JPH0374972B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to a device mainly composed of a composite transistor of a bipolar transistor and a field effect transistor.

[Prior art and its problems]

In the following description of each figure, the same reference numerals indicate the same or corresponding parts.

First, the problems of the prior art will be explained based on FIGS. 2 and 3.

Figure 2 is an example of a circuit consisting of this type of composite transistor called a so-called cascode connection, Figure 3A is an example of another circuit, and Figure B
shows an example of the characteristics of the Zener diode shown in FIG.

In Figure 2, Q1 is the main bipolar transistor (hereinafter also referred to as main transistor), Q2
is a field effect transistor (hereinafter referred to as FET),
ZD is a zener diode.

Collector C and emitter E of main transistor Q1
The drain D and source S of FETQ2 are connected in series at the emitter E and drain D to open and close a current (referred to as collector current for convenience) I _C supplied to a load not shown. Note that the transistors Q1 and Q2 are collectively referred to as a composite transistor for convenience. Further, a Zener diode ZD is connected between the base B of the main transistor Q1 and the source S of the FET Q2 so that the base B side is the cathode side.

A switching signal voltage eG that commands the ON/OFF of the collector current I _C is applied between the gate G and source S of the FET Q2 via a drive circuit (not shown), and between the base B of the main transistor Q1 and the source S of the FET Q2. Through the base power supply EB provided between the source S,
When FET Q2 is turned on, base current I _B is supplied to the base B.

This circuit focuses on the fact that the switching speed of FETs is generally faster than that of bipolar transistors, and attempts to obtain a high-speed, high-voltage composite switching element by combining a high-speed, low-voltage FET Q2 and a low-speed, high-voltage bipolar transistor Q1. This is a circuit that does this.

That is, first, let us describe the case where the composite transistors Q1 and Q2 are turned on. In this circuit,
Since the FET Q2 is connected to the emitter E of the main transistor Q1 and can switch the base current I _B , the base voltage e _B applied to the base B of the main transistor Q1 can be a relatively high voltage. When a switching signal voltage e _G to turn on the transistor Q2 is applied to Q2, the drain D-source S voltage V _DS drops sharply, causing the base current I _B to rise sharply, and the main Transistor Q1, and thus composite transistors Q1, Q2, can be turned on quickly.

On the other hand, when turning off the composite transistors Q1 and Q2, the transistor Q is connected to FET Q2.
When the switching signal voltage e _G that should turn off FET Q2 is given, the drain-source voltage V _DS of FET Q2
His energy rose sharply, cutting off the collector-style I _C flowing through him. At this moment, the collector current I _C flowing through the base and emitter of the main transistor Q1 is commutated to the Zener diode ZD. In this way, the stored carriers at the base of the main transistor Q1 are quickly released, so that the transistor Q1, and thus the composite transistors Q1, Q2, can be quickly turned off and the collector current I _C can be cut off quickly. .

Here, a Zener diode is connected to the commutation path.
The reason for using ZD is that, as will be explained later, during the commutation of the collector current I _C , the drain-source voltage V _DS of FET Q2 must be lower than the limit voltage (switching blocking voltage V _DS ) that can be turned off. and when the composite transistors Q1 and Q2 are on, the main transistor Q1 is connected from the base power supply E _B.
This prevents the base current I _B1 supplied to the base B side of the
This is to ensure that the base current I _B becomes effective.

By the way, when the composite transistors Q1 and Q2 are turned off, the collector current I _C flows from the base to the emitter of the main transistor Q1 to the Zener diode ZD.
The conditions for % commutation are Zener diode ZD
When a load current equal to the collector current I _C flows, the voltage across it is the loaded Zener voltage V _ZL , and the forward voltage between the base and emitter of the main transistor Q1 just before the collector current I _C disappears is the voltage across it. Assuming V _BE10 , the switching blocking voltage BV _DS is expressed by the following equation (1).

BV _DS V _ZL −V _BE10 ...(1) On the other hand, if the no-load Zener voltage of the Zener diode ZD is V _ZO and the operating resistance is R _Z , the collector current I _C flows into the Zener diode ZD in the blocking direction. The Zener voltage V _ZL under load in the above-mentioned state is expressed as V _ZL = V _ZO + R _Z・I _C ... (2) Therefore, by substituting equation (2) into equation (1), the following equation can be obtained.
It is expressed as (3).

BV _DS V _ZO +R _Z・I _C −V _BE10 ……(3) In general, the lower the switching blocking voltage BV _DS of a FET, the lower the drain-source voltage (simply called on voltage) V _DSON when it is on. Therefore, since the loss during on-time is reduced, an element with a low blocking voltage BV _DS is used in the circuit of FIG. 2. Therefore, from equation (3), for the Zener diode ZD,
Low no-load Zener voltage V _ZO and operating resistance R _Z
A low value is desired. However, a normal Zener diode with a low no-load Zener voltage V _ZO has the disadvantage of a high operating resistance R _Z , so a circuit equivalent to a Zener diode in which multiple diodes are connected in series in the forward direction as shown in Figure 3A is used. (hereinafter referred to as Zener diode ZD ₁ for convenience) is used.

However, the characteristics of the Zener current I _Z and Zener voltage V _Z of this Zener diode ZD ₁ are so-called soft constants in which the Zener current V _Z gradually increases on the curve as the Zener current I _Z increases, as shown in Figure 3B. Indicates the voltage characteristics and its no-load Zener voltage V _ZO
has a considerably lower value than the Zener voltage V _Z (Zener voltage under load V _ZL ) when a Zener current I _Z equal to the collector current I _C flows.

On the other hand, when the main transistor Q1 and FET Q2 are turned on, the base current I _B1 supplied from the base power supply EB (not shown, described above) is supplied to the main transistor Q1 as the base current IB without bypassing to the Zener diode _ZD1 side. In order for Q1 to be fully turned on, the base of main _transistor Q1 must be
Assuming that the emitter-to-emitter forward voltage is V _BE (Sat) and the on-voltage of FET Q2 is V _{DS ON} as described above, the Zener voltage V _ZO at no-load is: V _ZO V _BE (Sat) + V _DSON ……(4 ) conditions must be met.

Therefore, if the zener diode ZD ₁ is selected to satisfy the condition of equation (4), the zener voltage V _ZL under load will also be high as mentioned above, and therefore from equation (1),
The switching blocking voltage BV _DS of FET Q2 must also be selected with a high value, so FET Q
The on-voltage V _DSON of No. 2 is also high, and as a result, the loss cannot be made sufficiently small.

[Purpose of the invention]

The present invention eliminates the above drawbacks and provides a voltage corresponding to the no-load zener voltage V _ZO and a voltage corresponding to the on-load zener voltage V _ZL when a load current equal to the collector current flows during the commutation. A circuit _equivalent to an ideal Zener diode with a sharp constant voltage characteristic, in which Even if the condition is satisfied, the voltage corresponding to the load Zener voltage V _ZL in the above circuit can be kept as small as possible, and therefore, from equation (1), the switching blocking voltage
The object of the present invention is to select a FET with as small a BV _DS as possible, that is, to provide a semiconductor device in which the loss when turned on can be minimized as much as possible.

[Key points of the invention]

The gist of the present invention is to connect the emitter of a bipolar transistor and the drain of a field effect transistor (such as a composite transistor), and apply a switching signal voltage (for switching operation, etc.) between the gate and source of the field effect transistor. , a device for opening and closing a current supplied to the outside through a series circuit of the collector/emitter of the bipolar transistor and the drain/source of the field effect transistor, the base of the bipolar transistor and the source of the field effect transistor, Connect the collector (drain) and emitter (source) of each new transistor, and connect a Zener diode in parallel between the collector (drain) and base (gate) of this new transistor to block the base (gate) current of the transistor. By connecting the Zener diode and the new transistor to a polarity of
The point is that a circuit equivalent to an ideal Zener diode is configured.

[Embodiments of the invention]

Next, an embodiment of the present invention will be described based on FIG. In the figure, ZD ₂ is a Zener diode having a no-load Zener voltage V _ZO as low as possible within the range that satisfies equation (4), and Q3 is a new transistor (also referred to as an auxiliary transistor). This transistor Q
3 does not necessarily have to be bipolar.
Although it may be an FET, for convenience, it will be explained as a bipolar type.

If we now focus on the circuit consisting of the Zener diode ZD ₂ and the auxiliary transistor Q3, the Zener current I _Z will increase until the base voltage e _B exceeds the no-load Zener voltage V _ZO of the Zener diode ZD ₂ .
does not flow, and therefore the auxiliary transistor Q3, which uses the current I _Z as its base current, is also in an OFF state. When the base voltage e _B further increases and the Zener current I _Z begins to flow, the auxiliary transistor Q3 is turned on and the collector current IC ₃ begins to flow. Next, the base voltage e _B increases slightly and the collector current IC ₃ changes to the main transistor Q1.
Even if the collector current I _C is equal to the magnitude of
(This corresponds to the case where the base-emitter collector current I _C of the main transistor Q1 is commutated to the auxiliary transistor Q3 when the composite transistors Q1 and Q2 are turned off.) The zener current I _ZL (as _{the base} current _{of the} Can be a value. Therefore, the values of the base voltage e _B corresponding to the two Zener voltages V _ZL and V _ZO are also very close.

In other words, this circuit is equivalent to an ideal Zener diode that can flow up to the collector current I _C from a no-load state while maintaining a substantially constant Zener voltage as low as necessary.

Therefore, as for the FET Q2, it is possible to use one having a switching blocking voltage BV _DS as low as possible from equation (1), and therefore having a low on-voltage V _DSON , in other words, having a low loss.

Note that when a FET is used as the auxiliary transistor Q3, the collector, emitter, and gate of the bipolar transistor are the drain and gate of the FET, respectively.
All you have to do is replace it so that it corresponds to the source and gate.

In addition, the main transistor Q1 and FET Q in Figure 1
2. Although the auxiliary transistor Q3 has been described as an NPN transistor, an N-channel FET, an NPN transistor, or an N-channel FET, these may be replaced with a PNP transistor, a P-channel FET, a PNP transistor, or a P-channel FET, respectively. However, in the latter case, the Zener diode ZD ₂ must have reverse polarity. In other words, the polarity of the Zener diode ZD ₂ may generally be maintained at a polarity that blocks the base current or gate current.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, a bipolar transistor and a FET are connected in series at the emitter of the former and the drain of the latter, and the collector (drain) of a new transistor is connected to the base of the former and the source of the latter, respectively. ), the emitter (source) is connected, and a Zener diode is connected between the base (gate) and collector (drain) of the transistor with a polarity that blocks the base current (gate current). We decided to construct a circuit equivalent to the ideal Zener diode that can maintain a predetermined low voltage over a wide range of load currents (Zena current) with the Zener diode mentioned above, which enables high-voltage, high-speed switching, and low A lossy semiconductor device can be constructed.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention;
2 is a diagram showing an example of a conventional circuit, FIG. 3A is a circuit diagram showing another example, and FIG. 3B is a diagram showing an example of the characteristics of the Zener diode shown in FIG. Q1... Bipolar transistor (transistor), Q2... Field effect transistor (FET),
Q3...Transistor, ZD ₂ ...Zena diode.

Claims (1)

[Claims] 1. Connecting the emitter of a bipolar transistor and the drain of a field effect transistor, applying a switching signal voltage between the gate and source of the field effect transistor, and connecting the collector and emitter of the bipolar transistor to the field effect transistor. In a device for switching a current supplied to the outside through a series circuit with the drain and source of a new transistor, the base of the bipolar transistor and the source of the field effect transistor are connected to the collector (drain) and emitter (source ), and a Zener diode is connected in parallel between the collector (drain) and base (gate) of this new transistor with a polarity that blocks the base (gate) current of the new transistor.