JPH037060A - Variable overcurrent protective dc-dc converter - Google Patents

Abstract

PURPOSE:To protect a load against overload by additionally providing a voltage converting circuit in a conventional circuit then decreasing/increasing output voltage from the voltage converting circuit corresponding to increase/decrease of a voltage set value and providing the output voltage as an overcurrent set value to an overcurrent limiting circuit. CONSTITUTION:A control transistor Q2 constitutes an emitter follower circuit and voltage across a resistor R3 is applied onto the base of a transistor Q2, where the voltage across the resistor R3 is approximately equal to the voltage across a resistor R5. Since approximately identical current flows through the resistors R5, R4, voltages across the resistors R5, R4 are proportional to the resistances of the resistors R5, R4. Collector side terminal of the transistor Q2 is connected with >=-terminal of an overcurrent limiting circuit 2 which provides an overcurrent set value EIS.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a DC-DC converter that outputs a predetermined DC voltage by repeatedly opening and closing a DC power supply voltage through switching means such as a switching transistor, and has an overcurrent protection function that limits the output current to a predetermined value or less. In particular, the present invention relates to a variable overcurrent protection DC-DC converter having a function of varying a limiting current value according to a DC output voltage so as to protect a constant power load. Note that in the following figures, the same reference numerals indicate the same or corresponding parts. Also, logic or level “High”, “L”
ow" is simply written as "H" and "I L n.

[Conventional technology]

FIG. 3 shows a circuit example of a conventional DC-DC converter having an overcurrent protection function. In the same figure, 01 is a DC power supply, 0
2 is a load, Ql is a switching transistor for repeatedly turning on and off the DC power supply 01, 04 is a control circuit that controls opening and closing of this transistor (1), 03 is a DC power supply for this control circuit 04, and Ll is a transistor Q1 is a smoothing reactor for smoothing the intermittent output DC voltage, C1 is also a smoothing capacitor, Dl is a flywheel diode that becomes the current path for reactor L1 when transistor Q1 is off, and R1 is the output of this DC-DC converter. This is a current detection resistor for detecting the current ■0.Next, in the control circuit 04, l is a PWM (pulse modulation) comparator, and this comparator 1 is connected to its +(
A sawtooth-like high frequency (
In addition to inputting the voltage EIO of the carrier wave), a variable comparison voltage Ell is input to its two - (minus) terminals.
R12 is input, and during the period when the sawtooth wave voltage R100 value is lower than the lower of the comparison voltages Ell and R12, its output is set to "L" and the switching transistor Q1 is turned on, and vice versa. Sawtooth voltage EI
During the period when O exceeds the lower voltage, its output is set to “H”.
In this way, according to the increase or decrease of the comparison voltage Ell or R12 (whichever is lower) input to the PWM comparator 1, the ON time ratio of the switching transistor Q1 = (ON
time) / ((ON time) + (OFF time)) = (O
N time)/(carrier period), hence the transistor Q
The value of (output (average) voltage)/(voltage of DC power supply 01) of l is increased and decreased, respectively. 4 is a voltage adjustment circuit that compares the voltage setting value ES and the output voltage EO of the DC-DC converter, and adjusts the voltage so that EO becomes a predetermined value corresponding to BS, that is, the converter output voltage EO exceeds the predetermined value. When the converter output voltage EO tries to fall below the predetermined value, the comparison voltage Ell as the output voltage of the adjustment circuit 4 is lowered.
The input comparison voltage Ell to the PWM comparator 1 is variably adjusted so as to increase Ell. 2 is an overcurrent limiting circuit that compares the current detection voltage EOi generated in the current detection resistor R1 due to the output current 0 of the DC-DC converter with a predetermined voltage corresponding to the overcurrent setting value EIS, and the former EOi is determined as the predetermined voltage. When the voltage exceeds the limit circuit 2, the manual comparison voltage E12 to the PWM comparator 1 as the output voltage of the limiting circuit 2 changes from "H" (maximum input value) to "L" (
input minimum value). Figure 4 shows the output voltage EO of the DC-DC converter in Figure 3.
An example of characteristics of output current IO is shown. That is, while the output current 0 does not exceed the predetermined overcurrent limit value Is corresponding to the overcurrent setting value EIS, the comparison voltage E output from the overcurrent limiter circuit 2
Since 12 is "H" (maximum input value), the PWM comparator 1 compares the output comparison voltage Ell of the voltage adjustment circuit 4, which is lower than this comparison voltage E12, with the carrier wave EIO, and the converter output voltage EO is set at the voltage setting. Value ES of value ES
. It shows constant voltage characteristics according to BS2. However, when the converter output current ■0 increases and attempts to exceed the overcurrent limit value IS, the output comparison voltage E12 of the overcurrent limiter circuit 2 switches to "L" (minimum input value), and the PWM
The comparator l is connected to this comparison voltage E12 and the carrier wave input EI.
Converter output voltage E
O rapidly drops toward 0. In this way, regardless of the voltage setting value ES, a constant overcurrent limit value Is
A protective operation is performed.

[Problem to be solved by the invention]

However, with a C-DC converter as shown in Figure 3,
When trying to supply power to a load that requires constant power supply, such as a CRT, as shown in Figure 4, even if the overcurrent limit value Is is appropriate for the voltage setting value ESI, the voltage Even when the set value is increased to BS2, the overcurrent limit value remains at Is, so there is a habit of destroying load 02 due to overload. Therefore, the present invention adjusts the overcurrent set value EIS (therefore, the overcurrent limit value Is) to f depending on the increase or decrease of the voltage set value ES.
Ii=variable overcurrent protection gtDc- with the function of increasing
An object of the present invention is to solve the above problem by providing a DC converter.

[Means to solve the problem]

In order to solve the above problems, the DC-DC converter of the present invention repeatedly opens and closes a first DC voltage (voltage of DC power supply 01, etc.) via r switching means (switching transistor Q1, etc.) to generate a second DC voltage. Voltage (output voltage E
A DC-DC converter that outputs a first reference voltage (voltage setting value ES, etc.), wherein the second DC voltage
Voltage control means (such as a predetermined voltage proportional to
WM comparator 1. (voltage adjustment circuit 4, etc.) and the output current (IO, etc.) of the DCDC converter is
When a predetermined current value (overcurrent limit value Is, etc.) corresponding to a reference voltage (overcurrent setting value EIS, etc.) is exceeded, overcurrent limiting means (PWM comparator 1. 2 etc.) and a DC with
- The DC converter is provided with limited current variable means (voltage conversion circuit 3, etc.) that reduces the second reference voltage when the first reference voltage increases.

[For use]

In the present invention, a voltage conversion circuit is added to the conventional circuit, and the output voltage of the voltage conversion circuit 3 is changed to decrease or increase in response to an increase or decrease in the voltage setting value ES, and this output voltage is used as the overcurrent setting value EIS. Provided to overcurrent limit circuit 2.

【Example】

The present invention will be explained in detail below based on FIGS. 1 and 2. Figure 1 is a circuit diagram as an embodiment of the present invention.
2 is a diagram showing an example of the characteristics of the output voltage EO versus the output current 10 in FIG. 1, and corresponds to FIG. 4. In FIG. 1, in contrast to FIG. 3, a voltage conversion circuit 3 for converting a voltage set value ES into an overcurrent set value EIS is added to the input section of the overcurrent limiting circuit 2. That is, in this voltage conversion circuit 3, the control transistor Q
2 constitutes a so-called emitter follower circuit, and the voltage setting value ES is connected to the base of the transistor Q2 by a resistor R.
A voltage across the resistor R3 is applied, which is divided by 2 and R3, and the voltage across the resistor R3 is approximately equal to the voltage across the resistor R5 connected to the emitter of the transistor Q2. On the other hand, since approximately the same current flows through this resistor R5 and the resistor R4 connected to the collector of the transistor Q2, this resistor R5
The voltages across R4 and R4 have values proportional to their respective resistance values. Therefore, the voltage across resistor R4 is the voltage setting value ES
It has a value approximately proportional to . By the way, the terminal on the collector side of the line transistor Q2 in this resistor R4 is connected to the - (minus) input terminal of the overcurrent limiting circuit 2, and the overcurrent limiting circuit provides an overcurrent set value EIS. As a result, the overcurrent set value EIS decreases and increases as the voltage set value BS increases and decreases, respectively. Figure 2 shows the output voltage EO of the DC-DC converter in Figure 1.
An example of characteristics of output current IO is shown. In other words, when the voltage setting value ES is set to ESI, the overcurrent setting value EIS is E I
31 (corresponding overcurrent limit value TS is 15I), and a characteristic (curve) like the solid line is obtained.Similarly, when the voltage setting value is set to ES2, the overcurrent setting value is EI
S2 (the corresponding overcurrent limit value is 132), resulting in a characteristic (curve) like a broken line. In this way, the point PI at which the output power is maximum in each characteristic (curve) of the solid line and the broken line
. The trajectory followed by R2 is approximately like the dashed line in FIG. In practice, this is the case.C-DC converters are used within the range of the characteristics shown by the solid and broken lines, considering the general characteristics of the load. There is no need for major changes in
Load 02 that requires constant power supply with characteristics as shown in Figure 2
It is possible to protect just the right amount.

【Effect of the invention】

According to the present invention, there is provided a DC-DC converter that repeatedly opens and closes the DC voltage of a DC power source 01 via a switching transistor Q1 to output an output DC voltage EO, the output voltage EO being a predetermined value corresponding to a voltage setting value ES. the switching transistor Q1 so that the voltage is equal to
When the output current IO of the DC-DC converter exceeds a predetermined overcurrent limit Is corresponding to the overcurrent set value EIS, the switching transistor Q
In the DC-DC converter equipped with an overcurrent limiting circuit 2 that opens the overcurrent limit circuit 2, when the voltage setting value ES increases, the overcurrent setting value EI increases.
Since the voltage conversion circuit 3 that reduces S is provided, overcurrent protection for a load that requires constant power supply can be appropriately performed.

[Brief explanation of the drawing]

FIG. 1 is a configuration circuit diagram as an embodiment of the present invention, and FIG. 2 is the DC-DC converter constant value and EIS of FIG. 1.
(EISI, EIS2): Overcurrent setting value, Is (IS
I, l52): Overcurrent limit value, EO: Output voltage, IO:
Output current. FIG. 4 is a diagram showing an example of the characteristics of the DC-DC converter shown in FIG. 3, and corresponds to FIG. 2. OI: DC power supply, 02: Load, 04: Control circuit, Q1 switching transistor, 1: PWM comparator,
2: Overcurrent limiting circuit, 3: Voltage conversion circuit, 4: Voltage adjustment circuit, ES (ESl, ES2): Voltage setting (EIS2) (EISI) Fig. 2 Fig. 4

Claims (1)

[Scope of Claims] 1) A DC-DC converter that outputs a second DC voltage by repeatedly opening and closing a first DC voltage through a switching means, wherein the second DC voltage is a first reference voltage. voltage control means for controlling the opening/closing ratio of the switching means so as to be equal to the voltage, and when the output current of the DC-DC converter exceeds a predetermined current value corresponding to a second reference voltage, the switching means overcurrent limiting means for opening the DC-DC
A variable overcurrent protection DC-DC converter, characterized in that the converter includes variable limit current means that reduces the second reference voltage when the first reference voltage increases.