JPS6040218B2 - switching circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improving the switching speed of a switching circuit using a transistor as a switching element.

Rapid turn-off of a switching transistor in a saturated "on" state is achieved by rapidly releasing the storage carrier injected into the base. Conventionally, there are various switching circuits constructed based on this idea for the purpose of increasing the switching speed, and the circuit diagram shown in FIG. 1 is one example thereof.

In Figure 1, QI is a switching transistor, Q
2 is a control transistor, TI is a drive transformer, RI
The base current limiting resistor RL of the switching transistor Q1 is a load, and 1 and 2 are DC voltages Vc, respectively.
c, a power supply terminal to which Vc is supplied, 3 a terminal to which a control signal supplied as a current or voltage pulse is input, 4
is the grounding terminal. The operation of the switching circuit shown in FIG. 1 will be explained with reference to FIG. 2, which is a diagram showing control signals and current waveforms.

When the level of the control signal rises at time To and the control transistor Q2 turns on, the base current iB of the switching transistor QI flows through the drive transformer T1 and the base current limiting resistor RI, turning the switching transistor QI on. . The drive transformer TI continues to be excited by the excitation current during the time Tc when the control transistor Q2 is turned on by the control signal. and time t
The level of the control signal falls to l, and the control transistor Q2
When QI is "off", the base current iB from the drive transformer TI disappears, and the excitation energy stored in the drive transformer TI generates a flyback voltage, making the base potential of the switching transistor QI negative compared to the emitter potential. As a result, the storage carrier at the base of the switching transistor QI is pulled out, and the time from time tl to time t2 when the switching transistor QI is turned off and the collector current IC disappears, that is, the storage time Tst, is shortened and the switching speed is increased. It gets faster. However, in the switching circuit of FIG. 1, the storage time st' is influenced by the time Tc during which the control transistor Q2 is "on" by the control signal.

In other words, if the inductance of the winding of the drive transformer TI through which the excitation current flows is LI, then the drive transformer T
The excitation energy Et stored in I is expressed by the following formula. Et=V cow 3rd therefore the control transistor is “on”
In this case, the storage time Tst becomes longer because the excitation energy becomes smaller and the effect of pulling out the storage carrier becomes weaker. When a switching circuit is used in a power supply device such as a switching regulator, this is not desirable because the conditions of the control signal, for example, the duty ratio, vary over time.

The present invention provides a switching circuit that can set a fixed storage time Tst no matter how the control signal changes, and that also enables high-speed switching operation. The switching circuit of the present invention has a collector of a switching transistor and a primary winding of a transformer having three windings.
The emitter, the load, and the power source are connected in series, and the collector and emitter of the control transistor are connected in series between the secondary and tertiary windings, and the emitter is connected to the base of the switching transistor. 1 when the switching transistor is turned on by the control signal input to the base of the control transistor.
Feedback is applied in the direction of turning the switching transistor "on" by the next winding and the secondary winding, and feedback is applied in the direction of turning the switching transistor "off" by the primary winding when turning off. It is characterized by:

The following is a circuit diagram showing an embodiment of a switching circuit of the present invention.

This will be explained with reference to FIG. In FIG. 3, the same parts as in FIG. 1 are given the same reference numerals. T2 in Figure 3
is a current transformer, and the current transformer T2 is provided with three windings.

One end of the primary winding NI is connected to a power supply terminal via a load RL, and the other end is connected to the collector of a switching transistor QI.

The secondary winding N2 and the tertiary winding N3 are connected in series,
One end of the secondary winding N2 is connected to the collector of the control transistor Q2, and one end of the tertiary winding N3 is connected to the emitter of the control transistor Q2 via a diode D.

Further, the emitter of the control transistor Q2 is connected to the base of the switching transistor QI, and the connection point between the secondary winding N2 and the tertiary winding N3 is connected to the emitter of the switching transistor QI and the ground terminal 4. A control signal is input from terminal 3 to the base of control transistor Q2. In addition, in order to make the feedback loop gain more than 1, the current amplification factor hM of the switching transistor QI, the primary winding N,
, the number of turns NI1 and N22 of the secondary winding N2, respectively.
The turns ratio rl is in the relationship r1=N22/NII<hfe. The operation of the switching circuit of the present invention configured as described above will be explained with reference to FIG. 4, which is a diagram showing control signals, current, and voltage waveforms. In Fig. 4, the axis commonly represents time t, and the vertical axis represents the control signal, voltage v.
N2, voltage vN3, base current iB of switching transistor QI, and collector Yuka current IC. First, when the level of the control signal input to the base of the control transistor Q2 rises at a certain time, the base current iB of the switching transistor QI is supplied through its base emitter. Therefore, the collector current ic begins to flow through the primary winding NI of the current transformer TI, and the voltage vN flows through the secondary winding N2.
2 is induced. The voltage vN2 becomes the collector voltage of the control transistor Q2 in the "on" state, and a larger base current iB flows through the switching transistor QI. Such a positive feedback operation is performed instantaneously, and the switching transistor QI is rapidly turned on and saturated. Note that when the voltage vN2 due to the collector current IC is applied to the secondary winding N2, the voltage vN3 is also applied to the tertiary winding N3.
occurs, but the level is shifted by Dioto D so that no current flows. Next, at time tl, the level of the control signal decreases and the control transistor Q2 is turned off, stopping the supply of the base current iB of the switching transistor QI and increasing the induced voltage in each winding.

In this state, the diode D is biased in the forward direction, and the storage carrier injected into the base of the switching transistor QI is extracted through the diode D and the tertiary winding N3. Therefore, a feedback is applied between the tertiary winding N3 and the primary winding NI in the direction of turning off the switching transistor QI, and the switching transistor QI is quickly turned off and the collector current IC disappears. The storage time Tst of the switching transistor QI is set by the turns ratio r2 between the primary winding NI and the tertiary winding N3.

The numbers of turns of the primary winding N1 and tertiary winding N3 are NI1 and N3, respectively.
Expressed as 3, the smaller the value of u2=N33/NIl, the shorter the storage time Tst.

Thus, according to the invention, the switching transistor QI
When turns on, feedback is applied between the primary winding NI and the secondary winding N2 in the direction of turning on the switching transistor QI, and when it turns off, the feedback is applied between the primary winding NI and the tertiary winding N3. Switching transistor QI between
The return takes place in the direction of "off".

Therefore, unlike the conventional switching circuit of FIG. 1, which simply utilizes the flyback energy of the drive transformer TI to turn off the switching transistor QI, the turns ratio is independent of the "on" time Tc of the control transistor Q2. The storage time st can be set using only r2, and high-speed switching operation can be performed regardless of the control signal conditions. Further, since positive feedback is used when the switching transistor QI is turned on, there is an advantage that the power required for the control signal can be reduced. The application range of the switching circuit of the present invention is extremely wide, and it is suitable for use in various devices including power supplies such as switching regulators.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional switching circuit, FIG. 2 is a diagram showing control signals and current waveforms of the switching circuit of FIG. 1, and FIG. 3 is a diagram of the switching circuit of the present invention. FIG. 4 is a circuit diagram showing an embodiment, and FIG. 4 is a diagram showing control signals and current/voltage waveforms of the switching circuit of FIG. 3. 1, 2: Power supply terminal, 3: Terminal to which a control signal is input, 4
: Grounding terminal, T2: Current transformer, NI: Primary winding,
N2: Secondary winding, N3: Tertiary winding, QI: Switching transistor, Q2: Control transistor, RL: Load. Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1 The collector-emitter of the switching transistor is connected to the primary winding of a transformer having three windings, the load and the power supply are connected in series, and the collector-emitter of the control transistor is connected between the secondary winding and the tertiary winding. The diodes are connected in series and their emitters are connected to the base of the switching transistor, and when the switching transistor is turned on by a control signal input to the base of the control transistor, the primary winding and the secondary winding are connected. The wire provides feedback in the direction of turning on the switching transistor,
The switching circuit is characterized in that, when the switching transistor is turned off, feedback is applied by the primary winding and the tertiary winding in the direction of turning off the switching transistor.