JPS6142274A - Spike voltage absorbing circuit - Google Patents

Abstract

PURPOSE:To feed back energy by a spike voltage to a power source side without loss by connecting an absorbing capacitor to switch over the passage of a current between charging time and discharging time. CONSTITUTION:The connection of an absorbing capacitor 6 is switched by switching elements 16, 17. When a spike voltage is charged, the capacitor 6 is connected between the high voltage side terminal of the primary side DC power source and the collector terminal of a main transistor 1, and when discharged from the capacitor 6, the capacitor 6 is connected between the low voltage side terminal of the primary side DC power source and the high voltage side terminal of the primary side DC power source.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a spike voltage absorption circuit (snubber circuit) for protecting a switching transistor of a switching power supply from spike voltage.

(Conventional configuration and its problems) Traditionally, switching type stabilized DC power supplies, so-called switching power supplies, which have advantages such as high power efficiency and easy miniaturization, are rapidly becoming more popular in power supply devices. There is. However, in addition to the frequent switching noise problem, switching power supplies also have the problem of high voltage spikes appearing at the collector terminal of the switching transistor. In recent years, relatively high voltage transistors and FETs have been developed for switching, and efforts have been made to reduce spike voltage by increasing the degree of coupling between the primary and secondary sides of transformers. ,
After all, it is impossible to prevent the transistor from being destroyed without some kind of protection circuit. That is, if the maximum conduction time ratio of a switching transistor is 1/2, the maximum value of the DC voltage applied to its collector terminal is ideally twice the power supply voltage. However, since a spike voltage actually occurs, the collector voltage will be 3 to 5 times the power supply voltage if no protection circuit is inserted.

Figure 1 shows the configuration of a conventional spike voltage absorption circuit, where 1 is the main transistor for switching, 2 is the base drive circuit of the main transistor 1, and 3 is the primary side DC power supply (hereinafter referred to as DC). A transformer connected to a power source (not shown) and whose secondary side is connected to a rectifier circuit (not shown),
4 is a spike voltage absorption circuit consisting of a diode 5, an absorption capacitor 6, and a resistor 7, and the spike voltage input to this spike voltage absorption circuit 4 is charged and absorbed by the absorption capacitor 6 via the diode 5. Above, it is discharged by the resistor 7.

In the conventional example configured in this way, the primary side DC power supply voltage (hereinafter referred to as power supply voltage) is 140V (the value when AC 100V is double-wave rectified), and the conduction time ratio of the main transistor 1 is set to 1/3. For example, when the main transistor 1 is in the cut-off state, the stabilized collector voltage is 210V, and the diode 5
The cathode voltage of the diode 5 rises by at least 30V from the collector voltage due to the spike waveform (the spike voltage does not disappear completely), and reaches about 240V. 10
Approximately 0V is constantly applied. This relationship is illustrated in FIG. 2.

Now, if the resistance value of the resistor 7 is set to 5Ω, the power loss WL becomes an extremely large value of WL = 1002/5000 = 2 (W), which hinders the improvement of the power efficiency of the switching power supply and makes it difficult to improve the heat dissipation design. Make it difficult and resist 7
As the allowable power loss increases, the shape of the resistor 7 becomes larger, which impedes the miniaturization of the device.Furthermore, if the resistance value of the resistor 7 is increased to lower the allowable power loss, the effect of absorbing the spike voltage increases. It also had the disadvantage of lowering its performance.

(Objective of the Invention) An object of the present invention is to provide a spike voltage absorption circuit that absorbs spike voltages without causing allowable power loss in resistors, has good power efficiency, and generates little heat.

(Structure of the Invention) According to the present invention, when charging a spike voltage, an absorption capacitor is connected between the high potential side terminal of the primary DC power supply and the collector terminal of the main transistor, and when discharging from the absorption capacitor, By switching the connection of the absorption capacitor using a switching element so that the absorption capacitor is connected between the low potential side terminal of the primary side DC power supply and the high potential side terminal of the primary side DC power supply, the spike voltage can be reduced. The energy obtained by absorbing the energy is returned to the DC power supply without loss.

(Explanation of Embodiment) FIG. 3 shows the principle of one embodiment of the present invention.
The same reference numerals as those in the figure indicate the same parts, 8 is a commercial AC power supply, 9 is a rectifier diode 10.
11. A rectifier circuit consisting of 12 and 13, and 14 a smoothing capacitor. The AC power supply 8, the rectification circuit 9, and the smoothing capacitor 14 constitute a primary DC power supply (hereinafter referred to as DC power supply). The AC voltage output from 8 is
After being rectified in the rectifier circuit 9, the smoothing capacitor 1
4 to become the primary side DC power supply voltage (hereinafter referred to as power supply voltage). 15 is a spike voltage absorption circuit consisting of a diode 5, an absorption capacitor 6, and switching elements 16 and 17; this spike voltage absorption circuit 15 is connected to the collector terminal of the main transistor 1; When the movable contact of is in the position shown in FIG. When the movable contacts 17 and 17 are on the opposite side from the position shown in FIG. 3, the negative side of the DC power source becomes a low potential, and the positive side becomes a high potential, and the absorption capacitor 6 is placed in a discharged state.

In this embodiment configured in this way, the charging/discharging timing of the absorption capacitor 6, that is, the switching element 16
The switching timing of main transistor 1 and 17 is
The switching timing is synchronized with the switching timing, and when the main transistor 1 is cut off and generates a spike voltage, it is a charging period for the absorption capacitor 6, and when the voltage at the collector terminal of the main transistor 1 decreases and it is turned on, it is a discharging period. Become.

That is, when the main transistor 1 is conducting, one terminal of the absorption capacitor 6 is connected to the positive side of the DC power supply.
Since the other terminals of the absorption capacitor 6 are connected to the negative side of the DC power supply, the potential difference between the terminals of the absorption capacitor 6 is naturally equal to the power supply voltage. At the moment when the main transistor 1 is cut off, the absorption capacitor 6 is switched to a charging state, and the high potential side terminal of the absorption capacitor 6 is connected to the collector of the main transistor 1, and the low potential of the absorption capacitor 6 is connected to the collector of the main transistor 1. Since the side terminal is connected to the positive side of the DC power supply, the voltage at the high potential side terminal of the absorption capacitor 6 is further increased by the power supply voltage, and becomes twice the power supply voltage. At this time, the main transistor 1 tries to generate a large spike voltage, but when the voltage at the collector terminal of the main transistor 1 rises to twice the power supply voltage, a charging current flows to the absorption capacitor 6, causing a spike. The increase in voltage can be kept small. Therefore, after the potential difference between both terminals of the absorption capacitor 6 rises to a value slightly higher than the power supply voltage, when the charging current is stopped, the potential difference is maintained at that value. Next, when the main transistor 1 becomes conductive again, the absorption capacitor 6 switches to a discharge state and is discharged until the potential difference between both terminals of the absorption capacitor 6 returns to the power supply voltage, and this discharge current is transferred to the smoothing capacitor 14. Flow into. This means that the energy of the blockage and spike voltage is fed back to the DC power supply without loss. In this case, the capacitance of the smoothing capacitor 14 is sufficiently larger than that of the absorption capacitor 6 (usually 10oo times or more).
, the influence on the type difference between both terminals of the smoothing capacitor 14 is negligible. By the above second operation, the maximum voltage at the collector terminal of the main transistor 1 can be suppressed to a value slightly higher than twice the power supply voltage. For example, if the power supply voltage is 1.40V, the maximum voltage at the collector terminal is 140×2+α, and if we substitute the 30V used in the explanation of the conventional example in Figure 1 as the value of α, the maximum voltage becomes 310v. Therefore, this relationship is illustrated in FIG. 4. However, as can be seen from FIG. 4, the present invention cannot be used when the steady-state value of the collector voltage when the main transistor 1 is cut off is more than twice the power supply voltage. In other words, this means that the maximum conduction time ratio of the main transistor 1 needs to be 172. This is because, if the conduction time ratio is expressed as r, the steady-state value of the collector voltage mentioned above will be 1/(1-r) times the power supply voltage. By the way, in a normal switching power supply device, in principle, the conduction time ratio of the switching transistor is at most 1/2, and in most products, the maximum conduction time ratio is set to about 40%, with some margin.

Therefore, it can be seen that the above condition of 1/2 or less does not practically narrow the scope of application of the present invention.

FIG. 5 shows a specific example of one embodiment of the present invention, and the same reference numerals as those in FIG. 3 indicate the same parts.
Further, 18 is a bias conversion circuit 1 that outputs a bias voltage obtained by shifting the control signal of the main transistor 1 as a signal.
9 and a discharge transistor 2o functioning as a switching element, the collector of which is connected between the diode 5 and the absorption capacitor 6, the emitter connected to the positive side of the DC power supply, and the base connected to the bias conversion circuit 19. , a spike voltage absorption circuit consisting of a diode 21 connected between the absorption capacitor 6 and the positive side of the DC power supply, and a diode 22 connected between the absorption capacitor 6 and the negative side of the DC power supply. 1 and the discharging transistor 20 operate to conduct or cut off simultaneously.

In this specific example configured in this way, the main transistor 1
is cut off, the high potential side terminal of the absorption capacitor 6 is connected to the collector of the main transistor 1 via the diode 5, and the low potential side terminal of the absorption capacitor 6 is connected via the diode 21 to the collector of the main transistor 1. Connected to the positive side of the DC power source, the absorption capacitor 6 is placed in a charged state, and the absorption capacitor 6 is charged with a spike voltage.

When the main transistor 1 is conductive, one terminal of the absorption capacitor 6 is connected to the positive side of the DC power supply via the discharge transistor 20, and the other terminal of the absorption capacitor 6 is connected to the diode 22. is connected to the negative side of the DC power supply through the absorption capacitor 6.
enters a discharge state and is discharged until the potential difference between both terminals of the absorption capacitor 6 returns to the power supply voltage, and this discharge current flows into the smoothing capacitor 14. However, in this spike voltage absorption circuit 18, some loss occurs due to the forward voltage drop of the diodes 5, 21, 22 and the saturation voltage between the collector and emitter of the discharge transistor 20.
This is extremely small compared to the essential loss caused by conventional methods. Further, the discharge transistor 20 has a relatively small capacity 1
(0.01 to 0.1 pF) since the absorption capacitor 6 is simply discharged, a capacitor with a small allowable collector loss may be used.

(Effects of the Invention) As explained above, according to the present invention, the spike voltage absorption circuit is configured with an absorption capacitor and a switch, and the spike voltage absorption circuit is configured such that the current path is switched between charging and discharging. By connecting the power supply capacitor, the energy due to the spike voltage can be fed back to the power supply without causing loss, and there is an advantage that the limit voltage of the spike waveform can be set to approximately twice the power supply voltage. Furthermore, since the loss is reduced, there are advantages in that the switching power supply itself can be made smaller and the heat dissipation design can be simplified.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of a conventional spike voltage absorption circuit, Fig. 2 is an output waveform diagram of each part of the conventional spike voltage absorption circuit, Fig. 3 is a principle diagram of an embodiment of the present invention, and Fig. 4 is a diagram of the present invention. FIG. 5 is a diagram showing the output waveforms of each part of an embodiment of the invention. FIG. 5 is a configuration diagram of a specific example of an embodiment of the invention. 1... Main transistor, 5, 2], 22K diode, 6... Absorption capacitor, 16, 17...
・Switching element (20...discharge transistor)
. Patent applicant Matsushita Electric Industrial Co., Ltd. Figure 1 Figure 2 Figure 3/15 Figure 4

Claims (4)

[Claims] (1) In a spike voltage absorption circuit that protects the main transistor by absorbing the spike voltage appearing at the collector of the main transistor that switches the primary side DC power supply voltage of the switching power supply device, the spike voltage generated at the collector terminal of the main transistor An absorption capacitor that charges and absorbs a spike voltage is provided, and when charging the spike voltage, the absorption capacitor is connected between a high potential side terminal of the primary side DC power supply and a collector terminal of the main transistor, When discharging from the absorption capacitor, the absorption capacitor is connected between the low potential side terminal of the primary side DC power supply and the high potential side terminal of the primary side DC power supply. A spike voltage absorption circuit characterized by switching the connection of a capacitor using a switching element. (2) Claim (1) characterized in that the conduction time ratio of the main transistor is set to 1/2 or less.
Spike voltage absorption circuit as described in section. (3) The absorption capacitor includes a first diode whose anode terminal is connected to the collector terminal of the main transistor and whose cathode terminal is connected to one terminal of the absorption capacitor, and a first diode whose anode terminal is connected to the collector terminal of the main transistor. a second diode connected to the other terminal, the cathode terminal of which is connected to the high potential side terminal of the primary DC power source, and the anode terminal connected to the low potential side terminal of the primary DC power source; a third diode whose cathode terminal is connected to a connection point between the absorption capacitor and the second diode;
Charging and discharging are controlled by a circuit configured with the switching element installed between a connection point between the first diode and the absorption capacitor and a terminal on the high potential side of the primary power supply voltage. A spike voltage absorption circuit according to claim (1). (4) The spike voltage absorption circuit according to claim (2), wherein the switching element is controlled by a control signal of the main transistor.