JPH01125018A - Inverter circuit - Google Patents

Abstract

PURPOSE:To prevent the damage of a semiconductor switch element by bringing a resistance to parallel connection to a first diode of an arm which has brought the second diode to anti-parallel connection to a series circuit of the semiconductor switch element having a parasitic diode and a floating capacitor, and the first diode. CONSTITUTION:To block diodes 25, 35, 45 and 55 of each arm, discharge resistances 28, 38, 48 and 58 are connected in parallel, respectively. Therefore, by discharging the charge which is accumulated in a floating capacitor, it is prevented that a voltage is applied to a block diode. Also, by selecting a resistance so that it does not interfere with discharge from the floating capacitor, and also, becomes larger value than a reflux diode and its wiring impedance, a current value which is brought to flow division to the parasitic diode whose reverse recovery characteristic is not satisfactory is suppressed. In such a way, the possibility of damage of a MOSFET is prevented in advance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an impark circuit using a semiconductor switch element having a parasitic diode and a floating capacitor.

(Prior Art) A metal oxide semiconductor field effect transistor is a typical semiconductor switch element having a parasitic diode and a floating capacitor. Therefore, in the following description, a metal oxide semiconductor field effect transistor (hereinafter abbreviated as MOSFET) will be used as this semiconductor switch element.

FIG. 2 is a main circuit connection diagram showing a first conventional example of an inverter using MOSFETs, and shows a single-phase inverter that converts DC power into single-phase AC power. This second
Reference numeral 20 in the first phase upper arm of the inverter in the figure is a MOSFET, reference numeral 21 is its parasitic diode, and reference numeral 22 is its floating capacitor. This MOS
A block diode 25 as a first diode is connected in series to the FET 20.
A freewheeling diode 26 as a second diode is connected in parallel with the reverse polarity to the series connection circuit of MOSFET 20 and MOSFET 20. The first phase lower arm is also MOSFET30
．． Parasitic diode 31. Floating capacitor 32. Block diode 35. A freewheeling diode 36 is constructed with similar connections to the upper arm. Furthermore, the upper and lower arms of the second phase also have MOSFETs 40 and SO, parasitic diodes 41 and 51, respectively. Floating capacitors 42 and 5
2. Block diodes 45 and 55. Reflux Dyed 46
and 56. These four MOs
3FE T 20,30. By turning on and off the '40.50 as appropriate, the DC power from the DC power source 2 is converted into single-phase AC power and supplied to the load 4.

The voltage or current applied to the load 4 is controlled by pulse width modulation control by turning off the MO3FET 40 and turning off the MO3FET 40.
MOSFETs 20 and 30 are alternately turned on and off at high speed while T50 is on, and this is achieved by changing the on/off ratio. Also MO
If MO3FET40 and 50 are turned on and off alternately with 3FET20 off and MO3FET30 on,
The load 4 is given power opposite to the above.

Considering the mode of supplying positive power to load 4, first M
When O3FET50 is on, MO3FET40 is off, MO3FE720 is on, and MO3FET30 is off, DC power supply 2 → block diode 25 → MO3F
Current flows through the path of ET20 → load 4 → MO3FET50 → block diode 55 → DC power supply 2.
When the ON and OFF states of O3FETs 20 and 30 are both reversed, current continues to flow through the path of load 4 → MO3FET 50 → block diode 55 → freewheeling diode 36 → load 4. The block diode 35 is for preventing the current from flowing to the parasitic diode 31 at this time, and without the block diode 35, the parasitic diode 31 would be destroyed due to its small withstand capability during reverse recovery. .

When a gate signal is applied to a MOSFET to turn it on and off, the drain current turns on and off with a certain time delay with respect to the gate signal. Therefore, if the delay time when off is Toff and the delay time when on is Ton, generally Toff>Ton. Therefore, as shown in Fig. 2, MO3FET20 and MO3FET30 are
Both MO3FEs are connected in series circuit to
A gate signal that turns off this MO3FET 20 so that there is no period in which T20 and 30 are on at the same time,
Between the gate signal that turns on MO3FET30,
A waiting time Tw must be provided, and a waiting time Tw is also required in the opposite case. Note that the length of this waiting time T- may be determined as follows.

T-≧[Toff of one MOSFET) (Ton of the other MOSFET) However, in reality, considering the variation in operating time that exists in each MOSFET, in the worst case, the upper and lower arms of the same phase are Since the length of the waiting time Tw is set so that they do not turn on at the same time, both MO3 of the upper and lower arms
There will be a period when both FETs are in the off state.

As mentioned above, when MO3FET20 is on, MO5FE
When T30 is off, DC current B2 → MO3FET2
0→Load 4→MO3FET 50→DC current 'a2, and at this time, the floating capacitor 32 of MO3FET 30 is charged to the polarity shown in the figure to the voltage E of DC amount a2, so when MO3FET 20 is turned off next time, As mentioned above, since the freewheeling diode 36 becomes conductive, the MOSF
During the period when both MO3FETs 20 and 30 are off until ET 30 is turned on, the DC power supply voltage E charged in the floating capacitor 32 is applied to the block diode 35. This phenomenon also occurs in other arms, so other block diodes 25, 45.
55 also each have a period in which the voltage E having the same value as the power supply voltage is applied.

For the reasons mentioned above, the block diode 25.35°4
5.55 must each use a high voltage diode that can withstand the power supply voltage.

The original mission of these block diodes is MOSF
Since it is used to protect the parasitic diode of the ET, it should be a low-loss diode, but since low-voltage products such as Schottky diodes cannot be used, the loss generated is large and the efficiency of the inverter device is affected. This has disadvantages in that not only the loss of heat is reduced, but also that the cooling device for removing the heat generated by this loss becomes large and the cost also increases.

Therefore, Japanese Patent Laid-Open Publication No. 157274/1983 proposes a means for solving the above-mentioned problem by connecting discharge diodes in antiparallel to each block diode.

FIG. 3 is a main circuit connection diagram showing a second conventional example of an inverter using MOSFETs, and is
This was proposed in Publication No. 157274.

The DC power supply 2 shown in FIG. Load 4. MOS F
E T 20,30.40,50. parasitic diode 21
, 31°41.51. Floating capacitor 22, 32.42
．． 52. Block diode 25.35.45.5
5 and the freewheeling diodes 26, 36 and 46, 56 are the same as in the case of FIG. 2, and therefore their explanation will be omitted. In FIG. 3, block diodes 25, 35, 45, and 55 are connected to discharge diodes 27,
．． By connecting 37, 47, and 57 in antiparallel, it is avoided that the charging voltage of the floating capacitors 22, 32, 42, 52 as described above is applied to the block diode 25, 35, 45, 55. In addition, the code 26Z, 36Z
, 46Z, 56Z are freewheeling diodes 26.36, respectively.
, 46.56 wiring impedance, code 27
2,372.47Z and 51Z are the impedances when wiring the discharge diodes 27.37 and 47.57 added this time, respectively.

[Problem that the invention seeks to solve]

As shown in Figure 3, in an inverter in which a discharge diode is connected in antiparallel to a block diode, MO3FET
MO with 40 off and MO3FET 50 on.
Turn on 3FETs 20 and 30 alternately at high frequency.
Let us consider the case where pulse width modulation control is performed by turning off. First, by turning on MO3FET20, DC power supply 2 → MO3FET20 → load 4 → MO3FET50
→ Current flows in the path of DC power supply 2, but at the next moment MO
When the 3FET 20 is turned off, the freewheeling diode 36 becomes conductive in order to allow the current ■ flowing through the load 4 to circulate as it is, and the current continues to circulate through the path of the load 4 → MOSFET 50 → diode 36 → load 4.

At this time, the wiring impedance 36 of the freewheeling diode 36
The value of Z is the wiring impedance 3 of the discharge diode 37.
If the value is larger than 7Z, this circulating current ■ becomes
The current 136 flowing through the wiring impedance 36Z and the freewheeling diode 36, and the current 12 flowing through the wiring impedance 37Z, the discharge diode 37, and the parasitic diode 31? It will be divided into two parts. Here the current 1st
If the value of is large, the reverse recovery characteristics of the parasitic diode 31 will be poor, resulting in the inconvenience of damaging the MO3FET 30. Therefore, if the number of discharge diodes 37 connected in series is increased in order to suppress the current rst, the cost will increase. Note that even when power is regenerated from the load 4 to the DC power source 2, there is a risk that current will similarly flow through the parasitic diode, which may damage the MO3FET.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to make a first diode connected in series to the semiconductor switch element with low withstand voltage and low loss when an inverter is configured with a semiconductor switch element having a parasitic diode and a floating capacitor, and to By limiting the current flowing through the parasitic diode, damage to this semiconductor switch element can be prevented, and
The object is to reduce the cost of an inverter device.

[Means for solving problems]

In order to achieve the above object, the inverter circuit of the present invention forms an arm by connecting a second diode in anti-parallel to a series circuit of a first diode and a semiconductor switch element having a parasitic diode and a floating capacitor. ,
In an inverter circuit configured by bridge-connecting a plurality of arms, a resistor is connected in parallel to the first diode of each arm.

[Function] When an inverter is configured with a semiconductor switch element having a parasitic diode and a floating capacitor, the present invention provides a means for discharging the charge of the floating capacitor, thereby reducing the number of inverters connected in series with the semiconductor switch element. 1 diode can be made to have low withstand voltage and low loss, but since current flows to the parasitic diode through this discharge means, the discharge means is replaced with a resistor, and the value of this resistance is set as the value of the floating capacitor. By selecting a value that can discharge the charge and sufficiently suppress the current flowing to the parasitic diode through the parenthesis resistance, damage to the semiconductor switch element can be prevented, and a low-cost product can be used as the first diode. I'm trying to make it possible.

〔Example〕

FIG. 1 is a main circuit connection diagram showing an embodiment of the present invention.

FIG. 1 shows the case of a single-phase inverter that converts DC power from a DC power supply 2 into single-phase AC power and supplies it to a load 4. The first phase upper arm of this inverter has a parasitic diode 21 and a floating MOSFET 20 as a semiconductor switch element having a capacitor 22, and this MOSFET
A block diode 25 as a first diode connected in series with ET20. and a freewheeling diode 26 as a second diode connected in antiparallel to this series circuit, which is the same as in the conventional circuit described in FIGS. 2 and 3. Note that the impedance when wiring the freewheeling diode 26 is denoted by 26Z.

Further, in this single-phase inverter, MOS FET as semiconductor switching elements forming the first phase lower arm, the second phase upper arm, and the second phase lower arm.
30.40.50. Parasitic diode 31, 41° 51
, floating capacitors 32.42.52, block diodes 35, 45.55., as first diodes. Freewheeling diode 36, 46.56 as a second diode and wiring impedance 36Z of this freewheeling diode.

The names, uses, and functions of 46Z and 56Z are also the same as in the conventional example shown in FIG. 2 or 3, so their explanation will be omitted.

In the present invention, the block diode 25 of each arm
．． 35.45.55 respectively discharge resistance 28.38,4
8°58 are connected in parallel. This discharge resistor replaces the conventional discharge diode and prevents voltage from being applied to the block diode by discharging the charge accumulated in the floating capacitor.

Therefore, it is no longer necessary to use an element with a high breakdown voltage as the block diode, so it is possible to select an element with low loss even if it has a low breakdown voltage. Furthermore, the disadvantage that the freewheeling current or regenerative current that should flow through the freewheeling diode is shunted to the parasitic diode is that the resistance value of this discharge resistor connected in parallel with the block diode cannot be easily discharged from the above-mentioned floating capacitor. , and by selecting a value larger than the impedance of the freewheeling diode and the wiring in step 7, the current value that is shunted to the parasitic diode with poor reverse recovery characteristics is suppressed, thereby reducing the risk of damage to the MOSFET. prevent it from happening.

(Effects of the Invention) According to the present invention, a discharge resistor is connected in parallel to a block diode for preventing damage to a parasitic diode that is connected in series to a semiconductor switching element such as a MOSFET having a parasitic diode and a floating capacitor. bridge connections to form an inverter circuit.

If the circuit is configured in this way and the resistance value of this discharge resistor is appropriately selected, the voltage of the floating capacitor charged to the power supply voltage value during inverter operation will not be applied to the block diode, and the newly installed Since the discharge occurs through the discharge resistor, there is no need to use a single high-voltage block diode. As a result, the cost of the block diode becomes low, so the overall cost can be reduced. Furthermore, this block diode has low breakdown voltage and
Low-loss diodes can be selected, which not only improves the efficiency of the inverter device, but also reduces the heat generated by the block diodes, which simplifies the heat dissipation device, making it smaller and lighter, thereby reducing costs. This can be expected, and by making the resistance value of this discharge resistor larger than the freewheeling diode and its wiring impedance, the value of the current shunted to the parasitic diode can be controlled. This also provides the effect of preventing the switch element from being damaged.

[Brief explanation of the drawing]

Fig. 1 is a main circuit connection diagram showing an embodiment of the present invention, and Fig. 24!
FIG. 3 is a main circuit connection diagram showing a first conventional example of an inverter using MOSFETs, and FIG. 3 is also a main circuit connection diagram showing a second conventional example of an inverter using MOSFETs. 2...DC power supply, 4...Load, 20.30,40.
50... MOSFET as a semiconductor switch element having a parasitic diode and a floating capacitor, 21.31,
41.51... Parasitic diode, 22, 32.42.
52...Floating Ki, Yanoshita, 25, 35.45.5
5...Fink diode as the first diode, 26.36.46.56...Teno freewheeling diode as the second diode, 26Z, 27Z, 36Z, 37Z
. 46Z, 47Z, 56Z, 57Z... Wiring impedance, 27.37, 4T, 57... Discharge diode,
28.38.48.58...Discharge resistance. Figure 1 Sub Figure 2 $3 Figure procedure amendment form) 1. Incident display special wurs12-2J)2j? tK3,
Person making the amendment d. Address related to the applicant π: 1-1 Tanabe Shinden, Kawasaki-ku, Kawasaki City Name (
8m$1 Fuji Electric Co., Ltd. 4, agent

Claims (1)

[Claims] 1) An arm is formed by connecting a second diode in antiparallel to a series circuit of a first diode and a semiconductor switch element having a parasitic diode and a floating capacitor, and a plurality of arms are connected in a bridge. An inverter circuit characterized in that a resistor is connected in parallel to the first diode of each arm.