JPH04217865A - Resonance switching power supply - Google Patents

Abstract

PURPOSE:To suppress switching loss and switching noise upon turn ON of a switching element. CONSTITUTION:Upon turn OFF of a switching element SW, a resonance circuit is formed of the primary coil L1 of a transformer (50) and a resonance capacitor CRE and the voltage between the electrodes to be controlled of a switching element varies limitedly. A voltage detecting circuit comprising a diode D2, a resistor R9 and a transistor Q0 detects a minimal point of the voltage between the electrodes to be controlled and triggers the CT terminal of a control circuit (20). The control circuit (20) outputs a high level at the OUT terminal for a predetermined time after triggering of the CT terminal thus turning the switching element SW ON. Since the switching element SW is turned ON at a time point when the voltage between the electrodes to be controlled has a minimal value, switching loss can be neglected.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit that utilizes series resonance between an isolation transformer and a capacitor.

0002

2. Description of the Related Art A conventional switching power supply circuit will be explained with reference to FIG. The main circuit of the switching power supply circuit is a transformer (72) that isolates its output DCOUT from the input power supply DCIN, and the primary coil L of this transformer (72).
Switching element SW that controls switching of the current of 1
, a control circuit (60) that drives this switching element SW at a predetermined timing, and includes a resistor R22 to prevent noise due to large current switching and high voltage switching, or to prevent destruction of the switching element SW etc. due to this noise. , a snubber circuit consisting of a capacitor C22 and a diode D22, a snubber circuit consisting of a resistor R21 and a capacitor C21, a resistor R23, and a capacitor C.
24, and inductances such as bead cores (80) are added to predetermined locations of the circuit for the purpose of suppressing steep rises and falls and suppressing the generation of noise.

[0003] The switching element SW is a power MOSF
An ET or bipolar transistor is used; if a bipolar transistor is used, a sufficiently large base current is provided by the transformer drive to reduce its on-resistance. Further, as shown in the figure, the power MOSFET usually includes a flywheel diode D23.

The control circuit (60), which is often provided as an integrated circuit, includes a pulse generator (62), an AND gate (64), a flip-flop (66), and a buffer (
68) and a comparator (70). The pulse generator (62) operates at a constant frequency determined by the CR time constant of the capacitor and built-in resistor connected between the terminal TPIN and ground, or synchronized with the trigger pulse when the trigger pulse is supplied from an external source. Operate on frequency.

Next, the operation of this switching power supply circuit will be explained. The control circuit (60) is activated by the input power DCIN supplied to the terminal VCC via the resistor R20, and after activation is operated by the output of the tertiary coil L3 of the transformer (72), which is rectified and smoothed by the diode D20 and capacitor C20. .

Now, if the output of the comparator (70) is high level, the flip-flop (66) is set by the AND gate (64) at the rising edge of the pulse generator (62) output, and the flip-flop (66) is set by the AND gate (64) at the rising edge of the pulse generator (62) output. It is reset at the falling edge. The switching element SW is driven by the buffer (68) based on the set output Q of this flip-flop (66). The primary circuit current of the transformer (72) changes at a predetermined time constant based on the switching operation of the switching element SW.
Secondary coil L2 and tertiary coil L of transformer (72)
3 to induce a voltage. Therefore, the output voltage DCOUT of this switching power supply circuit is determined by the pulse generator (62).
Determined by the output duty of

Further, the primary circuit current of the transformer (72) is detected by a current detection resistor RSEN, and the primary circuit current of the transformer (72) is detected by a current detection resistor RSEN.
is input to the inverting input terminal of Therefore, input power D
When the primary circuit current detected by the current detection resistor RSEN exceeds the set value VS due to a decrease in the voltage at CIN or an increase in the current at the output DCOUT, the comparator (70
) outputs a low level to the AND gate (64), and this AND gate (64) causes the pulse generator (62
) to the flip-flop (66) is cut off. As a result, overcurrent control of the primary circuit current is performed.

[0008]

[Problems to be Solved by the Invention] Conventional switching power supply circuits require multiple snubber circuits and bead cores composed of diodes, resistors, and capacitors with good switching or frequency characteristics, so there is a demand for miniaturization of switching power supply circuits. However, it has the disadvantage that it cannot meet the demand for lower prices. The addition of the snubber circuit and bead core not only limits the operating frequency of the switching power supply circuit, but also has the disadvantage that power is consumed in the snubber circuit and the conversion efficiency of the switching power supply circuit is reduced. That is, when trying to reduce the noise generated from the switching power supply circuit, the conversion efficiency decreases, and when trying to improve the conversion efficiency of the switching power supply circuit, the noise generation increases. For this reason, it has not been possible to realize a switching power supply circuit that solves the conflicting technical issues of noise reduction and high conversion efficiency.

Furthermore, since a large current and a high voltage are switched, a large amount of power is consumed inside the switching element which turns on and off with a finite slope, resulting in a disadvantage that the conversion efficiency of the switching power supply circuit is reduced. In particular, when an IGBT with a low on-resistance is used as a switching element, a current based on residual minority carriers flows during reverse bias, which has the disadvantage of increasing power consumption inside the switching element.

Furthermore, when changing the output frequency of the pulse generator to a high value in order to greatly change the input/output voltage ratio, it is difficult to change the duty due to the frequency characteristics of the switching element (IGBT) for the pulse generator. This has the disadvantage that voltage control is difficult.

[0011]

[Means for Solving the Problems] The present invention has been made in view of the above problems, and provides a transformer for insulating its output from an input power source, a switching element for controlling the primary coil current of this transformer, and a switching element for controlling the primary coil current of this transformer. It consists of a control circuit that is driven at a predetermined timing and a resonant capacitor connected between the controlled electrodes of the switching element, and detects the oscillating voltage of the resonant capacitor due to series resonance between the isolation transformer and the capacitor, and adjusts the voltage of this resonant capacitor to a predetermined value. By configuring the control circuit to turn on the switching element by triggering the control circuit at the timing, all of the problems of the conventional switching power supply circuit described above are solved.

0012

[Operation] The oscillating voltage of the resonant capacitor is detected and the switching element is turned on at the timing when the voltage of this resonant capacitor reaches a predetermined value. (The voltage level at which the switching element is driven can be set to an arbitrary low value.) Power consumed inside the switching element is reduced. Also,
This reduces the level of switching noise and eliminates the need for a snubber circuit and bead core.

Furthermore, since the switching element is turned on at the minimum point of the charge level of the resonant capacitor, the charge energy of the resonant capacitor is effectively transmitted to the secondary side.
The charging energy consumed inside the switching element is reduced.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A resonant switching power supply circuit (hereinafter simply referred to as a switching power supply circuit) of the present invention will be described with reference to FIGS. 1 to 8. Referring to FIG. 1, the switching power supply circuit of the present invention connects its output DCOUT to the input power supply DC
Transformer (50) isolated from IN, power MOSFE
Switching element S using T or IGBT etc.
W, a resonant capacitor CRES connected between the controlled electrodes of the switching element SW, a control circuit (20) that drives the switching element SW at a predetermined timing, an output voltage detection circuit (52), and a residual voltage of the resonant capacitor CRES detected. It is composed of a voltage detection circuit (transistor Q0, resistor R9, diode D2), etc. Incidentally, a switching power supply circuit in which a capacitor as a snubber circuit is connected in parallel between the controlled electrodes of the switching element SW is known, but the operation of the switching power supply circuit of the present invention is clearly different from that type. be.

As shown in the block diagram of FIG. 2, the control circuit (20) includes a comparator (22) that detects the rise of the power supply voltage at the terminal VCC, and a A reference voltage circuit (34) that outputs 5V reference voltages VREF and VS, a 3-input OR gate (24)
), capacitor C3 and resistor R4 (
A clock generator (26), a flip-flop (28), a differential amplifier (30), a comparator (
32), push-pull connected transistors Q1, Q
Consists of 2.

In this embodiment, an output voltage detection circuit (52) is provided in the output circuit on the secondary side of the transformer (50), and this output via the photocoupler PC is used to detect the switching element SW.
The timing at which the switching element SW turns OFF is determined, and the timing at which the switching element SW turns ON (VDS
(minimum point of 20
) is controlled. As a result, the switching power supply circuit of the present invention operates on and off at a frequency according to the input/output conditions and the resonance conditions of the transformer (50) and the capacitor CRES.

The basic switching operation of the embodiment will first be explained with reference to FIGS. 1 and 2. Control circuit (20)
is the input power supply D supplied to the terminal VCC via the resistor R1.
Activated by CIN. During the unstable operation period in which the voltage at the terminal VCC changes, the voltage at the terminal VCC and the reference voltage V
A comparator (22) that compares ST with a 3-input OR gate (24) that inverts and inputs the comparison output turns off the transistor Q1 and turns on the transistor Q2, keeping the output OUT of the control circuit (20) at a low level. It's dripping. Therefore, the switching element SW is off at this timing.

[0018] When the switching element SW is off,
A resonant capacitor CR connected in parallel between the controlled electrodes
After the ES is charged at a predetermined time constant, the transformer (5
0) The circuit current oscillates sinusoidally due to the series resonant circuit formed by the primary coil L1 and the resonant capacitor CRES. Therefore, the voltage detection circuit (transistor Q0, resistor R9, diode D2) detects the minimum point of this oscillating voltage, and at that timing, the clock generator (26) is triggered from the terminal CT.

When the clock generator (26) is triggered, its narrow pulse causes the flip-flop (
28) is set, and its inverted output becomes low level. Subsequently, when the output of the clock generator (26) becomes low level, the output of the 3-input OR gate (24) turns on transistor Q1 and turns off transistor Q2 (
(see FIG. 2), the switching element SW is turned on.

When the switching element SW is turned on, a current that rises with a predetermined time constant flows through the primary coil L1 of the transformer (50), and the secondary coil L2 and the tertiary coil L3 flow.
A voltage is induced in The current in the primary coil L1 rapidly decreases to a predetermined current after a period of time corresponding to the load on the secondary coil L2.

In the switching power supply circuit of the present invention, the period during which the switching element SW is turned on is constant and determined by the resonance conditions. Further, the switching power supply circuit of the present invention is controlled at a constant voltage by a feedback system via an output voltage detection circuit (52) and a photocoupler PC. Furthermore, the primary coil L1 current of the transformer (50) is detected as ISEN by the current detection resistor RSEN, and the comparator (
32), it is compared with the reference voltage VS. Therefore,
The flip-flop (28) is reset at the timing when ISEN becomes larger than the reference voltage VS, the switching element SW is turned off, and overcurrent control is performed.

The present invention will be explained in more detail with reference to FIGS. 3 and 4. Note that FIGS. 3 and 4 schematically show the waveforms of the controlled inter-electrode voltage VDS and current IDS of the switching element SW of the conventional switching power supply circuit and the switching power supply circuit of the present invention, respectively. When referring to these waveforms, it should be noted that the voltage VDS between the controlled electrodes of the switching element SW and the current IDS are out of phase, and the voltage/current product of the switching element SW becomes small.

In a conventional switching power supply circuit that controls the current of a high inductance circuit by forcibly turning on and off the switching element SW at any time, as shown in FIG.
It is understood that a high level of noise is generated at the falling edge of S. In addition, since the switching element SW operates at a finite speed, the voltage between the controlled electrodes VDS and the current I at the rising and falling timings of VDS.
It is also understood that the internal loss of the switching element SW, expressed as the product of DS, increases. On the other hand, in the present invention in which a series resonant circuit is formed by the primary coil L1 of the transformer (50) and the resonant capacitor CRES, since the relatively large-capacity resonant capacitor CRES is connected in series with the circuit, the switching element SW The high inductance circuit is not opened when the switch is off, and noise is suppressed as shown in FIG. Also, since the switching element SW is turned on after VDS falls,
Extremely low noise generation during turn-on. Similarly, since the internal loss during turn-on of the switching element SW can be ignored, the switching loss during turn-on is large;
IGBTs that were difficult to apply to switching power supply circuits
can be used as a switching element SW.

Now, with reference to FIGS. 5 and 6, IGB
Let us briefly explain T. As shown in FIG. 5, the IGBT has N
- It has a cross-sectional structure similar to that of a known power MOSFET, except for the P+ region formed in the base region. While the N-base region of a power MOSFET without a P+ region acts as a resistor when turned on, in an IGBT a PNP transistor is formed by the P-base region, N-base region, and P+ region (see Figure 6). , the resistance of the N-base region when turned on becomes negligibly small. On the other hand, since a PNP transistor is formed, N
- It has a drawback that a current IM (see FIGS. 3 and 4) flows based on residual minority carriers in the base region. Also, P+
Since it has a large area, it has a characteristic that it cannot incorporate a flywheel diode.

However, according to the present invention, since the switching of the switching element SW is performed at a timing when no current/voltage product occurs, the internal loss due to the current IM due to residual minority carriers is offset by the low on-resistance. Further, since a flywheel diode is not built-in due to the structure, the flywheel diode D4 can be connected between the high voltage side controlled electrode of the switching element SW and the ground. For this reason, the flywheel diode D
The current flowing through the current sensing resistor RSEN is no longer consumed by the current sensing resistor RSEN.

Voltage detection circuit of the present invention (transistor Q0
, resistor R9, and diode D2) may be any arbitrary element having the function of detecting the minimum point of the oscillating controlled interelectrode voltage VDS, preferably the first minimum point of vibration, and turning on the switching element SW at this timing. A voltage detection circuit can be used to detect when the energy transfer from the resonant capacitor CRES to the secondary coil L2 of the transformer (50) is complete.

When the switching element SW is turned on at this timing, the energy charged in the resonant capacitor CRES is less consumed inside the switching element SW, and at the timing of the rise and fall of VDS, the energy between the controlled electrodes is reduced. Voltage VDS
The internal loss of the switching element SW, which is represented by the product of the current IDS and the current IDS, is suppressed.

The embodiment uses a transistor Q0 whose emitter is connected to the power supply terminal VCC of the control circuit (20) as a voltage detection circuit, a resistor R9 connected to the controlled electrode of the base switching element SW of this transistor Q0, and a high voltage switching diode. A specific circuit consisting of D2 is illustrated. Next, the detection level of this voltage detection circuit and the transformer (50
) will be explained with reference to FIGS. 7 and 8.

The controlled interelectrode voltage VDS oscillates as shown in FIG. 7, and the first extreme value VR of the oscillation becomes the minimum value. As shown in Figure 8, this extreme value VR is mainly caused by the transformer (
50) turns ratio of secondary coil and primary coil R=L2/L
It has been known through research by the inventors of the present invention that this is dominated by 1.

FIG. 8 is a characteristic diagram showing the coil turns ratio R=L2/L1 in a power supply circuit with an output voltage of about 120 to 130 V used for, for example, a CTV or CRT display. is the turns ratio R=L2/L1=
The controlled interelectrode voltage VDS when the voltage is 1.0, that is, the residual voltage VR of the resonant capacitor CRES is shown, and the graph ■ shows the resonant capacitor C when the turns ratio R=L2/L1=0.75.
It shows the residual voltage VR of RES. From the same figure, in order to reduce the power consumed by the switching element SW, the turns ratio R of the transformer (50) should be reduced and the resonant capacitor C
It is easily understood that it is preferable to lower the residual voltage VR of RES.

On the other hand, as a reaction to the above, the voltage applied to the switching element SW increases, but the voltage of the secondary side high-speed diode D3 decreases.
There is an advantage that the code can be used.

According to the present invention, the oscillating voltage of the resonant capacitor is detected and the switching element is turned on at the timing when the voltage of the resonant capacitor reaches a predetermined value. The power dissipated inside the switching element is reduced. Additionally, this reduces the level of switching noise, making the snubber circuit and bead core unnecessary.

Furthermore, since the switching element is turned on at the minimum point of the charge level of the resonant capacitor, the charge energy of the resonant capacitor is effectively transmitted to the secondary side.
This has the advantage that charging energy consumed inside the switching element is reduced. In addition, when implementing the present invention,
It is realized by mounting a predetermined circuit configuration on an integrated circuit board. In this case, for example, as shown in FIG.
If 0) is integrated, it can be used in a resonant type switching power supply circuit, and as shown in FIG. 9, it has the advantage that it can be used in common with a synchronous type switching power supply circuit.

[0034]

[Effects of the Invention] As described above, according to the present invention, (1
) Since the switching elements are turned on and off at timings when the voltage level and current level are low, the level of switching noise is reduced. Therefore, a snubber circuit and a bead core are not required, and the switching power supply circuit can be made smaller and lower in price. (2) Since the switching element is turned on at the minimum point of the charging charge level of the resonant capacitor, the charging energy of the resonant capacitor is effectively transmitted to the secondary side, and the charging energy consumed inside the switching element is reduced. (3) Turning on and off of the switching element depends on the voltage level,
Since the switching is performed at a timing when the current level is low and the temporal phase relationship is shifted from each other, the internal loss of the switching element expressed by the voltage/current product is reduced. (4) When using an IGBT as a switching element, good on-resistance characteristics can be fully utilized.

[0035]

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of a control circuit used in the embodiment.

FIG. 3 is a voltage/current waveform diagram of a conventional switching element.

FIG. 4 is a voltage/current waveform diagram of the switching element of the example.

FIG. 5 is a sectional view of an IGBT suitable for the present invention.

FIG. 6 is an equivalent circuit diagram of an IGBT.

FIG. 7 is a diagram illustrating the residual voltage of a resonant capacitor.

FIG. 8 is a diagram illustrating the relationship between the residual voltage of a resonant capacitor and the turns ratio of an isolation transformer.

FIG. 9 is a block diagram of a modification of the present invention.

FIG. 10 is a block diagram of a conventional example.

[Explanation of symbols]

20 Control circuit 50 Insulation transformer 52 Output voltage detection circuit

Claims (6)

[Claims] 1. An isolation transformer, a switching element that controls a primary coil current of this isolation transformer, a control circuit that drives this switching element at a predetermined timing, and a resonant capacitor connected between controlled electrodes of the switching element. and a voltage detection circuit that triggers the control circuit to turn on the switching element at a timing when the voltage of the resonance capacitor reaches a predetermined value. 2. The resonant switching power supply circuit according to claim 1, wherein the first minimum point of the oscillating voltage of the resonant capacitor is detected by the voltage detection circuit. 3. The resonant switching device according to claim 1, wherein the output circuit on the secondary side includes an output voltage detection circuit, and the output of the output voltage detection circuit controls the control circuit independently of the output of the voltage detection circuit. power circuit. 4. The switching element has a power M.
2. The resonant switching power supply circuit according to claim 1, wherein an OSFET is used. 5. IGBT as the switching element.
2. The resonant switching power supply circuit according to claim 1, wherein the resonant switching power supply circuit uses: 6. The resonant switching power supply circuit according to claim 5, further comprising a diode connected between the high voltage side controlled electrode of the switching element and ground.