JP2656070B2 - Constant current input type DC-DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] A switching element is connected in parallel to a series circuit of a load and a rectifying / smoothing circuit, and a driving power supply circuit and the driving power supply circuit are based on a current flowing through the load via a boosting coil. Constant-current input type in which the switching element is turned on / off by a drive circuit connected to the power supply to control a load voltage from a constant current source.
For DC-DC converters, the purpose is to prevent the load voltage from becoming excessive even before the input current from the constant current source to the converter reaches a steady state, and to prevent excessive voltage stress on the load components. A current detection circuit connected in series with the driving power supply circuit to detect a predetermined value of the converter input current; and a current detection circuit connected in parallel to the driving power supply circuit, short-circuiting the driving power supply circuit before the detection of the predetermined location, and An open / close circuit controlled by the current detection circuit so as to bring the drive power supply circuit into an operating state upon detection;
The converter is configured such that the predetermined value of the converter input current is a value at which the converter input voltage generated when flowing through the drive power supply circuit at a value lower than the constant current value is equal to or lower than the voltage stress threshold.

[Industrial applications]

The present invention relates to a constant current input type DC-DC converter, in particular, a switching element is connected in parallel to a series circuit of a load and a rectifying / smoothing circuit, and the switching element is connected based on a current flowing through the load via a boosting coil. The present invention relates to a constant current input type DC-DC converter that controls on / off and controls a load voltage from a constant current source.

There are two types of DC-DC converters: constant-voltage type and constant-current type.The former constant-voltage type is generally used as the power supply circuit for communication equipment, but the latter is a constant-current type. Have also been studied in various ways.

[Conventional technology]

A conventional constant current input type DC-DC converter is shown in FIG. 5, where 1 is an FET as a switching element.
(May be a transistor), PT is a constant current power supply (low current output converter) of FET1, Z is a load, C is a smoothing capacitor connected in parallel with the constant current power supply PF, L is a load from the constant current power supply PF to the load Z. A step-up coil for applying a step-up voltage (boost pressure). Voltage V _Z across load _Z = capacitor C
At both ends voltage V _L across the voltage V _C + coil L of.

D1 is a rectifying diode, C1 is a capacitor and forms a rectifying / smoothing circuit together with the diode D1, R1 and R2 are resistors for detecting the voltage of the load Z, and CTL is an on / off of FET1 based on the voltage of the resistor R2. Control circuit to perform on / off control, ZD
Is a Zener diode that converts the load current _IPF into a voltage,
2 is a drive power supply circuit for driving the FET1 by generating a predetermined DC voltage from the voltage across the Zener diode ZD and storing the voltage across the Zener diode ZD in a capacitor C3.
By controlling the transistors Q1 and Q2 with an external control signal, a predetermined voltage is generated across the capacitor C2 from the secondary side of the transformer T via the diode bridge circuit DB. The voltage across the capacitor C2 is determined by the transistor Tr1
, Tr3, resistors R3 to R5, and a diode D2, which are sent to a drive circuit 3 for driving the FET1. This drive circuit 3
The transistor Tr2 is turned on / off by a control circuit CTL.

 Next, the operation of this conventional example will be described.

The current supplied from the constant current source PF is a Zener diode
The voltage is converted into a voltage by the ZD, and a predetermined DC voltage is generated between both ends of the capacitor C2 by on / off control of the transistors Q1 and Q2.

Now, assuming that the transistor Tr2 is turned off by the control circuit CTL, the voltage generated in the capacitor C2 turns on the transistor Tr3 and turns off the transistor Tr1, so that the voltage between the gate G and the source S of the FET 1 Apply voltage to turn on FET1.

Thereby, energy is stored in the coil L. Next, when the transistor Tr2 turns on, the transistor Tr3 turns off, the transistor Tr1 turns on, and the FET1 turns off. The energy stored in the coil L is smoothed by the smoothing capacitor C1 via the rectifier diode D1 and released to the load Z. Then, FET1 is controlled by a control circuit CTL so that the constant output voltage E _0.

As described above, the control circuit CTL turns on / off the FET 1 at a predetermined duty ratio, whereby a predetermined current voltage is obtained.

[Problems to be solved by the invention]

In the above-mentioned conventional constant current input type DC-DC converter, when the constant current power supply PF is powered on and before the converter input current _IPF reaches a steady state, the Zener diode ZD generates a constant voltage. Therefore, the driving power supply circuit 2 operates and the driving circuit 3 drives the FET 1.

In this case, if trying to supply constant power P ₀ to the load Z, P ₀ = steady output voltage E _OS × constant output current I _OS, and the input power P _i of approximately equal the following formula in the output power P ₀ need Becomes

P _i = steady input voltage E _is × steady input current I _PFS Therefore, from both equations, P ₀ = P _i = E _is × I _PFS = constant, so that the current I _PF from the constant current source _PF is shown in FIG. As shown in FIG. _6A , if half of the steady value I _PFS is I _PFS / 2, the input voltage E _i (= V _C ) is twice the voltage of the steady value E _is as shown in FIG. 2E _is required. Therefore, the constant current source PF generates twice the voltage at this time.

The input voltage E _i becomes even larger than enough smaller the current I _PF.

As a result, there is a problem that excessive stress is applied to each component constituting the load, and if the startup time of the input current _IPF is long, the excessive stress is applied for a long time.

Therefore, the present invention provides a constant current source in which a switching element is connected in parallel to a series circuit of a load and a rectifying / smoothing circuit, and the switching element is turned on / off based on a current flowing through the load via a step-up coil. In a constant current input type DC-DC converter that controls the load voltage from the load, it prevents the load voltage from becoming excessive even before the input current from the constant current source reaches the steady state, and causes excessive voltage stress on the load components. The purpose is not to multiply.

[Means for solving the problem]

In the constant current input type DC-DC converter according to the present invention, as conceptually shown in FIG. 1, current detection for detecting a predetermined value of an input current _IPF of a converter connected in series with the drive power supply circuit 2 Circuit and the driving power supply circuit 2 in parallel,
Before the detection of the predetermined value, the switching circuit 5 is controlled by the current detection circuit 4 so as to short-circuit the driving power supply circuit 2 and to bring the driving power supply circuit 2 into an operating state when the predetermined value is detected.
Then, the predetermined value of the converter input current I _PF in this case, is a value converter input voltage E _i that occurs when flowing through the drive power source circuit 2 at a lower value than the constant current value I _PFS is less voltage stress threshold .

[Action]

In the constant current input type DC-DC converter according to the present invention shown in FIG. 1, as shown in the waveform diagram of FIG. 2, a current detection circuit 4 connected in series with the drive power supply circuit 2 has a converter input. The current _IPF is detected, and when the input current _IPF reaches a predetermined value _IPF1 , the switching circuit 5 connected in parallel with the drive power supply circuit 2 is controlled from the closed state to the open state.

Thus, the drive power source circuit 2 flows the converter input current I _PF to the driving power source circuit 2 since the working state, the input current I converter input voltage generated for the value of _PF E _i1 (= load voltage E _O1 ) Is below the voltage stress threshold.

Therefore, the input voltage E _i1 generated when the input current _IPF rises to the predetermined value can be suppressed to be equal to or less than the excessive voltage stress threshold applied to the load.

〔Example〕

Hereinafter, an embodiment of a constant current input type DC-DC converter according to the present invention will be described.

FIG. 3 (a) is a constant current input type of the present invention shown in FIG.
1 shows an embodiment of a current detection circuit 4 and a switching circuit 5 used in a DC-DC converter. Here, ZD is a Zener diode of the drive power supply circuit 2.

In this embodiment, the current detection circuit 4 includes a resistor 41 inserted in the input power supply line and connected in series with the zener diode ZD so that the input current _{IPF of the} converter flows, and an emitter-base between both ends of the resistor 41. The switching circuit 5 includes a transistor 42 having a transistor 42 connected therebetween and a Zener diode 43 connected between the base of the transistor 42 and one end of the resistor 41. The switching circuit 5 includes a collector of the transistor 42 and an anode of the Zener diode ZD. And a transistor 52 having an emitter connected between the collector and the base between both ends of the resistor 51 and a transistor 52 connected to the cathode side of the Zener diode ZD.

 Next, the operation of this embodiment will be described.

When the constant current source PF is started, a small current _IPF flows through the resistor 41,
The transistor 52 is turned on by the base current flowing,
The transistor 53 is turned off because no base current flows. Therefore, both ends of the Zener diode ZD constituting the drive power supply circuit 2 are short-circuited between the emitter and the collector of the transistor 52, so that the switching element serves as a switching element.
FET1 has no drive power supply and no switching operation is performed.
Converter input current = load current _IPF flows through the resistor 41. Accordingly, as shown in FIGS. 2B and 2C, the input voltage E _i and the output (load) voltage E ₀ increase in proportion to the load current _IPF .

Thereafter, the load current _IPF is increased to a predetermined value ( _IPF shown in FIG. 2 (a)).
_{When PF1} is _reached , the voltage drop I of the resistor 41 due to this current I _PF1
PF1 × r (resistance value of the resistor 41) is equal to the Zener diode 43
Breakdown emitter voltage V _D and the transistor 42 of the - base voltage
If V _EB and I _PF1 × r ≧ V _D + V _EB , the Zener diode 43 breaks down and turns on the transistor.

Therefore, the load current _IPF is equal to the
Since the resistor 51 flows through the collector, the transistor 52
Is turned off and the Zener diode ZD is activated.

As a result, the Zener diode ZD turns on / off the FET 1 at a predetermined duty ratio via the driving power supply circuit 2 and the driving circuit 3 as described above, and applies a predetermined voltage to the load Z.

FIG. 3 (b) is a constant current input type of the present invention shown in FIG.
This shows another embodiment of the current detection circuit 4 and the switching circuit 5 used in the DC-DC converter.

In this embodiment, the current detection circuit 4 includes a relay 44 connected to the Zener diode ZD series to flow input power I _PF of the inserted converter input power line,
The switching circuit 5 includes a contact 45 of the relay 44 connected between both ends of the Zener diode ZD.
44a.

 Next, the operation of this embodiment will be described.

In this embodiment, when the constant current source PF is started, the minute power supply I _PF
Flows through the relay 44. During this time, the Zener diode ZD is short-circuited by the contact 44a of the relay 44, and the on / off operation of the FET 1 cannot be performed.

Thereafter, when the load current _IPF reaches the above-mentioned predetermined value _IPF1 ,
The contact 44a is opened by the relay 44 that is operated for the first time by the current _IPF1 , and the Zener diode ZD is activated.

Accordingly, the FET 1 is similarly turned on / off at a predetermined duty ratio, and a predetermined voltage is applied to the load Z.

In addition, as shown in FIG. 4, in addition to the above embodiment, a start circuit ST may be added.

The starting circuit ST has been disclosed by the present applicant in Japanese Patent Application No. 63-16426.
And a voltage dividing circuit 6 comprising two resistors r1 and r2, a drain-source connection between the connection point of the resistors r1 and r2 and the driving circuit 3, and a gate connected to the driving power supply circuit 2.
And an FET 7 for inputting the output voltage of

In the operation of the start-up circuit ST, at the start-up time before the output voltage of the drive power supply circuit 2 is established, the FE
T1 is turned on, and after starting, when no drive output of the drive circuit 3 is generated, the FET7 is turned on and the current flows through the resistor r1, the FET7 and the drive circuit 3, thereby turning off the FET1. And
The drive circuit 3 performs the on / off operation of the FET 1 after the startup via the FET 7.

By using the starting circuit ST, it is possible to avoid excessive voltage stress when the load Z is open.

〔The invention's effect〕

As described above, according to the constant current input type DC-DC converter of the present invention, since the on / off control of the switching element is not performed until the converter input current at the time of starting reaches a predetermined value, the converter The overshoot of the input voltage is eliminated, and the voltage stress applied to each component constituting the load can be reduced, and the converter itself can be downsized by reducing the stress. Further, since the starting time of the constant current is not related to the length of the starting time, there is an effect that the design becomes easy.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram conceptually showing a constant current input type DC-DC converter according to the present invention, and FIG. 2 is an operation of the constant current input type DC-DC converter according to the present invention. FIG. 3 is a circuit diagram showing an embodiment of a current detection circuit and a switching circuit used in a constant current input type DC-DC converter according to the present invention, and FIG. 4 shows another embodiment. FIG. 5 is a diagram showing an example of a conventional constant current input type DC-DC converter, and FIG. 6 is a waveform diagram for explaining a problem of the conventional example. In FIG. 1, 1 ... switching element, 2 ... drive power supply circuit, 3 ... drive circuit, 4 ... current detection circuit, 5 ... switching circuit, Z ... load, PF ... constant current source. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. A switching element (1) is connected in parallel with a series circuit of a load (Z) and a rectifying / smoothing circuit (11), and a current flowing through the load (Z) via a step-up coil (12) is reduced. A constant current for controlling on / off of the switching element (1) by a driving power circuit (2) and a driving circuit (3) connected to the driving power circuit (2) based on the driving power circuit (2) to control a load voltage from a constant current source. In the input type DC-DC converter, a current detection circuit (4) connected in series with the drive power supply circuit (2) to detect a predetermined value of the converter input current (I _PF )
And connected in parallel to the drive power supply circuit (2) to short-circuit the drive power supply circuit (2) before the detection of the predetermined value, and to activate the drive power supply circuit (2) when the predetermined value is detected. A switching circuit (5) controlled by the current detection circuit (4), wherein the predetermined value of the converter input current (I _PF ) is lower than the constant current value (I _PFS ). 2) A constant current input type DC-DC converter characterized in that a converter input voltage (E _i ) generated when the current flows is equal to or lower than a voltage stress threshold.