JPS596624A - Circuit for driving gate of mosfet - Google Patents

Abstract

PURPOSE:To reduce the loss of switching, by making the charging current of capacitance between drain and gate electrodes flow out from the gate electrode to the external. CONSTITUTION:If voltage applied between the drain and source is raised when an MOSFET 1 is in nonconductive state, charging current is made flow into the feedback capacity inside the MOSFET 1. A shunt circuit 4 makes the charging current flow out from the gate electrode to the outside. Therefore, the nonconductive state of the MOSFET 1 can be kept. The value of a resistor RGS constituting the shunt circuit 4 is determined to a value enabling the MOSFET 1 to be precisely nonconductive when the MOSFET 1 is turned off from the values of turning-on time of the MOSFET 1 and feedback capacity. When an MOSFET having the rating of about 400V and 8V is used, the value of RGS is <=87.5OMEGA. Care should be taken so that loss due to the resistors is increased when the RGS is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gate drive circuit for a MOSFET, and particularly to a gate drive circuit for a MOSFET.
The present invention relates to a gate drive circuit that can reliably maintain the non-conducting state even if the voltage applied between the main electrodes increases during the non-conducting period of an OSFET.

Figure 1 shows the equivalent capacitance inside the MO8FET and the MO8FET.
An example of an SFET gate drive circuit is shown. In the figure 1
is MOSFET XD, S, G are each MOSFET
The drain, source, and gate electrodes of Comal
cr'ss l C11g is an equivalent capacitance existing inside and is called an output capacitance, a feedback capacitance, and an input capacitance, respectively.

2 is a gate drive circuit, which includes a gate current limiting resistor 21,
It is composed of a transformer 22 and a transistor 23. Ea is a gate driving power source.

To change MO8F'ET l' from non-conducting state to conducting state, connect MOS to the polarity shown in the input capacitor 1tCt-'e.
It is sufficient to charge the battery to a value higher than the gate threshold voltage Vth determined by the characteristics of the FET.

Moreover, in order to change from a conductive state to a non-conductive state, it is sufficient to lower the voltage to below CI-kVtb, which is charged to the polarity shown in the figure.

In the gate drive circuit 2, the conduction of the transistor 23 causes the resistor 21, CI, to pass through the transformer 22.

Charge the voltage above Clear k Vtb with the current passing through the
Turn on all MOSFET 1.

When the transistor 23 is turned off, the excitation i current flowing in the transformer 23 is discharged through the source electrode, gate electrode, and resistor 21 of the MOSFET, and is charged to a voltage with the opposite polarity to that shown in the CI-'e diagram, and the entire MO8FET is charged. Turn off.

Figure 2 shows an example of MOSFET' used in a bush-pull type inverter.

In the figure, EM is the main power supply that supplies power to the load 3 via the main transformer TM, and 11.12 is the MOSFE in Figure 1.
This is a MOSFET similar to T1.

Since the operation of the push-pull type I/verter is widely known, a description of the operation will be omitted.

When both MOSFETs 11 and 12 are in the off state, the voltage of the main power supply EV is applied to both MO8FETs. Input capacity 1. Assuming that the charging voltage of is OV, at this time, the output capacitors 1C01 and feedback capacitor Cras are each charged to a voltage of EM with the drain electrode side of the positive polarity.

Next, when MOSFET 12 is turned on, the entire operation is completed. When rν108FET 12 turns on, a voltage twice as high as EM is applied to MOSFET 11, which is in a non-conducting state. Current flows. At this time, since the current charging Cram' flows into Cl M M , C16 is charged to the polarity shown in FIG. As a result, when C1, . is charged to Vth or higher, MOSFET 11 becomes conductive, transformer TM becomes short-circuited, and a short-circuit current flows through both MOSFETs. In particular, in the MO8L"ET 11, a current flows during the process of increasing the voltage from EM to twice the voltage of EM, so an extremely large loss occurs.

When MOSFET 11 turns on, a similar operation causes a large loss in MOSFET 12.

This loss increases as the MOSFET'e is driven at a higher frequency, making it difficult to drive the MOSFET at a higher frequency.
The disadvantage is that the high-speed switching characteristics, which are characteristic of , cannot be fully utilized.

The gate drive circuit of the conventional MO8F'ET is the above-mentioned M
It is not possible to prevent the loss caused by the equivalent capacitance inside the O8FET.

The purpose of the present invention is to provide a gate drive circuit that prevents a conductive state caused by the equivalent capacitance inside the MO8F'ET from occurring during a period in which the MO8F'ET is in a non-conductive state, thereby reducing the total loss of the MOSFET. Our goal is to provide the following.

When the MOSFET is in a non-conducting state, when the voltage applied between the drain and the source increases, the feedback capacitance C inside the MO8FET increases.
A charging current flows through 11. The present invention allows this charging current to flow from the gate electrode to the gate drive circuit, and
The input capacitance Cl # I of T is the gate threshold voltage V
The gate drive circuit has the function of completely preventing charging beyond sh, which makes it possible to reduce the loss of the MOSFET.

FIG. 3 shows an embodiment of the present invention. In this embodiment, a shunt circuit 4 is added to the conventional gate drive circuit shown in FIG. The shunt circuit 4 is composed of a resistor Ras. Conventionally, a resistor was sometimes connected between the gate electrode and source electrode of a MOSFET, but the main purpose of this was to prevent damage between the gate and source due to surge voltage.
The resistance value was often selected to be several hundred ohms to several ohms.

In order to completely prevent conduction of the MOSFET due to the equivalent capacitance inside MO8FET, it is better to select a small value for the resistance Rag'e in Figure 3, but if you select a small value for 'fLG11, the loss caused by Ros will become large. Become.

Therefore, it is important to select the input capacitance CI m s to an appropriate value that can be maintained below the gate threshold voltage Vth by the charging current of the feedback capacitor C21.

The appropriate value of the resistor R(]I will be discussed, taking as an example the case where the entire gate drive circuit shown in FIG. 3 is applied to the bush-pull type inverter shown in FIG.

When MOSFET 11 is turned on, the voltage applied between the drain and source of MO8Ji''ET 12, which should be in a non-conducting state, is the voltage increase rate dV/, which is expressed by the following equation, if the leakage inductance of the main transformer TM and the wiring inductance are ignored. It rises at dt.

(1) Nionioi-71:', t,, (7) MOSF
ET 11 (D is the turn-on time. By dV/dt in equation (1), the charging current tr
am is given by the following formula.

bs i ram ” Cram・-1...Clap...(2
)t, Il 1! 16 flows into CI m m and CI a
As the voltage across m increases, the resistance increases. Current flows through S.

MOSFET 11 (D turn-on time, MO8F
The electric power ff1Qos flowing through the resistor RGll provided in the gate drive circuit of the ET12 is expressed by the following equation.

Also, MOSFET 11(r) turn-on time KC,
,.

The amount of electric charge Qr of the current for total charging is as follows from equation (2).

EM Q□=/ C□5・□・di ' ton Since the difference between Qra and QcsO is the charge amount of Cl@a, MOSFET 12F, to ensure it is in a non-conducting state, if the next winter crop is satisfied. good.

Q, Hatake -Qcs cl,, -(v... °゛°°町°) Currently commercially available 400V, 8. Using A grade MO8F'ET gold,
Find Ros k that satisfies equation (5). This MO8F
'ET is Cram I Com5 20p each
F.

800pF', turn-on time t and u are 5oμS
.

Vth is IV. In addition, EM is set to 140V in consideration of the DC voltage obtained from a commercial 100V frog current.

When calculating Ros f using these conditions, Ras is 8
Must be selected to be 7.5Ω or less.

From the above results, it is clear that the MOSFET cannot be reliably kept in a non-conducting state unless the resistance value is selected to be about 1/10 of the resistance value conventionally provided between the gate and source for the purpose of protecting from surge voltage. I understand.

According to this embodiment, even if the voltage applied to the MOSFET increases during the non-conducting period, the input capacitance CIms does not exceed the gate threshold voltage Vth, so the MOSFET is reliably
Since the off-state of the MOSFET can be maintained, the loss of the MOSFET can be significantly reduced.

FIG. 4 shows another embodiment. In this embodiment, the shunt circuit 4 shown in FIG. 3 is composed of a transistor 26. In the figure, 25 is a base current limiting resistor of the transistor 26, 24 is a transformer for driving the transistor 26, 27 is a transistor for switching the entire base current of the transistor 26, 5IGI, 8IG2
are driving signals for transistors 23 and 24, respectively.

The operation of switching the MOSFET i is similar to that shown in FIG. 3, and the transistor 23 may be driven by the SIO1. When the voltage applied between the drain and the source increases during the non-conducting period of MOSFET 1, SIO2 makes transistor 27 conductive and turns on transistor 26. Due to the conduction of the transistor 26,
The gate electrode of MOSFET 1 is fixed at a potential higher than the source electrode by the saturation voltage Vcg of transistor 26.

If you ignore all the internal resistance of MOSFET 1, the input capacitance C
,,, will be charged to Vcg, so the VCE? of the transistor 26? If you set the following conditions, MOSFET
1 = can be reliably maintained in a non-conductive state.

VcE<Vth ・・・・・・・・・(
6) The case where the gate drive circuit shown in this embodiment is applied to the bush-pull type inverter shown in FIG. 2 will be described next.

When MO8FE'r 12 turns on, MO81
FisIGl is input to the gate drive circuit of i'ET12.

If this signal 'eMO8 is input as SIO2 to the gate drive circuit of the FET 11, the gate electrode of the MOSFET 11 will be at a voltage Vcm higher than the source electrode by the transistor 26 shown in FIG. M.O.
When SFET 12's ter7 is on, MO8F'BT11
Although the voltage between the drain and source of MOSFET 11 increases, the relationship shown in equation (6) allows MOSFET 11 to remain non-conductive.

When MOSFET 11 turns on, the MOSFET
Signal 8 IGI input to the gate drive circuit of T11
, MOSFET 12 gate drive circuit KS IO2
By inputting as , the non-conducting state of MO8F'ET can be maintained.

As described above, according to this embodiment, as in the embodiment shown in FIG.
In addition to preventing conduction of SFET and reducing the loss of MO8F'ET, since the transistor 26 is made conductive only when the voltage applied to the MOSFET increases, the loss of the resistor Ram can be reduced as in the embodiment shown in FIG. This has the effect of completely reducing the loss of the gate drive circuit.

FIG. 5 shows another embodiment. In this example, MOSFET
When the voltage applied to the MOSFET increases, the gate electrode of the MOSFET is reverse biased so that the charging current corresponding to the equivalent capacity is forced to flow out to the gate circuit.

In the figure, El is MOSFET l When fully conductive,
A power supply for biasing the gate electrode to positive polarity through the transistor 5 and the resistor 7, 8 a current limiting resistor for the base of the transistor 50, and E2 a power supply for reverse biasing the gate electrode of the MOSFET through the transistor 6 and the resistor 9. The resistor 10 is a base current limiting resistor of the transistor 6. Transistor 13 switches transistor 5 via transformer 11, transistor 14 switches transistor 6 via transformer 12, and E3 supplies base current to transistors 5 and 6. It is the power source.

To make MOSFET l conductive, signal 5IGI
turns on transistor 13 and Et
The voltage 'e is applied to the gate of MO8FET.

When turning off MOSFET lk and MOS
When the voltage applied to FET 1 increases and a charging current flows through the feedback capacitor, transistor 14 is turned on by 5IG2, and MOSFET is turned on by the voltage of power supply E2.
The gate electrode of No. 1 is reverse biased.

According to this embodiment, the gate electrode of the MOSFET is connected to the power source E.
2, the input capacitor C16 does not reach Vth and can reliably maintain the non-conducting state of the MOSFET 1. Therefore, as the capacity of MOSFET increases, the internal feedback capacitance ic increases, and even if the charging current of %I increases, the MO8FET'
It has the effect of maintaining the non-conducting state.

FIG. 6 shows another embodiment. This embodiment is similar to the actual example shown in FIG. 5 by adding resistances R and s'(r).

The effect of adding Rsi will be described using FIG.

MO8I at the time to! When the turn-on signal of 'ET is input, the input capacitors i c , . . . begin to be charged.

Charging starts from the state where the charging voltage of Cl@@@ is OV, and at a time point tl reaches Vth, and the MOSFET starts to turn on. However, as shown by the broken line, Cl
From the state where l @ is reverse biased to the voltage of E, MO
When a turn-on signal is input to SFET, CI m
The time point when the charging voltage of s reaches Vtb is 12, and M
The turn-on time of the OSFET becomes longer.

As shown in FIG. 5, when the MOSFET is turned off and
When a charging current flows through the feedback capacitance Cr@a, when charging the input capacitance C,,, + reverse polarity, when all the MOS FETs are turned on, as shown by the broken line in Fig. 7, Cl
Since MO8FET't is turned on with a reverse biased, turn-on time becomes longer and high frequency drive is required. It will be preventable. Resistor R8 prevents this. In other words, during the non-conducting period, MO8F'
The applied voltage of ET increases, and the feedback capacitance G' r a a
The signal 5IG3 is pulsed for the time that the charging current flows.
is input, and while transistor 6 is conducting, the input capacitance c,...' 2 is reverse biased to maintain the non-conducting state of MO8F'ET, but even after transistor 6 is turned off, CIms remains reverse biased. It is in a state of being This CI-'r: MO8F'ET k is discharged by the resistor R8 until it is turned on next, and as shown by the solid line in Fig.
It is designed to enable the turn-on operation of 'l'.

The L7'j 400V, 8A class MO8FET 'e mentioned above is applied to the bush-pull type inverter shown in Fig. 2, and the output frequency is 200 k1. (The RaO value when driving so as to be z will be described next.

Assuming that C1) is charged to -15V by power supply E2, 95% will be charged by the next time the MOSFET turns on.
C at the time of turning on when applying pressure to discharge
For 1 mg, 0.75 V of reverse bias voltage remains. In the bush-pull type inverter shown in Figure 2, the output frequency is '1200k) (to set it to Z, MOSFET
There is a time of 2.5 μs after MOSFET 12 is turned on until MOSFET 1 + is turned on. M.O.S.
After FET 11 turns on, MOSFET 1
The same holds true for the time it takes for 2 to turn on. Therefore 8
C2 of 009F.

from a charging voltage of 15V to 0.75V in 2.5μs
The resistance Rs' required for discharging up to
几8 can be obtained from the following equation.

From equation (7), R8 becomes 61Ω.

If Rsi is determined in this manner, when the MO8F'ET starts turn-on operation, charging of elms starts at almost oV, which has the effect of preventing the turn-on time from becoming long.

FIG. 8 shows another embodiment. In this embodiment, the main transformer TM of the bush-pull type inverter shown in the filcz diagram is
It is provided with windings n2+*nzzk. In the figure, nII + 012 is the primary winding of the main transformer TM, and the gate circuit 10 is the gate drive transformer 101,
It is composed of a gate current limiting resistor 1o2, a gate driving transistor 103, a resistor 104, a diode 105, and a resistor Rs.

Now, consider the case where MOSFET 12 is turned on. When MOSFET 12 turns on, the main transformer T
A voltage whose polarity is positive as shown by the black circle in the figure is generated in each winding of w.

At this time, the winding n22 is connected to the gate and source of MOSFET 11, and since the gate is biased to negative polarity, the input capacitor C11 is also charged to the opposite polarity, and MO8FE'r
The conduction of MOSFET 11 is completely blocked when MOSFET 12 is turned on. Moreover, the voltage generated in the winding n21 is MOS
No current flows through the winding gnat because it is blocked by the diode 105 in the gate drive circuit 10 provided in the FET 12. 7tMO is reversely charged by the voltage generated at 122.
The input capacitance CI a m of 8FET 11 is MOS
It is discharged through resistor Rs' until FET 11 is turned on.

When MOSFET 11 is turned on, the conduction of MOSFET 12 can be completely blocked by a similar operation.

As stated above, according to this embodiment, when the voltage applied to the MOSFET increases, the power to reverse bias the gate electrode can be obtained from the main power source EV, so the power of the gate drive power source Ea can be reduced, and Reliably MO8li' with a simple configuration
It has the effect of completely blocking ET conduction. Although this embodiment has been described using a bush-pull type inverter as an example, it goes without saying that it can also be applied to a bridge type inverter, a half 7' bridge type inverter, etc.

According to the present invention, when the applied voltage to the MOSFET that should be in the non-conducting period increases, the M
This is effective in preventing conduction of the OSFET and reducing loss in the MOSFET.

According to the inventor's experiments, when driving at 200kHz2, the switching loss of the MOSFET without taking the measures according to the present invention is 13.7W, while when the measures according to the present invention are taken, the switching loss is 4.2W. Then,
It was found that the loss could be suppressed to less than 1/3.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of an equivalent container inside an MO8FET and a conventional gate drive circuit, Fig. 2 is a diagram illustrating problems when a conventional gate drive circuit is applied to a bush-pull type inverter, and Fig. 3 is a diagram showing an example of a conventional gate drive circuit. 4.5 and 6.8 are diagrams showing other embodiments of the present invention.
This figure is a diagram explaining the operation of the embodiment of FIG. 6 of the present invention. 1...MOSFET, Rm...resistance for discharging input capacitance, EM...main power supply, TM...main transformer, 几G8.
...Resistance, answer 7m 'ttrt, t2

Claims (1)

[Claims] 1. A MOSFE characterized by a shunt circuit that drains the charging current of the feedback capacitance existing between the drain and gate electrodes of the MOSFET to the outside from the gate electrode.
T gate drive circuit. 2. In claim 1, the shunt circuit comprises:
A MOSFET gate drive circuit configured by connecting a resistor between the gate and source electrodes of the MO8FET and having nine features. 3. In claim 1, the shunt circuit comprises:
1. A gate drive circuit for MO8F'ET, characterized in that all transistors that are closed during a non-conducting period of the MO8FET are connected between both gate and source electrodes. 4. In claim 1, the shunt circuit comprises:
A power source that applies a reverse bias between the gate and source electrodes of the MO8li"ET, and a power source that is connected in series with this power source and the M
1. A gate drive circuit for a MOSFET, characterized in that a transistor that is closed during a non-conducting period of O8li'ET is connected between both gate and source electrodes. 5. In claim 4, the MO8FET
A gate drive circuit for a MOSFET, characterized in that a resistor is added between the gate and source electrodes of the MOSFET.