JPH0578274B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a flyback type DC-DC converter used as a DC power source for various electrical devices, and particularly to a flyback type DC-DC converter that is used as a DC power source for various electrical devices. This relates to a converter configured as a current resonance type to eliminate the problem.

[Conventional technology]

FIG. 4 is a principle diagram of a DC-DC converter circuit, where E is a DC voltage source and R _L is a load. Assuming that the voltage applied to the switch S is V _s and the current that flows is I, in this circuit the current I rises rapidly, so the voltage V _s and the current I change when the switch S is turned on and off, as shown in Figure 5A. There is a period in which the
The switching loss becomes large as shown in the figure (b).

In order to reduce this switching loss, it has been considered to use a resonant circuit to make the current or voltage waveform into a sine wave, and turn the switching element on and off at its zero point. In the resonant converter configured in this manner, the switch elements can be turned on and off during periods when no current flows. Therefore, a converter circuit with extremely small energy loss during switching and low noise can be obtained, which is extremely advantageous in increasing the frequency of the converter circuit.

FIG. 6 shows an example of such a conventional single-ended current resonant flyback converter.
DC power supply E with voltage V _i and primary winding of transformer T
_N1 and the switching transistor Q form a closed loop, and the transistor Q is driven on and off by a drive pulse signal output from the pulse signal generator P. When the transistor Q is on, an exciting current flows through the primary winding _N1 and the transformer T
When transistor Q is turned off, this electromagnetic energy is stored in diode D.
It is rectified and smoothed by capacitor C and then supplied to load R _L. This point is similar to a general flyback converter.

A series circuit of an inductor L _R and a capacitor C _R connected in series to the secondary winding N ₂ of the transformer T is
This corresponds to the resonant circuit for reducing the switching loss mentioned above. Of the two circuit constants, the inductor L _R is partially or completely connected to the transformer T.
It may be configured to obtain a leakage inductance of .

Next, the operation of this resonant circuit will be explained with reference to FIG.

If no current flows through the secondary winding _N2 of the transformer T, when the transistor Q turns on, the input voltage
The seventh is determined by V _i and the inductance of the primary winding N ₁ .
The excitation current shown in Figure A will flow through the primary winding _N1 .

However, when the transistor Q is turned on, a voltage corresponding to the input voltage V _i and the turns ratio of the transformer T is actually generated in the secondary winding _N2 , so a sinusoidal resonant current flows in the secondary winding _N2 . I ₂ flows. Since the polarities of the primary winding _N1 and secondary winding _N2 of the transformer T are as shown by the dots in Fig. 6, the resonant current _I2 is
A is the connection point between inductor L _R and capacitor C _R
Flows in the direction that makes the potential at a point negative. Figure 7 B is A
Shows the change in potential at a point. The direction of flow of the resonant current I ₂ changes to the opposite direction from the time when the potential at point A exceeds the negative maximum value, and the capacitor C _R is charged again.
At 3/4 of the resonance period of the inductor L _R and the capacitor _CR , this resonant current I ₂ in the opposite direction becomes the largest.

The amplitude of the resonant current I ₂ is determined by the characteristic impedance of the inductor L _R and the capacitor C _R and the applied voltage. Furthermore, the current I ₁ flowing through the primary winding N ₁ becomes a composite current of the excitation current and the resonant current I ₂ . Therefore, at time t ₂ which is the final point of the on-time of the transistor Q, the resonant current I in the opposite direction If the peak of ₂ is determined to coincide with the excitation current, the current I ₁ flowing through the primary winding N ₁ becomes zero at time t ₂ as shown in the figure.

That is, the composite current _I1 is zero when the transistor Q is on, and is also zero when the transistor Q is off, making it possible to switch the transistor Q at zero current. Figure 8 shows the relationship between voltage and current during switching in the same way as Figure 5.
Figure A and B. As is clear from A.
Collector-emitter voltage V _CE of transistor Q
The change in current I 1 is steep, but in this circuit, the change in current I ₁ is gentler than in the non-resonant type. For this reason,
When the transistor Q is on or off, the period during which the voltage V _CE and the current I ₁ overlap is reduced.
As a result, the loss during switching is improved as shown in (b).

[Problem that the invention seeks to solve]

However, the conventional circuit shown in FIG. 6 has a problem in that the conditions for realizing switching with zero current are limited. That is, once the on-time T of the transistor Q, the inductance L of the primary winding _N1 , and the ratio n of the primary winding _N1 and the secondary winding _N2 are determined, the values of the inductor L _R and capacitor C _R are as follows. It is determined that

L _R = 2L / 3πn ² C _R = 2T ² n ² / 3πL Generally, in a converter, the number of turns n is determined by the values of the input voltage and output voltage, and the inductance L of the primary winding _N1 is determined by the output power. ing. For this reason, conventional resonant converters had the disadvantage that resonant circuits with different circuit constants had to be designed for each model with different specifications.

[Purpose of the invention]

The purpose of the present invention is to reduce switching loss by turning on and off the switching element when the current value is zero, and also to increase the current so that even converters with different specifications can use a resonant circuit with the same configuration. An object of the present invention is to provide a resonant flyback converter.

[Means for solving problems]

In the present invention, a tertiary winding is provided in the transformer, and a series resonant circuit consisting of an inductor and a capacitor is connected in series to this tertiary winding, and the current of this series resonant circuit changes the ON period of the switch element to 3/4 period. It is characterized by a structure in which the current flowing through the primary winding when the switch element is turned off is almost zero by resonating at the natural frequency of the switch element.

〔Example〕

An embodiment of a current resonant flyback converter according to the present invention is shown in FIG. In this figure as well, parts corresponding to those in the conventional example shown in FIG. 6 are given the same reference numerals.

The transformer T is provided with a tertiary winding N ₃ in addition to the conventional primary winding N ₁ and secondary winding N ₂ . At both ends of the tertiary winding _N3 , there is an inductor L _R connected in series.
A series resonant circuit consisting of a capacitor CR and a capacitor C _R is connected. Point B, which is the connection point between the tertiary winding N ₃ and the capacitor _CR , is connected to one end of the load R _L in order to fix the potential of the tertiary winding N ₃ . Since no current flows between this point B and one end of the load _RL , the point B may be connected to another location.

A rectifying diode D and a smoothing capacitor C are connected in series to both ends of the secondary winding _N2 , and the polarity of the rectifying diode D is conductive when the transistor Q is turned off.A load R _L is connected to both ends of the capacitor C. It is designed to connect.

As shown in Figure 2 E and D, when the transistor Q is turned on at time _t1 by the drive pulse voltage V _P output from the pulse signal generator P, the primary winding
Current I ₁ begins to flow through N ₁ . The diode D connected to the secondary winding N ₂ is connected to the terminal voltage V _N2 of the secondary winding N ₂
becomes negative as shown in C, so it becomes reverse biased and turns off, and a sinusoidal resonant current _I3 flows in the tertiary winding _N3 at a period determined by the values of inductor L _R and capacitor C _R , as shown in B. start.

The values of the inductor L _R and the capacitor C _R in the converter of the present invention are such that the 3/4 period of the resonant current I ₃ corresponds to the on-period of the transistor Q, that is, the period from time t ₁ to time t ₂ in FIG. , and the amplitude of the resonant current I ₃ is chosen to be equal to the excitation current flowing through the primary winding N ₁ at the end of the drive pulse, ie at time t ₂ . By configuring the series resonant circuit connected to the tertiary winding N ₃ in this way, the resonant current I ₃ at the end of the drive pulse, i.e. at time t ₂
is made to match the transformer excitation current, and as a result, the current I ₁ actually flowing through the primary winding N ₁ can be made zero. Therefore, even if transistor Q is turned off at this time, no energy loss occurs. After this, inductor L _R and capacitor C _R of the resonant circuit
The energy stored in the transformer T is transferred to the capacitor C when the terminal voltage of the secondary winding _N2 increases.
and is consumed by the load R _L.

In addition, from t ₁ to t ₃ in Fig. 2 is the inductor L _R
This is one oscillation period of the resonant current _I3 determined by the value of the capacitor C _R , and in the present invention, it is 3/4 of this period.
The period from t ₁ to t ₂ is designed to approximately match the on-period of the transistor Q. However, the current that actually flows into the tertiary winding N ₃ after t ₂ is such that even if the transistor Q is turned off, the rise in the terminal voltage V _N2 of the secondary winding N ₂ is delayed due to the presence of the capacitor _CR .
Since the diode D does not conduct immediately, the resonant current _I3 is slightly delayed in time, as shown by the broken line in the figure.

The position where the tertiary winding _N3 connected to the resonant circuit is provided is not limited to the load side, but may be provided on the primary circuit side insulated from the load side as shown in FIG. In this example, the tertiary winding N ₃ and the capacitor
The connection point of C _R is connected to the emitter of transistor Q so that the combined current flowing through primary winding _N1 in the primary circuit is zero. Note that D ₂ is a diode for preventing reverse voltage from being applied to the transistor Q.

〔Effect of the invention〕

Since the present invention utilizes the resonance of a circuit separate from the secondary output circuit, the conditions of the resonant circuit can be designed independently of the voltage of the secondary circuit. Therefore, even if the converters have different specifications, a resonant circuit having the same configuration can be used in common, and there is no need to redesign the converter for each model. Also, the inductor L _R of the resonant circuit
Even when using the leakage inductance of a transformer, the number of turns of the tertiary winding _N3 can be fixed, so
A configuration is possible in which a constant leakage inductance is obtained regardless of the number of turns of the secondary winding _N2 .

Although more loss in a resonant circuit occurs in an inductor than in a capacitor, the present invention can configure a low-loss resonant circuit in which less current flows through the inductor, regardless of the value of the output voltage.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
Figures A to E are operating waveform diagrams of each part, Figure 3 is a circuit diagram showing another embodiment of the present invention, Figure 4 is a circuit diagram for explaining the principle of energy loss generation in the converter, and Figure 5 is an illustration of the operating waveform diagram of each part. is a timing diagram showing the relationship between voltage and current during switching of the circuit in Figure 4, B is a diagram showing the loss at that time, Figure 6 is a circuit diagram of a conventional resonant converter, and Figure 7 is a resonant converter. Figure 8A shows the current waveform diagram, B shows the voltage waveform diagram, Figure 8A shows the timing diagram showing the relationship between voltage and current during switching of the circuit in Figure 6, and B shows the loss at that time. FIG. N ₁ ...Primary winding, Q, S...Switch element,
N ₂ ... Secondary winding, D ... Diode, N ₃ ... Tertiary winding, C, C _R ... Capacitor, T ... Transformer, L, L _R ... Inductor, E ... Voltage source.

Claims (1)

[Claims] 1 A transformer having a primary winding, a secondary winding, and a tertiary winding, a voltage source, a switch element that intermittently connects the voltage source to the primary winding, and between the secondary winding and the load. comprising a diode connected in series and oriented to conduct when the switch element is off, and a series resonant circuit consisting of an inductor and a capacitor connected in series with the tertiary winding;
The current in the series resonant circuit resonates at a natural frequency that corresponds to 3/4 period of the on period of the switch element, and the current flowing through the primary winding when the switch element is turned off becomes almost zero. A current resonant flyback converter.