JPH05137326A - Flyback type switching regulator - Google Patents

Abstract

PURPOSE:To raise the efficiency of a switching regulator by reducing the loss of a diode for rectification on the secondary side of a step-down transformer. CONSTITUTION:In a flyback type switching regulator, a rectification circuit is composed at least of a field effect transistor for rectification, which is connected in series to one end of the secondary terminal of a step-down transformer 4, and a capacitor 6 for smoothing, which is connected in parallel between the other end of this field effect transistor 5 and the other end of the secondary terminal of the step-down transformer 4. Hereby, as compared with the one wherein a rectification diode such as a Schottky barrier diode, etc., is used, the loss of the field effect transistor becomes small, and the efficiency can be raised.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator, and more particularly to a flyback type switching regulator suitable for low voltage output such as DC3v.

[0002]

2. Description of the Related Art Conventionally, a flyback type switching regulator of this type has a rectifying diode bridge 1, a smoothing capacitor 2 and a power MOS as shown in FIG.
Power transistor 3 such as FET, and step-down transformer 4
And a rectifying diode (mainly a Schottky barrier diode) 20, smoothing capacitors 6 and 7, and a smoothing reactor 8.

This operation is performed, for example, by rectifying an AC input V _{IN of} AC100v by a rectifying diode bridge 1 and applying a DC high voltage V _C1 (for example, DC100) to a smoothing capacitor 2.
v) is created. Next, the pulse width controlled power transistor (power MOSFET or the like) 3 of the control circuit (not shown) is turned on, and energy represented by the following equation is stored in the exciting inductance L _T of the step-down transformer 4.

[0004] ^{_{1/2 · L T · I 1 2}} = 1/2 · L T · _{[(V C1 / L T) ·} t ] _{^{2 = (V C1 2 · t}} 2) / (2 · L T)

However, I ₁ is the peak value of the primary winding current of the step-down transformer 4, and t is the ON period of the power transistor 3. At this time, on the secondary side of the step-down transformer 4, (N ₂ /
N ₁ ) · V _C1 (However, N ₁ / N ₂ is 1 of the step-down transformer 4 respectively.
Indicates the number of secondary / secondary windings. ) Is generated, but no current flows because it is in the blocking direction of the rectifying diode 20.

Next, by turning off the power transistor 3, the energy accumulated in the exciting inductance L _T of the step-down transformer 4 is released, and the step-down transformer 4 is released.
Current from the secondary winding N ₂ (initial current I ₂ is (N ₁ / N ₂ )
I ₁ ) flows and charges the smoothing capacitor 6 via the rectifying diode 20. Further, it is smoothed by the smoothing reactor 8 and the smoothing capacitor 7, and its DC output V
For example, a DC voltage of 3v is obtained as _OUT .

[0007]

[SUMMARY OF THE INVENTION However, such a conventional flyback type switching regulator, the use of the rectifying diode 20, forward voltage drop V _F small Schottky barrier diode (V _F ≒ 0. Four
Even in the case of v), there is a problem that the efficiency becomes poor when a low voltage (for example, 3v) DC output is obtained. For example, with only the rectifying diode 20, 0.4v / 3v = 13 of DC output
You will lose as much as%.

[0008]

In order to solve the above-mentioned problems, the present invention comprises the following means. That is, in the above flyback switching regulator, at least one end of the secondary side terminal of the step-down transformer is connected in series, and the other side of the field-effect transistor and the secondary side of the step-down transformer. A rectifying circuit is composed of a smoothing capacitor connected in parallel with the other end of the terminal.

[0009]

Therefore, by using the field effect transistor instead of the rectifying diode such as the Schottky barrier diode, the loss can be reduced and the efficiency of the switching regulator can be increased.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention, in which a rectifying diode bridge 1, a smoothing capacitor 2 and
Power transistor 3, step-down transformer 4, rectifying field effect transistor 5, smoothing capacitors 6, 7
And a smoothing reactor 8. That is, this embodiment differs from the conventional example shown in FIG. 7 in that the drain terminal is connected to one end of the secondary winding N ₂ of the step-down transformer 4 and the auxiliary secondary winding N ₃ of the auxiliary winding N ₃ is connected. The rectifying field-effect transistor 5 having the gate and source terminals connected between both ends is replaced by the diode 20. The same reference numerals in the drawings indicate the same or corresponding parts.

Next, the operation of the above embodiment will be described. This operation is similar to the conventional example.
_IN is rectified by the rectifying diode bridge 1 to generate a DC high voltage V _C1 in the smoothing capacitor 2. Then, the power transistor 3 whose pulse width is controlled is turned on to store energy in the exciting inductance of the step-down transformer 4. At this time, voltages of (N ₂ / N ₁ ) · V _C1 and (N ₃ / N ₁ ) · V _C1 are generated on the secondary side of the step-down transformer 4, but between the gate and source of the field effect transistor 5. Since a negative voltage [-(N ₃ / N ₁ ) · V _C1 ] is applied to the field effect transistor 5 and the field effect transistor 5 is in the off state, no current flows.

Next, by turning off the power transistor 3, the energy accumulated in the exciting inductance of the step-down transformer 4 is released, and a positive voltage is applied between the gate and source of the field effect transistor 5, The field effect transistor 5 is turned on. When the field effect transistor 5 is turned on, it has a resistance characteristic as shown in FIG. 2, for example (N channel FE
In the case of T), the voltage drop V _F can be reduced.

For example, at 10 w (3.3 A) output, by using the field effect transistor 5 having an on-resistance of 20 mΩ, a loss of 0.5 w, that is, a direct current output of 0.5 w.
The loss was reduced to / 10w = 5%. In FIG. 2, the horizontal axis represents the drain-source voltage −V _DS of the field effect transistor 5, and the vertical axis represents the drain current I _D thereof. In the figure, the broken line is on and the solid line is off. The characteristics of time are shown respectively.

In FIG. 1, which is an embodiment of the present invention, the power transistor 3 is turned on (FIG. 3) before the continuous mode shown in FIG. 3, that is, before the energy stored in the exciting inductance of the step-down transformer 4 is completely discharged. 3 (b)) In the case of the control method of starting energy storage in the exciting inductance, a voltage is applied between the gate and the source of the field effect transistor 5 so that no current flows from the drain to the source in the N channel. Is applied to operate (Fig. 3
(c) and (d)).

However, in the discontinuous mode shown in FIG. 4, that is, after the energy accumulated in the exciting inductance of the step-down transformer 4 has been exhausted, the power transistor 3 is turned on (FIG. 4 (b)) to change to the exciting inductance. In the case of the control method that newly starts the storage of energy, a positive voltage is applied between the gate and the source of the field effect transistor 5 before the end of the release of the energy stored in the exciting inductance (see FIG. (c) and (d)) Since a voltage is applied from the capacitor 6 to the secondary winding N ₂ of the step-down transformer 4 immediately after the discharge, a positive voltage is applied between the gate and source of the field effect transistor 5. The voltage is continuously applied, and the field effect transistor 5 continues to maintain the ON state.

Therefore, the current shown by the diagonal lines in FIG. 4 (c) flows, and energy is accumulated in the exciting inductance of the step-down transformer 4. Therefore, there is a disadvantage that the loss of the field effect transistor 5 increases due to the current that does not contribute to the DC output. In addition, the step-down transformer 4 is shown in FIG.
The voltage V _C1 of the primary winding N ₁ of each of the above is shown. What solves such a drawback is described in claims 2 and 3 of the present invention, and respective embodiments are shown in FIGS. 5 and 6.

In the embodiment of the present invention shown in FIG. 5, a voltage rise detection circuit 9 for detecting the voltage rise is connected in parallel to the smoothing capacitor 6, and this voltage rise detection circuit 9 is used.
, NPN type transistors 10 and 11 and diode 1
2, 13, a capacitor 14, and a resistor 15. Here, in the voltage rise detection circuit 9, the base of the transistor 10 is connected to one end of the smoothing capacitor 6 via the anode of the diode 12 and is connected to the other end of the smoothing capacitor 16 via the capacitor 14. ing. The collector of the transistor 10 is connected to the gate of the field effect transistor 5 and the emitter of the transistor 11 via the anode of the diode 13, and its collector is connected to the base of the transistor 11 and
The source of the field-effect transistor 5, the collector of the transistor 11 and the other end of the resistor 15 are connected to the resistors 15, respectively, and are connected to one end (−) of the DC output terminal.

Next, the operation shown in FIG. 5 will be described. In this operation, when the current stored in the smoothing capacitor 6 flows through the parasitic diode of the field effect transistor 5 and the voltage rises (dV _cb / dt) due to the release of the energy accumulated in the exciting inductance of the step-down transformer 4, the transistor 10 is turned on. A current (C ₁₄ × dV _cb / dt) flows from the base to the capacitor 14 having the capacitance C ₁₄ via the emitter. Therefore, the transistor 10 is turned on, a voltage is applied between the gate and the source of the field effect transistor 5, and the field effect transistor 5 is turned on.

Next, when the release of the energy stored in the exciting inductance of the step-down transformer 4, the voltage rise of the smoothing capacitor 6 stops and the base current of the transistor 10 becomes zero. Therefore, the transistor 10 is turned off, and the charge accumulated between the gate and the source (input capacitance C _iss ) of the field effect transistor 5 first flows through the emitter and base of the transistor 11 into the resistor 15 to turn on the transistor 11. This transistor 11
Are discharged and the field effect transistor 5 is turned off. Therefore, due to this operation, the current shown by the diagonal lines in FIG. 4 (c) stops flowing, and the loss of the field effect transistor 5 does not increase due to the current that does not contribute to the DC output.

Another embodiment of the present invention shown in FIG. 6 is one in which a voltage detection circuit 16 for detecting the positive / negative of the voltage between the terminals is connected between the drain and source terminals of the field effect transistor 5. In this case, the voltage detection circuit 16 is composed of a comparator 17, a capacitor 18, and a diode 19.

According to the embodiment shown in FIG. 6, the operation of the comparator 17 is performed while the energy stored in the exciting inductance of the step-down transformer 4 is being discharged, that is, while the current is flowing from the source to the drain of the field effect transistor 5. Since the potential of the non-inverting input (+) is higher than the potential of the inverting input (-), the output becomes a positive voltage and the field effect transistor 5 is turned on.

When the energy stored in the exciting inductance of the step-down transformer 4 is released and a current is about to flow from the smoothing capacitor 6 to the field effect transistor 5, the potential of the non-inverting input (+) of the comparator 17 is inverted. The potential becomes lower than the input (-) potential, the output becomes a negative voltage, and the field effect transistor 5 is turned off. Due to this operation, the electric current shown by the diagonal lines in Fig. 4 (c) stops flowing,
The loss of the field effect transistor 5 does not increase due to the current that does not contribute to the DC output.

[0023]

As described above, according to the present invention, after rectifying an AC input, it is converted into AC by using the on / off operation of the transistor element, and the AC voltage is stepped down by a step-down transformer and turned back to DC. In the flyback switching regulator for conversion, since the field effect transistor is used as the rectifying diode on the secondary side of the step-down transformer, the loss can be reduced and the efficiency of the switching regulator can be improved.

According to another invention of the present invention, in the above, a voltage rise detection circuit for detecting a voltage rise of the smoothing capacitor on the secondary side of the step-down transformer, or a drain terminal of a rectifying field effect transistor. By providing the voltage detection circuit for detecting the positive / negative of the voltage between the source terminal and the source terminal, the loss of the field effect transistor is not increased by the current that does not contribute to the DC output, and the loss can be further reduced.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of a flyback type switching transistor according to the present invention.

FIG. 2 is a characteristic diagram of a field effect transistor used in the present invention.

FIG. 3 is an operation waveform (continuous mode) diagram of the embodiment circuit shown in FIG.

FIG. 4 is an operation waveform diagram (discontinuous mode) of the circuit shown in FIG.

FIG. 5 is a circuit configuration diagram showing another embodiment of the present invention.

FIG. 6 is a circuit configuration diagram showing still another embodiment of the present invention.

FIG. 7 is a circuit configuration diagram showing an example of a conventional technique.

[Explanation of symbols]

 1 Diode Bridge 2 Smoothing Capacitor 3 Power Transistor 4 Step Down Transformer 5 Rectifying Field Effect Transistor 6,7 Smoothing Capacitor 8 Smoothing Reactor 9 Voltage Rise Detection Circuit 16 Voltage Detection Circuit

Claims (3)

[Claims] 1. A flyback type switching in which after rectifying an AC input, the ON / OFF operation of a transistor element is used to convert the AC voltage into an AC voltage, which is stepped down by a step-down transformer and converted into a DC voltage again by a rectifier circuit. In the regulator, the rectifier circuit includes at least one rectifying field-effect transistor connected in series to one end of a secondary side terminal of the step-down transformer, the other end side of the field-effect transistor, and the secondary side of the step-down transformer. A flyback switching regulator having a smoothing capacitor connected in parallel with the other end of the terminal. 2. The flyback switching regulator according to claim 1, further comprising a voltage rise detection circuit that detects a voltage rise of the smoothing capacitor. 3. The flyback switching regulator according to claim 1, further comprising a voltage detection circuit that detects whether the voltage between the drain terminal and the source terminal of the rectifying field-effect transistor is positive or negative.