JP2912978B2 - Drive circuit for switching power supply - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a drive circuit used for a flyback type switching power supply.

[Conventional technology]

FIG. 9 is a circuit diagram showing a conventional switching power supply drive circuit of the flyback type, and FIG.
FIG. 4 is an operation waveform diagram of the circuit in FIG.

Between the drive power source + E ₁ ~-E _1, the drive transistor 1 and the drive transformer 2 is inserted in series, the resistance between the secondary winding of the drive transformer 2 3 and the resistor 4
Are connected in series. The connection point of the resistor is connected to the base of the main transistor 5. The emitter of the main transistor 5 is connected to the main power supply -E _2, the main power source -E ₂
Is connected to the secondary winding output of the drive transformer 2 on the resistor 3 side. The collector of the main transistor 5 is connected to the mains + E ₂ through the primary winding of the main transformer 6. The secondary winding of the main transformer 6 has a diode 7 for rectification.
Is connected to the capacitor 8 for smoothing.

In the above configuration, when a pulse voltage is applied to the base of the drive transistor 1 while the main power supplies + E ₂ and −E ₂ and the drive power supplies + E ₁ and −E ₁ are applied, the drive transistor 1 is turned on. A current corresponding to the ON operation flows to the primary side of the drive transformer 2, and a voltage is induced on the secondary side accordingly. By this induced voltage, the resistance 4 → the base of the main transistor 5 → the emitter → the drive transformer 2
The current flows through the path. Base current I _B and the collector current I _C of the main transistor 5 at this time is represented by the following formula.

E ₁ : Power supply voltage of drive circuit n ₁ : Primary winding of drive transformer ₂ n ₂ : Secondary winding of drive transformer 2 V _BE : Base-emitter voltage of main transistor 5 R _B : Resistance of resistor 4 Value E ₂ : Main power supply voltage L _P6 : Primary inductance of main transformer 6 L _P2 : Primary inductance of drive transformer 2 t _on : ON time of drive transistor 1 t _stg : Storage time of main transistor 5 V _out : Drive transformer In the above configuration, the main transistor 5 has a base current
It flows is I _B, the same period in synchronization therewith (t _on) only the main transistor 5 is turned on, the collector current I _C of the main transistor 5 in synchronization with the start of the on increases gradually. Base current I _B is turned off when the generated counter electromotive force of the drive transformer 2 simultaneously, whereby a reverse current -I _B occurs. The collector current I _C becomes zero in a certain time after the base current I _B is lost (after the lapse of t _stq after t _on completion).

FIG. 11 is a circuit diagram showing another example of a conventional switching power supply drive circuit, and FIG. 12 is an operation waveform diagram of the circuit of FIG. Note that in FIG. 11, the same reference numerals are used for the same components as those in FIG. 9, and therefore, duplicate description will be omitted below.

Between main power supply E _2, base-collector and emitter of the transistor 9, 10 commonly connected complementary emitter follower configuration is connected through a resistor 11 and a resistor 12. A pulse signal is applied to the bases of the transistors 9 and 10 via a capacitor 13.

In this configuration, when a pulse signal is applied, the transistor 9 or the transistor 10 is turned on according to the polarity. When the transistor 9 is on, the main transistor 5
Is turned on, and the current I _C starts to flow through the main transformer 6.

At this time, the base current I _B , the collector current I _C, and the reverse current −I _B are as follows.

E ₁ : Power supply voltage of drive circuit V _BE : Base-emitter voltage of main transistor 5 R _B1 : Resistance value of resistance 11 R _B2 : Resistance value of resistance 12 E ₂ : Main power supply voltage L _P6 : Main transformer voltage 6 primary inductance t _on: the drive transistor 1 on-time t _stg: storage time V _out of _the main transistor 5: output voltage of the drive transformer 2 in this configuration, as shown in FIG. 11, corresponding to the energization start of I _B the collector current I _C is gradually increased from 0 Te, 0 by -I _B
Is returned to. Is similar also FIG. 9, the base current I _B is
As is clear from the equations (1) and (4), the resistance 4 and the resistance
The current waveform is limited by 11 and does not change with time.

[Problems to be Solved] However, in the conventional switching power supply drive circuit described above, since the base current I _B is the flow I _B corresponding to the I _Cmax relative incremental change in the collector current I _C becomes a binary change rather than I _C / h _FE, flow extra base current I _B, it has been increasing the drive loss.

In the configuration of Figure 9, it determines the base current by the primary winding n ₁ and the secondary winding n ₂ of the ratio of the drive transformers 2 and resistor R _B, reverse current -I _B drive transformer 2 Is determined by the primary inductance L _P2 and the base resistance,
Hardly decided to balance the base current I _B and the reverse current, the storage time t _stg is prolonged, it is impossible to rapidly perform the OFF operation of the main transistor 5. In addition, a base resistance loss occurs.

Similarly, in the configuration of FIG.
It takes a long time for the transistor 10 to turn on and the transistor 10 to turn on, so that the storage time t _{stg also} becomes long, and the main transistor 5 cannot be turned off steeply. In addition, a base resistance loss occurs. For this reason, the high-speed operation is restricted, and the power supply efficiency is deteriorated.

An object of the present invention is to provide a drive circuit for a switching power supply that can achieve low loss and high speed.

[Means for solving the problem]

In order to achieve the above object, the present invention provides a main transistor connected between a power supply through a primary winding of a main transformer having a secondary winding serving as an output, and a control signal applied to a base of the main transistor in response to a control signal. A drive circuit for a switching power supply having a drive transistor for periodically supplying a base current to the drive transistor, comprising a primary winding and a secondary winding, and an inductor in which the primary winding is connected in series to the drive transistor. And control means for applying flyback energy generated when the base current is cut off to the secondary winding of the inductor as an inverse base current to the main transistor.

[Action]

According to the above-described means, since the drive transistor is connected in series to the primary winding of the inductor having the primary winding and the secondary winding, the current amplification factor of the drive transistor is inversely proportional to the inductor, and Since the transistor operates in proportion to the on-time of the transistor, a base current waveform similar to the collector current waveform of the main transistor can be generated, so that no extra base current flows and there is no base resistance loss. In addition, since the flyback energy generated in the secondary winding of the inductor is supplied to the main transistor as the reverse base current, it is possible to steeply cut off the collector current of the main transistor, reduce loss, Switching speed can be improved. Further, the drive energy can be used for turning off the main transistor, and the drive loss can be reduced. The drive energy is determined by E ₁ + and the primary inductance L _P , so that the design becomes easy.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply drive circuit according to the present invention. Also, FIG.
FIG. 4 is a waveform chart showing the operation of the embodiment shown in FIG. In FIG. 1, the same components as those in FIG. 9 are denoted by the same reference numerals, and a duplicate description will be omitted below.

In this embodiment, using the drive transistor 14 of NPN type driving transistor, connect the emitter to drive power source + E _1, the base pulse signal is applied via a diode 15 and a capacitor 16. Between the base and the drive power source -E ₁ of the drive transistor 14, the resistor 17 for bias supply is connected.

Further, one end of each of a primary winding 18a and a secondary winding 18b of a drive transformer 18 (inductor) is connected to the base of the transistor 5, and the other end of the primary winding 18a is connected to a collector of the drive transistor 14. ing. Also, the secondary winding
The other end of 18b are connected to the drive power source -E ₁ via the diode 19 and the diode 20.

In the above configuration, when no pulse signal is applied, a bias is applied to the base of the drive transistor 14 via the resistor 17, so that the drive transistor 14
Turns on. When the drive transistor 14 is turned on, the current of the drive power source + E ₁ is, the collector of the emitter → the drive transistor 14 of the drive transistor 14 →
Flows through the route of the drive transformer 18 primary winding 18a → the main transistor 5 based → main transistor 5 emitters → drive power source -E _1, increases linearly with time.

In this state, when a pulse signal is applied, a positive voltage rectified by the diode 15 is applied to the drive transistor 14, and the drive transistor 14 is turned off. When the drive transistor 14 is turned off, the current flowing through the primary winding 18a of the drive transformer 18 is abruptly becomes zero, the base current I _B is cut off.

At this point, the drive transformer 18 stores the energy of the following equation (10). This energy is released from the secondary winding 18b via the diodes 19 and 20 simultaneously with the interruption of the primary current, giving a negative current to the base of the main transistor 5 and turning off the main transistor 5 sharply.

By the above operation, a base current similar to the form of the collector current of the main transistor 5 can be obtained, an unnecessary base current does not flow, and the loss of the drive circuit can be reduced.

Further, as a result of the main transistor 5 being able to be turned off sharply, the fall time of the collector current (fall time)
And the switching loss of the main transistor 5 can be reduced.

The base current I _B and the collector current I _C in the embodiment of FIG. 1 is expressed by the following equation.

E ₁ : Power supply voltage of drive circuit V _BE : Base-emitter voltage of main transistor 5 E ₂ : Main power supply voltage L _P18 : Primary inductance of primary transformer t _on : On time of drive transistor 14 t _stg : Main transistor 5 storage time V _out: output voltage type drive transformer 18 (8) as is clear from ~ (10), the base current of the main transistor 5 when on the drive transistor 14 I _B
And the collector current I _C changes with time (t _on or t _on
+ T _stg dependent) That now can show the base current I _B of increasing characteristic as shown in FIG. 2, so that it is not necessary to shed excess base current.

Moreover, as it is clear from equation (7) and (9), the base current I _B in the main transistor 5, -I _B is possible both to increase, as a result, can be steep off of the main transistor 5 become.

FIG. 3 is a circuit diagram showing a second embodiment of the switching power supply drive circuit according to the present invention. FIG. 4 is a waveform chart showing the operation of the embodiment of FIG. In the present embodiment, the same reference numerals are used for the same components as those in FIG. 1, and duplicate description will be omitted below.

In the present embodiment, a PNP transistor is used for driving in the embodiment of FIG. 1, but an NPN drive transistor 21 is used, which is connected to the primary winding 18a of the drive transformer 18 and the main transistor. 5 between the bases. According to this configuration, since a pulse signal can be directly input to the base of the drive transistor 21, the diode 15, the capacitor 16, and the resistor 17 required in FIG. 1 can be eliminated.

The operation and effects of this embodiment are the same as those of the above-described embodiment, and a description thereof will be omitted. FIG. 5 is a circuit diagram showing a third embodiment of the switching power supply drive circuit according to the present invention. FIG. 8 is a waveform chart showing the operation of the embodiment of FIG. In the present embodiment, the same reference numerals are used for the same components as those in FIG. 1, and duplicate description will be omitted below.

This embodiment is different from the embodiment shown in FIG.
And a bias circuit 22 connected in parallel. As shown in FIG. 6 or 7, the bias circuit 22 has a configuration in which a resistor and a diode are connected in series, or a capacitor is connected in series with the two.

By providing the bias current circuit 22, 1) Since the current amplification factor h _FE of the main transistor is a function of the current, the increase of the base current in the process of increasing the collector current I _C directly corresponds to h _FE. It needs to be corrected.

2) FIG. 6 shows an initial current setting circuit, and FIG. 7 shows a circuit for injecting a necessary amount of electric charge when the main transistor is turned on.

As described above, according to the above embodiment, by inserting a member having an inductance (drive transformer) in series with the drive transistor and controlling the main transistor by its output, the collector current I _C because of it is possible to obtain a base current I _B waveform shape corresponding to the waveform, it is not flowing extra base current, it is possible to reduce the loss.

Also, a secondary winding is provided on the drive transformer to
By causing reverse current (-I _B) to this after the disappearance of I _B, it is possible to sharply off operation of the main transistor 5. As a result, it is possible to reduce the loss and increase the switching speed.

〔The invention's effect〕

According to the present invention configured as described above, it is possible to generate a base current waveform similar to the collector current waveform of the main transistor, no extra base current flows, and the collector current of the main transistor is reduced. Can be sharpened, thereby reducing the loss,
Switching speed can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply drive circuit according to the present invention, FIG. 2 is a waveform diagram showing the operation of the embodiment of FIG. 1, and FIG. 3 is a switching power supply drive circuit according to the present invention. FIG. 4 is a waveform diagram showing the operation of the embodiment of FIG. 3, FIG. 5 is a circuit diagram showing a third embodiment of the switching power supply drive circuit according to the present invention, 6 and 7 are circuit diagrams showing details of the bias circuit shown in FIG. 5, FIG. 8 is a waveform diagram showing the operation of the embodiment of FIG. 5, and FIG. 9 is a conventional switching power supply of the flyback type. FIG. 10 is an operation waveform diagram of the circuit of FIG. 9, and FIG. 11 is a circuit diagram showing another example of a conventional switching power supply drive circuit.
FIG. 12 is an operation waveform diagram of the circuit of FIG. 3,17… Resistance, 5… Main transistor, 6… Main transformer, 7,15,19,20… Diode, 8… Capacitor, 1
4,21 …… Drive transistor, 16 …… Capacitor, 18
…… Drive transformer, 18a …… Primary winding, 18b …… Secondary winding, 22 …… Drive transformer.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Masae Sugawara Inventor Masamichi Sugawara, Shibata-cho, Miyagi Prefecture, 3rd name, Shinmei-do, Tomei Ricoh Co., Ltd. (72) Inventor Tsuneo Ikegami Ichibancho, Aoba-ku, Sendai City, Miyagi Prefecture 1-14-6 (56) References JP-A-63-73873 (JP, A) JP-A-62-78088 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) H02M 3 / 00-3/44

Claims (1)

(57) [Claims] 1. A main transistor connected between power supplies via a primary winding of a main transformer having a secondary winding serving as an output;
A drive circuit for a switching power supply having a drive transistor for periodically supplying a base current to a base of the main transistor in accordance with a control signal, comprising a primary winding and a secondary winding, wherein the drive transistor An inductor having a winding connected in series, and control means for applying flyback energy generated when the base current is cut off to the secondary winding of the inductor to the main transistor as a reverse base current. Characteristic drive circuit for switching power supply.