JPS6070973A - Dc/dc converter - Google Patents

Abstract

PURPOSE:To improve the conversion efficiency by providing a winding for a transformer exclusive for an FET, and self-oscillating under resonance condition of a floating capacity intrinsic for the FET and the inductance of the transformer. CONSTITUTION:When a DC voltage Vcc is applied between the drains and the sources of FET1, FET2, the voltage is applied also through resistors R1, R2 and capacitors C1, C2 to the gates, and since the used FETs are of MOS enhancement type, the drain current is not flowed when the voltage between the gate and the source is 0V, but when the voltage between the gate and the source of either FET exceeds the threshold of the FET, it becomes ON, and an oscillation is continued under the resonance condition of the floating capacity intrinsic for the FET and the inductance of the transformer by the start.

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a DC-DC converter using a self-commutated inverter used in a switching regirator, etc., and particularly relates to a stray capacitance unique to a field effect transistor as a switching element and a transformer. DC- using a self-excited inverter circuit that performs self-excited oscillation under resonance conditions due to inductance.
This relates to a DC converter.

(Prior Art) Conventionally, a self-excited inverter is a self-excited oscillation circuit that utilizes the saturation characteristics of a transformer, and a typical example thereof is the Royer circuit shown in FIG. As can be seen from the figure, the Royer circuit is the same as changing the CR coupling of a multipipulator circuit to an M coupling, and it uses two transistors Ql with the same characteristics.
, Q2 alternately perform switching operations, and generate power on the secondary side through a transformer.

Now, when DC voltage vco is suddenly applied between the collectors and emitters of transistors Ql and Q2, at the same time the starting resistance R
8, a forward bias is applied to transistors Ql and Q2, and a current flows between the collector and emitter. However, even if the transistors have the same characteristics, the current is exactly the same for both transistors due to differences in internal stray capacitance, etc. It is normal for one of the currents to be higher than the other, causing an imbalance.

On the other hand, if the current through Ql is large, the core is excited and the voltage induced in the winding NB is applied to the base of Ql (in the direction of increasing by the collector current of h, on the other hand, the voltage induced in the NB2 winding is The applied voltage acts in a direction to keep Q2 OFF.

This positive feedback action quickly turns Ql ONI Q2 OFF. At this time, the core is magnetized with a linear change, and the time changes in the core inductance and collector current remain constant unless Ql becomes unsaturated.

- When the core eventually reaches its saturation magnetic flux, the inductance decreases rapidly, and as a result, the time change in the collector current becomes infinite, as shown in the waveform in FIG. 2 (B).

That is, when the collector current reaches h□ times the base current, Ql
is unsaturated, and the induced voltage in each winding of the transformer decreases, so the supplied base current also decreases, and the collector current also begins to decrease accordingly.

Even when the core is saturated, it still has some residual inductance, and when the collector current decreases, this residual inductance generates an induced voltage opposite to the previous one and commutates, rapidly turning off Ql and turning off Q2. Oscillation is started by inverting it to ON.

In this way, the Royer circuit uses the collector currents of the two transistors to increase the magnetic flux level of the core to the saturation magnetic flux +
Oscillation is maintained by reciprocating between Φ□ and one Φ□.

However, in this Royer circuit, in order to magnetize the core to the saturation magnetic flux density, the excitation power of the core is large, and the transistor becomes unsaturated at the maximum value of the collector current, so the collector power consumption is large. That is, there is a drawback that core loss and collector loss during commutation are large. It is fine as long as this loss is very small compared to the output capacitance and does not have much effect on the conversion efficiency under load.
In an inverter circuit of W or less, the conversion efficiency deteriorates, resulting in a loss that cannot be ignored.

For example, it has been difficult to implement a DC-DC converter with an output power of 150 mW and an input power limited to 200 mW or less using a conventional inverter circuit.

(Objective of the Invention) In order to eliminate loss during commutation, which is a drawback of the Royer circuit, the present invention employs a field effect transistor (hereinafter referred to as F'ET) as a switching element, and also uses a dedicated transformer for each FET. The present invention provides a DC-DC converter using an inverter circuit (5) which has a winding and has good conversion efficiency that self-oscillates under resonance conditions due to the stray capacitance unique to the FET and the inductance of the transformer.

(Structure of the Invention) In order to achieve the above object, the drain of the first FET which is one switching element is connected to one end of the primary winding of the first transformer, and the drain of the first FET which is one switching element is connected to one end of the primary winding of the first transformer. The drain of the FET No. 2 is connected to one end of the primary winding of the second transformer, and the drain of the FET No. 2 is connected to one end of the primary winding of the second transformer. The sources of the second FETs are connected to each other, and the sources of the first and second FETs are connected to each other. The other ends of the primary windings of the second transformer are connected to each other, and the first...
Each drain of the second FET is connected to the second FET via a resistor and a capacitor connected in parallel with the resistor. The first FET is connected to the dart of the first FET. The connection point of the primary winding of the second transformer and the connection point of the primary winding of the second transformer. By applying a DC voltage between the sources of the 20th FET, the 1st. A self-excited inverter in which the second FET performs switching operations alternately, self-excited oscillation occurs under the resonance conditions of the stray capacitance unique to the FET and the inductance of the transformer, and generates AC power to the secondary side via the transformer. (6) A DC-DC converter characterized by having an evening circuit and converting the output into DC power through a smoothing circuit.

(Example) FIG. 3 is a circuit diagram showing one embodiment of the present invention.

Now, when DC voltage vcc is applied between the drain and source of FETI and FET2, it is also applied to each dart through resistor R1*R2 and capacitor C11c2,
Since the FETs used are of the MO8 type enhancement type, when the dart-source voltage is Ov, no drain current flows, but the gate-source voltage of either FET is equal to the threshold of that FET ( When the threshold value is exceeded, the circuit turns on, and upon activation, this circuit continues to oscillate due to resonance conditions caused by the stray capacitance unique to the FET (see FIG. 4) and the inductance of the transformer.

Now, if FET 1 is turned on, FE
T.

Since the drain voltage of FET2 becomes 0 level, the dirt voltage of FET2 also becomes OV level through R2C2.
is in the OFF direction. The drain-source voltage waveform of FETz becomes a resonant waveform as shown in FIG. 5(B), and the change in potential becomes a dart voltage of FET l through RIC, which appears as shown in FIG. 5(C).

The drain current of the FET is equal to the winding Nil of the transformer TI.
is excited to generate and output an induced voltage in the secondary winding N12. The drain current at this time increases linearly due to the inductance of the winding N11 as shown in FIG. 5), and the core of the transformer Tl does not reach saturation.

Eventually, the gate voltage of the FET decreases and when it reaches the cutoff voltage, the FET is turned off and no drain current flows. At the same time, the drain potential of FET 1 begins to rise under resonance conditions. Accordingly, the dart voltage of FET2 also gradually increases through R2C2, and when it exceeds a threshold, FET2 turns ON.

The drain current of FET2 excites the N21 winding of the transformer T2 and induces a voltage in the winding N22, thereby producing a large output.

Also, if the drain voltage of FET2 is ovtz-=L,
Since the dirt voltage of FET l also becomes the ov level through RICl, FET l remains OFF.

In this way, FET 1 is turned OFF and FET 1 is turned ON, and by repeating this, oscillation is continued.

Note that in this embodiment, FET1. FET2 is provided with transformers 'r, and transformer T2, but even if the primary and secondary windings are wound in an inner iron shape on each of the two legs except one E+ type cocoa center leg, or the EE type 2 on cocoa center leg
It goes without saying that the primary and secondary windings of the set may be wound together.

Furthermore, the oscillation conditions are determined by the stray capacitance unique to the FET and the inductance of the transformer, but if you want to change the oscillation frequency, you can do so by changing the inductance of the transformer, for example, as shown by the dotted line in Figure 3. Like FET
This can be easily done by adding a C3°C4 capacitor between the dart and source.

As an example, when this circuit was implemented in a power supply circuit with an output capacity of 150 mW, the conversion efficiency of the inverter circuit was 94%. Incidentally, when the conventional circuit was implemented in a power supply circuit with an output capacity of 150 mW like the present circuit, the conversion efficiency was around 50%. Even with this, it is clear how the circuit of the present invention was able to improve the conversion efficiency.

The AC power obtained by the self-excited inverter circuit of the present invention described above is transmitted through the diode D11D2 and the capacitor C.
75: is converted into DC power by a rectifying and smoothing circuit consisting of:

It goes without saying that other known circuits may be used for this rectifying and smoothing circuit.

(Effects of the Invention) As explained above, according to the present invention, an inverter that self-oscillates at a desired oscillation frequency determined by the stray capacitance unique to the FET and the inductance of the transformer and obtained by adjusting these. As a result, a DC-DC converter with high conversion efficiency was obtained, with significantly reduced loss during commutation compared to conventional self-excited inverter circuits.
If applied to circuits with even larger capacity, it can be expected to have lower loss and higher efficiency conversion, bringing about great effects.

[Brief explanation of the drawing]

Fig. 1 is a conventional Royer circuit diagram, Fig. 2 is a waveform diagram of each part thereof, Fig. 3 is a circuit diagram showing an embodiment of the present invention, Fig. 4 is an equinodal circuit showing the stray capacitance unique to FET, FIG. 5 is a waveform diagram of each part of an embodiment of the present invention. FET, , FET2...field effect transistor,
Cts8・tCoss r Cras --- Stray capacitance of FET, T0n T2-) Lance, Nlt +NH
...Primary winding, N121 N22...Secondary winding, D
I, D2...diode, CI + C2+ Cs
r C4+ C...Capacitor, Rt J R2*
R3+ R4"'Resistance, Vcc...'DC power supply. Patent applicant Oki Electric Industry Co., Ltd. Nippon Telegraph and Telephone Public Corporation Tohoku Oki Electric Co., Ltd. (11)

Claims (1)

[Claims] The drain of the first field effect transistor, which is one switching element, is connected to one end of the primary winding of the first transformer, and the drain of the second field effect transistor, which is the other switching element, is connected to one end of the primary winding of the first transformer. connected to one end of the primary winding; The sources of the second field effect transistors are connected to each other, and the sources of the first field effect transistors are connected to each other.
The other ends of the primary winding of the second transformer are connected to each other,
Said 1st. Each drain of the second field effect transistor is connected to the second field effect transistor through a resistor and a capacitor arranged in parallel with the resistor. 1st
is connected to the gate of the field effect transistor of the first field effect transistor. The connection point of the primary winding of the second transformer and the first
．． By applying a DC voltage between the sources of the second field effect transistor, the first field effect transistor. The second field-effect transistor alternately performs switching operations, self-oscillates due to resonance conditions caused by the stray capacitance unique to the field-effect transistor and the inductance of the transformer, and generates AC power to the secondary side via the transformer. A DC-DC converter comprising an excited inverter circuit and converting the output power into DC power through a smoothing circuit.