JPH0334694B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a Darlington transistor circuit with a protection function.

[Technical background of the invention and its problems]

In recent years, as a semiconductor device for high power switches,
The use of high-power Darlington transistors is being actively developed, and in particular, the use of Darlington transistors in high-voltage, high-power switches is progressing. For example, a Darlington transistor for a high power switch with an input voltage of 480V (AC) has a Forward Safe Operating Area.
Area) characteristics, especially the load short circuit safe operating area (Lord Short Circuit Safe Operating Area)
In terms of characteristics, it is essential that the product does not break down for several tens of microseconds or more at 800V (DC).

By the way, it has been conventionally known that the breakdown resistance in the load shot safe operating characteristics depends on the collector resistance R _C of the Darlington transistor.
By increasing this R _C , the breakdown resistance is improved. For example, in the case of a transistor with an NPIN structure, the vertical thickness of the intrinsic region is increased to increase R _C . However, this means that the forward characteristics of the transistor are degraded. In other words, the collector-emitter saturation voltage V _CE (sat) increases, the switching time increases,
Both of these problems, such as a decrease in collector peak current, are undesirable for use in high-power switches. Above V _CE (sat)
If only the characteristics and collector peak current were to be improved, this could be achieved by increasing the emitter area several times that of the conventional one, but such an increase in chip size would be far from commercially viable. Furthermore, even if the chip size is increased in this manner, the switching time characteristics are not improved, and a practical high-power device for a high-power switch cannot be obtained.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a Darlington transistor circuit in which the load shot safe operating area of the Darlington transistor can be expanded without increasing the chip size.

[Summary of the invention]

Darlington transistor for switching is
When a load is short-circuited, a high bias of several 100 V is applied between the collector and emitter, but when the base current is transferred in this state, the collector current times h _FE of the Darlington transistor instantly flows into the final stage transistor, and at that time Silicon melts due to concentrated power. On the other hand, according to the present invention, while a high bias is applied between the collector and emitter, a bypass circuit is formed between the base of the first-stage transistor and the base of the last-stage transistor. Even if a base current is transferred in a biased state, there is no amplification effect as a Darlington transistor, and the collector current flowing into the final stage transistor is at most h _FE times that of a single transistor, which can be kept to a small value. Therefore, it can withstand a higher bias collector-emitter voltage. That is, it is possible to expand the load shot safe operation area.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. Figure 1 shows the main circuit of a Darlington transistor for high power switching, where Q ₁ is the first stage transistor, Q ₂ is the last stage transistor, I _C , I _C1 , I _C2
is the collector current, _IE is the emitter current I _B1 , I _B2 is the base current. The circuit body shown in FIG. 1 generally includes a stabilizing resistor, a speed-up diode, and a fast recovery diode, but these are omitted.

FIG. 2 shows an embodiment circuit of the present invention.
The circuit body shown in the figure is provided with a Darlington-configured bypass-forming transistor _Q3 consisting of transistors _Q31 and _Q32 , whose collector is connected to the base of transistor _Q1 , and whose emitter is connected to the base of transistor _Q2 . The base is connected to the collector of the circuit body of the Darlington transistor via a diode D ₁ and a resistor R ₁ .

In the circuit shown in Figure 2, when the load L is shot, etc., the collector of the Darlington transistor
When a base current I _B1 is transferred with a high bias voltage V _CC applied between the emitters and the collector C to emitter E voltage is greater than the voltage of the diode D ₁ , the current I _C3 ≒ (V _CC −
V _Z )/R ₁ (where V _Z is the reverse breakdown voltage of diode D ₁ ) is supplied, and transistor Q ₃ turns on.
The current I _B1 flows as a collector current of the transistor Q ₃ up to I _C3 ×h _FE3 according to the current amplification factor h _FE3 of the transistor Q ₃ and does not become a base current to the transistor Q ₁ . Therefore, by appropriately setting the resistor R ₁ and diode breakdown voltage V _Z , the base current I _B2 to the transistor Q ₂ can be calculated as follows: I _B2 = I _E3 = I _B1 + I _B1 /h _FE3 ...(1) Furthermore, h _FE3 If we choose a sufficiently large value, I _B2 ≒ I _B1 . Therefore, the collector current I _C2 flowing to the transistor Q ₂ can be kept smaller than the current amplification factor h _FE2 of the transistor Q ₂ as I _C2 = I _B1 × h _FE2 (2). On the other hand, in the case without a conventional bypass circuit, the current amplification factor h _FE1 of transistor Q ₁
As I _C2 ′=I _B1 ×h _FE1 ×h _FE2 ……(3), it becomes huge.

FIG. 3 shows the I _C -V _C characteristics of the Darlington transistor with a bypass circuit according to the present invention. However, R ₁ =
This is the case of 1MΩ, V _Z = 10V, h _FE3 10K. The broken line in the figure is the I _C -V _C characteristic without a bypass circuit. Figure 3a shows the characteristics at low voltage, Figure 3b
is the characteristic at high voltage. The characteristic line in the destruction region of FIG. 3b is an estimate since the element is destroyed.
The border line between destruction and non-destruction in FIG. 3b depends on the collector resistance R _C as described above, and changes depending on the thickness of the intrinsic region in the NPIN structure. FIG. 4 shows the dependence of the borderline on the intrinsic region thickness. Here, the specific resistance in the intrinsic region is 110Ω·cm, and the I _B pulse width is 50 μsec. In practical use, curve B is the limit due to other characteristics of the transistor. Figure 3b
As shown in Figure 2, 600V is applied between collector C and emitter E with conventional current I _B = 2A being transferred.
When applied, a _current of about 280A flows instantaneously,
The silicon melts within several microseconds, resulting in short-term destruction between C and E. On the other hand, if the bypass circuit of the present invention is used, the current I _C is about 75 A at most, and destruction will not occur within several tens of microseconds. In the device of the present invention, even when 800 V is applied for the same I _B =2 A, the current I _C is about 130 A, which is still in the non-destructive range even when the current is applied for several tens of microseconds.

As described above, the load shot safe operation area of the Darlington transistor according to the present invention is greatly expanded. Also at this time, the collector resistance
Since the characteristics can be improved without increasing R _C , the transistor's characteristics such as V _CE (sat), I _C peak current, and switching time will not be impaired in any way as long as it is considered as a switching device.

Note that the above resistor R ₁ , diode D ₁ , and transistor Q ₃ can be selected arbitrarily, but for example, R ₁ may be selected depending on the power supply voltage V _CC applied to the device during actual use.
Determine I _C3 = V _CC /R ₁ . This passes through the transistor _Q3 when it is off and acts like a leakage current, so it is desirable that _R1 be extremely large.
At this time, the current I _C3 becomes a small value, so if the current amplification factor h _FE3 of the transistor Q ₃ is small, “I _C3 × h _FE3
At this time, “I _B1 −I _C3 ×h _FE3 ” branches off as the base current _of transistor Q ₁ , and the base current I _B2 to transistor Q ₂ becomes “I _B2
= I _C3 × h _FE3 + (I _B1 − _{I C3} × h _FE3 ) × h _FE2 ″, and since the second term easily becomes large, I _B2 increases as a result, which is undesirable. Therefore, transistor Q ₃ of
h _FE3 needs to be large, and a Darlington transistor or a single transistor with a high current amplification factor is preferable. The reverse withstand voltage _VZ of the diode _D1 determines the collector-emitter voltage at which the bypass circuit of the present invention turns on, and if the V _CE (sat) voltage of the Darlington transistor in the main body is small enough, it can be used as a switching transistor. It is unnecessary and does not necessarily need to be added. Moreover, in the above, transistor 2
Although the case of a two-stage Darlington transistor in which two Darlington transistors are connected is taken as an example, the present invention can be similarly applied to a three-stage Darlington or even a multi-stage Darlington. In any case, it is preferable that the bypass be formed between the first base and the last base, but the present invention does not specify this. Furthermore, although the transistors Q ₁ , Q ₂ , and Q ₃ are NPN transistors, they are not limited to these. It is also possible to connect the bypass directly from the base of the first stage to the emitter of the Darlington transistor, but in this case, when the switching operation is about to transition from the OFF state to the ON state, if "I _B1 I _C3 × h _FE3 ", the base input current all flows through the bypass circuit, and the Darlington transistor never turns on. However, if R ₁ and Q ₃ are set appropriately for the collector-emitter voltage and base input current during actual use, and "I _B1 > I _C3 ×h _FE3 ", switching operation can be made possible.

〔Effect of the invention〕

As explained above, according to the present invention, by adding inexpensive transistors, resistors, diodes, etc. to the conventional Darlington transistor, it is possible to improve the breakdown strength without increasing the chip size. .

[Brief explanation of drawings]

Figure 1 is a Darlington transistor circuit diagram, Figure 2 is a circuit diagram of an embodiment of the present invention, Figures 3 and 4.
The figure is a characteristic diagram of the same circuit. Q ₁ ... Front stage transistor, Q ₂ ... Later stage transistor, Q ₃ ... Bypass formation transistor,
_D1 ...Diode, _R1 ...Resistor.

Claims (1)

[Claims] 1. A Darlington transistor formed by connecting at least two transistors in Darlington is provided in series with a load, and the collector of the transistor,
If the potential between the emitters exceeds a certain value and the front stage transistor base current of the Darlington transistor is applied, the base input current to the front stage transistor is directly input to the base input current of the rear stage transistor, and A Darlington transistor circuit characterized in that the base thereof has a terminal for providing an independent base input to the base. 2. Claim 1, wherein the bypass is such that the collector, emitter, and base of a bypass-forming transistor are connected to the base and collector of the transistor in the preceding stage and the transistor in the succeeding stage of the Darlington transistor, respectively. Darlington transistor circuit as described in . 3. The Darling transistor circuit according to claim 2, wherein the bypass forming transistor is a Darlington transistor with a high current amplification factor. 4. The Darlington transistor circuit according to claim 2, further comprising a diode between the base of the bypass forming transistor and the collector of the Darlington transistor. 5. The Darlington transistor circuit according to claim 2, further comprising a resistor between the base of the bypass forming transistor and the collector of the Darlington transistor.