JPH036695B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a switch circuit using a semiconductor, and particularly to a switch circuit suitable for application to a high voltage circuit configured by connecting a plurality of MOSFETs in series.

[Background of the Invention] With the development of higher voltage resistance of semiconductor elements, the conventional vacuum tubes are being replaced with switches in high voltage circuits by series-connected circuits of semiconductor elements.
A typical example is a power conversion device that consists of several hundred thyristors connected in series, and the voltage withstand capacity is
A high voltage switch of 250kV and 1.5kA current has been realized. However, since thyristors do not have a damping function, vacuum tubes are still used in high voltage switch circuits that require a damping function.

One of the semiconductor elements that has a cutting function is
There is a MOSFET. Because MOSFETs are voltage-controlled elements, their driving power is low, there is no current concentration and they are resistant to destruction, and they have no carrier accumulation effect and have excellent switching characteristics. However, MOSFETs have only been commercialized so far that can withstand voltages up to about 1kV, and in order to achieve high voltage resistance, it is necessary to connect multiple MOSFETs in series.

FIG. 1 shows a so-called totem pole switch circuit in which n MOSFETs are connected in series.
When no gate signal is applied to MOSFET 1 ₁ , MOSFET 1 ₁ is almost cut off and the current is 0. At this time, the MOSFETs _{1 2} to 1 _o connected in series are also in a cut-off state, and the voltage divided by the resistors _{2 1} to 2 _o is applied to the gates of the MOSFETs 1 ₂ to 1 _o .
There is almost a resistance between the drain and source of _{1 1} to ₁ 2 ₁
~ _2o A fixed voltage is applied. Next MOSFET1 ₁
When a signal is applied to the gate of MOSFET 1 ₁ starts conducting, and at the same time MOSFET _{1 1} to 1 _o also start conducting.
It starts conducting following _MOSFET11 , and the switch circuit turns on.

In this way, in the switch circuit of Figure 1,
On/off control can be performed by applying and stopping the gate signal to _MOSFET11 , but in order to turn on _{MOSFET12 to MOSFET1} , a sufficient voltage must be applied to maintain the on state.
It is necessary to apply it between the gates and sources of MOSFETs _{1 2} to 1 _o . Therefore, in the circuit system shown in Figure 1, the drain potential of 1 ₂ to 1 _o increases by the voltage necessary for driving in order to maintain the on state, increasing switch circuit loss and increasing the voltage applied to the load circuit. This has the disadvantage of causing a drop in the voltage applied.

[Purpose of the invention]

The purpose of the present invention is to reduce switch circuit loss in a wide range from small currents to large currents, and
Our goal is to provide a switch circuit using MOSFETs that can be turned on and off stably.

[Summary of the invention]

In order to perform on/off operations stably with little loss, it is necessary to apply and stop a sufficient voltage between the gate and source of the MOSFET that operates in accordance with the MOSFET. For this reason, the tracking operation
The MOSFET is equipped with a capacitor circuit that absorbs circuit energy when it is off and supplies drive energy when it is on, and operates by following the MOSFET.
Now controls MOSFET.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIG. In the figure, symbols 1 ₁ to 1 _o are MOSFETs, symbols 3 ₁ to 3 _o are capacitors, and symbols 4 ₁ to 4 _o are resistors.
Code 5 ₁ ~ 5 _o-1 is a Zener diode, code 6 ₁
6 _o are diodes, 7 ₁ to 7 _o are resistors, and 8 ₁ to 8 _o are Zener diodes.

When a positive gate signal is not applied to MOSFET _{1 1} , MOSFET _{1 1} is in a cut-off state, and MOSFETs _{1 2} to ₁ that operate accordingly become cut-off, and the power supply voltage E _s is applied to the entire switch circuit. Ru.
The voltage applied between the drain and source of _MOSFET11 ~ _1o is determined by the _{resistance 71 ~ 7o} .
If the values of are made equal, approximately equal voltages will be applied. Next, when a positive gate signal is applied to MOSFET 1 ₁ , MOSFET 1 ₁ starts conducting. When 1 ₁ starts conducting, the charge on capacitor 3 ₁ becomes 4 ₁ and MOSFET _{1 2}
gate, source and drain of MOSFET1 ₁ ,
Discharge begins through the source, and a voltage sufficient to operate MOSFET 1 ₂ is applied between the gate and source of MOSFET 1 ₂ , and MOSFET 1 ₂ begins to conduct. Note that the Zener diode 5 ₁ is a Zener diode for suppressing the voltage between the gate and source of the MOSFET 1 ₂ to a predetermined value or less. MOSFETs _{1 3} to 1 _o sequentially start conducting in the same manner as MOSFET 1 ₂ , and the switch circuit is turned on. MOSFET1 ₁ is a MOSFET that maintains the on state with a small voltage drop because a sufficiently large positive gate signal continues to be applied during the on period.
In 1 ₂ , a voltage determined by the Zener diode 5 ₁ or 8 ₁ continues to be applied between the gate and the source, and as in 1 ₁ , the on state is maintained with a sufficiently small voltage drop.
MOSFETs _{1 3} to 1 _o are similar to MOSFET 1 ₂ . Therefore, even if a large number of switch circuits are connected in series, the voltage drop in the switch circuit when turned on can be sufficiently reduced.

Next, when the application of the gate signal to MOSFET 1 ₁ is stopped, MOSFET 1 ₁ becomes disconnected.
The drain current of _MOSFET11 becomes 0. Therefore, the current in the load circuit is the source of _MOSFET12 ,
It flows through the gate, diode 6 ₁ , and capacitor 3 ₁ , and extracts the charge applied between the gate and source of MOSFET 1 ₂ when it is on. At this time, if the diode 6 ₁ is not present, the current in the load circuit will flow through the resistor 4 ₁ , so a step voltage determined by the current and the resistance value of the resistor 4 ₁ will be applied. In order to prevent the application of this step voltage, a diode 6 ₁ is connected in parallel to the resistor 4 ₁ ; if the step voltage is not a problem, the diode 6 ₁ can be removed. MOSFET1 ₂ gate,
When the charge is extracted between the sources, the MOSFET
1 ₂ becomes cut off and the current becomes 0. The current in the load circuit flows through the source, gate, diode 6 ₂ and capacitor 3 ₂ of MOSFET _{1 3} ,
Charge is extracted between the gate and source of _MOSFET13 . In the same way as MOSFET1 ₂
MOSFETs 1 _{and 3} become disconnected. Similarly,
The MOSFETs _{1 4} , 1 ₅ , . . . 1 _o are turned off in sequence, and the switch circuit is turned off.

Since the MOSFET has a very short switching time, the unbalance of the shared voltage due to the difference in switching time between on and off operations described above is small. Unbalance of shared voltage and power supply voltage due to off operation
In order to equalize the shared voltages by absorbing voltage fluctuations caused by fluctuations in E _s , there is a circuit including low resistors 7 ₁ to 7 _o and Zener diodes 8 ₁ to 8 _o . The capacitor 3 ₁ discharges via the resistor 4 ₁ , the Zener diode 8 ₁ , and the resistor 7 ₁ , and the capacitors 3 ₂ to 3 _o discharge in the same way as the capacitor 3 ₁ . For this reason,
It operates so that the voltage sharing is always approximately at the value determined by the resistors 7 ₁ to 7 _o to maintain a stable off state. The Zener diodes 8 ₁ to 8 _o prevent the discharge of charges in the capacitors 3 ₁ to 3 _o during the on period, and the voltage is the same as that of the Zener diodes 5 ₁ to 5 _o.
It is chosen to be equal to or higher than _-1 .

FIG. 3 shows another embodiment. The difference from the embodiment shown in FIG. 2 is that a series circuit of a Zener diode 8 and a resistor 7 is connected in parallel to each of the capacitors 3 ₁ to 3 _o . This embodiment also has the same effect as the embodiment shown in FIG.

Incidentally, in the embodiments shown in FIGS. 2 and 3, the Zener diode _8o connected to the top stage can also be removed.

〔Effect of the invention〕

According to the present invention, it is possible to realize a small signal driven switch circuit that has a small voltage drop and can stably turn on and off in a wide range from small currents to large currents.

[Brief explanation of the drawing]

Figure 1 shows a conventional switch circuit that uses MOSFETs connected in series to perform follow-up firing, Figure 2 shows an example of a switch circuit according to the present invention, and Figure 3 shows another example of a switch circuit according to the present invention. An example is shown. 1 ₁ ~ 1 _o ... MOSFET, 3 ₁ ~ 3 _o ... Capacitor, 4 ₁ ~ 4 _o ... Resistor, 5 ₁ ~ 5 _o-1 ... Zener diode, 6 _{1 ~} 6 _o ... Diode, 7 ₁ ~ _7o
...Resistor, 8 ₁ ~ 8 _o ... Zener diode.

Claims (1)

[Claims] 1. A MOSFET whose conduction is controlled by a control signal applied to its gate, and one or more MOSFETs that are connected in series to the drain side of this MOSFET and operate in accordance with the operation of this MOSFET.
In a switch circuit consisting of a MOSFET, conduction is controlled by a control signal applied to the gate.
connected in series with the source of the MOSFET.
MOSFET gate, and each serially connected
The source of the MOSFET and each
A series circuit consisting of a capacitor and a resistor and a series circuit consisting of a resistor and a Zener diode are connected in parallel between the MOSFET gates, and the MOSFET is located at the end of the MOSFETs connected in series.
A series circuit consisting of a capacitor and a resistor is connected between the source and drain of the MOSFET, and a Zener diode is connected between the source and gate of each MOSFET connected in series to the MOSFET whose conduction is controlled by a control signal applied to the gate. A switch circuit characterized by the following. 2 In claim 1, the drain of each MOSFET connected in series and the
MOSFET source and connected to this
A switch circuit characterized in that a diode is connected between the connection point of a capacitor and a resistor in a series circuit of a capacitor and a resistor connected between the gates of a MOSFET.