JPH01218352A - Switching power source circuit - Google Patents

Abstract

PURPOSE:To reduce the noise of an apparatus with a simple circuit constitution by connecting a third diode between the input side terminal of a resonating inductor and a power source negative electrode. CONSTITUTION:A switching power source circuit is composed of a switching element SW, a first diode 6, resonating inductor 8 and capacitor 9, a second diode 11, and load side choke coil 12 and capacitor 13 to supply power from output terminals 3, 4 to a load L. In this case, a third diode 10 is provided between the junction and terminals of said resonating inductor 8 and load side choke coil 12, and respective inductor and capacitor are formed so that there exists the relation of L1<<L2 and C1<<C2 between them. Thus, at the point of time when the switching element SW is turned OFF and a current source for a circuit current iL1 disappears, said third diode 10 is turned ON and said circuit current decreases gradually so that arc is self-extinguished. As a result, it is possible to prevent generation of a surge voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary of the Invention] The present invention relates to a switching power supply, and particularly to a simple and low-noise circuit.

[Contents and problems of conventional technology]

In switching power supplies, it is effective to reduce switching loss in switching elements and increase conversion efficiency. One way to do this is to use a series resonant circuit to change the current waveform when the switching element conducts to a sine wave. A circuit method was used to do so.

FIG. 4 shows an embodiment of the prior art described above, and FIG. 5 shows an example of its operation chart. An overview of the conventional techniques will be described below with reference to these.

In FIGS. 4 and 5, a DC input voltage Vin is connected to the input terminal 1.2, and a load whose output current is 0 is connected to the DC output terminal 3.4. -Here, when the switching element SW5 is first turned on, the circuit current i flowing through the first diode Di6 and the resonant inductor L18
L1 begins to flow at a slope of Vin/Ll. Then, when the output current becomes equal to 0 at time T1, the diode Di6 is cut off, and the resonance capacitor C19
At the same time, a series resonance operation with a natural vibration period of 2π is started, and at time T3, the circuit current iL1 flowing through the resonance inductor 8 reaches zero.

Therefore, if the switching element SW5 is turned off at this point, the circuit current iL1 at the time of turn-on and turn-off can be suppressed and a low-loss switching operation can be realized.
The output was controlled by adjusting the repetition period.

However, in the above conventional technology, in order to turn off the switching element SW5 when the circuit current IL1 becomes zero, it is necessary to provide a detector for the circuit current and also provide a dedicated circuit function for turning off the switching element SW5. there were. Therefore, the circuit configuration becomes complicated, and there is a drawback that an inexpensive switching power supply with a simple configuration cannot be provided.

In addition, if a method is adopted in which the on period of the switching element 5 is fixed in order to configure a simple and inexpensive switching power supply with the conventional configuration as described above, a choke input smoothing circuit is connected to the resonance capacitor C19. Since the output circuits including the
As shown in FIG. 5, after reaching <-zero, it starts to gradually increase again.

Therefore, if the switching element is turned off at this point, the current flowing through the resonant inductor becomes discontinuous, generating an excessive surge voltage across it, and propagating an extremely large spike voltage to the input and output each time. This had the disadvantage that it was impossible to implement.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and provide a switching power supply with a simple circuit configuration and low noise.

[Structure of the invention]

The present invention provides the following features: (1) In a switching power supply circuit that uses a switching element to convert DC input into alternating pulses directly or via a transformer or the like, and obtains a DC output by rectifying and smoothing the DC input, a first diode is connected to the pulse voltage. A series circuit of a resonant inductor, and a unidirectional series resonant circuit is constructed by connecting the inductor and a resonant capacitor in series. 2
The positive terminal of the third diode is connected to the negative terminal of the second diode, and the negative terminal of the second diode is connected to the negative terminal of the power supply voltage, and a DC output is obtained by connecting a choke input smoothing circuit consisting of a choke coil and a smoothing capacitor. By connecting the positive pole to the input side terminal of the resonant inductor and connecting the negative pole of the diode to the negative pole of the power supply, a current path is configured to flow through the resonant inductor even after the pulse voltage disappears. This is a switching power supply circuit.

(2) The switching power supply circuit according to claim 1, wherein the switching element is configured to have a constant conduction time and adjust a repetition period to keep the output voltage constant.

[Explanation based on examples]

Next, a switching element using a series resonant circuit according to the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a circuit configuration of a first embodiment according to the present invention,
Figure ♀ shows an operation chart for one cycle.

In FIG. 1, the DC input voltage V1n is the input terminal l, 2
Also, a load whose output current is Io is connected to the DC output terminal 3.4. Furthermore, between the resonance inductor Ll 8 and the load-side choke coil L212, and between the resonance capacitor C19 and the load-side capacitor C213 that smoothes the output, there are connections between Ll<(L2, (:1<(C2
The configuration is such that the relationship holds true. First, the circuit current passing through the switching element SW5 is changed to the switching element SW5.
5 is turned on, the circuit current iL1 is Vin through the resonant inductor Ll 8 and the first diode Di 6.
/ Starts to flow with a slope of LL. Then, at time T1, the output current becomes equal to 0, and the second diode D2
Cut off 11.

At this point, the resonant inductor Ll8 and the resonant capacitor C19 constitute a series resonant circuit, and the circuit current iL1 opens the series resonant operation as shown below, and π
At time T2 after L〒5 has elapsed, the circuit current iL1 becomes equal to the output current IO again, and the resonance capacitor C
The terminal voltage ■C1 of 1 becomes equal to 2 Vln.

Further, at time T3, the circuit current becomes zero and the resonant operation stops due to the unidirectional function of the first diode Di6. Also, during this period T3-TI, as shown below, at this point, the current source flowing through the choke coil L212 is entirely due to the residual charge of the resonant capacitor C19, and its terminal voltage vcI decreases linearly and becomes zero at time T5. , simultaneously the second diode D2 becomes conductive.

On the other hand, since the switching element Sw is in the on state during this period, the current IL1 flowing through the resonant inductor L1 is also Since the current iL1 performs series resonant operation centered on the output current 0, if the maximum value of the output current is Io (Max), the maximum period from when the switching element is turned on until iLl reaches zero current t wax is shown below.

Therefore, in this circuit, if the on period of the switching element Sw is fixed within the range of the above t wax value, time T6
At this point, switching element SW is turned off. Then, due to the turn-off of the switching element, the circuit current iL1 that gradually increases through the resonant inductor L1 after the above-mentioned time T4 loses its current source.

Therefore, in the conventional technology, the current becomes discontinuous, and an excessive surge voltage is generated across the resonant inductor L18 every time it is turned off, which is a fatal drawback.

On the other hand, in the switching power supply circuit according to the present invention, the switching element SW5 is turned off,
When the current source of the circuit current iL1 disappears, the third diode D310 becomes conductive, and after securing a current between the input and output return wires, the circuit current gradually decreases and self-extinguishes. . Therefore, it is obvious that the present invention is highly effective in that it is possible to prevent the generation of surge voltage as in the conventional example described above, and also to regenerate the current flowing through the inductor L1 to the negative electrode side.

Furthermore, even if the output current Io is small, the terminal voltage of the capacitor C1 is small even if the value at time T3 is small. Since the downward slope of time 1 becomes gentle, the time difference between time T4 and time T6 does not become very large.

It is clear that this value is sufficiently small compared to the above, and the switching loss can also be made sufficiently small.

From the above, in the switching power supply using series resonance according to the present invention, even if the on-period of the switching element SW5 is fixed within the above-mentioned twax and only the operating cycle is adjusted, the switching power supply can be simply configured. It does not increase noise as in the conventional case and can operate with low loss.

FIG. 3 shows a second embodiment using a series resonant circuit according to the present invention.

In this embodiment, the DC input voltage is input to the input terminals 111 and 112.
A series circuit of the primary winding N1 of the transformer T1 and the switching element SW5 is connected, and the alternating pulse voltage is applied to the secondary winding $IN2 according to the on/off times of the switching element by turning it on and off. The only difference from the first embodiment is that the second embodiment
At time 1=0 in the figure, the switching element is turned on to output a forward pulse voltage according to the transformation ratio of the primary and secondary windings of the transformer, and at time t=T6. By configuring the device to output pulses in the opposite direction, the same operation and effect as in the first embodiment can be obtained. Therefore, the explanation of its operation will be omitted.

〔Effect of the invention〕

As described above, by using the switching power supply circuit according to the present invention, the resonant inductor that occurs in the conventional example when the ON period of the switching element is fixed and only the operating cycle is adjusted is provided. It is possible to prevent the occurrence of excessive noise due to discontinuity in the current flowing through the circuit, and furthermore, it is possible to operate with low loss, so it can be said that the industrial benefits are extremely large.

Note that the resonant inductor in the present invention is not necessarily limited to a lumped constant, and of course includes wiring inductance, leakage inductance of wire parts, etc., and also the resonant capacitor.
Stray capacitance due to wiring or windings may also be used.

[Brief explanation of the drawing]

FIG. 1 is a switching power supply circuit diagram of a first embodiment of the present invention. FIG. 2 is an operation chart of the switching power supply shown in FIG. 1. FIG. 3 is a switching power supply circuit diagram showing a second embodiment of the present invention. FIG. 4 is a circuit diagram of a conventional switching power supply of this type. FIG. 5 is an operation chart of the switching power supply circuit shown in FIG. 4. 1.2, 111, 112...input terminals. 3.4...Output terminal. 5... Switching element Sw. 6... First diode D1゜8... Resonant inductor L1゜9... Resonant capacitor C1゜10... Third diode D3゜11... Second diode D2゜12. ...Load side choke coil L2゜13...Load side capacitor C2゜14...Direction of output current Io. 15...Transformer T0 Vln...DC input voltage. L...Load. N1...Primary winding. N2... Secondary winding. Io...output current. iL+...Circuit current. Patent applicant Tohoku Metal Industry Co., Ltd. Figure 1 Figure 2-J Figure 4 Figure 5 11 1j 15

Claims (2)

[Claims] (1) In a switching power supply circuit that uses a switching element to convert DC input into alternating pulses directly or via a transformer, rectifies and smoothes it, and obtains a DC output, a first diode and a resonant inductor are connected to the pulse voltage. A unidirectional series resonant circuit is constructed by connecting the inductor and a resonant capacitor in series, and a second diode is connected in parallel to both ends of the resonant capacitor in the positive direction of the power supply voltage. The positive terminal of the second diode is connected to the negative terminal of the power supply voltage, and a DC output is obtained by connecting the choke input smoothing circuit consisting of a choke coil and a smoothing capacitor. By connecting the third diode to the input side terminal of the resonant inductor and connecting the negative pole of the third diode to the negative pole of the power supply, a current path is provided that flows through the resonant inductor even after the pulse voltage disappears. A switching power supply circuit featuring: (2) The switching power supply circuit according to claim 1, characterized in that the output voltage is kept constant by keeping the conduction time of the switching element constant and adjusting the repetition period.