JPH08317639A - Ringing choke converter of synchronous rectification system - Google Patents

Abstract

PURPOSE: To obtain a ringing choke converter by which a resistance value in an on- period is reduced and whose efficiency is improved by a method wherein the width of a pulse applied across the gate and the source of a MOSFET is controlled by a resistance, a saturable inductor and a reset-signal control circuit. CONSTITUTION: A reset-signal control circuit 15 detects an output voltage. When it is higher than a set value, a current which is output from a diode 15f is made large, and, when it is low, the current is made small. In addition, the current which is output from the diode 15f is passed through a saturable inductor 17 when the voltage of an auxiliary winding 11c is changed to a direction in which a MOSFET 14 is reverse- biased. It flows through a resistance 16, the auxiliary winding 11c and a secondary winding 11b so as to reset the saturable inductor 17. Then, when a reset current is large, the time elapsed until the saturable inductor 17 is saturated becomes long when the voltage of the auxiliary winding 11c is changed in a direction in which the MOSFET 14 is forward-biased, and the on-period of the MOSFET 14 becomes long. As a result, it is possible to obtain a ringing choke converter in which an output voltage is kept constant and whose efficiency can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ringing choke converter which is one type of power supply device.

[0002]

2. Description of the Related Art A general ringing choke converter has a constant ratio of an on period and an off period under a constant input / output voltage condition, and the output voltage is changed by changing an oscillation cycle with respect to a change in output current. Is kept constant. If the output current is small, the oscillation cycle becomes short, and therefore the ON period and the OFF period become short.

Under load conditions in which the output current changes from the maximum value to zero, the ON period changes over a wide range, but as the output current approaches zero, it becomes shorter and control becomes more likely to be out of control. Therefore, problems such as intermittent oscillation and overvoltage may occur.

Therefore, the present applicant previously provided a ringing choke converter capable of passing a current in the reverse direction in the secondary winding (Japanese Patent Application No. 6-294051). FIG. 3 shows this circuit.

[0005]

The winding 11 shown in FIG.
A voltage proportional to the output voltage is generated in c, and this voltage is M
When the threshold voltage of OSFET14 is exceeded, the MOS
The FET continues to be in the ON state even after the flyback current from the secondary winding 11b is discharged, and conversely the capacitor 13
Is applied to the secondary winding 11b. Therefore, the secondary winding 1
A current flows in the direction 1b in the direction opposite to the direction of the flyback current. This current becomes a current for exciting the transformer 11 in the opposite direction.

The exciting energy accumulated in the transformer 11 by the exciting current flowing through the secondary winding 11b is the primary winding 1
When the switching element 21 connected in series to 1a turns on in the next cycle, it becomes a flyback current passing through the primary winding 11a, and this flyback current returns to the capacitor 20 as charging energy, so there is almost no loss.

The exciting current flowing through the secondary winding 11b continues until the voltage of the auxiliary winding 11c becomes smaller than the threshold voltage of the MOSFET 14. Therefore, the output voltage becomes substantially equal to the threshold voltage multiplied by the winding ratio of the windings 11b and 11c.

As described above, in the circuit shown in FIG. 3, the output voltage is determined by the threshold voltage of the MOSFET 14. Therefore, the control circuit 22 of the MOSFET 21 connected in series to the primary winding 11a only needs to limit the maximum value of the exciting current flowing through the primary winding or the maximum ON period of the MOSFET 21, and the feedback control function can be performed by the output voltage detection value. You don't have to have.

However, since the MOSFET maintains the ON state in the vicinity of the threshold voltage, the ON resistance of the MOSFET cannot be made sufficiently small.

Therefore, according to the present invention, the auxiliary winding 11c is connected to the MO
By controlling the width of the pulse applied to the gate of the SFET, it is possible to keep the output voltage constant and at the same time set the amplitude of the pulse applied to the gate of the MOSFET sufficiently higher than the threshold voltage. Is aimed at improving efficiency.

[0011]

In order to achieve the above object, in the invention according to claim 1, the pulse applied between the gate and source of the MOSFET connected in parallel with the secondary side rectifying diode from the auxiliary winding. The width is controlled by a resistor, a saturable inductor and a reset signal control circuit.

[0012]

According to the first aspect of the present invention, the period until the saturable inductor is saturated is substantially equal to the ON period of the MOSFET, and the flyback current and the exciting current flow in the secondary winding during this ON period.

Since the voltage applied to the saturable inductor from the auxiliary winding is in only one direction by the diode connected in series with the saturable inductor, once the saturable inductor saturates, the reset signal is applied by another circuit. As long as it is not, it keeps a state close to a short circuit, and even if a pulse voltage is generated in the auxiliary winding, it is stepped down by a resistor,
The voltage exceeding the threshold voltage is not applied between the gate and the source of the MOSFET, and the MOSFET cannot be turned on.

The resistor connected in series with the auxiliary winding is
Immediately after the saturable inductor is saturated, it limits the inrush current flowing from the auxiliary winding to the saturable inductor for a short period.

Next, when a reset signal is applied to this saturable inductor by another circuit, the MOSFET also remains in the ON state for a period corresponding to the strength of the reset signal.

Depending on the strength of the reset signal, the MOSFET
Since the ON period is controlled, the output voltage can be kept constant by changing the strength of the reset signal according to the output voltage detection value.

In the invention of claim 2, if the on-resistance of the MOSFET is sufficiently small and the drop voltage due to the current in the secondary winding is smaller than the forward voltage of the rectifying diode, this rectifying diode can be excluded. If the MOSFET does not turn on when the voltage of the auxiliary winding drops during overload or short circuit, MOSF
The ET parasitic diode acts as a rectifier.

[0018]

FIG. 1 is a circuit diagram showing an embodiment of the invention described in claim 1. The same reference numerals are given to the same or equivalent portions as those of the circuit of FIG. 3 showing a conventional example.

FIG. 2 is a circuit diagram showing an embodiment of the invention described in claim 2. The same reference numerals are given to the same or equivalent portions as those of the circuit of FIG. 3 showing a conventional example.

FIG. 4 shows the secondary winding 11b of the circuit shown in FIG.
The current waveforms of the voltage waveforms at both ends, the drain current of the MOSFET 14 and the current of the diode 12 are measured on the same time axis.

In the circuit of FIG. 1, the reset signal control circuit 15 detects the output voltage, and when the value is higher than the set value, increases the current output from the diode 15f,
When the value is lower than the set value, the current output from the diode 15f is reduced. As for the current output from the diode 15f, the voltage of the auxiliary winding 11c is MOSFET.
When the direction of reverse biasing 14 is changed, the current flows through the saturable inductor 17, the resistor 16, the auxiliary winding 11c, and the secondary winding 11b. This current resets the saturable inductor 17. That is, when the output voltage becomes higher than the set value, the reset current becomes large.

When this reset current is large, the auxiliary winding 1
When the voltage of 1c changes to forward bias the MOSFET 14, the saturable inductor 17 causes the auxiliary winding 11 to move.
The voltage of c increases the time to reach saturation,
The ON period of the MOSFET 17 also becomes long.

The exciting current flowing in the reverse direction through the secondary winding 11b increases as the ON period of the MOSFET 17 increases. Since this current is supplied by the discharge of the capacitor 13, the output voltage drops. In this way the output voltage is kept constant.

The positive part of the drain current waveform of the MOSFET 14 shown in FIG. 4 is the flyback current, and the negative part is the exciting current. When the output current is zero, the areas of these two currents are almost half and when the output current is maximum, the area of the exciting current is almost zero.

The total area of the flyback current and the exciting current is constant while the output current changes from zero to the maximum value, and the total of each period is also constant. Therefore, the oscillation cycle is also almost constant.

The fact that the oscillation period is almost constant is different from the general ringing choke converter.

In the circuit of FIG. 2, the MOSFET 17 is the only rectifier circuit connected to the secondary winding. MOSFE
If the on resistance of T17 is sufficiently small, the efficiency of the converter can be increased.

Further, in the circuit of FIG. 2, the reset signal supplied to the saturable inductor 17 is obtained from the output voltage different from the detection voltage. In such an application, when the detection voltage is relatively high, the power by the reset current is increased. It is effective in saving loss.

[0029]

As described above, according to the present invention, a converter having a stable operation and a high efficiency, in which the oscillation frequency is substantially constant, can be formed with a simple structure.

[Brief description of drawings]

FIG. 1 is an embodiment of the invention described in claim 1.

FIG. 2 is an embodiment of the invention described in claim 2;

FIG. 3 is a conventional circuit diagram.

FIG. 4 is an operation waveform of the circuit shown in FIG.

[Explanation of symbols]

 11 Transformer 12 Diode 13 Capacitor 14 MOSFET 15 Reset Signal Control Circuit 16 Resistor 17 Saturable Inductor 18 Diode 20 Capacitor 21 MOSFET 22 Gate Control Circuit 23 Diode 24 Capacitor 11a Primary Winding 11b Secondary Winding 11c Auxiliary Winding 11d Positive Feedback Winding 11e Second secondary winding 15a Voltage detection IC 15b Resistor 15c Resistor 15d Resistor 15e Resistor 15f Diode 15g Transistor

Claims (2)

[Claims] 1. A ringing choke converter comprising a transformer having a primary winding and a secondary winding, and a diode connected in series to the secondary winding of the transformer,
A MOSFET is connected in parallel to the diode, an auxiliary winding is added to the transformer, terminals are connected to the gate and the source of the MOSFET, respectively, and the auxiliary winding and the MOSF are connected.
A resistor is inserted in series in the circuit connecting the gate and source of the ET, a series circuit composed of a saturable inductor and a diode is connected between the gate and source of the MOSFET, and the reset signal corresponding to the output voltage detection value is applied to the circuit. A ringing choke converter of a synchronous rectification type, characterized in that a reset signal control circuit for supplying to a saturation inductor is connected. 2. A synchronous rectification type ringing choke converter according to claim 1, wherein a diode connected in series to said secondary winding is deleted.