JPH08336280A - Current mode control converter - Google Patents

Abstract

PURPOSE: To obtain a small-sized current mode control converter in which the circuit efficiency is enhanced while reducing the power loss by controlling a main switch based a current signal, on which a DC voltage is superposed through first and second DC bias means, and the output voltage signal from a rectifying/smoothing circuit. CONSTITUTION: A voltage drop across a current detection resistor 11 is passed through a low pass filter comprising a resistor 12 and a capacitor 14 in order to remove noise therefrom and then it is applied to the current signal input B of a control circuit 5. On the other hand a bias current flows from a power supply 16 through a route of a resistor 15 → a resistor 12 → the current detection resistor 11 and a bias voltage is applied to the current signal input B. A voltage obtained by dividing the drive voltage of a main switch 4 by the resistors 13, 12 is also applied, as a bias voltage, to the current signal input B. Consequently, a current mode control converter can be operated with a desired drooping overcurrent protection starting current by selecting the resistance of the current detection resistor 11 such that a signal voltage of 1.2V will appear at the current signal input B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current mode control converter and, more particularly, to generation of a current signal thereof.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram showing a configuration of a conventional current mode control converter. In the figure, 21 is an input terminal of a power supply, 22 is a bypass capacitor, 23 is a transformer, 24 is a main switch, 25 is a control circuit, 26 and 2
7 is a rectifying diode, 28 is a choke coil, 29 is a smoothing capacitor, 30 is an output terminal, and 31 is a current signal detection resistor.

Next, the current mode control of the conventional current mode control converter will be described. First, the control circuit 25 repeatedly turns on and off the main switch 24 to generate a square wave voltage in the secondary winding of the transformer 23. Then, this square wave voltage is applied to the diodes 26, 2
7. A rectifying / smoothing circuit composed of a choke coil 28 and a capacitor 29 is used to convert to a direct current and output a desired direct current voltage.

At this time, as a means for stabilizing the output voltage, the power supply input current is detected by the resistor 31, and this current signal and the output voltage are input to the control circuit 25 so that the control circuit 25 has a constant output voltage. The main switch was driven by the output of the control circuit with on-duty.

[0005]

However, when the maximum output current is normally extracted, a voltage of about 1 V is required as the peak value of the current signal, so that the power loss in the resistor 31 increases and the efficiency of the circuit deteriorates. However, there was a problem that miniaturization was also an obstacle.

[0006]

SUMMARY OF THE INVENTION A current mode control converter according to the present invention is a main circuit that interrupts current in a transformer and a primary winding of the transformer to generate a square wave voltage in a secondary winding of the transformer. A switch, a rectifying / smoothing circuit that converts a square wave voltage into a direct current and output it, a current detection resistor that detects the current flowing through the main switch as a voltage value, and a direct current voltage superimposed on the current signal detected by the current detection resistor. A first direct current bias means for causing the current signal detected by the current detection resistor to superimpose a part of the drive voltage of the main switch on the current signal; and a direct current by the first and second direct current bias means. And a control circuit for controlling the main switch based on the current signal on which the voltage is superimposed and the output voltage signal of the rectifying and smoothing circuit.

[0007]

In the present invention, the main switch interrupts the current in the primary winding of the transformer to generate a square wave voltage in the secondary winding of the transformer, and the rectifying and smoothing circuit converts the square wave voltage into a direct current. Is output as a voltage value, and the current flowing through the main switch is detected as a voltage value by the current detection resistor.
The first DC bias means superimposes the DC voltage on the current signal detected by the current detection resistor, and the second DC bias means adds the DC voltage to the current signal detected by the current detection resistor. The control circuit controls the main switch based on the current signal and the output voltage signal of the rectifying / smoothing circuit in which the DC voltage is superimposed by the first and second DC bias means.

[0008]

1 is a circuit diagram showing the configuration of a current mode control converter according to an embodiment of the present invention. In the figure, the power supply terminal 1 is connected to the bypass capacitor 2 and the hot side of the primary winding of the transformer 3 (black circles are added in the figure),
The other end of the bypass capacitor 2 is grounded. Also,
The drain of the main switch 4 is connected to the cold side of the primary winding of the transformer 3, the gate is connected to the output C of the control circuit 5, and the source is grounded at one end.
Connected with.

A resistor 13 is connected between the current signal input B of the control circuit 5 and the gate of the main switch 4, and a resistor 12 is connected between the current signal input B and the source of the main switch 4.
Is connected, and a capacitor 14 is connected between the current signal input B and the ground.
Of the DC bias means of. Further, one end of the resistor 15 is connected to the current signal input B, the other end of the resistor 15 is connected to the positive electrode of the power source 16, and the negative electrode of the power source 16 is grounded. 1 constitutes a DC bias means.

The hot side of the secondary winding of the transformer 3 is connected to the anode of the diode 6, and its cathode is connected to the cathode of the diode 7, one end of the capacitor 9 and the output terminal 10, respectively. The cold side of the secondary winding of the transformer 3 is connected to the anode of the diode 7 and one end of the choke coil 8, and the other end of the choke coil 8 and the other end of the capacitor 9 are grounded.
The diode 6, the diode 7, the choke coil 8 and the capacitor 9 form a rectifying / smoothing circuit 17. Also,
The output terminal 10 and the voltage input A of the control circuit 5 are connected.

Next, the operation of this embodiment will be described. 2A and 2B are waveform diagrams showing voltage waveforms of respective parts during operation of this embodiment. FIGS. 2A to 2C are waveforms at the time of rating, and FIGS. 2D to 2F are light. The waveform at the time of load is shown.

First, when the converter is operating, a current having a waveform as shown in FIG. 3 (b) or FIG. 2 (e) flows through the current detection resistor 11. The current signal of the control circuit 5 is obtained based on the voltage signal generated in the current detection resistor 11, and the current signal provided to the control circuit 5 also serves as the output current limit of the DC converter.
When the peak value of this current signal exceeds 1.2 V, for example, the on-duty of the main switch is narrowed to perform a so-called slow drooping overcurrent protection operation in which the output voltage is reduced while keeping the output current at a constant value. ing.

In the current mode control method, the output voltage of the DC converter is also fed back, so that
Since the current signal is used for the purpose of obtaining the timing of the switching operation in the normal state where the overcurrent protection is not activated, the magnitude of the peak value is not related to the output voltage.

Here, the peak of the main switch current at the start of the operation of the drooping type overcurrent protection is IP, and the current detection resistor 1
If the resistance value of 1 is R11 (hereinafter, the resistance value of the resistor n is represented by Rn), the voltage drop at the current detection resistor 11 is IP.
It becomes R11. Then, this voltage is applied to the current signal input B of the control circuit 5 after the noise component is removed by the low-pass filter composed of the resistor 12 and the capacitor 14.

On the other hand, the power source 16 causes the resistor 15 to be the resistor 1
2 → Since a bias current flows through the route of the current detection resistor 11, the voltage of the power supply 16 is set to E2, and the resistance value of the current detection resistor 11 is smaller than the resistance values of the resistors 12 and 15 (for example, the current detection resistor The resistance values of the resistor 11, the resistor 12, and the resistor 15 are 0.8Ω, 1 kΩ, and 6.7, respectively.
kΩ), the current signal input B is roughly E2.
A bias voltage of {R12 / (R12 + R15)} is applied.

A voltage obtained by dividing the drive voltage of the main switch 4 by the resistors 13 and 12 is also applied to the current signal input B as a bias voltage. The resistance value of the detection resistor 11 is smaller than the resistance values of the resistors 12 and 13 (for example, the current detection resistor 11, the resistor 12, and the resistor 1).
Resistance values of 3 are 0.8Ω, 1kΩ and 100k respectively
Ω), E1 · {R12 / (R12 + R1
3)}. Therefore, the signal voltage VB (V) of the current signal input B of the control circuit 5 is given by the following equation.

VB = IP * R11 + E2 * {R12 / (R12 + R15)} + E1 * {R12 / (R12 + R13)} = IP * R11 + 0.519 + 0.119

Therefore, R is set so that VB = 1.2.
By selecting 11 (here, for example, 0.8Ω),
The operation with the desired drooping overcurrent protection start current can be performed. At that time, the voltage generated in the current detection resistor 11 is IP · R11 = 0.562 (V), and the drooping overcurrent protection is performed. That is, the voltage for starting the operation is lower than that of the conventional example. (In the conventional example, IP · R11 = 1.2
(V))

As described above, in this embodiment, by applying a DC bias to the voltage generated in the current detection resistor 11 connected to the source of the main switch 4, the voltage generated in the current detection resistor 11 becomes relatively large. The desired drooping characteristic can be obtained even with a small value, and the drive signal of the main switch 4 is divided by the resistor 12 and the resistor 13 and added to the input B of the control circuit to increase the DC bias level. If the absolute value of the current signal is increased without increasing it more than necessary and the output current of the DC converter as shown in FIG. 2 (e) is small and the amount of change in the current signal decreases, the circuit operates due to external noise or the like. It is designed to prevent instability.

Next, a specific example of the power loss in the current detection resistor 11 of this embodiment will be described. First, as described above, assuming that the current peak of the main switch 4 at the start of the operation of the drooping type overcurrent protection is IP, IP.R11 =
Must be 0.562. Here, considering the power loss in the current detection resistor 11, when the average current of the main switch at the rated output is IAVE, IAVE becomes IAVE
= KIP (k: proportional constant) Therefore,
Power loss P1 in the current detection resistor 11 of this embodiment
Is expressed by the following equation, where D is the on-duty ratio of the main switch 4.

P 1 = (IAVE) ² · R 11 · D = k ² · IP ² · R 11 · D = 0.562 · k ² · IP · D

On the other hand, in the conventional example, as described above, IP
Since R11 must be increased so that R11 = 1.2, the power loss in the conventional current detection resistor 11 is represented by the following equation.

P ² = (IAVE) ² · R 11 · D = K ² · IP ² · R 11 · D = 1.2 · k ² · IP · D

From the above, it can be seen that this embodiment can reduce the power loss in the current detection resistor 11 to about 47% as compared with the conventional example. Therefore, it is effective in improving the efficiency and downsizing of the converter.

[0025]

As described above, according to the present invention, the main switch intermittently interrupts the current in the primary winding of the transformer to generate a square wave voltage in the secondary winding of the transformer for rectifying and smoothing. The circuit converts the square wave voltage into direct current and outputs it, the current detection resistor detects the current flowing through the main switch as a voltage value, and the first direct current bias means converts it into a current signal detected by the current detection resistor. , Superimposing a DC voltage, superimposing a part of the drive voltage of the main switch on the current signal detected by the current detection resistor by the second DC bias means, and by the control circuit, the first and second DC biases. Since the main switch is controlled on the basis of the current signal on which the DC voltage is superimposed by the means and the output voltage signal of the rectifying / smoothing circuit, the voltage for starting the operation of the drooping overcurrent protection of the converter is controlled. It is possible to reduce the voltage drop across the detection resistor, it is possible to improve the efficiency of the converter,
It has an effect that it can be miniaturized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a current mode control converter according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing voltage waveforms of various parts during operation of the embodiment.

FIG. 3 is a circuit diagram showing a configuration of a conventional current mode control converter.

[Explanation of symbols]

 1 Power Supply Terminal 2 Bypass Capacitor 3 Transformer 4 Main Switch 5 Control Circuit 6, 7 Diode 8 Choke Coil 9, 14 Capacitor 10 Output Terminal 11 Current Detection Resistor 12, 13, 15 Resistor 16 Power Supply 17 Rectification Smoothing Circuit 18 Second DC Bias Means 19 First DC bias means

Claims (1)

[Claims] 1. A transformer, a main switch that interrupts a current in a primary winding of the transformer to generate a square wave voltage in a secondary winding of the transformer, and converts the square wave voltage into a direct current. A rectifying / smoothing circuit that outputs the current, a current detection resistor that detects a current flowing through the main switch as a voltage value, a first DC bias unit that superimposes a DC voltage on a current signal detected by the current detection resistor, Second DC bias means for superposing a part of the drive voltage of the main switch on the current signal detected by the current detection resistor; and DC voltage superposed by the first and second DC bias means. A current mode control converter comprising: a control circuit that controls the main switch based on a current signal and an output voltage signal of the rectifying and smoothing circuit.