JP2001178124A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the efficiency of a switching power supply by reducing the switching loss, when a switch element 8 is closed by discharging a snubber capacitor 9 for protecting the element 8 against a surge voltage in the power supply for generating a constant voltage DC power at the secondary side of a transformer 12 and supplying the power to an external load 15 by turning on/off the element 8 under a PWM control, so that the time point reaching the lowest voltage is set to the on-time point in advance with the charging/ discharging of an oscillation capacitor 2 repeating charging/discharging between prescribed highest voltage and lowest voltage. SOLUTION: An auxiliary winding 16 is provided in a transformer 12, a voltage generated in the winding 16 when the switch element 8 is turned off is applied to the oscillation capacitor 2 via a diode 17 for charging the capacitor 2 to the highest voltage. When this charging is finished, LC resonance of the transformer 12 and the snubber capacitor 19 and the voltage drop of the capacitor 2 due to the discharging power are started. Thus, when the voltage of the capacitor 2 arrives at the lowest voltage, the vibration voltage of the capacitor 9 is set to a minimum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a semiconductor switch element connected at least between an input DC power supply as an energy source via an inductive means such as a transformer or a reactor, and is repeatedly opened and closed by a PWM-controlled drive pulse. A so-called switching power supply device as a power supply device that generates a stabilized DC power supply and supplies the DC power to the outside, in particular, a snubber capacitor that protects the semiconductor switch element from a surge voltage generated when the semiconductor switch element is turned off is a semiconductor switch element. The present invention relates to a switching power supply device that minimizes switching loss of a semiconductor switching element due to discharge at an ON point, prevents heat generation of the semiconductor switching element, and improves device efficiency. In the drawings, the same reference numerals indicate the same or corresponding parts.

[0002]

2. Description of the Related Art FIG.
1 shows an example of a circuit configuration of a conventional switching power supply device using the same. In the figure, reference numeral 8 denotes a semiconductor switch element made up of an N-channel MOSFET (also simply referred to as a switch element).
And a control circuit for driving the switch element 8 on / off by a PWM output 7 as a drive pulse subjected to PWM (pulse width modulation) control so that the supplied DC voltage is constant while detecting the DC voltage supplied to the switch. It is a power supply control circuit.

In the power supply control circuit 5, reference numeral 3 denotes an oscillator which oscillates and outputs an oscillating wave (in this case, a triangular wave) using the oscillating capacitor 2, and 4 denotes an oscillating output of the oscillator 3 and the switching power supply. An output voltage detection input 6 as a detection value of a DC voltage supplied to the external load 15 by the device.
Is input to drive the switch element 8 on / off.
It is a PWM control unit that generates the WM output 7.

[0004] With this configuration, the switch element 8 uses the input DC power supply 1 at the frequency of the oscillator 3 and the DC output voltage on the secondary winding 11 side of the flyback transformer 12, that is, the output voltage detection input 6 becomes constant. Such a duty, that is, ON ratio = ON period / (ON period + OFF period) is intermittently applied to the primary winding 10 of the transformer 12. When the switching element 8 is turned off, a voltage having a polarity corresponding to the direction in which the current flowing through the primary winding 10 is maintained is generated in the secondary winding 11 of the transformer 12, so that the diode 13 is turned on. This voltage is smoothed by the capacitor 14 and supplied to the external load 15 as the output voltage of the switching power supply.

The capacitor 9 connected in parallel with the switch element 8 is called a snubber capacitor, and suppresses a surge voltage generated when the switch element 8 is turned off and noise accompanying the surge voltage. FIG. 7 shows an example of an operation waveform of each unit in FIG. 7A shows a voltage waveform of the oscillation capacitor 2 as an output waveform of the oscillator 3 in the power supply control circuit 5, and FIG. 7B shows a gate drive pulse given to the switch element 8 by the power supply control circuit 5. (FIG. 7C) shows a snubber voltage as a voltage between both ends of the snubber capacitor 9. The PWM output 7 is a signal indicating the ON / OFF state of the switch element 8 at the same time.

The power supply control circuit 5 includes a switch element 8
Is determined by the oscillator 3 using the charging and discharging of the oscillation capacitor 2. FIG. 8 shows an example of a basic circuit configuration of the oscillator 3. In the figure,
36 is a constant voltage internal power supply, 27 is a constant current source for charging the oscillation capacitor 2 (a resistor 28 may be used instead of the constant current source 27), and 29 is a constant current for discharging the capacitor 2 Source (note that instead of this constant current source 29, a resistor 3
0 may be used), 34 is a comparator having hysteresis, 35 is a reference voltage that can be switched between an upper limit value VU and a lower limit value VL, 33 is an inverter element that inverts the output of the comparator 34 and gives it to a switch 31, Reference numeral 32 denotes a switch composed of a transistor and opened and closed alternately by the output of the comparator 34.

Next, the operation of FIG. 8 will be described with reference to FIGS. Now, it is assumed that the switch 31 is closed and the switch 32 is opened by the output of the comparator 34. At this time, the reference voltage 35 input to the comparator 34 is at the upper limit value VU. Then, the oscillation capacitor 2 is charged via the constant current source 27 (or from the internal power supply 36 via the resistor 28), and the voltage of the capacitor 2 increases.

When the voltage of the capacitor 2 reaches the upper limit value VU, the output of the comparator 34 is inverted, the switch 31 is opened, the switch 32 is closed, and the reference voltage 35 is switched to the lower limit value VL. Thereby, the capacitor 2 is discharged via the constant current source 29 (or via the resistor 30 to the ground potential), and the voltage of the capacitor 2 decreases. Thereafter, when the voltage of the capacitor 2 reaches the lower limit value VL, the output of the comparator 34 is again inverted, and the switch 31 is closed,
Is opened, and the reference voltage 35 is switched to the upper limit value VU.

By repeating such charging / discharging operations, a triangular wave oscillation waveform shown in FIG.
This oscillation wave, that is, the voltage between both ends of the oscillation capacitor 2 is sent to the PWM control unit 4 as the output of the oscillator 3. An oscillation period Tosc comprising a charge period Tc and a discharge period Tdc
, The switch element 8 repeats on / off. P
In the WM control unit 4, as shown in FIGS. 7 and b), the voltage of the deviation between the output voltage detection input 6 and the reference value of the output voltage of the switching power supply corresponding to the detection input 6 is set to 0. A PWM output 7 having a pulse width of a variable ON period Ton is generated and output.

In this case, the beginning of the ON period Ton is set at the start of the charge period Tc (in other words, at the end of the discharge period Tdc). The termination of the on-period Ton fluctuates due to the control of the output voltage of the switching power supply. In this device, the discharge period Tdc is always turned off by the switch element 8 (dead time) for the purpose of protection in the event of an abnormality. , The maximum ON period Ton max
Is set to be equal to the charging period Tc.

Therefore, the normal on-period Ton is such that the on-period Ton increases within the maximum on-period Ton max if the output voltage of the switching power supply tries to drop too much compared with the reference value, while If the output voltage of the power supply is going to rise too high compared to the reference value, the on-period Ton is controlled so as to decrease.

[0012]

As described above, FIG.
c) shows an example of the voltage across snubber capacitor 9 in the switching power supply device of FIG. That is, when the transformer 12 finishes releasing energy to the secondary side after the switch element 8 is turned off, LC resonance occurs due to the inductance of the transformer 12 and the snubber capacitor 9 and the voltage oscillates.

During this vibration, the switch element 8 is turned on (turned on). At this time, the snubber capacitor 9 releases all the stored energy according to the voltage between both ends as the switching loss of the switch element 8. The turn-on timing is determined by the oscillator 3 of the power supply control circuit 5 and is independent of the voltage oscillation. For this reason, it often turns on when the voltage of the snubber capacitor 9 is large. In this case, the loss of the switch element 8 becomes very large, and the presence of the snubber capacitor 9 is effective in suppressing the surge voltage and the noise. However, on the other hand, there is a problem that the heat generation of the switching element 8 is increased and the efficiency of the switching power supply device is reduced.

Therefore, the present invention minimizes the loss of the switch element 8 due to the snubber capacitor 9 by turning on the switch element 8 when the voltage always becomes the minimum during the voltage oscillation of the snubber capacitor 9 described above. It is an object of the present invention to provide a switching power supply device that can suppress the switching power supply.

[0015]

According to a first aspect of the present invention, there is provided a switching power supply device which is charged to a predetermined maximum voltage (maximum value VM) and then charged to a predetermined minimum voltage (lower limit value VL). ) And an inductive means consisting of a transformer (12) or a reactor (18) between the input DC power supply (input voltage 1) and the oscillation capacitor (2) which is repeatedly discharged, and in parallel with the snubber capacitor (9). And a semiconductor switch element (8) that is turned on / off such that a point in time when the voltage of the oscillation capacitor reaches the minimum voltage is set as an on point in each period of charging and discharging of the oscillation capacitor. A DC power supply of a predetermined voltage to be supplied to an external load (both ends of the smoothing capacitor 14) is generated by using energy released from the inductive means when the semiconductor switch element is turned off. As described above, in a switching power supply device in which a ratio between an ON period and an OFF period of the semiconductor switch element is controlled (by a PWM control unit 4), an auxiliary winding (16) is provided in the inductive means, A voltage generated in the auxiliary winding during an off period of the semiconductor switch element is applied to the oscillation capacitor via a diode (17) to charge the oscillation capacitor to the maximum voltage, and the oscillation capacitor is charged at least during the charging period. Is connected to a discharge power source consisting of a constant current source (29) or a constant voltage source (ground potential) having a predetermined resistor (30) in series, and the voltage of the oscillation capacitor is changed from the highest voltage to the lowest voltage via the discharge power source. Time until voltage falls (voltage fall time TF)
Is set to の of the period of the voltage oscillation generated by the inductive means and the snubber capacitor.

According to a second aspect of the present invention, in the switching power supply of the first aspect,
The auxiliary winding is also used to generate a power supply (such as the control circuit power supply 22) in the switching power supply device. The operation of the present invention is as follows.
That is, an auxiliary winding is provided in an inductive means such as a transformer 12 or a reactor in which current is interrupted by the switch element 8, and the oscillation capacitor is pulled up by a voltage generated when the switch element 8 is turned off.

The time during which the voltage of the oscillation capacitor falls from the maximum value after the pull-up charge to the lower limit value by the discharge power supply is set to 1 / cycle of the voltage oscillation (LC resonance) generated by the inductive means and the snubber capacitor 9. By setting the value to 2, the oscillation voltage of the snubber capacitor 9 is also minimized when the voltage of the oscillation capacitor reaches the lower limit value when the switch element 8 is turned on (turned on).

As a result, switching loss and heat generation due to the discharge of the snubber capacitor 9 when the switching element 8 is turned on are reduced, and the efficiency of the switching power supply is improved.

[0019]

FIG. 1 shows a circuit configuration of a main part of a switching power supply according to a first embodiment of the present invention. FIG. 1 corresponds to FIG. FIG. 3 is a principle circuit diagram of the oscillator 3 shown in FIG. 1, and FIG. 3 corresponds to FIG. In FIG. 1, an auxiliary winding 16 is added to the transformer 12 as compared with FIG. 6, and the voltage of the auxiliary winding 16 is
As shown in FIG. 3, it is added to the oscillation capacitor 2 via a diode 17. Note that the oscillator 3 in FIG.
Is the same as that of FIG.

FIG. 2 shows operation waveforms of each part in FIG. Next, the operation of the main parts of FIGS. 1 and 3 will be described with reference to FIG. The auxiliary winding 16 has a polarity that charges the oscillation capacitor 2 when the switch element 8 is turned off, in other words, FIG.
Are connected so that the positive polarity portion of the voltage shown in FIG. When the switch element 8 is turned off by applying the voltage of the auxiliary winding 16 to the oscillation capacitor 2, the voltage of the oscillation capacitor 2 as the output voltage of the oscillator 3 is supplied from the constant current source 27 (or the internal voltage via the resistor 28). From time t1, when the voltage on the auxiliary winding 16 exceeds the conventional rising curve due to charging (from the power supply 36), the voltage on the auxiliary winding 16 is further pulled up during the pull-up period Tpu shown in FIG. .

When the voltage of the oscillation capacitor 2 reaches the upper limit value VU of the reference voltage 35 input to the comparator 34 of FIG. 3 at time t2 during the rise, the output of the comparator 34 is inverted, and the switch of FIG. 31 is open, 32
Is closed, and a constant current source 29 (or a ground potential via a resistor 30) is connected to the capacitor 2 as a power source for discharging.

However, thereafter, as shown in FIG. 2 (c), the voltage of the capacitor 2 is pulled up by the auxiliary winding 16 during the period in which the transformer 12 is discharging energy to the secondary side until time t3. Does not fall. At this time, the maximum value VM (flat portion) of the voltage of the oscillation capacitor 2 is the voltage of the secondary winding 11 in a conductive state, that is, the voltage of the secondary smoothing capacitor 14, and therefore the external load from the switching power supply. It is proportional to the constant-voltage controlled output DC voltage supplied to 15 and is practically constant.

When the transformer 12 has finished emitting energy to the secondary side at time t3, the voltage of the snubber capacitor 9 starts oscillating. Then, the voltage of the auxiliary winding 16 also decreases due to vibration. Therefore, the auxiliary winding 16 is separated from the oscillation capacitor 2 by the diode 17, and the voltage of the capacitor 2 starts to decrease by the above-described discharge power supply.

As described above, when the voltage of the oscillation capacitor 2 reaches the lower limit value VL of the reference voltage 35 input to the comparator 34 at time t4, the comparator 3
The output of 4 is inverted again, and the power supply for charging described above is connected to the oscillation capacitor 2, and at the same time, the switch element 8 is turned on. In the present invention, the time t when the pull-up of the auxiliary winding 16 is released by adjusting the capacity of the oscillation capacitor 2 or the like.
3 to the time point t4 when the switching element 8 is turned on (in other words, the voltage falling time TF from when the voltage of the oscillation capacitor 2 falls from the maximum value VM to the lower limit value VL by the discharging power source) is equal to the voltage falling time TF of the snubber capacitor 9. It is set to coincide with a half cycle of the voltage oscillation due to the LC resonance.

Then, at time t4, the snubber capacitor 9
Is also minimized, and the switch element 8 is turned on at this point, so that the turn-on loss (switching loss) of the switch element 8 due to the discharge of the snubber capacitor 9 can be minimized. Further, the period of the voltage oscillation is determined by the inductance L and the capacitance C of the circuit such as the transformer 12 and the snubber capacitor 9 and does not depend on the input voltage or the load. Therefore, even if the input or the load changes, this effect does not change.

FIG. 4 shows a circuit configuration of a main part according to a second embodiment of the present invention. In the figure, the transformer 1 of FIG.
A switching power supply device is configured in a step-up chopper system using a reactor 18 instead of 2. The auxiliary winding 16 is wound around the reactor 18. As described above, the present invention can be applied to a switching power supply device that does not use a transformer.

FIG. 5 shows a circuit configuration of a main part according to a third embodiment of the present invention. In this example, the commercial power supply 19 is rectified through a diode bridge circuit 20 and smoothed through a capacitor 21 to obtain an input DC voltage of a switching power supply. In general, when the input DC voltage of the switching power supply is high as in the case of FIG.
Is generated using the auxiliary winding of the transformer 12. Therefore, in this embodiment, the auxiliary winding for generating the power supply 22 is also used as the auxiliary winding 16 used in the present invention.

That is, the power supply 22 of the power supply control circuit 5 is made by rectifying the voltage of the auxiliary winding 16 via the diode 23 and smoothing it via the capacitor 24. On the other hand, the voltage of the auxiliary winding 16 is applied to the oscillation capacitor 2 via the diode 17 and the resistor 25 for implementing the present invention. Note that a zener diode 26 is inserted in parallel with the oscillation capacitor 2.

The resistor 25 and the Zener diode 26
Is not necessary for this operation, but because the auxiliary winding 16 is also used for generating the power of the power supply control circuit 5 and for the present invention, the voltage applied to the oscillation capacitor 2 from the auxiliary winding 16 is 3 is used for protection so as not to exceed the rated voltage.

[0030]

According to the present invention, the point in time when the voltage of the oscillation capacitor reaches the minimum voltage is defined as the ON point in synchronization with the charging and discharging of the oscillation capacitor in which charging and discharging are repeated between a predetermined maximum voltage and a minimum voltage. A semiconductor switch element connected in series with an inductive means consisting of a transformer or a reactor and connected in parallel with a snubber capacitor between input DC power
In a switching power supply device which is turned on / off by M control and supplies a DC power of a predetermined voltage to an external load using energy released from the inductive means when the semiconductor switch element is turned off, an auxiliary winding is connected to the inductive means. The voltage generated in the auxiliary winding is applied to the oscillation capacitor via a diode during the off-period of the semiconductor switching element to charge the oscillation capacitor to the maximum voltage, and at least the constant current is supplied to the oscillation capacitor during the charging period. Source or a constant voltage source having a predetermined resistance connected in series, and the inductive means and the snubber capacitor determine the time until the voltage of the oscillation capacitor falls from the highest voltage to the lowest voltage via the discharge power source. 1 / of the period of the voltage oscillation caused by
2, the voltage of the snubber capacitor when the semiconductor switch element is turned on is minimized, the switching loss of the semiconductor switch element due to the discharge of the snubber capacitor is minimized, the heat generation of the semiconductor switch element is reduced, and the switching is performed. The efficiency of the power supply can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a main part according to a first embodiment of the present invention.

FIG. 2 is an operation waveform diagram of each part in FIG. 1;

FIG. 3 is a circuit diagram showing a principle configuration of an oscillator part of FIG. 1;

FIG. 4 is a circuit diagram showing a configuration of a main part according to a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing a configuration of a main part according to a third embodiment of the present invention.

FIG. 6 is a conventional circuit diagram corresponding to FIG.

FIG. 7 is an operation waveform diagram of each unit in FIG. 6;

FIG. 8 is a circuit diagram showing a basic configuration of an oscillator part of FIG. 6;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Input voltage 2 Oscillation capacitor 3 Oscillator 4 PWM control part 5 Power supply control circuit 6 Output voltage detection input 7 PWM output 8 Semiconductor switch element (Switch element) 9 Snubber capacitor 10 Primary winding 11 Secondary winding 12 Transformer 13 Diode 14 Smoothing capacitor 15 Load 16 Auxiliary winding 17 Diode 18 Reactor 19 Commercial power supply 20 Diode bridge circuit 21 Smoothing capacitor 22 Control circuit power supply 23 Diode 24 Smoothing capacitor 25 Resistance 26 Zener diode 27 Constant current source 28 Resistance 29 Constant current source 30 Resistance 31, 32 Switch 33 Inverter 34 Comparator 35 Reference voltage 36 Internal power supply VU Upper limit value of reference voltage VL Lower limit value of reference voltage VM Maximum value of oscillation capacitor voltage TF Falling time of oscillation capacitor voltage

Claims (2)

[Claims] 1. An oscillation capacitor which is repeatedly charged to a predetermined maximum voltage and then discharged to a predetermined minimum voltage, and a snubber capacitor in series with an inductive means comprising a transformer or a reactor between input DC power supplies. A semiconductor switch element that is connected in parallel, and is turned on / off such that a point in time when the voltage of the oscillation capacitor reaches the minimum voltage is set to an on point in each period of charging and discharging of the oscillation capacitor; The ratio between the on-period and the off-period of the semiconductor switch element so that a DC power of a predetermined voltage to be supplied to an external load is generated by using energy released from the inductive means when the semiconductor switch element is turned off. In the switching power supply device, an auxiliary winding is provided in the inductive means, and the auxiliary winding is provided during an off period of the semiconductor switch element. A voltage generated is applied to the oscillation capacitor via a diode to charge the oscillation capacitor to the maximum voltage, and at least during this charging period, a constant current source or a constant voltage source having a predetermined resistance in series with the oscillation capacitor. And the time required for the voltage of the oscillation capacitor to fall from the highest voltage to the lowest voltage via the discharge power source is set to の of the period of the voltage oscillation generated by the inductive means and the snubber capacitor. A switching power supply device characterized in that: 2. The switching power supply according to claim 1, wherein said auxiliary winding is also used to generate a power supply in said switching power supply.