JPS5910945Y2 - voltage isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in voltage isolators.

In analog signal transmission circuits, isolators are used as part of common mode noise countermeasures.

Although various types of isolators are in use, conventional examples of voltage isolators include those shown in FIGS. 1 and 2.

In Fig. 1, a series circuit of a diode D1 and a capacitor C1 is connected to the primary coil Np of a transformer T, a resistor R1 is connected in parallel to the capacitor C1, and an input DC voltage V1 is connected across the resistor R1. , diode D1
The polarity is opposite to that of .

A similar circuit is formed in the secondary coil N5 of the transformer T, using a diode D2, a capacitor C2, and a resistor R2, and a DC output voltage is generated from both ends of the resistor R2.

is taken out.

The turns ratio between the primary coil Np and the secondary coil N5 is 1.

A constant voltage DC power supply is connected to both ends of the secondary coil Ns.

An excitation circuit including a switching element SW is connected to the excitation circuit.

Direct power supply ■. The polarity of the excitation voltage is determined by the diode D
2, and the switching element SW is turned on and off at a predetermined duty ratio.

In such a circuit, when the switching element SW is on, the DC power supply is 1V.

voltage is applied to the secondary coil N5, which generates an induced voltage in the primary coil Np, but both voltages are blocked by the diodes D1 and D2 and are not transmitted to the input side and output side.

When the switching element SW is turned off, the electromagnetic energy stored in the transformer T generates a flyback voltage in the primary coil Np and secondary coil N5, and these voltages are applied to the capacitors C1 and C1 through diodes Di and D2, respectively. It is applied to a parallel circuit of resistor R1 and a parallel circuit of capacitor C2 and resistor R2.

The input voltage ■ to the parallel circuit of capacitor C1 and resistor R1 is
1 is given, the flyback voltage of the primary coil Np is amplitude limited to a value equal to the human power voltage ■1 (
(Assume that the voltage drop across diode D1 is negligible).

Therefore, the flyback voltage of the secondary coil N5 also has the same value, and this is rectified and smoothed by a circuit consisting of a diode D2, a capacitor C2, and a resistor R2, resulting in an output voltage ■.

As a result, the output voltage ■ matches the input voltage V1.

is obtained. In such a voltage isolator,
When the amplitude of the flyback voltage is limited, a current flows from the primary coil Np toward the input voltage source based on the excess energy.

This current causes a voltage drop in the internal resistance of the human voltage source and the resistance of the signal line, increasing the apparent input voltage and causing an error.

The ratio of this error becomes larger as the human power voltage v1 becomes smaller and the current flowing into the input voltage source becomes larger.

Figure 2 shows a solution to these problems.
The voltage of the excitation DC power source -1 is changed in accordance with the output voltage Vo, and a suitable compensation resistor is used as the resistor RA.

In this circuit, as the input voltage (1) decreases, the output voltage (1) decreases.

The excitation voltage is lowered by taking advantage of the fact that By appropriately determining
A certain amount of current is drawn from the input voltage source to offset the current due to the flyback voltage.

However, even in such a circuit, the excitation DC power source -■o includes a fixed bias source in order to overcome the voltage drop across the diodes D1 and D2, so that the input voltage V1 is When it reaches a value comparable to the voltage drop of D2, it approaches a fixed excitation state and an increase in error occurs.

Further, as a voltage isolator, the problem is that the input impedance is small.

An object of the present invention is to provide a voltage isolator with small errors and large input impedance even at low input voltages.

The present invention uses a comparator to compare the rectified and smoothed value of the flyback voltage with the input voltage, and absorbs excess energy so that the two match.

The present invention will be explained below with reference to the drawings.

FIG. 3 is an electrical connection diagram of an embodiment of the present invention.

In Figure 3, CF is a comparator, D3 is a diode, and R
3 is a resistor, and the other parts are the same as in the case of FIG.

The comparator CF compares the input voltage ■1 with the rectified and smoothed value ■, of the flyback voltage of the primary coil Np, and when it becomes ■,>■1, it generates a negative output voltage, and the primary coil Np absorbs excess flyback voltage.

Since the gain of comparator CF is sufficiently large, this results in
The voltage (2) is controlled to match the input voltage V+.

When VF<Vl, the output voltage of the comparator CF becomes positive and is blocked by the diode D3, so that the flyback voltage is not affected.

When the amplitude of the flyback voltage of the primary coil Np is limited in this way, the output voltage V
.

corresponds to the input voltage ■1. In such a circuit, the excess flyback voltage is absorbed by the comparator CF, so no current flows into the input voltage source.

Therefore, errors due to such currents do not occur, and accurate operation is performed even when the input voltage (1) is small.

Furthermore, since the comparator CF can have a high input impedance by using an operational amplifier, it can be made into a voltage isolator with a high input impedance.

The power supply voltage of the comparator CF can be obtained by providing another coil in the transformer T and rectifying and smoothing the induced voltage when the switching element SW is turned on.

In addition, if the voltage (2) input to the comparator CF is extracted from the middle of the resistor R1, the output voltage (2) is obtained by expanding the human voltage V1 to the comparative example.

is obtained.

Further, the excitation of the transformer T may be performed by providing a dedicated coil.

The amplitude of the flyback voltage can also be limited as shown in FIG.

In FIG. 4, a transistor Q is connected in parallel to the primary coil Np, and this transistor Q is controlled by the output of a comparator CF.

Comparator CF turns on transistor Q when VF>1, absorbs excess energy, and limits the amplitude of the flyback voltage.

As described above, the present invention uses a comparator to compare the rectified and smoothed value of the flyback voltage with the input voltage, and absorbs excess energy so that the two values match.

Therefore, it is possible to realize a voltage isolator with small errors and large input impedance even when the input voltage is small.

[Brief explanation of the drawing]

1 and 2 are electrical connection diagrams of the conventional example, and FIGS. 3 and 4 are electrical connection diagrams of the embodiment of the present invention. T...Transformer, Np...Primary coil, N8...Secondary coil, D1, D2, D3...
...Diode, R1, R2, R3...Resistor, -Vc...DC power supply for excitation, SW...
...Switching element, Q...Transistor.

Claims (1)

[Scope of utility model registration request] A transformer having at least one pair of coils insulated from each other; excitation means for intermittently exciting the transformer with a DC voltage; a pair of rectifying and smoothing circuits each connected to the pair of coils and rectifying and smoothing the flyback voltage;
and a voltage isolator equipped with a comparator that compares the output voltage of one of these rectifying and smoothing circuits with the input voltage and absorbs excess energy of the transformer during flyback so that the two match.