WO1993023915A1

WO1993023915A1 - Voltage measuring circuit

Info

Publication number: WO1993023915A1
Application number: PCT/FI1993/000210
Authority: WO
Inventors: Matti Tuominen
Original assignee: Nokia Telecommunications Oy
Priority date: 1992-05-21
Filing date: 1993-05-19
Publication date: 1993-11-25
Also published as: FI922315A; FI94686B; SE511049C2; AU4071093A; GB9423240D0; GB2283115B; SE9403984L; DE4392104T1; FI922315A0; SE9403984D0; GB2283115A; FI94686C

Abstract

The invention relates to a voltage measuring circuit, especially for measuring the output voltage of a switched-mode power supply. According to the invention a first switching transistor (TR2) periodically switches the output voltage to a primary winding (W4) of a measuring transformer (T2). The induced secondary voltage of the transformer (T2) is rectified and switched to a control circuit by a second switching transistor (TR3). The first and second transistors are synchronized and exhibit a resistive voltage/current behaviour in a conducting state.

Description

Voltage measuring circuit

Field of the invention

The present invention relates to a voltage measuring circuit especially for measuring the output voltage of a switched-mode power supply, comprising a transformer with a primary and secondary winding, a first switch means connected in series with the primary winding of the transformer between the input terminals of the ea- suring circuit for periodically switching the measuring voltage to the primary winding of the transformer.

Background of the invention

Switched-mode power supplies typically comprise a power transformer Tl with a primary winding Wl and a secondary winding W2, the power transformer dividing the switched- mode power supply into two galvanically isolated portions, i.e. a primary circuit and a secondary circuit, as illustrated in Figures 1 and 2. The DC switched-mode power supply comprises on its secondary side at least a rectifier D2 and a filter capacitor Cl. The primary side comprises typically a switching tran¬ sistor TRl, which switches the direct voltage DC to the primary winding of the transformer Tl at the operating frequency of the switched-mode power supply. The output voltage DC of the switched-mode power supply is dependent on the switching frequency of the transistor TRl. Therefore, the switching frequency is adjusted depending on the output voltage DC so as to maintain a desired level of the output voltage. The problem with the circuit is the measuring of the output voltage DC on the secondary side of the power supply and transfer¬ ring the measuring result or adjustment to the primary side, which is galvanically isolated from the secondary side, for controlling the transistor TRl. In one known solution the control information to be feedbacked is derived from a power transformer, as illustrated in Figure l. In this case the power trans- former Tl is provided with an extra secondary winding

W3. The winding W3 provides a measuring voltage which is rectified with a rectifier Dl and fed into a control circuit 11 of the switching transistor __„ . This is a

^ TRl simple solution but it causes measuring errors as a result of the stray inductance of the transformer Tl and the voltage drop across the rectifier.

In another known solution, illustrated in Figure 2, the output voltage DC is measured directly from the out- put of a switched-mode power supply by means of a control circuit 21 positioned on the secondary side of the switched-mode power supply. The control signal is fed to the primary side by means of isolation unit 21. The solution demands, however, relatively many extra components.

GB patent application No. 2 153 113 discloses a DC-DC converter, in which the output voltage of the converter is connected to the primary winding of the isolation transformer by means of a transistor, which is operated by the switching frequency on the secondary winding of the power transformer of the converter. The induced voltage across the secondary winding of the isolation transformer is rectified and feedbacked to the control electronics of the converter. In this connection, too, the voltage drop across the rectifier diode and the switching transistor cause measurement errors. The faster the circuit is made, the lower the accuracy of the measuring circuit is. Disclosure of the Invention

The object of the present invention is a ciruit for measuring the output voltage of a switched-mode power supply in a manner that the above problems are avoided and other advantages are obtained.

This is achieved by a voltage measuring circuit of the type disclosed in the introduction, which according to the invention is characterized in that the measuring circuit further comprises a second switch means connec¬ ted in series with the secondary winding between the output terminals of the measuring circuit for rectifying the voltage induced in the secondary winding of the transformer and for switching the rectified voltage to the output terminals of the measuring circuit, and means for synchronizing the first and the second switch means with each other, and the first and the second switch means having resistive voltage/current behaviour in a conducting state.

In the present invention the output voltage of the switched-mode power supply is measured by a series con¬ nection of a measuring transformer and a switch means having a preferably resistive voltage/current behaviour in a conducting state. The switch means switches the voltage to be measured periodically to the primary winding of the measuring transformer, whereby the vol¬ tage is induced to the secondary winding of the measuring transformer. The voltage of the secondary winding of the measuring transformer is rectified by a switch means similar to that used on the primary side of the transformer. The rectified voltage is switched to the output terminals of the measuring circuit to be fed as a measuring value to the control circuit of the switched-mode power supply on the primary side of the power suppl . The measuring circuit is bidirectional, since the switch means allow the current to flow in both directions during operation with resistive voltage drops. Because the operation of the measuring circuit according to the invention is bidirectional, it trans¬ fers measuring information essentially faster than the measuring circuit according to GB patent application No. 2 153 113, since the secondary winding of the measuring circuit during one operating cycle exhibits a voltage equal to or directly proportional to the primary voltage. The switch means are synchronized with each other, whereby the signals occurring in the power supply may be utilized. The errors caused by the com¬ ponents on the secondary side of the power supply are eliminated, because the actual output voltage is being measured. In GB patent application No. 2 153 113 and the known solutions according to Figure 1 the voltage in the terminals of the transformer is being measured, but the actual output voltage is as much lower due to the vol- tage drop across the rectifier diode and the switching transistor. The voltage drop across the rectifier diode, on the other hand, is temperature dependent and moreover there are variations due to component tolerances. When synchronized switching elements, which do not have any PN-junction, are being used in the invention, the errors caused by the temperature dependence of the PN-junction are eliminated because the resistances of the switching elements can be made several times lower than the impe¬ dance of the measuring circuit. In addition, the measur- ing circuit according to the invention can be put into practice with a smaller number of components than the known measuring circuits.

Brief Description of the Drawings The invention will be described in the following by means of examples with reference to the attached drawings, in which

Figures 1 and 2 illustrate the principal schematic diagram of two known measuring and control circuits for a switched-mode power supply,

Figure 3 illustrates the principal schematic diagram of the measuring circuit according to the invention, and Figure 4 illustrates the principal schematic diagram of the measuring circuit of Figure 3 when applied to a switched-mode power supply.

Detailed description of the Invention

The measuring circuit according to the invention can be applied in any measuring application in which exact voltage measuring is required and the measuring result is wished to be transferred between galvanically iso¬ lated device sections.

The basic construction of the invention is illustrated in Figure 3, which shows only those components that are most essential with regard to the operation of the measuring circuit according to the invention.

The measuring circuit according to the preferred embodiment of the invention shown in Figure 3 comprises a measuring transformer T2 with a primary winding W4 and a secondary winding W5. A switch component TR2 having a resistive voltage/current behaviour in a conducting state is connected in series with the primary winding W4 of the transformer T2. The series connection of the winding W4 and the switch component TR2 is connected between the measuring or input terminals A and B of the measuring circuit. A second switch component TR3 of the same type . as the switch component TR2 used on the primary side is connected in series with the secondary winding W5 of the transformer T2. The series connection of the secondary winding W5 and the switch component TR3 is connected between the output terminals C and D of the measuring circuit. The switch components TR2 may be any switch component which conducts only in one direction in order to achieve the rectifying action and which exhibits the desired resistive voltage/currentbehaviour in a conducting state, for example semiconductor switch components. In the preferred embodiment of the invention TR2 and TR3 are MOSFET-switch components controlled at gate electrodes Gl and G2.

The measuring circuit according to Figure 1 operates as follows. The measuring voltage occurring across the input terminals A and B is periodically switched to the primary winding W4 of the measuring transformer T2 by the switch component TR2 under control of a control signal applied to the gate electrode Gl of TR2. As a result, the voltage occurring across the primary winding W4 induces a corresponding voltage across the secondary winding W5. The induced voltage across the secondary winding W5 is rectified and the rectified secondary voltage is switched to the output terminals C and D of the measuring circuit by the switch component TR3 under control of a control signal applied to gate electrode G2. The switch components TR2 and TR3 are synchronized with each other so that they are in a conducting state and a non-conducting state, respectively, essentially concurrently.

Figure 4 illustrates the application of the measuring circuit of Figure 3 to a DC-DC switched-mode power supply. Figure 4 shows only those circuit components which are essential in order to explain the invention. As is obvious for one skilled in the art, the switched- mode power supply may comprise also other additional components.

In Figure 4 the switched-mode power supply comprises a transformer Tl, which divides the power supply into a primary side (input side) and a secondary side (output side) . The transformer comprises a primary winding Wl and a secondary winding W2. On the primary side a switching transistor TRl is connected in series with the primary winding Wl of the power transformer Tl. The transistor TRl connects at the switching frequency f the incoming direct voltage across the series connection to the primary winding Wl, and the voltage across said winding induces a corresponding voltage across the secondary winding W2. The voltage across the secondary winding W2 is rectified by a rectifier diode D2 and filtered by a filter capacitor Cl, whereby the voltage across the capacitor Cl provides the direct voltage output DC of the power supply. The input terminals A and B of the measuring circuit 41 according to the invention shown in Figure 3 are connected to the power supply output terminals across the capacitor Cl. The output terminals C and D of the measuring circuit 41 are connected to a control circuit 42 on the primary side of the power supply. The control circuit 42 provides the control signal for the transistor TRl. In this way it is possible with the measuring circuit according to the invention to measure the output voltage on the secon¬ dary side of the switched-mode power supply and to transfer the result of the measurement in a galvanically isolated way to the control circuit 42 on the secondary side. In its simplest form the control circuit 42 may consist of a differential amplifier, which compares the voltage produced by the measuring circuit 41 to a pre- set reference voltage (which corresponds to the desired output voltage DC ) and produces their difference or error voltage, and a modulator circuit which is con¬ trolled by said error voltage.

For the control and mutual synchronizing of the switch components TR2 and TR3 of the measuring circuit 41 according to the invention, signals already existing in the power supply may be utilized. In the embodiment shown in Figure 4, a control signal is derived for the gate Gl of the switch component TR2 by the drive circuit 44 from the voltage across the secondary winding W2 of the power transformer Tl. Correspondingly, a control signal is derived for the gate electrode G2 of the switch component TR3 by the drive circuit 43 either from the primary voltage across the primary winding Wl of the power transformer Tl or alternatively from the control signal of the transistor TRl. In this way a galvanic isolation is maintained between the drive cir¬ cuits 44 and control signals of the measuring circuit 41, because they are derived from different sides of the power transformer Tl and fed to different sides of the measuring transformer T2. However, the switching frequency of the transistor TRl occurs as the primary switching frequency in all control signals, whereby the switch components TR2 and TR3 operate synchronized with each other.

In the alternative embodiment shown in Figure 5 a control signal for the gate Gl of the switch component TR2 is produced by the drive circuit 54 and a control signal for the gate electrode G2 of the switch component TR3 by the drive circuit 53. The drive circuits 53 and 54 are synchronized with each other through the trans¬ former T3, whereby the drive circuits 53 and 54 and the control signals produced by them are galvanically iso- lated from each other. The transformer T3 is totally independent of the power transformer Tl and its signals, thus the drive circuits 53 and 54 can control the measuring circuit at a higher operating frequency than the operating frequency of the switched-mode power supply, whereby the circuit may be implemented with components of essentially smaller size.

The invention can be applied also to switched-mode power supplies carried out in some other way.

The accompanying figures and the description related thereto are only intended to illustrate the present invention. The measuring circuit according to the inven- tion may to its details vary within the enclosed claims.

Claims

1. Voltage measuring circuit, especially for measuring the output voltage of a switched-mode power supply comprising a transformer (T2) with a primary and a secondary winding (W4, W5), a first switch means (TR2) connected in series with the primary winding (W4) of the transformer between the input terminals (A, B) of the measuring circuit for periodically switching the measuring voltage to the primary winding (W4) of the transformer, c h a r a c t e r i z e d in that the measuring circuit further comprises a second switch means (TR3) connected in series with the secondary winding (W5) between the output terminals (C, D) of the measuring circuit for rectifying the voltage induced in the secondary winding of the transformer and for switching the rectified voltage to the output ter- minals (A, B) of the measuring circuit, and means (43, 44) for synchronizing the first and the second switch means with each other, and the first and the second switch means (TR2, TR3) having a resistive voltage/current behaviour in a conducting state.

2. Measuring circuit according to claim 1, c h a r a c t e r i z e d in that the first and the second switch means (TR2, TR3) are semiconductor switch components, preferably MOSFET-switch components.

3. Measuring circuit according to claim 1 or 2, c h a r a c t e r i z e d in that the means for synchro¬ nizing the first and the second switch means (TR2, TR3) controls the switch means with control signals derived from the switched-mode power supply.

4. Measuring circuit according to claim 3, c h a r a c t e r i z e d in that the switched-mode power supply comprises a power transformer (Tl) with a primary and a secondary winding (Wl, W2), and that the means for synchronizing the first and the second switch means (TR2, TR3) comprises a first drive circuit (44) controlling the first switch means by a first control signal derived from the secondary side of the power transformer, and a second drive circuit (43) controlling the second switch means by a second control signal derived from the primary side of the power transformer, the first and second control signals having equal fre¬ quencies.

5. Measuring circuit according to claim 4, c h a r a c t e r i z e d in that the first control signal is derived from the voltage of the secondary winding (W2) of the power transformer and the second control signal is derived from the voltage of the primary winding (Wl).

6. Measuring circuit according to one of claims 3, 4 or 5, c h a r a c t e r i z e d in that the frequency of the control signals is the operating frequency of the switched-mode power supply.

7. Measuring circuit according to claim 1 or 2, c h a r a c t e r i z e d in that the means for synchronizing the first and the second switch means (TR2, TR3) comprises a first drive circuit (44) controlling the first switch means, a second drive cir¬ cuit (43) controlling the second switch means, and a transformer, through which the drive circuits are synchronized with each other.

8. Measuring circuit according to one of the previous claims, c h a r a c t e r i z e d in that the switched- mode power supply comprises a power transformer (Tl) with a primary and a secondary winding (Wl, W2), and a control circuit (42, TRl) connected to the primary side of the power transformer and controlling the primary voltage, the output terminals (C, D) of the measuring circuit being connected to said control circuit.