JPS5858851B2 - Switching circuit for inductance load - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching circuit using a transistor as a switching means for supplying power to an inductance load.

Switching transistor (with inductance load) When turning on and off the supplied power, the switching loss associated with the on/off operation causes phenomena such as secondary breakdown, which often leads to damage to the switching transistor. It's easy.

In particular, when the switching transistor is cut off, the current flowing through the inductance continues to flow through the junction capacitance formed between the collector and base of the switching transistor, and this leakage current flows between the base and base of the switching transistor. If it were to flow through the emitter junction, the switching transistor's cut-off operation would be unreliable, and as a result, current would flow from the collector to the emitter, causing switching losses and damaging the switching transistor as secondary breakdown.

Conventionally, it has been considered to absorb the above leakage current from other transistor circuits connected to the base and prevent it from flowing to the base/emitter of the switching transistor.
For this purpose, it is necessary to use a transistor having approximately the same current capacity as the switching transistor, which is not a good idea in terms of cost.

The present invention provides a switching circuit which eliminates the drawbacks of the conventional circuit, and will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the switching circuit of the present invention, including a transistor QS and an inductor L.

, diode D. and capacitor C are known step-down DC
-Constitutes a DC converter circuit.

That is, by turning on and off the switching transistor Q8, the primary power source E1-G is connected in series between the transistor Q8 and the inductor L inserted therein.

The changing voltage that occurs at the connection point with diode D.

and rectification and smoothing by capacitor C, and inductor L.

The secondary power source E2 is taken out from the output terminal OUT#.

A junction capacitance C8b between the collector and base of the switching transistor Qs is shown as a dotted line.

The primary power source E1 represents a positive potential, and G1 represents a reference potential such as ground.

A third winding N3 forming the secondary side of the transformer T is connected between the base and emitter of the switching transistor Q8, with one end connected to the base via a series circuit of a resistor R1 and a forward polarity diode D2, and the other end connected to the emitter T. A fourth winding N4 has one end connected to the base via a diode D3 of opposite polarity, and the other end connected to the emitter.

A resistor R2 for noise prevention, which will be described later, is inserted between the base and emitter of the transistor Q8.

The base of the drive transistor Qd is connected to the input terminal IN to which a unidirectional input pulse signal is applied, and the collector thereof is connected to the terminal l of the first winding N1 forming the primary side of the transformer T.

The primary side of the transformer T is divided into a first winding N1 and a second winding N2 forming an intermediate tap n connected to one E3 of the drive power supplies E3-G3.

E3 of the drive power source E3-G3 is a positive potential, G3 is a reference potential such as ground, and the terminal m of the second winding N2 is connected to the drive power source E3-03 through a diode D1 of opposite polarity. is connected to the reference potential G3, and the diode D1
) A resistor R3 is connected in parallel.

Since the above-mentioned structure is specially formed, the second
Figure a (When a pulse input signal whose duty ratio is controlled as shown in this figure is applied, the drive transistor Q at time t11
d conducts to the first winding N.

A voltage E3 is applied.

The winding direction of each winding of the transformer T is determined so that it has the respective polarity indicated by the symbol . Therefore, the third winding N3 has a voltage value -E3 having a polarity of 3 to turn on the switching transistor Q. occurs.

(For convenience, the symbols N1 to N4 of the first to 41st windings are regarded as the respective winding numbers.

)3 The above voltage value -E3 is approximately 3V.1 The number of turns of the first and third windings N1. N3 is determined, because if this value is too large, the drive loss will increase, and if it is too small, the base-emitter voltage of the switching transistor Q8 will change due to temperature changes, and the appropriate value will be determined. It is impossible to supply a base current of .

The resistor R1 is a base current limiting resistor, and has an appropriate resistance value so that fluctuations in the base current due to temperature change i are not large.

In this way, the voltage -E3 generated in the third winding N3 is set to a value that can supply a base current sufficient to saturate the maximum load current of the switching transistor.

When switching transistor Q8 is turned on, the voltage between its base and emitter is about 0.8V, but
On the other hand, since a voltage of the same polarity as that of the third winding N3 is generated in the fourth winding N4, by setting the number of turns N4 of the fourth winding so that this value is about 0.5 V higher than that of the third winding N3. A forward voltage of approximately 0.3 V, which is the difference between the two terminals, is applied to both ends of the diode D3 of opposite polarity, but the diode D3 does not conduct at this level of voltage.

Therefore, the current supplied from the third winding N3 is not dissipated through the diode D3, and all but the switching transistor Q
, becomes the base current of .

This base current l flows from the primary power supply E1 to the switching transistor Qs1 inductor L.

A secondary power source E2 that supplies the maximum load current to a load (not shown) is obtained through the output terminal OUT#.

The current flowing through the first winding N1 is N3/N of the current flowing through the resistor R1 as shown in Figure 2. A peaceful current with a value of 1 times b1 and the inductance of the first winding N1 are □ 1, when □161...-837i, and excitation current 8 having a defined slope.

Next, at time t2i in FIG. 2a, no pulse signal is applied, so the drive transistor Qd turns off, and the first
The current flowing through the second winding N1 becomes O, but the current flowing through the second winding N2
An excitation current flows backward from the reference potential G3 toward the positive potential E3 via the diode D1.

At this time, voltage E3 is applied across the second winding N2, so a voltage -E32 is generated across the fourth winding MN4 across the switching transistor QJ, and this is applied to the transistor Q5 via the diode D3. Applied to the base-emitter of transistor Q8
will be turned off.

When the switching transistor Q is turned off, its emitter voltage falls, and therefore its base voltage also falls, so that a charging current flows through the collector-base junction capacitance C8b.

OB flows.

This current [. OB has a value approximately equal to the maximum load current of the transistor Q8, and when this flows into the base, the transistor Q5 is not turned off completely, but the current flows from the collector to the emitter and instantaneously (as a result of causing a large loss, The above charging current (2) is passed through the series circuit of the diode D3 connected to the base and the fourth winding N4, although this causes secondary breakdown and damage to the transistor Q8.

Since oB is bypassed, it does not flow into the base of transistor Qs.

This current is transmitted to the second winding N2 as a current with a value of N4 ooB, but this current is transferred from the reference potential G3 to the positive potential E31 through the diode D1 and becomes an exciting current that flows backwards. included.

FIG. 2C shows the current ■n2 flowing through the second winding N2I.

That is, when the drive transistor Qd is turned off at time t2, the value of -■ The excitation current in the opposite direction 2 is about to flow, but in reality, as mentioned above, the charging current 2 is about to flow.

oB + Ko base Volume 2 4 Current -I transmitted through line N21.

2 minus OB 4 Current will flow.

-2 corresponding to IooB The current is shown as the bunched portion of the figure.

Furthermore, this exciting current flowing through the second winding N2
When the inductance of mN2 is L2, L2d I
, ↓=The slope i is determined by E3, and it becomes 09 koi. However, when this happens, the current no longer flows through the diode D1t, and the voltage at the terminal m of the second winding N2 becomes 1.1 2 E3 (1e L2) E therefore rises and finally reaches voltage E3.

Resistor R3 is selected to have an appropriate value I to slow the rise in voltage across terminal ml, thereby suppressing the generation of oscillating voltages due to the stray capacitance of each winding of the transformer.

The resistor R2 is provided to bypass the noise current so as to prevent the switching transistor Q8 from being turned off uncertainly due to the noise current I.

Figure 2d shows the state of the collector voltage of the drive transistor Qd at each time.The voltage is O while the pulse signal is input from time t1 to time t2 and it is on, but immediately after it is turned off. 1 E3 (1+-) voltage E increases.

Therefore, the driving transistor Qd is required to withstand this voltage tC.

The voltage-time products in the on and off states of this figure (φ1 and φ2I trowels are shown, respectively).

) are naturally equal.

FIG. 2e shows the voltage waveform at the terminal ml of the second winding N2, which is opposite to the collector voltage waveform of the drive transistor Qd in FIG. It is determined by the ratio of the number of turns of the second winding N2 to the number of turns of N1, and is a small value in this embodiment.

Fig. 2 f shows the current waveform of the third winding N3, Fig. 2 g shows the current waveform of the fourth winding N3.
The current waveforms of the winding N4 are shown respectively.

The second diagram shows the voltage waveform at the emitter of the switching transistor Qs, and the second diagram g shows the inductor L.

When the switching transistor Q8 is turned on, the primary power supply E and the inductor L are
.

The slope is determined by the inductance I of the inductor L.

(It flows.

Next, when the switching transistor Q is turned off, a charging current flows through its collector-base junction capacitance Cob.

OB flows for a short time, followed by diode D.

It is shown that a gradually decreasing current ■Do flows through it.

When the pulse input signal is further applied to the input terminal [NI], the above operation is repeated.

As described above, according to the circuit of the present invention, an intermediate tap is provided on the primary side of the transformer inserted in the collector and emitter circuits of the drive transistor, so that when the drive transistor is conductive, the excitation flowing through the winding of this side is The two primary windings are coupled to each of the primary windings so as to obtain a positive voltage which conducts the switching transistor by the current, and a reverse excitation current which flows through the other winding when the drive transistor is cut off, thereby producing a reverse voltage which turns off the switching transistor. By using a transformer with a secondary winding, there is no need to form a separate reverse drive means, so there is no hassle of setting this reverse drive means to a proper operating state, and the construction is simple and cost-effective. Despite being inexpensive, it is possible to reliably prevent damage to the switching transistor and increase the switching speed. It is a kill thing to realize the switching circuit.

Although the step-down DC-DC converter circuit has been described in Embodiment E in FIG. The circuit of the invention is extremely useful.

[Brief explanation of drawings]

Figure 1 shows a step-down DC-DC converter using the switching circuit of the present invention.
One embodiment of the DC converter circuit, FIG. 2 shows waveform diagrams at various parts of the circuit of FIG. 1, respectively. Lo...Inductor, Q8...Switching transistor, Qd...Drive transistor, T...Transformer, N1...First winding , N2... second winding, N3...
Third winding, N4...Fourth winding, Dl...
...First diode s D2...Second diode, D3...Third diode.

Claims (1)

[Claims] 1. A switching transistor that turns on and off a power supply supplied to a load including an inductance, a drive transistor that turns on and off depending on the presence or absence of an input pulse signal, and the drive transistor and the switching transistor. The transformer has an intermediate tap connected to one of the drive power supplies, one end connected to the drive transistor, and the other end connected to the drive power supply E through a first diode of opposite polarity. A first winding and a second winding are separated from the intermediate tap I of the primary winding, which are both connected to the other of the drive power supply, and a forward polarity second winding is connected between the base and emitter of the switching transistor.
a third winding connected through a diode, and a fourth winding connected between the base and emitter of the switching transistor through a third diode of opposite polarity; The winding No. 3 is energized by the first winding through which current flows when the drive transistor is turned on, and supplies a current that conducts the second diode and turns it on to the switching transistor. , the fourth winding is energized by the second winding through which excitation current flows when the drive transistor is turned off, conducts the third diode, and turns the switching transistor into the off state. A switching circuit for an inductance load, which is characterized by supplying a reverse voltage. 2. The inductance load according to claim 1, wherein the third winding is formed to generate a voltage sufficient to saturate at least the maximum load current of the switching transistor. 3. The fourth winding applies a reverse voltage to the third diode at a required level that does not cause the third diode to conduct when the switching transistor is turned on. The switching circuit for an inductance load according to item 1.