JP2854010B2 - Semiconductor switch circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor switch circuit for disconnecting a load current, and more particularly to a switch circuit that enables high-efficiency and high-speed switching.

2. Description of the Related Art Conventionally, as shown in FIG. 3, this type of semiconductor switch circuit uses a power transistor Q1, _one end of a switch control voltage source ei is connected to an emitter, and the other end is a base current limiting series resistor Rx. via is connected to the base, and the said base, when oN of the power transistor Q ₁ with respect to the maximum envisaged the load current Io, accelerate the oN rate is sufficiently saturated,
It is set to give a sufficient base current to reduce the ON resistance. Furthermore, in order to increase the OFF speed at the time of OFF, a resistor Ry for rapidly extracting the base charge stored at the time of ON is connected between the base and the emitter.

Problems to be Solved by the Invention By the way, in the conventional circuit, at the time of ON, the base current is applied to the power transistor irrespective of the load current, so that when the load is light, that is, when the load current is small, the base current more than necessary is necessary. Given, the accumulated charge on the base increases,
Not only is the speed slow, making high-speed switching difficult, but also the base current given unnecessarily reduces the efficiency of the switch circuit significantly. Therefore, the present invention has a simple circuit configuration, and a variable impedance circuit that saturates the base current according to the collector-emitter saturation voltage at the time of ON, but does not flow excessively, is used as the base of the power transistor. To solve the above problem.

Means for Solving the Problems In a semiconductor switch circuit of the present invention, a power transistor is used as a switching element, and in a semiconductor switch circuit driven by a signal of a switch control voltage source, a base accumulated charge between a base and an emitter of the power transistor. with connecting circuits pulling, the base connected to the emitter of the power transistor, a collector connected to said switch control voltage source, a collector connected to the base by a resistor R _1, a series of base resistance R ₃ and the diode A variable impedance circuit comprising a second transistor connected to the collector of the power transistor via a circuit, and controlling a base current according to the collector-emitter saturation voltage when the power transistor is turned on; Saturate the transistor but base too much Flow, characterized in that said on avoid flow.

Function With the above configuration, the resistors R ₁ and R ₃ are selected in accordance with the ON resistance characteristics of the power transistor and the voltage of the switch control voltage source, and according to the collector-emitter saturation voltage of the power transistor even when the load is light when the power transistor is ON. A variable impedance circuit for controlling the base current is provided so as to saturate the power transistor but prevent the base current from flowing excessively, so that the accumulated charge in the base can be minimized. Since a circuit for extracting the accumulated charge of the base is connected between the emitters, it is easy to extract the charge, the OFF speed becomes faster, and the base current enough to saturate the power transistor flows, so the ON
Speed is fast and ON resistance is small. In addition, the power efficiency of the switch circuit can be improved in combination with not allowing an excessive base current to flow.

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

FIG. 1 is a semiconductor switch circuit diagram of one embodiment of the present invention. In FIG, Q ₁ is a variable impedance circuit provided NPN power transistor for switching, A is according to the present invention. The emitter of the power transistor Q ₁ is to GND, the base is connected to one end of the switch controlled voltage source ei through a variable impedance circuit A, the other end of the voltage source ei is connected to GND. And between the base and emitter of the power transistor Q ₁ is connected by a resistor R ₂ as a circuit for withdrawing the base of the accumulated charge.

Variable impedance circuit A collector switch control voltage source ei side, the emitter is an NPN transistor connected to the base of the power transistor Q ₁ (second transistor), between the collector-base connected by a resistor R ₁ , base connected to the collector of the power transistor Q ₁ via a series circuit of a diode D to the resistance R ₃ and the power transistor Q ₁ side and the cathode.

Next, the operation of the switch circuit having the above configuration will be described.

Now, it is assumed that the switch control voltage source ei outputs the positive voltage E _ON . The base-emitter voltages of transistors Q ₁ and Q ₂ are V _BE1 and V _BE2 respectively, and the collector-emitter voltages are V
_CE1 , V _CE2 and the voltage between the anode and cathode of the diode D
When V _D, the emitter current I ₁ of the transistor Q ₂ is given by the following equation.

_{I 1 = {(E ON -V} BE1 -V BE2) / R 1 - (V BE1 + V BE2 -V CE1 -V D) / R 3} * h fe2 ...... Here, h _fe2 the transistor Q ₂ current Amplification rate.

Also, the base current I _B1 of the transistor Q ₁ is given by _{I B1 = I 1 -V BE1 /} R 2 .......

It can be seen from the equation that if V _CE1 is small, that is, if the power transistor is saturated with a small voltage, the effect of reducing the emitter current I ₁ of the transistor Q ₂ is obtained.

As a special case, if the resistances R ₁ and R ₃ are equal and R ₁ = R ₃ = R, the equation is: I _B1 = [{E _ON -2 (V _BE1 + V _BE2 ) + V _CE1 + V _D } / R] * h _fe2 −V _BE1 / R ₂ ……

Here, assuming that the load current of the switch is I _O and the saturation ON resistance of the transistor Q ₁ is r _SC1 , V _CE1 ≒ r _SC1 * I _o .

Wherein more I _B1 becomes I _B1 = _{[{E ON -2 (V BE1 +} V BE2) + r SC1 * I o + V D} / R ] _{* h fe2 -V BE1 / R 2} ....... Here, it is assumed that the fluctuations corresponding to the fluctuations of the load current _Io of the base-emitter voltages V _BE1 , V _BE2 of the transistors Q ₁ , Q ₂ and the anode-cathode voltage V _D of the diode D are negligibly small. When obtaining the relevant for the load current I _o of the base current I _B1 of the transistor Q ₁ performs a rough approximation.

d (I _B1 ) / d (I _o ) = (r _SC1 / R) * h _fe2 , and the saturation ON resistance r of the transistor Q ₁ indicating the saturation state
_SC1 changes in the base current I _B1 as a proportional constant d (I _B1) is proportional to the change of the collector current I _o d (I _o). That is, current corresponding to the load current I _o It can be seen that flows as the base current of the transistor Q _1.

Therefore, since there is no flow of excessive base current to the transistor Q _1, the base accumulated charge is suppressed small, relatively charge withdrawal of a resistor R ₂ of high resistance is facilitated, not only the OFF speed increases, excessive This has the effect of eliminating power loss due to base current.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention. This embodiment, implementation of the first embodiment, in place of the resistor R _2, in order to pull out more quickly based accumulated charge, add the transistor Q ₃ and the inverter Q _4, further enhanced switching characteristics It is an example.

In operation, when the output voltage of the switch control voltage source ei is switched to 0V from the switch ON voltage E _ON, the output of the inverter Q ₄ is drive becomes High, the base of the transistor Q _3, ON transistor Q ₁ very low saturation oN of the transistor Q ₃ than the at accumulated based charges to the resistor R ₂
Since it is pulled out by a resistor, there is an advantage that extremely high-speed switching is possible.

As described above, according to the present invention, a variable impedance circuit that automatically controls the base current of the switching transistor according to the load current is provided as a relatively small-scale circuit, so that the high-speed, high-speed It has a great effect, for example, it is easy to incorporate an efficient semiconductor switch into an integrated circuit.

Although an NPN transistor is used as an example of a switching element in this embodiment, it is needless to say that a PNP transistor can similarly be used.

[Brief description of the drawings]

FIG. 1 is a semiconductor switch circuit diagram according to a first embodiment of the present invention, and FIG. 2 is a semiconductor switch circuit diagram according to a second embodiment of the present invention. FIG. 3 is a circuit diagram of a conventional semiconductor switch. Q ₁ , Q ₂ , Q ₃ ... transistor, R ₁ , R ₂ , R ₃ , Rx, Ry ... resistor, ei ... switch control voltage source, Q ₄ ... inverter, A ... variable impedance circuit.

Claims (1)

(57) [Claims] 1. A semiconductor switch circuit comprising a power transistor as a switching element and driven by a signal from a switch control voltage source, wherein a circuit for extracting a charge stored in a base is connected between a base and an emitter of the power transistor. Connect the emitter to the base of the transistor,
A collector connected to said switch control voltage source, a collector connected to the base by the resistors R _1, made of a second transistor connected to the collector of the power transistor base via a series circuit of a resistor R ₃ and a diode A variable impedance circuit for controlling the base current in accordance with the collector-emitter saturation voltage when the power transistor is turned on to saturate the power transistor but prevent excessive base current from flowing. A semiconductor switch circuit characterized by the above-mentioned.