle of free oscillations in the serial circuit composed of the capacitor 4 and the total leakage inductance Ls1 + Ls2 (Figure 2) is equal to the time required to transfer the energy of the main stream to the load ( interval t4-t5).

Information sources

1. Integrated circuits: Integrated circuits for switching power supplies and their application. Ed. 2nd. - M .: DODEKA, 2000 .-- 608 p., P. 191.

2.7 H02M 3/28. JP 3237633 B2. 12/02/98. Switching Power Supply (73) Murata Mfg Co Ltd.

Claims (1)

A method of converting direct voltage to direct voltage with galvanic isolation, which consists in the fact that energy is stored in the primary winding of the transformer from the input power source, at the end of the process of energy storage by the magnetizing current, the first capacitor is charged, at the end of the charge, at the reverse polarity of the voltage on the transformer windings, the accumulated energy is transferred to the load and part of the energy to the second capacitor, which is connected to the primary winding at the moment of equal voltage on the primary voltage and voltage at the second capacitor, and then transfer the energy received by the second capacitor through the transformer and rectifier to the load, characterized in that when the voltage polarity is reversed, the magnetization current of the opposite polarity is generated by the energy of the second capacitor, the signal obtained as the difference of the signals is compared, proportional to the primary and secondary currents, with the difference of the signals proportional to the input voltage and the voltage of the second capacitor, if the difference is positive, or zero, if the difference is negative, when they are equal, the second capacitor is turned off and the first capacitor is discharged using the magnetization current of reverse polarity until the sign of the primary winding voltage changes and its input voltage is equal, the remainder of the energy of the magnetization current of reverse polarity is returned to the input source and the voltage conversion cycle is repeated.

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