JPH0572192B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an inverter circuit with improved switching characteristics at turn-off of a field effect transistor (hereinafter referred to as MOSFET) used as a switching element.

[Prior Art] MOSFETs can be turned on and off at high speed, but the turn on and off between the drain and the source depends on the rise and fall speed of the voltage between the gate and the source. In reality, there is a relatively large capacitance Ciss between the gate and source, so the charging speed directly regulates the turn-on and turn-off speed of the MOSFET.

Further, between the control voltage application circuit 1 and the gate of the MOSFET 2, as shown in FIG. 1, a small resistor 3 for preventing parasitic vibration is usually inserted. This parasitic vibration prevention resistor 3 is provided to prevent parasitic vibration between the gate-source capacitance Ciss of the MOSFET 2 and the control voltage application circuit 1.

[Problems to be Solved by the Invention] In FIG. 1, even if we try to speed up the voltage application between the gate and source of MOSFET 2 by inserting transistors 4 and 5, the parasitic vibration prevention resistor 3 Therefore, the rate of change of the gate-source voltage Vgs is limited. In addition, in Figure 1, 6
is the main transformer, and 7 is the rectifier and smoothing circuit on the secondary side.
Further, the diode in parallel with the resistor 3 is according to the present invention, and detailed explanation will be given later.

"Means for Solving the Problems" The present invention has been made to solve the problems of the conventional art as described above, and is composed mainly of a field effect transistor 2 and a main transformer 6, This is an inverter circuit that starts oscillation of an effect transistor 2 using a starting means to trip a DC power supply, and includes an auxiliary winding 9 provided on the main transformer 6 between the gate and source of the field effect transistor 2. A control voltage application circuit 1 consisting of a resistor 10 and a saturation transformer 11 is inserted, and a parasitic vibration is generated between one end of the secondary winding 13 of the saturation transformer 11 of the control voltage application circuit 1 and the gate of the field effect transistor 2. In an inverter circuit formed by inserting a small resistor 3 for prevention, a diode 14 is inserted in parallel with the resistor 3 with its anode facing toward the gate of the field effect transistor 2, and the 2 of the saturation transformer 11 is A diode 15 is inserted between the other end of the next winding 13 and the source of the field effect transistor 2, with the anode facing toward the source of the field effect transistor 2.
This is an inverter circuit characterized in that a capacitor 20 is inserted in parallel with the inverter circuit.

[Function] When the diode 14 of the present invention is coupled to both ends of the resistor 3, vibrations are prevented by the resistor 3, and it is possible to increase the speed of change of the gate-source voltage Vgs to the maximum. That is, first, the saturation and voltage drop of the saturation transformer 11 are the same as in the conventional example, but the charge Ciss between the gate and the source is rapidly discharged through the diode 14, and a steep discharge current flows, which causes the gate, The source-to-source voltage drops rapidly, and the drain-to-source voltage quickly begins to bear the load, which significantly improves the switching characteristics over the conventional system.

[Example] Hereinafter, an example of an inverter circuit using MOSFETs according to the present invention will be described based on the drawings starting from FIG.

FIG. 2 shows a basic circuit diagram of the forward type according to the present invention, and FIG. 3 shows a specific circuit diagram of FIG. 2. In these figures, + and - are positive and negative DC input power supply terminals, which are connected from one power supply terminal + to the other power supply terminal - through the primary winding 8 of the main transformer 6, the drain and source of MOSFET 2. There is. Furthermore, a control voltage application circuit 1 is coupled to the gate and source terminals of the MOSFET 2. This control voltage application circuit 1 includes the main transformer 6
Both ends of the primary winding 12 of the saturation transformer 11 are connected via a resistor 10 to both ends of the auxiliary winding 9 provided at the 13 to the gate of MOSFET 2, and the other end is connected to the source of MOSFET 2.

In the present invention, in particular, a diode 14 is inserted in parallel with the resistor 3 so that its anode faces toward the gate of the MOSFET 2.

Further, the secondary winding 13 of the saturation transformer 11 and
A diode 15 is inserted between the sources of MOSFET 2, and between the cathode side of this diode 15 and the positive side terminal +, there is a resistor 16 and an electronic switching element 18 that is opened and closed by an auxiliary winding 17. A means 19 for eliminating the influence of the activation side is added later. Further, a capacitor 20 having a capacitance larger than the charge Ciss between the gate and the source is coupled in parallel with the diode 15, and an electronic switching element 22 that is opened and closed by an auxiliary winding 21 is connected in parallel to the diode 15 after startup. Means 23 for extinguishing the voltage are coupled.

The secondary winding 24 of the main transformer 6 is connected to the output terminal 2 via a rectifying and smoothing circuit 7 and an error amplifying circuit 25.
6 and 27 are combined.

Next, the operation of the circuit shown in FIG. 3 will be explained with reference to the waveform diagram during normal operation shown in FIG.

First, an outline of the self-oscillation operation by the saturation transformer 11 will be explained.

At startup, when the electronic switching element 18 is turned on,
Positive side terminal + → electronic switching element 18 → resistor 16 → secondary winding 13 of saturation transformer 11 → resistor 3 →
A circuit from the gate and source of MOSFET2 to the negative terminal is formed. Then, MOSFET 2 turns on, and the positive terminal + → primary winding 8 of main transformer 6 →
A circuit from the drain and source of MOSFET2 to the negative terminal is formed and starts up.

After startup, the electronic switching element 18 is turned off by the induced voltage in the auxiliary winding 17 of the main transformer 6 → the primary winding 8 of the main transformer 6 → the drain and source of MOSFET 2 →
Drain current flows to the negative terminal.

Next, voltage V ₉ is taken out from auxiliary winding 9 of main transformer 6 in FIG. 3 and supplied to saturation transformer 11 via current limiting resistor 10. saturation transformer 1
The secondary winding 13 of 1 is connected to the MOSFET through the resistor 3.
Since it is connected between the gate and source of No. 2, the fifth
Between T ₁ and T ₂ in the figure, the voltage V ₉ is applied as the gate-source voltage Vgs via the saturation transformer 11, and the conduction of the MOSFET 2 is maintained. Note that since almost no gate current Ig flows during conduction, it is safe to assume that V ₉ ≈Vgs.

Near time T ₂ , when the saturation transformer 11 experiences a constant voltage-time product, the saturation transformer 11 becomes saturated. When saturated, the saturation transformer 11 becomes as if it were short-circuited, so the gate-source voltage Vgs is lost and it is cut off at _T2 . At this saturation,
The current from the auxiliary winding 9 is limited by a current limiting resistor 10.

During T ₂ - T ₃ , MOSFET 2 remains cut off, but when the main transformer 6 turns on MOSFET 2 (T ₁ -
When it receives a constant voltage time product of the same amount as that received at T ₂ o'clock) and reaches T ₃ o'clock, inversion starts, and the gate,
The source-to-source voltage Vgs becomes a positive voltage, which
When the gate threshold voltage of MOSFET2 is exceeded, it turns on and starts conducting, and the oscillation operation is repeated in the same manner.

Next, the operation of each part during turn-on and off-time in the case of the conventional circuit without the diode 14 of the present invention will be explained with reference to FIG.

First, when the saturation transformer 11 becomes saturated and short-circuited at time t ₁ , the saturation transformer 11 rapidly increases its voltage.
V ₁₁ disappears. However, due to resistance 3,
The charge Ciss stored between the gate and source of MOSFET 2 becomes a finite current Ig and flows through the resistor 3, the secondary winding 13 of the saturation transformer 11, and the capacitor 20, thereby increasing the voltage Vgs between the gate and source. begins to descend at a relatively slow speed, as shown by the dashed-dotted line in Figure 6. At _T2 , when the voltage Vgs between the gate and the source becomes less than the gate threshold voltage, the voltage Vds begins to be applied between the drain and the source. In other words, start turning off. Below, this gate,
Following the decrease in the source voltage Vgs, the drain-source voltage Vds increases and reaches time _t3 , at which point turn-off is completed.

As explained above, the drain-source voltage
The speed of increase in Vds, that is, the turn-off speed, is proportional to the change in the gate-source voltage Vgs. For this reason, it is desirable to make the resistance 3 small. However, this resistor 3 needs to have a certain capacity or more in order to prevent vibration between the gate and the source capacitance Ciss.

Next, in the present invention, the diode 14 is connected to the resistor 3.
Since the resistor 3 is coupled to both ends of the resistor 3, vibrations are prevented by the resistor 3, and the speed of change in the gate-source voltage Vgs can be increased to the maximum. The above situation will be explained with reference to FIG.

First, the saturation of the saturation transformer 11 and the drop in the voltage _V11 at time _t1 are the same as in the conventional example shown in FIG.
However, in the present invention, the charge Ciss between the gate and source of MOSFET 2 is transferred through the diode 14.
Secondary winding 13 of saturation transformer 11, capacitor 20
It is rapidly discharged by the circuit of t ₁ , t ₂ ,
A steep discharge current similar to the gate current between _t3 flows, which causes the gate-source voltage Vgs to drop rapidly, which causes the drain-source voltage to drop.
Vds begins to bear the voltage rapidly, which improves its switching characteristics significantly over the conventional method. By the way, the current Idc between t ₂ and _{t 3} in Figure 7 is when the capacitance 20 between the drain and the source is charged, and this is returned to the power supply at turn-on, so no power loss occurs and the overall Efficiency is also greatly improved. Here, the capacitance of the capacitor 20 is the charge Ciss between the gate and source of MOSFET 2.
It needs to be larger than .

Although the present invention does not improve turn-on, in converters where power and voltage are controlled on the output side as shown in Figures 3 and 4, the drain current at turn-on is zero. In combination, the improvement only at turn-off is sufficient for the entire circuit. This means that even in the flyback system shown in FIG. 1, the turn-on current is almost zero, so by inserting the diode 14 in parallel with the resistor 3, the same effect at turn-off as in the above case can be obtained.

The present invention uses two MOSFETs as shown in Figure 4.
Even when 2a and 2b are coupled in a push-pull manner, it can be used as is, and its operation is substantially the same as described above.

[Effects of the Invention] (1) The electric charge Ciss accumulated between the gate and source of the field effect transistor 2 is transferred through the diode 14, the secondary winding 13 of the saturation transformer 11, and the capacitor 20, without passing through the resistor 3. The switching characteristics at turn-off can be significantly improved. In particular, it works most effectively on ultra-high frequency inverter circuits that use field effect transistors 2 as switching elements, and utilizes this oscillation frequency to the limit of the characteristics of field effect transistors 2 to create ultra high frequency converters of 1000 to 1500 KHz. This can be achieved without any decrease in efficiency.

(2) Since the electric charge charged in the capacitor 20 is returned to the power source when the field effect transistor 2 is turned on, no power loss occurs and the overall efficiency is greatly improved.

(3) Insert a diode 14 in parallel with the parasitic vibration prevention resistor 3, and
A diode 1 is connected between the other end of the next winding 13 and the source of the field effect transistor 2 with the anode facing toward the source of the field effect transistor 2.
5 and a capacitor 20 in parallel with this diode 15, the initial purpose can be achieved with only a slight increase in the number of components.
There is almost no cost increase.

[Brief explanation of the drawing]

Figure 1 is a flyback type inverter circuit diagram using MOSFETs, Figure 2 is a basic electric circuit diagram of a forward type inverter circuit, Figure 3 is a basic electric circuit diagram of an inverter circuit according to the present invention, and Figure 4 is a basic electric circuit diagram of a forward type inverter circuit.
Figure 5 is an electric circuit diagram of another embodiment of the present invention, Figure 5 is an output waveform diagram during self-excited oscillation, Figure 6 is a waveform diagram with the time axis enlarged at turn-off in the conventional circuit, and Figure 7 is FIG. 3 is a waveform diagram showing an enlarged time axis during turn-off in the present invention. 1... Control voltage application circuit, 2, 2a, 2b...
MOSFET, 3, 3a, 3b...resistance, 6...main transformer, 7...rectifier smoothing circuit, 9...auxiliary winding,
10...Resistor, 11...Saturation transformer, 14,14
a, 14b...Diode.

Claims (1)

[Claims] 1 Mainly composed of a field effect transistor 2 and a main transformer 6, the field effect transistor 2
The inverter circuit starts oscillation by a starting means and chops the DC power supply, and the inverter circuit includes an auxiliary winding 9 and a resistor 10 provided on the main transformer 6 between the gate and source of the field effect transistor 2. A control voltage application circuit 1 consisting of a saturation transformer 11 is inserted, and a parasitic vibration prevention circuit is installed between one end of the secondary winding 13 of the saturation transformer 11 of the control voltage application circuit 1 and the gate of the field effect transistor 2. small resistance 3
In the inverter circuit, a diode 14 is inserted in parallel with the resistor 3 with its anode facing toward the gate of the field effect transistor 2, and a diode 14 is inserted in the secondary winding 1 of the saturation transformer 11.
A diode 15 is inserted between the other end of the field effect transistor 3 and the source of the field effect transistor 2, with the anode facing toward the source of the field effect transistor 2, and a capacitor 2 is connected in parallel with the diode 15.
An inverter circuit characterized by inserting 0.