JPH05168234A - Switching power unit - Google Patents

Abstract

PURPOSE:To reduce the noise at turn on and at turn off by adding a delay circuit to the control circuit of a switching power unit, and connecting a capacitor in parallel with a switching element. CONSTITUTION:When the secondary current I2 of a transformer 5 becomes zero, a diode 6 checks a reverse current, and on the primary side, an LC resonance circuit is constituted of the exciting inductance of the transformer 5 and a capacitance 2, and the voltage VDS of the switching element 4 drops in the shape of full sine waves. Here, while a secondary current is flowing, a display circuit 8 does not turn on the switching element 4 and turns off the switching element 4 at the point of time when the VDS has dropped completely. Accordingly, the turn on voltage is low enough, and it reduces turn-on loss and turn-on noise. Moreover, at turn off, the rise of the voltage is gentled to reduce the turn on noise.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to provide a low noise switching power supply device.

[0002]

2. Description of the Related Art FIG. 1 shows an example of a conventional flyback converter. 1 is a DC power supply, 2 is a capacitor, 3 is a diode, 4 is a switch element, 5 is a conversion transformer, and 5-1.
Is its primary winding, 5-2 is a secondary winding, and 5-3 is an auxiliary winding. 6 is a diode and 7 is a capacitor. Figure 1
FIG. 3 shows the operation waveform of the circuit of FIG. As shown in the VDS waveform of this figure, the switching element 4 has an input voltage Vin
Is turned on with the voltage applied.

This is because the auxiliary winding voltage Vsub is applied to the gate of the switching element torque 4, and the switching element torque 4 turns on at the moment when this voltage becomes positive from zero. The winding voltage of zero means that the voltage of the DC power supply 1 is applied to the switching element 4.

[0004]

As described above, in the conventional circuit, since the switch element 4 turns on while having a voltage, there is a problem in that loss and switching noise are generated by short-circuiting the capacitor 2.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a low noise switching power supply device by solving the above problems of the prior art.

[0006]

FIG. 2 shows an embodiment of the present invention, in which 1 is a DC power source, 2 is a capacitor, 3 is a diode, 4 is a switch element, 5 is a conversion transformer, and 5-1 is its primary winding. Line, 5-
2 is a secondary winding, 5-3 is a tertiary winding, 6 is a diode, 7
Is a capacitor, and 8 is a delay circuit.

An example of the delay circuit 8 is shown in FIG. Further, FIG. 4 shows waveforms of respective parts according to FIG. In FIG. 4, when the transformer secondary current I2 becomes zero at time t1, the reverse current is blocked by the diode 6 and does not flow, and the secondary side becomes open. Therefore, on the primary side, the exciting inductance of the conversion transformer 5 and the capacitor 2 form an LC resonance circuit, and the voltage VDS of the switch element 4 drops in a cosine wave shape.

An equivalent circuit at this time is shown in FIG. Since the DC power supply 1 has no impedance in terms of AC, the capacitor 2 is equivalent here even if it is connected in parallel with the exciting inductance of the conversion transformer 5. Next, VDS is Vin
Until it goes down, the operation is the same as the conventional circuit. VDS is V
A voltage lower than in means that a positive voltage appears in VSUB, but the delay circuit 8 prevents this voltage from being applied to the gate of the switch element 4, and the switch element 4 is turned off. Do not turn on. Therefore, VDS continues to drop further, and if the resistance component in the circuit is ignored, as shown in FIG. 7, it oscillates with an amplitude of (initial voltage −Vin) around Vin. This is clear from the equivalent circuit of FIG.
When the number of turns of the transformer is n, the turns ratio of the transformer is n: 1, the input voltage is Vin, and the output voltage is Vo as shown in FIG. 2, the initial voltage of VDS at the start of LC resonance is represented by Vin + nVo. This is clear from the fact that the primary winding is clamped at nVo because the secondary winding voltage is clamped at Vo while the secondary current is flowing.

As shown in FIG. 4, by turning on the switch element 4 at a point (time t2) when VDS is completely lowered, it is possible to turn on the switch element 4 at a voltage lower than that of the conventional type by nVo. This makes it possible to reduce the turn-on loss and the noise generated at this time. Next, VDS is also the voltage applied to the capacitor 2, and the decrease of VDS means that the energy −1 / 2CV2 of the capacitor 2 decreases. The reduced amount of energy is returned to the DC power source 1 and is not lost at all. That is, FIG.
In the circuit shown in FIG. 3, the energy consumed by the conventional switch element 4 is fed back to the DC power supply 1.

As is clear from the above, VDS can be reduced to zero by satisfying the condition of nVo> Vin. Figure 5
Is the waveform in that case, and the switch element 4 is turned on at a completely zero voltage. In this case, in principle, no turn-on loss and noise occur.

FIG. 2A shows an embodiment of a delay circuit, 11 is a capacitor, 12 is a resistor, 13 is a first transistor, 14 is a resistor, 15 is a second transistor, 16
And 17 are resistors, 18 is a third transistor, and 19 is a diode. This delay circuit detects the point where the differential value of the voltage applied between terminals A and B becomes zero and
The operation of changing the non-conducting state between -C and the conducting state is performed. VDS in FIG. 2 is represented by the sum of the input voltage and the primary winding voltage. Since the input voltage is constant, its differential value is always zero. To detect the point where the differential value of VDS becomes zero, the differential value of the primary winding voltage may be detected. Since the waveforms of the winding voltages are similar, the same result can be obtained by detecting the tertiary winding voltage. That is, the terminal AB of FIG.
Is connected to the auxiliary winding, the point where the differential value of VDS becomes zero is detected, and then the operation between the non-conducting state and the conducting state between terminals A and C is performed. The third winding voltage is applied to the gate of the switching element torque 4 shown in FIG. 2 by the conduction between A and C, and the switching element torque 4 is turned on.

The advantage of the circuit according to FIG. 2 is that the input voltage Vi
n, the winding ratio of the transformer is n: 1, and the turn-on can always be performed at the optimum timing regardless of the value of the output voltage Vo. If VDS does not drop to zero as shown in Fig. 4, VDS
Turns on when is the smallest, and when it reaches zero as shown in FIG. 5, it turns on when it reaches zero. This is VD
It is obvious from turning on at the point where the differential value of S becomes zero.

Next, the operation of the circuit shown in FIG. 2-1 will be specifically described. In FIG. 2A, the voltage at the terminal AB is VAB, the voltage at the capacitor 11 is VC, and the base-emitter voltage of the transistor 13 is VBE. The base current of the transistor 13 is IB, the resistance value of the resistor 12 is R,
When the capacitance of the capacitor 11 is C, IB = C dVc / dt-VBE / R = Cd (VAB-VBE) / dt-VBE / R = C dVAB / dt-C dVBE / dt-VBE / R transistor 13 Since VBE is constant due to the nature of the transistor while it is on, dVBE / dt = 0 ∴IB = C dVAB / dt-VBE / R VBE / R is so small that if ignoring it, IB will also be IB. It will be zero. When IB becomes zero, the first transistor 13 turns off, a current flows through the resistor 14 to the base of the second transistor 15, and the second transistor 15 turns on. Then, a base current flows through the third transistor 18, the third transistor 18 turns on, and the terminals A and C become conductive.

As described above, the delay circuit shown in FIG. 2-1 detects the point where the voltage change of VAB becomes zero, and operates to bring the terminals A and C from the non-conducting state to the conducting state. Generally, the switch element generates noise not only at turn-on but also at turn-off, but the turn-off noise can be reduced by arranging a capacitor in parallel to slow the rise of voltage. The same thing can be said for a general converter, but the mere addition of a capacitor improves the turn-off noise, but at the cost of increasing the turn-on loss, it is not realistic. In the circuit of the present invention, while reducing the turn-off noise by attaching a sufficient capacitor, the increase in the turn-on loss is suppressed by returning the energy stored in the capacitor to the input power source. Noise can be reduced at the time of turn-off, and a low noise converter can be realized as a whole.

FIG. 2-2 shows another embodiment of the delay circuit, in which 30 is a capacitor, 31 is a resistor, and 32 is a transistor. This delay circuit detects the point where the differential value of the voltage applied between terminals A and B becomes zero, and then detects
The voltage between them is transmitted between terminals CB. The operation of the transistor of this delay circuit is exactly the same as that of the first transistor of the delay circuit of FIG. 2A. When the differential value of the voltage between the terminals A and B is positive, it is conductive, and when it becomes zero, It becomes conductive. If the transistor conducts, terminal B-
When the voltage between C becomes zero and the switch element of FIG. 2 does not conduct and the transistor becomes non-conductive, the voltage between terminals A and B appears between terminals CB and the switch element of FIG. 2 conducts. .. Thus, this delay circuit operates exactly the same as the delay circuit of FIG. 2-1 and 2-2
2-2 and FIG.
2-2 has a merit that the number of parts is about 1/3, which is small compared to the embodiment of FIG. 2B, but has a demerit that loss due to short circuit of the transistor 32 occurs in FIG. 2-2. Therefore, it is necessary to use them properly according to the purpose.

Next, the actual noise reduction effect will be described based on data obtained by experiments.
FIG. 8 is a circuit diagram used in the experiment. In principle, the circuit is the same as in FIG. The input voltage is 216 to 390 VDC, the output is 15V2 ADC, and the circuit of FIG. 2-1 is used as the delay circuit 8. When the capacitor 22 is removed and the delay circuit is short-circuited, the conventional ringing choke converter (R
It becomes the same circuit as CC). Input feedback noise is compared between a conventional RCC circuit and a state in which a capacitor and a delay circuit are added. As an experimental parameter, the DC input voltage 39
0V, DC output 15V 2A, conventional RCC,
Add a capacitor 22 (1000PF) as shown in
Further, comparing the input feedback noise with the case where the delay circuit 8 is inserted, when a capacitor is added and the delay circuit is inserted, the frequency is 1 MHz to 30 MHz, and
It can be seen that -30 dB noise is reduced, and there is a remarkable noise reduction effect.

Next, the loss reducing effect will be calculated. A converter with an input voltage of 200 VAC and an output of 100 W is assumed. If the efficiency is 80%, the total loss is 25W. Now, assuming that the capacitor 2 in FIG. 2 is 1000 PF and the frequency is 100 KHz, the loss due to short-circuiting the capacitor 2 with the input voltage is: loss = 1/2 CV2f = 1/2 × 1000 × 10-12 × ( √2 × 200) 2 × 100 × 10 -3 = 4W where C: capacitance of the capacitor V: voltage applied to the capacitor f: frequency Since this loss can be reduced, 16% of the total loss is reduced. It becomes a calculation that makes things possible. The efficiency at this time is 8
It will rise 2.6% at 2.6%. Moreover, in the conventional RCC,
There is a problem that the loss of the switch element increases and the temperature rises because the frequency increases at light load. In order to solve this, if a dummy resistor is attached to the output to suppress the frequency from rising, the loss of the dummy resistor will reduce the overall efficiency. Since the loss of the switch element is originally reduced in the circuit of FIG. 2, there is no such problem, and this point also contributes to high efficiency.

[0018]

The noise at turn-on can be reduced by adding a delay circuit to the control circuit of the switching power supply device, and the noise at turn-off can be reduced by connecting a capacitor in parallel with the switch element. The present invention is for communication equipment that requires a small and low noise power source, E
Use as a power source for DP and OA equipment is effective.

[Brief description of drawings]

1 is a circuit example of a conventional flyback converter.

FIG. 2 shows an embodiment of the present invention

[Fig. 2-1] One example of a delay circuit

FIG. 2-2 is another embodiment of the delay circuit.

FIG. 3 is an operation waveform of a conventional flyback converter (FIG. 1)

FIG. 4 is an operation waveform of one embodiment (FIG. 2) of the present invention.

[Fig. 5] Waveform of VDS when nVo> Vin

FIG. 6 is an equivalent circuit between times t1 and t2 according to FIG.

7 is a VDS waveform when the turn-on of the switch element 4 according to FIG. 2 is delayed.

FIG. 8 is a circuit used in an experiment for confirming the effect of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2 Capacitor 3 Diode 4 Switching element 5 Conversion transformer 5-1 Primary winding of conversion transformer 5-2 Secondary winding of conversion transformer 5-3 Tertiary winding of conversion transformer 6 Diode 7 Capacitor 8 Delay circuit 11 Capacitor 12 Resistor 13 First transistor 14 Resistor 15 Second transistor 16 Resistor 17 Resistor 18 Third transistor 19 Diode 21 DC power supply 22 Capacitor 23 Diode 24 Transistor 25 Conversion transformer 26 Diode Do 27 Capacitor 30 Capacitor 31 Resistor 32 Transistor Vin DC input Vo DC output A terminal B terminal C terminal

Claims (3)

[Claims] 1. A primary winding of a conversion transformer and a switch element are connected in series to a DC power source, and a diode and a capacitor are connected in parallel with the switch element, and a secondary winding of the conversion transformer is connected. A converter in which a rectifying / smoothing circuit is connected to obtain a direct current output, by providing a delay circuit between the tertiary winding of the conversion transformer and the gate terminal of the switching element, the switching element A switching power supply device, wherein a part or all of the energy stored in a capacitor connected in parallel with the DC power supply is fed back to the DC power supply. 2. The switching power supply device according to claim 1, wherein a parasitic diode existing parasitically on the switch element is used instead of the diode connected in parallel with the switch element. 3. The method according to claim 1, wherein the parasitic capacitance parasitically present in the switch element is used instead of the capacitor connected in parallel with the switch element, or the capacitor is connected in parallel with the primary winding. Alternatively, a switching power supply device characterized in that the capacitor is optionally used together.