JPS631592Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a push-pull converter circuit, and particularly relates to protection of a first-stage transistor.

There is a converter that has a transformer winding in the load circuit of the switching element, and induces a DC voltage on the secondary side by intermittent or commutating the current flowing through the transformer.There is a converter in which the transistor is formed into a push-pull type based on the switching action. . Conventionally, there is a converter shown in the circuit diagram of FIG. 1 as such a converter. This will be explained below with reference to the figures.

First, in the figure, 1 is a transformer, 1a and 1a' are primary coils, 1b is a secondary coil, 2a is an input terminal of this circuit, and 2b is an output terminal. Next, 3a and 4a are both front-stage transistors of the Darlington circuit, the collectors of which are connected to the first terminal of the primary coil 1a of the transformer, and
The base is connected to the first terminal of another primary coil 1a' of the transformer, and the emitter is grounded. On the other hand, the subsequent NPN transistors 3b and 4b forming a Darlington circuit together with the Darlington-connected NPN transistors 3a and 4a have their respective collectors connected to the first terminal of the primary coil 1a of the transformer together with the collector of the preceding transistor. , the bases of each transistor are connected to the emitters of the adjacent preceding transistors, and the emitters are grounded. Then, an input voltage E _IN is applied between the neutral terminal of the primary coil 1a of the transformer and the neutral terminal of the other primary coil 1a' (grounded and connected to the emitter of the subsequent transistor). is applied, and the secondary coil 1b of the transformer generates an output voltage E _OUT .

The operation of the push-pull converter circuit described above is as follows.

Now, suppose that when the input voltage E _IN is applied, the 4a and 4b sides of the Darlington-connected transistors are turned on. Eventually, magnetic saturation of the transformer occurs due to the increase in magnetic flux, and the induced voltage in the primary winding connected to the base of Darlington transistor 4a becomes zero, turning transistors 4a and 4b into an OFF state.

Conversely, when the magnetic flux begins to decrease, the polarity of the induced voltage is reversed, and the transistors 3a on the opposite side,
The 3b side will be turned on.

These operations are repeated, resulting in so-called self-excited oscillation.

FIG. 3 shows the collector current waveforms of the Darlington-connected transistors 3a, 3b and 4a, 4b in this oscillation operation. This is based on actual measurements.

As is clear from the figure, transistors 3a and 3b
Although the sides 4a and 4b are turned on alternately, it can be seen that a rush current Ic, a so-called spike current, flows at the moment each turns off.

Accordingly, it can be seen that a negative spike current flows through the transistor on the other side. Although the spike current was confirmed through actual measurements,
This is because at the moment when a transistor that has been on turns off, the charge accumulated in the base region (P conductivity type region) of the transistor element when it is on is instantly released, and this flows as a spike current.For example, transistors 4a and 4b When the side changes from on to off, as shown by the dotted line in FIG.
This is thought to be because the water circulates through the water. This circulating current is a reverse mode current for transistors 3a and 3b.

Furthermore, in the Darlington transistor connection, the front stage transistors 3a and 4a are of smaller power capacity than the rear stage transistors 3b and 4b, but a situation may occur where the transistor at the front stage of the Darlington transistor is destroyed when the current is in the reverse mode as described above. It was becoming a problem.

It is considered that the breakdown of the transistor in the preceding stage of the Darlington transistor in the reverse mode also indicates that a spike current flows through the transistor in the preceding stage of the Darlington transistor.

This invention provides a converter circuit that improves the above-mentioned conventional drawbacks that lead to reverse mode destruction.

The push-pull converter circuit of this invention has circuits made of Darlington-connected transistors arranged in parallel, the collectors and bases of each column are connected to respective power supplies of opposite phase, and the emitters are commonly grounded. A diode is inserted in the collector circuit in the forward direction with respect to the collector to reduce reverse mode breakdown of the first stage transistor.

This invention will be explained in detail below with reference to the drawings for one embodiment. The basic parts of the converter circuit are the same as before, so only the improved parts will be explained. Figure 2 shows a push-pull converter circuit according to one embodiment, in which 1 is a transformer.
a, 1a' are the primary coils, 1b is the secondary coil,
2a is an input side terminal of this circuit, and 2b is an output side terminal. Next, 13a and 14a are both front-stage transistors constituting the Darlington circuit, and their collectors are connected to the first terminal of the primary coil 1a of the transformer through forward diodes 23 and 24, respectively. The base is connected to the first terminal of the other primary coil 1a' of the transformer, and the emitter is grounded. On the other hand, the Darlington-connected NPN transistors 13a and 14a
together with the subsequent stage forming the Darlington circuit.
The NPN transistors 13b and 14b each have their collectors connected to the first and second terminals of the primary coil 1a of the transformer, their bases connected to the emitters of the adjacent preceding transistors, and their emitters grounded. . An input voltage is applied between the neutral terminal of the primary coil 1a of the transformer and the neutral terminal of the other primary coil 1a' (grounded and connected to the emitter of the subsequent transistor).
E _IN is applied, and the secondary coil 1b of the transformer generates an output voltage E _OUT .

According to the above circuit, the diodes 23 and 24 are connected with a polarity that blocks the reverse mode current of the front stage transistors 13a and 14a, so that the rush current generated when switching the magnetic flux of the coil in the oscillation transformer is caused by the push-pull front stage transistors. Since the rush current does not flow through the base-collector of the previous stage transistor, there is a significant advantage that reverse mode damage caused by the rush current flowing to the base-collector of the previous stage transistor can be completely prevented. This idea also has the advantage of being extremely easy to implement. Furthermore, if a diode is inserted between the emitter of the front-stage transistor and the base of the rear-stage transistor, the base input voltage for driving the Darlington-connected transistor must be increased by the forward voltage V _F of the diode. In the configuration of the present application, it is possible to prevent rush current without requiring such an increase.

Note that this invention is not limited to the embodiments, but can also be applied to circuits using PNP transistors, FETs, etc.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional push-pull converter circuit, Figure 2 is a circuit diagram of an embodiment of this invention,
FIG. 3 is a waveform diagram of the positive and reverse mode transistors in the converter circuit. 1 ...transformer, 1a, 1a'...transformer 1
Secondary coil, 1b...Secondary coil of the transformer,
13a, 14a...transistors at the front stage of the Darlington circuit, 13b, 14b...transistors at the rear stage of the Darlington circuit, 23, 24...diodes.

Claims (1)

[Scope of utility model registration request] The emitters of the rear-stage transistors of each of the first and second circuits having Darlington-connected transistors are commonly connected, and the primary winding of a transformer is connected between the collectors of the rear-stage transistors. An output voltage is obtained from the secondary winding of the transformer by supplying voltages of opposite phases to the bases of the front-stage transistors, and supplying an input voltage between the neutral point of the primary winding and the common connection of the emitter. 1. A push-pull converter circuit, characterized in that a forward diode with respect to the collector is inserted in the collector circuit of the preceding stage transistor.