JPH0570189U - Switching power supply - Google Patents

Abstract

(57) [Abstract] [Purpose] To prevent cross current, transformer saturation, and an increase in onregulatory voltage due to changes in ambient temperature. A dead time control circuit 15A for clamping the output voltage VB 'at a predetermined voltage level is provided in a feedback circuit for stabilizing the DC output voltage Vo. The dead time control circuit 15A has a diode D1 as an element whose impedance changes with temperature, and the voltage level of the output voltage VB 'changes with temperature. At this time, if the diode D1 is appropriately selected, the maximum on-time of the switching element 4 is substantially fixed regardless of the influence of the ambient temperature, and the trouble caused by the temperature change of the storage time T of the switching element 4 is eliminated. it can.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a switching power supply device including a dead time control circuit that limits the maximum duty for a switching element.

[0002]

[Prior Art]

 Generally, a DC input voltage is switched by a switching element and applied to the primary winding of a transformer, and the voltage induced in the secondary winding of this transformer is rectified and smoothed and output, while stabilizing the output voltage. A switching power supply device provided with a pulse width control circuit for controlling the pulse conduction width of a switching element as a feedback circuit is disclosed in, for example, JP-A-64-12859. In the feedback circuit of such a switching power supply device, in order to prevent destruction of the switching element due to the conduction width of the pulse being unnecessarily widened, the dead circuit for limiting the maximum duty to this switching element is provided. A time control circuit is provided.

FIG. 2 shows a forward type switching power supply device of this type. The commercial power source 1 is rectified by a rectifying circuit 2 such as a diode bridge, and the rectified DC input voltage Vi is smoothed by a smoothing capacitor 2. Then, the DC input voltage Vi is switched by the switching element 4 composed of an NPN type transistor and applied to the primary winding 6 of the transformer 5 which insulates the primary side from the secondary side. Then, the voltage induced in the secondary winding 7 of the transformer 5 is rectified and smoothed by the rectifier diode 8, the flywheel diode 9, the choke coil 10 and the smoothing capacitor 11, and a predetermined DC output voltage Vo is output terminal + V,- It is supplied between V.

As a feedback circuit for stabilizing the DC output voltage Vo, a series circuit of voltage dividing resistors 12 and 13 is connected between the output terminals + V and −V, and a DC output voltage Vo is connected. Is divided by the resistors 12 and 13 and applied as an output detection voltage to the inverting input terminal of the operational amplifier 14 for voltage feedback. Then, the output detection voltage is compared and amplified with a reference voltage connected to the non-inverting input terminal of the operational amplifier 14, and this comparatively amplified voltage is passed through the dead time control circuit 15 to the inverting input terminal of the PWM comparator 16. The switching element 4 is controlled to be turned on and off via the driver circuit 18 by a control signal VD having a pulse conduction width based on the comparison result compared with the triangular wave signal applied and output from the triangular wave oscillation circuit 17. Is composed of. A pulse width control circuit 19 has the PWM comparator 16, a triangular wave oscillating circuit 17, and a driver circuit 18.

The dead time control circuit 15 is inserted and connected between the output terminal of the operational amplifier 14 and the inverting input terminal of the PWM comparator 16, and is composed of an inverted L-shaped circuit composed of resistors R1 and R2. In order to avoid the output of the control signal VD that exceeds the maximum duty of the switching element 4, the maximum value of the voltage level output from the operational amplifier 14 is clamped when the input voltage Vi drops more than necessary. The switch element 4 is prevented from being broken. That is, if the operating voltage of the operational amplifier 14 is VA, at least the voltage level output from the operational amplifier 14 does not exceed Vo, so that the dead time control circuit 15 applied to the PWM comparator 16 outputs. The output voltage VB is clamped at a value of VB = VA × (R1 / (R1 + R2)), which limits the maximum duty of the switching element 4.

[0006]

[Problems to be solved by the device]

 In the above prior art, as shown in the waveform diagram of FIG. 3, when the control signal VD applied to the base falls as a characteristic of the switching element 4, the collector current Ic causes a certain delay time, that is, the storage time T. Moreover, it is known that the storage time T changes depending on the ambient temperature, and the storage time T becomes long when the ambient temperature is high and the storage time T becomes short when the ambient temperature is low. Therefore, when the ambient temperature rises, even though the maximum duty of the switching element 4 is limited by the dead time control circuit 15, the maximum on-time of the switching element 4 is further extended and the cross current and the transformer 5 are saturated. On the other hand, when the ambient temperature becomes low, the maximum on-time of the switching element 4 becomes short. Therefore, when the direct-current input voltage Vi is lowered, the limit angle voltage at which the direct-current output voltage Vo can be maintained rises. However, there is a problem in that the smoothing capacitor 3 having a large capacity has to cope with an increase in the voltage of the Anreggi.

Therefore, the present invention solves the above-mentioned problems and provides a switching power supply device capable of preventing the occurrence of cross current and saturation of the transformer at high temperature and the rise of the angle voltage at low temperature. The purpose is to

[0008]

[Means for Solving the Problems]

 This invention applies a DC input voltage to a primary winding of a transformer by switching it with a switching element, rectifies and smoothes the voltage induced in the secondary winding of the transformer, and stabilizes this output voltage. In a switching power supply device having a dead time control circuit for limiting the maximum duty for the switching element in a feedback circuit for controlling the switching element, the dead time control circuit has an element whose impedance changes with temperature. The element is configured to change the maximum duty with respect to the switching element depending on the temperature.

[0009]

[Action]

 With the above configuration, an element whose impedance changes with temperature is selected as appropriate, and the maximum duty of this switching element is turned on by changing the maximum duty for this switching element with temperature while taking into consideration the change in storage time of the switching element with temperature. The time is fixed regardless of the influence of ambient temperature.

[0010]

【Example】

 An embodiment of the present invention will be described below with reference to the accompanying drawings. The same parts as those in FIG. 2 are designated by the same reference numerals, and detailed description of the common parts will be omitted.

In FIG. 1, the dead time control circuit 15A has the same configuration as that of FIG. 2 except that a diode D1 is inserted between the resistor R2 and the ground as an element whose impedance changes with temperature. Is.

Next, the operation of the above configuration will be described. When the power is turned on, the AC power supply voltage from the commercial power supply 1 is rectified by the rectifier circuit 2, and the rectified DC input voltage Vi is smoothed by the smoothing capacitor 3. The DC input voltage Vi is switched by the switching element 4 and applied to the primary winding 6 of the transformer 5, and the voltage induced in the secondary winding 7 of the transformer 5 is rectified by a rectifying diode 8, a flywheel diode 9 and a choke coil. The DC output voltage Vo is supplied between the output terminals + V and -V by being rectified and smoothed by the 10 and the smoothing capacitor 11. The DC output voltage Vo is divided by the resistors 12 and 13 and is amplified as an output detection voltage by the operational amplifier 14 in comparison with the reference voltage. Then, this comparatively amplified voltage is compared with the triangular wave signal from the triangular wave oscillating circuit 17 in the PWM comparator 16 through the dead time control circuit 15, and by the control signal VD having the pulse conduction width based on the comparison result. The fact that the switching element 4 is controlled to be turned on and off is the same as the operation in the conventional example.

On the other hand, when the input voltage Vi drops more than necessary, the dead time control circuit 15A clamps the voltage level of the output voltage VB ′ applied to the PWM comparator 16. That is, assuming that the operating voltage of the operational amplifier 14 is VA and the voltage drop of the diode D1 is VF, the output voltage VB 'is clamped by the value of the following formula.

[0014]

[Equation 1]

In the above formula, the operating voltage VA of the resistors R1 and R2 and the operational amplifier 14 has little change in value with temperature, while the voltage drop VF of the diode D1 has a large change in temperature. Therefore, the output voltage VB 'changes with temperature depending on the voltage drop VF of the diode D1. Therefore, the voltage drop VF of the diode D1 is appropriately selected, and the maximum duty of the control signal VD applied to the switching element 4 is changed according to the temperature in consideration of the change of the storage time T of the switching element 4 depending on the temperature. As a result, the maximum on-time of the switching element 4 is substantially fixed regardless of the influence of the ambient temperature.

As described above, according to the above-described embodiment, the dead time control circuit 15A for limiting the maximum duty with respect to the switching element 4 is provided with the diode D1 whose voltage drop VF changes depending on the temperature. By changing the maximum duty for the switching element 4 by the temperature, the maximum on-time of the switching element 4 becomes almost constant regardless of the temperature change. Therefore, as in the conventional example, when the ambient temperature is high, the cross current or the transformer is changed. There is no possibility of saturation of No. 5 or increase of the angle voltage when the ambient temperature is low, and the trouble in the power supply device caused by the temperature change of the storage time T of the switching element 4 can be eliminated. It will be possible. Further, particularly when a transistor is used as the switching element 4, the effect of the storage time T is further increased because the change of the storage time T with temperature is larger than that of an FET or the like.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the gist of the present invention. For example, a diode is used as an element whose impedance changes with temperature in the embodiments, but a temperature sensitive element such as a thermistor may be used. Further, the switching element is not limited to the transistor, and a MOS type FET may be used. Further, the switching power supply device is not limited to the one-stone forward type, and can be applied to various types such as a bridge type and a push-pull type.

[0018]

According to the present invention, the DC input voltage is switched by the switching element and applied to the primary winding of the transformer, and the voltage induced in the secondary winding of the transformer is rectified and smoothed and output. In a switching power supply device having a feedback circuit for stabilizing the output voltage and a dead time control circuit for limiting the maximum duty for the switching element, the dead time control circuit is an element whose impedance changes with temperature. This element is configured to change the maximum duty with respect to the switching element depending on the temperature by this element, and prevents the occurrence of cross current and transformer saturation at high temperature and the rise of unregulated voltage at low temperature. It is possible to provide a switching power supply device capable of doing so.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing a conventional example.

FIG. 3 is a waveform diagram showing an operating state of a switching element.

[Explanation of symbols]

 4 Switching element 5 Transformer 8 Rectifying diode 11 Smoothing capacitor 15A Dead time control circuit D1 Diode (element)

Claims (1)

[Scope of utility model registration request] 1. A DC input voltage is switched by a switching element to be applied to a primary winding of a transformer, a voltage induced in a secondary winding of the transformer is rectified and smoothed and output, and the output voltage is stabilized. In a switching power supply device including a dead time control circuit for limiting a maximum duty for the switching element in a feedback circuit for converting the dead time control circuit to an output circuit, the dead time control circuit includes an element whose impedance changes with temperature. The switching power supply device is characterized in that the maximum duty for the switching element is changed according to the temperature.