JPH0231912Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to an improvement of a circuit for driving a field effect transistor (hereinafter referred to as FET) of a switching power supply device.

Conventional technology In order to make switching power supplies smaller and lighter, the switching frequency is being increased to a higher frequency. However, increasing the frequency increases losses due to the rise time, fall time, and storage time of switching elements, and the Conversion efficiency deteriorates. To improve this, it is necessary to shorten the rise time, fall time, and accumulation time, and as a switching element, FETs with essentially no accumulation time and very short rise and fall times are used. Things are happening more and more.

However, even in this case, since there is a capacitance component between the gate and source of the FET, the rise and fall times cannot be shortened unless this is rapidly charged and discharged, making it difficult to achieve high frequencies.

FIG. 2 is an example of an FET drive circuit in a conventional device of this type. In the figure, 1 is an output transformer, 2 is a FET, 3 is a pulse transformer that insulates the FET 2 and the control circuit, 4 is a vibration absorption resistor, 5 is a transistor that drives the pulse transformer 3, and 6 is a diode for resetting the pulse transformer 3. , 7 is a rectifier and smoothing circuit, and 12 is a DC power supply.

In FIG. 2, when the transistor 5 is turned on, a voltage is generated on the secondary side of the pulse transformer 3,
FET2 is turned on, but as is well known, there is a capacitance component between the gate and source of the FET, so charges are accumulated there. Next, when transistor 5 turns off, pass transformer 3 is reset by diode 6, and a voltage in the opposite direction is generated on the secondary side, which is stored between the gate and source of FET 2. The charge is discharged through the secondary winding of the pulse transformer 3 and the resistor 4, and the FET 2 is turned off.
In order to shorten the fall time when FET 2 turns from on to off, it is necessary to rapidly discharge this charge, so the impedance of the resistor 4 and the secondary winding of the pulse transformer 3 in Fig. 2 should be made as small as possible. There must be. However, resistor 4 is FET2
A voltage of about 10V is applied to the gate to completely turn on the
Due to the relationship between the sources and the supply, it is impossible to reduce the value to an extremely small value, and since the secondary winding of the pulse transformer 3 essentially has an inductance component, the impedance of this discharge path There is a limit to how small it can be. Therefore, with such a conventional circuit, it is not possible to shorten the fall time, and the conversion efficiency of the power supply deteriorates due to the increase in frequency.

Means for Solving the Problems The present invention eliminates the drawbacks of the conventional technology and provides an FET drive circuit that does not deteriorate the power conversion efficiency even when the frequency is increased. A switch element operated by the reset voltage of the pulse transformer is inserted between the gate and source of the FET, and the switch element short-circuits the gate and source when the FET is off.

Embodiments Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 1 shows an embodiment of a switching power supply device equipped with a drive circuit according to the present invention. Components corresponding to those in the conventional circuit are given the same reference numerals, and explanations thereof will be omitted. In Fig. 1, a diode 8 is connected between one end of the secondary winding of the pulse transformer 3 and the gate of the FET 2.
The pulse transformer 3 side is connected to serve as an anode. Further, a diode 11 is connected between the other end of the secondary winding of the pulse transformer 3 and the source of the FET 2. The polarity of the diode 11 is such that the pulse transformer 3 side is the cathode. Furthermore, the transistor 10 is connected such that the cathode of the diode 11 serves as the base, the anode of the diode 11 serves as the emitter, and the cathode of the diode 8 serves as the collector. Further, a resistor 9 is connected between the emitter of the transistor 10 and the anode of the diode 8.

In this circuit, when transistor 5 turns on, a voltage sufficient to turn on FET2 is generated on the secondary side of pulse transformer 3, and this voltage is applied to FET2 through diodes 8 and 11.
is turned on. In this state, the transistor 10 is turned off because its base and emitter are reverse biased by the diode 11.

Next, when the transistor 5 is turned off, the pulse transformer 3 is reset by the diode 6, so the secondary side is in the opposite direction, that is, the base of the transistor 10 is positive, and the diode 8
A voltage is generated that makes the anode negative. Then, current flows through the path of the base of the transistor 10 → the emitter → the resistor 9, and the transistor 10
is turned on. As a result, the impedance between the collector and emitter of the transistor 10 becomes extremely small, and the charge stored between the gate and source of the FET 2 is instantly released through this.
FET2 turns off quickly.

According to experiments, in a switching power supply with an output of 120W and a conversion frequency of 100KHz, the efficiency was improved by about 5% compared to the conventional example, and the loss during switching was significantly reduced.

As mentioned above, although the present invention has a very simple circuit configuration, it is possible to significantly shorten the fall time of the FET, and prevent the deterioration of the conversion efficiency of the power supply due to higher frequencies. An excellent practical effect can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the switching power supply drive circuit of the present invention, and FIG. 2 is a circuit diagram showing an example of a conventional device of this type. 1... Output transformer, 2... Field effect transistor, 3... Pulse transformer, 4, 9... Resistor,
5, 10...transistor, 6, 8, 11...diode, 7...smoothing circuit, 12...DC power supply.

Claims (1)

[Scope of utility model registration request] A switching power supply in which a field effect transistor that performs a switching operation is connected between a DC power supply and a load, and control pulses generated by a control pulse generation circuit are applied to the field effect transistor through the pulse transformer, the pulse transformer; a first diode connected between one end of the secondary side of the pulse transformer and the field effect transistor so that the secondary side of the pulse transformer serves as an anode; a base connected to the other end of the secondary side of the pulse transformer; a transistor whose cathode is connected to the cathode of the first diode, a second diode whose cathode is connected to the base of the transistor and whose anode is connected to its emitter, and a resistor which is connected between the anode of the first diode and the emitter of the transistor. Power supply drive circuit.