JP4858815B2 - Forward converter - Google Patents

Description

  The present invention relates to a switching power supply, and more particularly to a soft switch.

The switching power supply generates a stable voltage by repeatedly turning on and off the current, and adjusting the ratio of the on period to the repeated cycle. Abrupt changes in current that occur in the transformer windings and wiring when turning the current on and off generate noise.
Noise at the time of turning on the current is called turn-on noise, but when the switch element is a MOSFET, it is also called gate noise. This noise occurs when the current rises from zero to a certain value instantaneously.
In the case of the forward converter, since the current proportional to the excitation energy remaining in the secondary reactor flows at the same time as the switching element is turned on, it is necessary to charge the gate instantaneously. If the charging of the gate is delayed, a current flows before the voltage across the switch element is sufficiently lowered, causing power loss.
This loss is called turn-on loss, but has a trade-off relationship with turn-on noise. That is, increasing the speed of charging the gate to improve the turn-on loss increases the turn-on noise, and decreasing the speed of charging the gate to improve the turn-on noise increases the turn-on loss.
Since noise must be cleared by a standard set by laws and public institutions, turn-on loss is slightly increased, but lowering noise is prioritized. As a conventional method for reducing turn-on noise, the method shown in FIGS. 4 and 5 is employed.
In FIG. 4, a series circuit of two resistors 101 and 102 is connected between the oscillation control circuit 500 and the switch element 100, and a diode 103 is connected in parallel to the resistor 101. The diode 103 has a cathode facing the oscillation control circuit side.
A signal for turning on the switch element 100 flows through two resistors, but a signal for turning off the switch element 100 flows through the resistor 102 and the diode 103.
For example, if the resistor 101 is selected to be 100Ω and the resistor 102 is selected to be 10Ω, the current at turn-on and turn-off differs by about 10 times, and the rise of the voltage of the control electrode of the switch element 100 at turn-on is about 10 times. Be late.
In FIG. 5, the output stage of the oscillation control circuit 500 includes a transistor 501 and a transistor 502. The transistor 501 functions as a high-side switch that charges the control electrode of the switch element 100 from the auxiliary power supply 200. It functions as a low-side switch that discharges the control electrode. Since the resistor 101 limits the current of the signal to be turned on and the resistor 102 limits the current of the signal to be turned off, the rise of the control electrode of the switch element is arbitrarily delayed by appropriately selecting the value of the resistor 101. And its value does not affect the speed of discharge of the control electrode.
4 and 5 can delay the rise of the voltage of the control electrode of the switch element 100 depending on the value of the resistor 101, but if the value is increased too much, the voltage of the control electrode can be sufficiently increased. The switching current flows through the switch element before, and the loss caused because the voltage across the switch element is not sufficiently low during the ON period becomes large.

The turn-on noise is proportional to the rate of change per hour of the current flowing at the moment when the switch element changes from the off state to the on state, and can be improved by controlling the rate of change.
In the conventional method, by inserting a current limiting resistor between the oscillation control circuit and the control electrode of the switch element, the slope of the rise of the voltage of the control electrode is relaxed and the current per unit time of the current flowing through the switch element is reduced. The rate of change is suppressed.
However, the resistance reduces the speed at which the switch element transitions to the fully on state, thus increasing turn-on loss.

  Therefore, the present invention is not a conventional method that sacrifices either the turn-on noise or the turn-on loss, but an object of the present invention is to provide means for simultaneously solving both the turn-on noise and the turn-on loss.

In order to achieve the above object, the invention according to claim 1 is a series circuit comprising a primary winding and a switching element, a rectifying / smoothing circuit for generating a DC voltage from the secondary winding and the secondary winding, and on / off of the switching element. In a forward converter including an oscillation control circuit that controls power and an auxiliary power source that supplies power to the oscillation control circuit, a first saturable inductor is inserted in series between the control electrode of the switch element and the oscillation control circuit, A diode was connected in parallel to one saturable inductor, and a second saturable inductor was inserted in series between the secondary winding and the rectifying / smoothing circuit.
The invention described in claim 2 is a series circuit composed of a primary winding and a switching element, a secondary winding, a rectifying / smoothing circuit for generating a DC voltage from the secondary winding, an oscillation control circuit for controlling on / off of the switching element, and oscillation In a forward converter having an auxiliary power source for supplying power to a control circuit, a first saturable inductor is inserted in series between a control electrode of a switch element and an oscillation control circuit, and oscillation control of the first saturable inductor is performed. The base is connected to the terminal on the circuit side, the emitter is connected to the terminal on the control electrode side of the switch element of the first saturable inductor, and the collector is connected to the electrode on the opposite side of the electrode connected to the primary winding of the switch element. A transistor was added, and a second saturable inductor was inserted in series between the secondary winding and the rectifying / smoothing circuit.
According to a third aspect of the present invention, there is provided a series circuit composed of a primary winding and a switching element, a rectifying / smoothing circuit for generating a DC voltage from the secondary winding and the secondary winding, and an oscillation control circuit for controlling on / off of the switching element. And a forward converter having an auxiliary power source for supplying power to the oscillation control circuit, the oscillation control circuit includes a high side switch for charging the control electrode of the switch element and a low side switch for discharging, and the auxiliary power source and the high side switch The first saturable inductor was inserted in series between the second winding and the second saturable inductor between the secondary winding and the rectifying / smoothing circuit.

When the oscillation control circuit outputs a signal for turning on the switch element, the voltage of the control electrode of the switch element rises slowly until the first saturable inductor is saturated. Meanwhile, since the second saturable inductor delays the flow of current due to the exciting energy of the reactor constituting the rectifying and smoothing circuit to the secondary winding, a large current does not flow through the switch element, and the turn-on loss is small.
Then, the two saturable inductors are saturated so that the second saturable inductor is saturated after the first saturable inductor is saturated and the voltage of the control electrode of the switch element is fully increased and the switch element is turned on. Is selected, the current at the time when the exciting energy of the reactor starts flowing through the secondary winding rises loosely, so the slope of the current flowing through the primary winding and the switch element also becomes loose, and the turn-on noise becomes small.
When the rising speed of the control electrode voltage of the switch element is limited by resistance, the voltage rises according to an exponential function and rises gradually with the largest slope first and gradually decreases, so the resistance that slows the turn-on speed is the control electrode after turn-on. Also slows the voltage rise. When a MOSFET is applied as a switch element, the gate voltage is turned on at 3 to 5 V, but the on-resistance of the switch element approaches a minimum when the gate voltage is close to 10 V. For this reason, the gate voltage is raised to 10 to 15 V even after turn-on, but when the resistance is large, the time until it reaches 10 V is lengthened, and a loss occurs during that time. This loss is a loss caused by the on-resistance of the MOSFET and is called on-loss.
On the other hand, if the rise of the voltage of the control electrode is limited by the saturable inductor, the slope of the voltage rise is close to zero at the beginning and becomes maximum when it gradually increases and becomes saturated. When a MOSFET is used as the switch element, optimum control can be obtained by selecting the saturable inductor to saturate immediately after turn-on, and the on-loss can be minimized.

In the first aspect of the present invention, the turn-on signal sent from the oscillation control circuit to the control electrode of the switch element causes resonance by the first saturable inductor inserted in series. This resonance occurs due to the inductance of the first saturable inductor before saturation and the capacitance of the control electrode, and the voltage of the control electrode rises in a triangular function waveform. The voltage of the control electrode that changes the switch element from the off state to the on state is called a threshold value. However, before and after the threshold value, there is a state in which the current is controlled by the voltage of the control electrode, not the on state. During an incomplete on-state transition period in which the current of the switch element changes depending on the voltage value of the control electrode, a voltage is applied across the switch element, and a current flows through the switch element.
The transition period becomes longer because the rise of the voltage of the control electrode is delayed by the first saturable inductor, but the steep rise of the current accompanying the release of the excitation energy of the secondary side reactor is caused by the secondary winding and the rectifying and smoothing circuit. It is suppressed by the second saturable inductor inserted in series therebetween, and the current of the switch element during the transition period is only the exciting current that starts to flow through the primary winding. Since the exciting current of the primary winding of the forward converter is small, the loss in the transition period is small.
Then, the transition period is lengthened by the first saturable inductor and the steep rise of current is suppressed by the second saturable inductor, so that the rate of change of the current flowing through the switch element is reduced and the turn-on noise is improved. The That is, it is possible to simultaneously solve two problems that are a trade-off between turn-on loss and turn-on noise.
A diode connected in parallel with the first saturable inductor is included to prevent the first saturable inductor from causing a delay when the switch element is turned off.
The invention according to claim 2 is the same as the invention according to claim 1 in that the first saturable inductor delays the turn-on of the switch element. At turn-off, the transistor is turned on using the voltage applied to the first saturable inductor so that the charge of the control electrode of the switch element can be quickly absorbed into the transistor.
According to a third aspect of the present invention, the first saturable inductor inserted in series between the high-side switch having the role of charging the control electrode of the switch element of the oscillation control circuit and the auxiliary power supply is the switch element. Since only the turn-on is delayed, the diode according to claim 1 and the transistor according to claim 2 are not required.
The first and second saturable inductors described in claims 1 to 3 are saturated at a predetermined current, but have high inductance until saturation. Therefore, even if a voltage is applied, the current rises slowly, and it takes time to reach a predetermined current. The product of the applied voltage and the time until saturation is called a voltage-time product and is one of the items indicating the characteristics of the saturable inductor. By appropriately selecting the voltage time product of each of the first saturable inductor and the second saturable inductor, a current can be passed through the switch element after the switch element is turned on.
Both the first saturable inductor and the second saturable inductor must be saturable inductors, not normal inductors. The reason is that if a normal inductor is inserted in series between the oscillation control circuit and the control electrode of the switch element, the resonance that occurs between the capacitive component of the inductor and the control electrode will not stop, so the voltage of the control electrode This is because the vibration becomes unstable. In the case of a saturable inductor, a predetermined voltage time product can be selected so as to be saturated in the middle of the rise of resonance.
In addition, when a normal inductor is inserted in series between the secondary winding and the rectifier circuit, the inductor functions in the same way as the leakage components of the primary and secondary windings of the transformer, impeding coupling. , Worse efficiency. In the case of a saturable inductor, a predetermined voltage-time product can be selected so that it has an inductance component only during the turn-on transition period and saturates when it is completely turned on and becomes close to a short state.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.
In the figure, immediately after the switch element 2 is turned on, the voltage of the DC power source 10 is applied to the primary winding 1a, and a voltage proportional to the number of turns is generated in the secondary winding 1b. In the case of a general forward converter, the current of the reactor 3c flows to the secondary winding 1b, and the current that is inversely proportional to the number of turns flows to the primary winding 1a, but until the second saturable inductor 8 is saturated. Current does not flow.
That is, a current flows through the switch element 2 after the switch element 2 is turned on and the second saturable inductor 8 is saturated.
Since the current for charging the control electrode of the switch element 2 from the oscillation control circuit 4 is delayed by the first saturable inductor 6, the transition period from the OFF state to the ON state of the switch element 2 is longer than that of a general forward converter. Meanwhile, the current is limited by the second saturable inductor 8. The charge stored in the capacitive component of the switch element 2 is also slowly discharged, which also has the effect of improving turn-on noise.
FIG. 2 is a circuit diagram showing an embodiment of the second aspect of the present invention.
In the figure, the working manner of the first saturable inductor 6 and the second saturable inductor 8 is the same as that of FIG. The transistor 9 does not work when a current for charging from the oscillation control circuit 4 to the control electrode of the switch element 2 flows, but conversely, when a current for discharging the control electrode flows, the transistor 9 continues until the saturable inductor is saturated. Since a current flows from the emitter to the base and the transistor is turned on, most of the discharge current flows from the emitter to the collector of the transistor, thereby increasing the turn-off speed.
FIG. 3 is a circuit diagram showing an embodiment of the third aspect of the present invention.
In the figure, the functions of the first saturable inductor 6 and the second saturable inductor 8 are the same as those in FIG. The two transistors at the output stage of the oscillation control circuit 4 are alternately turned on and off, the transistor 4a is turned on when charging the control electrode of the switch element 2, and the transistor 4b is turned on when discharging. The first saturable inductor 6 is inserted in series on the collector side of the transistor 4a to reduce the charging speed.

  The separately-excited forward converter with fixed frequency is applied to many power supplies having 3.3V and 5V as output voltages, including the power supply of desktop personal computers, and can be applied to those power supplies. Despite the fact that the components added to the conventional circuit are two saturable inductors, the noise reduction becomes easy and the development period is shortened, so the economic effect is great.

  FIG. 2 is a circuit diagram showing an embodiment of the invention as set forth in claim 1;   It is a circuit diagram which shows the Example of invention of Claim 2.   It is a circuit diagram which shows the Example of invention of Claim 3.   It is a circuit diagram which shows an example of a conventional system.   It is a circuit diagram which shows an example of a conventional system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transformer 1a Primary winding 1b Secondary winding 2 Switch element 3 Rectification smoothing circuit 3a, 3b Diode 3c Reactor 4 Oscillation control circuit 4a, 4b Transistor 5 Auxiliary power supply 6 First saturable inductor 7 Diode 8 Second possible Saturated inductor 9 Transistor 10 DC power supply 11 Load 100 Switch element 101, 102 Resistor 103 Diode 200 Auxiliary power supply 500 Oscillation control circuit 501, 502 Transistor

Claims (3)

  A series circuit composed of a primary winding of a transformer and a switch element, a secondary winding electromagnetically coupled to the primary winding, a rectifying / smoothing circuit for generating a DC voltage from a voltage generated in the secondary winding, and the DC voltage In a forward converter having an oscillation control circuit for controlling on / off of the switch element to keep the switching element constant and an auxiliary power source for supplying power to the oscillation control circuit, between the control electrode of the switch element and the oscillation control circuit A first saturable inductor is inserted in series, a diode is connected in parallel to the first saturable inductor, and a second saturable inductor is connected in series between the secondary winding and the rectifying and smoothing circuit. A forward converter characterized by insertion.   A series circuit composed of a primary winding of a transformer and a switch element, a secondary winding electromagnetically coupled to the primary winding, a rectifying / smoothing circuit for generating a DC voltage from a voltage generated in the secondary winding, and the DC voltage In a forward converter having an oscillation control circuit for controlling on / off of the switch element to keep the switching element constant and an auxiliary power source for supplying power to the oscillation control circuit, between the control electrode of the switch element and the oscillation control circuit The first saturable inductor is inserted in series with the terminal of the first saturable inductor on the oscillation control circuit side, and the base thereof is connected to the control electrode side terminal of the switch element of the first saturable inductor. A transistor having an emitter connected to the electrode opposite to the electrode connected to the primary winding of the switch element and a collector connected to the collector; Forward converter, characterized in that the insertion of the second saturable inductor in series between the rectifying and smoothing circuit.   A series circuit composed of a primary winding of a transformer and a switch element, a secondary winding electromagnetically coupled to the primary winding, a rectifying / smoothing circuit for generating a DC voltage from a voltage generated in the secondary winding, and the DC voltage In order to keep the switching element constant, an oscillation control circuit for controlling on / off of the switch element and a forward converter having an auxiliary power source for supplying power to the oscillation control circuit, the oscillation control circuit is configured such that the auxiliary power source is the switch element. A high-side switch for turning on and off a current supplied to the control electrode and a low-side switch for discharging the control electrode of the switch element are provided, and a first saturable inductor is connected in series between the auxiliary power source and the high-side switch. And a second saturable inductor is inserted in series between the secondary winding and the rectifying / smoothing circuit. Converter.

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