JPH0540711Y2 - - Google Patents

Description

[Detailed explanation of the idea] [Industrial application field]

The present invention relates to an output current detection circuit for observing the load condition of a switching power supply, and particularly to improving linearity in a low current region.

[Conventional technology]

In order to observe the load condition of the switching power supply,
Conventionally, an output current detection circuit has been provided. This application is for overcurrent detection.
FIG. 10 is a block diagram of a conventional switching power supply output current detection circuit. A DC voltage Vin is applied to the primary side of the transformer T, which is turned on and off by a switching element such as an FET. The switching element is turned on and off using PWM (pulse width
modulation) is performed by the control IC. Since a switching signal appears on the secondary side of the transformer T, it is rectified by a diode, smoothed by a capacitor, and the output current Iout is supplied to the load Z. The current voltage circuit uses a current transformer installed on the primary side.
Detect output current via CT. The detected current is voltage doubled and rectified by a pair of diodes D1 and D2, and this voltage is generated across resistor R1. The doubled voltage is rectified by capacitor C2 and diodes D3 and D4 and stored in capacitor C3. The stored voltage Vs is sent to the load side as a current detection signal Vs.

[Problem that the idea aims to solve]

FIG. 11 is a characteristic diagram of the current detection circuit of FIG. 10. Theoretically, if the inductance L is infinite ∞, the output current Iout and the voltage Vs are proportional.
However, the inductance of the main transformer is finite and is influenced by the excitation current of the current transformer CT, so at minute currents the output current Iout
There was a problem that there was a non-linear relationship between the voltage and the voltage Vs. In other words, it shows an almost linear response to a current higher than the critical current Icr, but in this response, the critical current
When extrapolated to a current below Icr, a residual voltage Vres occurs for zero current. This phenomenon will be explained in detail using waveforms. The switching waveform should be a rectangular wave because it is an on/off signal, but due to the influence of magnetism, the waveform is distorted and becomes a waveform in which a triangular wave △ is superimposed. When the output current Iout decreases, the rectangular wave component disappears and only the triangular wave component remains. The current at this time is called the critical current Icr, and below the critical current, only the triangular wave component is observed. Originally a square wave□
Since the waveform that should be a triangular wave becomes △, the rectified voltage Vs loses its linearity. When used only for overcurrent detection as in the past, nonlinearity did not cause any problems. However, in large power supplies, it is necessary to accurately monitor even minute currents for maintenance inspections, and there is a problem of improving nonlinearity. The present invention solves these problems and provides an output current detection circuit for a switching power supply that compensates for the nonlinearity of the current detection circuit in minute currents and provides a detection output proportional to the output current. With the goal.

[Means to solve the problem]

The present invention achieves this purpose by rectifying and smoothing an AC input voltage or directly inputting a DC voltage, and turning on and off the input DC voltage by receiving a switching control signal from a control circuit provided on the primary side. A switching element, a transformer T that inputs this switched signal to the primary side and outputs it to the secondary side in an insulated state, and a secondary that rectifies and smoothes the switching signal that appears on the secondary side and supplies it to the load. This is a switching power supply device having a side rectifying and smoothing section, and has the following configuration. That is, as an output current detection circuit, there is a current transformer CT that detects the switching current flowing to the primary side of the transformer, and a capacitor C that rectifies and smoothes the current that appears on the secondary side of the current transformer.
3, and a voltage dividing section 22 that outputs half the voltage of the output signal of this rectification and smoothing section.
The output current Iout to the load side of the switching power supply device and the output voltage Vs of this rectifying and smoothing section are proportional to the output current, which is proportional to the critical current Icr or more, and the output current becomes zero current until the output current becomes zero current. A voltage shift section 23 that applies a reverse bias voltage Vsh to the output voltage that cancels the residual voltage Vres that remains in the output voltage when the voltage is extrapolated, and a voltage that is higher between this voltage dividing section and the voltage shift section. It is characterized by having a selection section 24 for selecting. Note that in place of the aforementioned voltage dividing section, voltage shifting section, and selection section, the output current Iout to the load side of the switching power supply device and the output voltage of this rectifying and smoothing section are used.
Reverse bias voltage that cancels the residual voltage Vres that remains in the output voltage when the output voltage is extrapolated until the output current becomes zero current at the slope of the output current that is proportional to Vs with respect to the current that is greater than or equal to the critical current Icr
A voltage shift rectifier 26 may be employed that applies Vsh to the output voltage and outputs zero voltage in a region close to zero voltage where the polarity changes in the region below the critical current among the added output voltages. .

[Effect]

Each component of the present invention operates as follows. The switching power supply device is a common one, and is preferably a DC stabilized power supply. The current transformer detects the output current, and the output current is rectified and smoothed by the rectifier and smoother. The voltage dividing section calculates the output voltage at a minute current smaller than the critical current. The voltage shift section performs calculations in a current region larger than the critical current, and performs level shifting to maintain continuity with the calculated value of the voltage division section at the critical current. The selection section compares the output current with a critical current and switches between the voltage dividing section and the voltage shifting section. When using a voltage shift rectifier, if the polarity in the region below the critical current is the same as in the region above the critical current, the voltage of the voltage divider section with a reverse bias voltage applied will be output, and the polarity will change. When they differ, zero voltage is output.

【Example】

The present invention will be explained below using the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, parts having the same functions as those in FIG. 10 are given the same reference numerals and their explanations will be omitted. In the figure, the switching power supply device is a general type, and the input may be a commercial AC power supply or a DC power supply. For this power supply, the output voltage
It is preferable to feed back Vout and control the switching signal of the PWM control section to stabilize the output voltage. The current transformer CT and the rectifying and smoothing section 21 are approximately the same as those in FIG. Capacitor C1 connected in parallel to the secondary side of current transformer CT
has a small capacitance and excludes high frequency components.
22 connects the voltage Vs0 of the capacitor C3 to the resistor R2,
This is a circuit that divides the voltage by R3, and although it is halved here, it may also be amplified by half using an OP amplifier. 23 is a voltage shift unit that applies a reverse bias voltage to the voltage Vs0 of the capacitor C3, and this reverse bias voltage is determined by the slope of the output current Iout for a current exceeding the critical current Icr explained in FIG. 11 when extrapolated to zero current. The residual voltage Vres is set to a value that cancels out the residual voltage Vres, and the inductance L is equivalently set to infinity ∞. Here, the negative power supply V _EE is divided and supplied by resistors R4 and R5, and the power supply level shift amount V _sh
is expressed by the following equation. V _sh = {R4/(R4+R5)}×(Vs0−V _EE ) (1) 24 is a selection for selecting the higher of the output signal Vs1 of the voltage dividing section 22 and the output signal Vs2 of the voltage shift section 23. In this case, a voltage follower and diodes D5 and D6 are combined, and an output current signal Vs is output. The operation of the device configured in this manner will be described next. FIG. 2 is a relationship diagram between the output current Iout and the voltage signal Vs, where A indicates the output signal Vs1 of the voltage dividing section 22, B indicates the output signal Vs2 of the voltage shift section 23, and C indicates the output current signal Vs of the selection section 24. It shows. In the figure, the broken line indicates the signal value Vs0 stored in the capacitor C3 of the rectifying and smoothing section 21. With the critical current Icr as the boundary, if it is below this, the output voltage Vs1 is selected,
If it is higher than this, output voltage Vs2 is selected. FIG. 3 is a circuit diagram for explaining the operation of the present invention.
The DC voltage Ein is sent as a rectangular wave current IE by turning on and off a switch, and is output as a DC voltage Eout by a rectifying and smoothing circuit including a diode, an inductor L, and a capacitor. FIG. 4 is a waveform diagram when the rectangular wave current IE is also larger than the critical current Icr. If the average output current during the period Ton in which the switch is on is Iout, the peak current is Ip, and the wave height of the triangular wave is 2XIoc, the following equation holds true. Ip=Iout+Ioc (2) Here, the relationship between the critical current Icr and the triangular wave current Ioc becomes consistent if the critical current Icr is defined as the average output current during the period Ton in which the switch is on. Therefore, as shown in Figure 5, the output current
If the triangular wave current Ioc is removed from Iout, it overlaps with the straight line from the origin. That is, the shift voltage V _sh and the critical current
There is a one-to-one correspondence with ICR. FIG. 6 is a waveform diagram when the rectangular wave current IE is less than or equal to the critical current Icr, where A is equal and B is small. If the average current of the triangular wave △ is Iout, the peak current Ip can be expressed by the following formula. Ip=2√・ (3) In a special case, the square wave current IE becomes the critical current Icr
(equivalent to the case where the output current Iout is equal to the triangular wave current Ioc), equations (2) and (3) become equal, and Ip=2×Ioc (4) holds true. Therefore, it is desirable that the selection unit 24 performs switching using this critical current Icr. Figure 7 shows critical current
It is a characteristic diagram near Icr. At the critical current Icr, the current of the voltage shift section 23 is Ip=Icr. Therefore, in order to make the voltage of the voltage dividing section 22 equal and continuous, it is necessary to divide equation (3) by 2. As a result, although it is strictly a square root function below the critical current Icr, since the indication error of instruments is defined by the value relative to the full scale, it can be considered that for very small values, a value that is virtually linearly approximated can be obtained. As a result, the output voltage is proportional to the output current Iout.
Vs is obtained. Furthermore, when the circuit is configured in this way, even if the critical current Icr is unknown, the selection unit 24 can be adjusted by adjusting the voltage shift unit 23 so that the value at full scale is equal to the straight line passing through the origin. performs switching at critical current Icr. FIG. 8 is a block diagram showing a second embodiment of the present invention. Compared to FIG. 1, an amplification section 25 and a voltage shift rectification section 26 are provided in place of the voltage shift section 23 and selection section 24, thereby reducing the number of parts. In FIG. 8, parts having the same functions as those in FIG. 1 are given the same reference numerals and their explanations will be omitted. In the figure, the amplifying section 25 includes a buffer U1 that receives the output voltage Vs1 of the voltage dividing section 22 as an input, and an OP amplifier U2 that inverts and amplifies the output signal of the buffer U1 at a rate determined by resistors R6 and R7.
The voltage shift rectifier 26 receives the output signal of the OP amplifier U2 input via the resistor R9 and the reference voltage Vref.
OP amplifier U that adds the reverse bias voltage converted at a rate determined by resistors R10, R11, and R12.
3 and more. This reverse bypass voltage cancels the residual voltage Vres of the signal inverted by the amplifier section 25, similar to the voltage shift section 23 in FIG. The OP amplifier U3 inverts the output signal of the OP amplifier U2, and the diodes D5 and D6 connected to the OP amplifier U prevent the output voltage Vs from becoming a positive voltage and becoming a negative voltage. Figure 9 shows the 8th graph of output current Iout and voltage signal Vs.
FIG. 3 is a relationship diagram in the device shown in FIG. The output voltage Vs has a linear relationship with respect to the output voltage Iout which is larger than the critical current Icr. For output voltage Iout larger than critical current Icr, output voltage Vs decreases nonlinearly, and in the region below zero level, diode D5,
Due to the action of D6, it is limited to zero level and does not become a negative voltage.

[Effect of the idea]

As explained above, according to the present invention, the non-linearity of the output current detection circuit in the case of minute currents is compensated for, so the correct output current can be determined even at minute outputs, which has a great practical effect. . Further, in the case of setting the level to zero in a minute area as in the second embodiment, there is a practical effect that the number of parts can be reduced and the manufacturing cost can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a configuration block diagram showing an embodiment of the present invention, FIG. 2 is a relationship diagram between the output current Iout and the voltage signal Vs, and FIG. 3 is a circuit diagram for explaining the operation of the present invention.
Fig. 4 is a waveform diagram when the rectangular wave current IE is larger than the critical current Icr, Fig. 5 is a characteristic diagram in the region larger than the critical current Icr, and Fig. 6 is a waveform diagram when the rectangular wave current IE is less than the critical current Icr. FIG. 7 is a characteristic diagram near the critical current Icr. FIG. 8 is a block diagram showing a second embodiment of the present invention, and FIG. 9 is a diagram showing the relationship between the output current Iout and the voltage signal Vs in the device shown in FIG. Figure 10 is a configuration block diagram of the output current detection circuit of a conventional switching power supply.
FIG. 1 is a characteristic diagram of the output current detection circuit. CT... Current transformer, 21... Rectification and smoothing section, 22... Voltage dividing section, 23... Voltage shift section,
24... Selection section, 26... Voltage shift rectification section.

Claims (1)

[Claims for Utility Model Registration] (1) AC input voltage is rectified and smoothed, or DC voltage is directly input, and the input DC voltage is turned on and off in response to a switching control signal from a control circuit provided on the primary side. A switching element, a transformer T that inputs this switched signal to the primary side and outputs it to the secondary side in an insulated state, and a secondary that rectifies and smoothes the switching signal that appears on the secondary side and supplies it to the load. It is a switching power supply device having a side rectifying and smoothing section, which includes a current transformer CT that detects the switching current flowing to the primary side of the transformer, and a current appearing on the secondary side of the current transformer that is rectified and smoothed and stored in a capacitor C3. a rectifying and smoothing section 21; a voltage dividing section 22 that outputs half the voltage of the output signal of this rectifying and smoothing section; an output current Iout to the load side of the switching power supply device and an output voltage Vs of this rectifying and smoothing section. When the output voltage is extrapolated until the output current becomes zero current at the slope of the output current with respect to the current equal to or higher than the critical current Icr, which is proportional to A switching power supply characterized by comprising: a voltage shift section 23 that applies an output voltage; a selection section 24 that selects the higher voltage of either the voltage dividing section or the voltage shift section; and an output current detection circuit comprising: output current detection circuit. (2) The switching power supply device according to claim 1, and the current transformer and rectification smoothing section according to claim 1, and the output current Iout to the load side of the switching power supply device and the output voltage of this rectification and smoothing section. When the output voltage is extrapolated until the output current becomes zero current at the slope of the output current with respect to the current that is proportional to the critical current Icr, the reverse bias voltage Vsh that cancels out the residual voltage Vres that remains in the output voltage is determined. an output current detection circuit comprising: a voltage shift rectifier 26 that applies to the output voltage and outputs zero voltage in a region close to zero voltage where the polarity changes in a region below a critical current of the added output voltage; An output current detection circuit for a switching power supply, characterized by comprising: