JPH0556111B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to the improvement of a DC-DC converter, and in particular to a DC-DC converter that has variable overcurrent detection level according to a drive pulse and has a fold-back droop characteristic. Regarding converters.

(b) Conventional technology The number of portable OA devices with battery input, such as laptop computers, is increasing. Since the power supplies built into these OA devices use batteries as their energy source, it is important that they are highly efficient DC-DC converters. Furthermore, when the input voltage of the battery is low, it is desirable that the current detection element used for overcurrent protection be as small as possible, both from the standpoint of efficiency and from the standpoint of covering the low input voltage.

A conventional DC-DC converter will be explained with reference to FIG. 21 is a battery input terminal of a DC power supply (+8 to 16 V), 22 is a transformer having a primary winding 23 and a secondary winding 24, and 25 is a first winding 23.
26 is an overcurrent detection element, 27 is a control circuit formed of an integrated circuit that supplies pulse width modulated driving pulses, and 28 is a complementary circuit that drives switching element 25. A driver circuit formed of a transistor, 29 an error amplifier formed of an operational amplifier, 30 a Zener diode that supplies a reference voltage to the error amplifier 29, and 31 a current smoothing circuit connected to the second winding 24.

In such a DC-DC converter, when an overcurrent flows in the output current _I0 due to load shot, etc., the overcurrent detection is performed using a resistor of approximately 0.2Ω connected in series with the switching element 25 on the primary side. This voltage drop is indirectly detected by the element 26 and inputted to the inverting input terminal (-) of the error amplifier 29, and is driven by the control circuit 27 using an error signal with respect to the always constant reference voltage formed by the Zener diode 30. Overcurrent protection is achieved by narrowing the pulse width.

As such prior art, Japanese Patent Application Laid-Open No. 148563/1983
Publication No. (H02M3/335) etc. are known.

(c) Problems to be Solved by the Invention However, in such a DC-DC converter, the overcurrent detection level (OCP level) in the overcurrent detection element 26 is constant because the reference voltage of the Zener diode 30 is constant. Therefore, as shown in FIG. 5, when the output current I ₀ is large, the overcurrent detection element 2
The detection voltage generated at 6 has a large pulse width as shown on the upper side, and when the output current _I0 is small, the detection voltage has a small pulse width as shown on the lower side, but OCP
Since the level is constant, the peak current value does not change.
Therefore, as shown in Figure 6, the output protection characteristic of output voltage V ₀ - output current I ₀ has the problem that even if it exceeds the OCP level, the output voltage V ₀ decreases, but the output current I ₀ increases. was. As a result, especially in the output shot state, the output current I ₀ is also large, and there is also the problem that the components may heat up and be destroyed due to the output current.

(d) Means for Solving the Problems The present invention changes the output current I ₀ during overcurrent protection by varying the reference voltage input to the error amplifier according to the pulse width of the drive pulse applied to the switching element. By reducing this, a DC-DC converter having fold-back droop characteristics is realized.

(E) Effect According to the present invention, the voltage of the overcurrent detection element is
When the OCP level is exceeded, overcurrent protection is activated.
Narrow the pulse width of the driving pulse for the switching element. This reduces OCP levels and
When the OCP level is exceeded, a deeper overcurrent protection operation is entered and the pulse width of the drive pulse is further narrowed. As a result, the overcurrent protection characteristic can have a fold-back drooping characteristic.

(F) Embodiment An embodiment of the DC-DC converter according to the present invention will be described in detail with reference to FIGS. 1 to 3.

In Figure 1, 1 is a DC power supply (+8 to 16V)
2 is a transformer having a primary winding 3 and a secondary winding 4; 5 is an N-channel power supply connected in series with the first winding 3;
A switching element consisting of a MOSFET, 6 an overcurrent detection element consisting of a resistor, 7 a control circuit formed of an integrated circuit that supplies a pulse width modulated driving pulse, and 8 a complementary transistor that drives the switching element 5. 9 is an error amplifier formed by an operational amplifier, 10 is a Zener diode that generates a reference voltage, 11 is a rectifier and smoothing circuit connected to the second winding 4, and 12 is an output voltage V on the secondary side. ₀ (+5V)
This is an output terminal that outputs .

A feature of the present invention is the reference voltage bias to the error amplifier 9. That is, a constant reference voltage is applied by the Zener diode 10 to the non-inverting input terminal (+) of the operational amplifier, which is the error amplifier 9, as in the conventional case, and the drive pulse from the control circuit 7 is further converted to a constant DC value by the Zener diode 13. After cutting the voltage, a voltage proportional to the pulse width smoothed by the capacitor 14 is applied. Note that a detection voltage from the overcurrent detection element 6 is applied to the inverting input terminal (-) of the operational amplifier.

Next, the operating principle of the present invention will be explained. First, to explain the normal operation, a drive pulse from the control circuit 7 turns the switching element 5 on and off via the driver circuit 8, and the two of the transformers 2 are turned on and off.
A stabilized output voltage V ₀ is obtained from the next winding 4. Next, we will explain the overcurrent protection operation.
When an accident such as load shot occurs on the secondary side, the output current I ₀ increases. Along with this, an overcurrent flows through the switching element 5, and the overcurrent detection element 6 also flows.
A detection voltage exceeding the OCP level is generated and input to the error amplifier 9. This causes the error amplifier 9
An error signal is input to the control circuit 7, which acts to narrow the drive pulse width and limit the current flowing through the switching element 5. However, since the OCP level is not changing here, the output voltage V ₀ decreases, but the output current I ₀ tends to increase. Therefore, in the present invention, since a voltage proportional to the drive pulse is applied to the error amplifier 9 together with the reference voltage, the second
As shown in Figure a, when the pulse width of the drive pulse is large, the DC level of the OCP level increases, and the second
As shown in FIG. b, when the pulse width of the drive pulse is small, the OCP level works to decrease. This is because a DC voltage proportional to the pulse width is generated in order to smooth the DC component corresponding to the pulse width of the drive pulse using a capacitor. Therefore, when the pulse width of the drive pulse is large in the overcurrent detection element 6,
When the OCP level is exceeded, the control circuit 7 reduces the pulse width of the drive pulse in response to the error signal from the error amplifier 9, thereby reducing the output voltage _V0 .
As the pulse width of the drive pulse decreases, the OCP level decreases as shown in FIG. 3, and the error signal from the error amplifier 9 acts to further reduce the pulse width of the drive pulse in the control circuit 7. As a result, as shown in FIG. 3, it is possible to realize a fold-back drooping characteristic in which the output current I ₀ also decreases as the OCP level decreases.

(g) Effects of the Invention According to the present invention, by applying a voltage proportional to the pulse width of the drive pulse to the reference voltage of the error amplifier 9 and reducing the OCP level when the pulse width becomes smaller, foldback droop can be prevented. It has the advantage of realizing overcurrent protection with specific characteristics.

As a result, heat generation and destruction of components due to peak current caused by load shot etc. can be completely prevented.

Furthermore, when the OCP level is kept constant, the fold-back drooping characteristic shows input voltage dependence, and as the input voltage increases, the output voltage I ₀ also increases, but in the present invention, the OCP level is related to the drive pulse width. Therefore, it has the advantage of improving the input voltage dependence of the fold-back drooping characteristic.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram explaining the DC-DC converter according to the present invention, FIG. 2 is a waveform diagram explaining its operation, FIG. 3 is a characteristic diagram explaining its fold-back droop protection characteristic, and FIG. The figure is a circuit diagram illustrating a conventional DC-DC converter, and FIGS. 5 and 6 are waveform diagrams and characteristic diagrams illustrating its operation. 1 is a battery input terminal, 2 is a transformer, 3 is a first winding, 4 is a second winding, 5 is a switching element, 6 is an overcurrent detection element, 7 is a control circuit, 8 is a driver circuit, 9 is an error amplifier, 10 and 23 are Zener diodes, 11 is a rectifier and smoothing circuit, 12
is an output terminal, and 14 is a capacitor.

Claims (1)

[Claims] 1. A transformer having a first winding and a second winding, a switching element connected in series to the first winding, and control for supplying a drive pulse to turn on and off the switching element. a direct current circuit comprising: an overcurrent detection element connected in series to the switching element; an error amplifier for detecting an error in the voltage of the overcurrent detection element with respect to a reference voltage; and a current circuit connected to the second winding. - A DC-DC converter, characterized in that a voltage proportional to the drive pulse is applied to the error amplifier together with a reference voltage. 2. The DC-DC converter according to claim 1, wherein the DC value of the drive pulse is cut using a Zener diode, smoothed by a capacitor, and applied to the error amplifier together with a reference voltage.