JPH0314950Y2 - - Google Patents

Description

[Detailed explanation of the idea] [Industrial application field]

The present invention relates to converters that supply DC power to loads such as small computers and office automation equipment, and in particular to so-called flyback type DC-DC converters that control the primary current of a transformer by turning on and off switching elements. Concerning converters.

[Technical background]

In recent years, with the spread of small computers and office automation equipment, there is a strong demand for smaller and cheaper power supplies for these equipment. To meet this demand, flyback type converters are often used due to their simple structure and low cost. Conventionally, this converter has been controlled using a pulse width modulation control method that varies the switching duty according to the control input signal, but recently a simpler and more rational method has been used to control the transformer's primary current. A so-called current mode control method for direct control is being adopted. Since the switching circuit itself detects the primary current level and controls the peak current, it is generally resistant to fluctuations in power supply voltage, does not require an overcurrent protection circuit, and is in current mode, so it does not respond to comparison input signals. The output voltage becomes a transfer function with only a first-order lag, and
Features include large feedback gain and high performance. Figure 4 shows an example of this, where 1 is a transformer, with a primary winding 1a, a secondary winding 1b, and a tertiary winding 1c.
is wound around the core with the polarity shown. One end (negative pole) of the primary winding 1a of this transformer 1 is connected to the positive terminal of a DC power supply 2 whose negative terminal is grounded to the circuit, and the other end (positive pole) of the primary winding 1a and the circuit A current detection resistor Re is inserted in series between the drain and source of a switching element Q consisting of a MOSFET between the ground, and when the switching element Q is turned on, the primary winding 1a is excited. There is. 3 and 4 are the secondary and tertiary windings 1 of the transformer 1
These are rectifier circuits connected to terminals b and 1c, respectively.
The rectifier circuit 3 is formed by connecting a diode _D1 and a capacitor _C1 in series to both ends of the secondary winding 1b, and grounding one end (negative pole) of the secondary winding 1b.
A load 5 is connected via a connection terminal between the connection point between the diode D ₁ and the capacitor C ₁ and the circuit ground. Further, the rectifier circuit 4 also has the tertiary winding 1 as described above.
It is formed by inserting a diode D ₂ and a capacitor C ₂ in series between both ends of the capacitor C, and a load 6 is connected between the terminals of the capacitor C ₂ . 7 is an error amplification circuit that amplifies and outputs the difference between the output voltage V ₈ of the voltage divider circuit 8 that divides the output voltage of the rectifier circuit 3 to the load 5 using resistors R ₁ and R ₂ and a preset reference voltage Vref. It is. This is done by connecting the output terminal of the voltage divider circuit 8 via a resistor R ₃ to the inverting input terminal of the operational amplifier _{A 1 which} inputs the reference voltage Vref to the non-inverting input terminal. By inserting a resistor R ₄ and a capacitor C ₃ in series between the and output terminals, the output voltage is amplified by the difference between the input voltage V ₈ and Vref.
Vc is output to the inverting input terminal of a comparator circuit consisting of operational amplifier _A2 . Then, the connection point between the source of the switching element Q and the current detection resistor Re is connected to the non-inverting input terminal of the operational amplifier _A2 , and the current ip flowing through the resistor Re when the switching element Q is turned on is converted into a voltage. When the input voltage Vp exceeds the other input voltage Vc (Vp > Vc), the output signal of the operational amplifier _A2 is inverted to "H" level. It's summery. The output terminal of the operational amplifier _A2 serving as this comparison circuit is connected to the reset input terminal R of the flip-flop circuit _FF1 , and the set input terminal S is connected to a pulse generator that oscillates a narrow pulse signal at a constant high frequency (for example, 50 kHz). Connect the output end of the circuit OSC, connect the gate of the switching element Q above to the output end Q, and complete the pulse oscillation circuit OSC.
The flip-flop circuit _FF1 is set by the pulse signal of , and the output signal of the output terminal Q is inverted to "H" level to turn on the switching element Q, and the transformer 1 is turned on.
The primary side of the comparator circuit is excited and the excitation current increases, and as this excitation current increases, the input voltage Vp of the above comparison circuit increases.
When exceeds Vc, the flip-flop circuit _FF1 is reset and the output signal at its output terminal Q is set to "L".
level and turns off the switching element Q. As a result, the output voltage of the error amplifier circuit 7
The primary current of the transformer 1 can be controlled according to Vc. The comparison conditions for operational amplifier _A2 as a comparison circuit are
Since Vc=ip·Re, the peak current Ip flowing through the primary winding 1a is Ip=Vc/Re, and Vc and Ip are proportional. This is always true even if the power supply voltage Vs of the DC power supply 2 changes or the output current changes, so it can be said to be an extremely effective method as a current mode control method. Incidentally, in the case of the pulse width modulation control method, if the power supply voltage Vs changes, even if the rise (Vs/Lp) changes, the voltage will turn on and off in the same time width, so Ip will not be constant. When the switching element Q is turned on by the output signal from the output terminal Q of the flip-flop circuit _FF1 ,
The rise rate of the primary current of the transformer 1 is dip/dt=Vs/Lp, where Vs: the power supply voltage of the DC power supply 2, Lp: the primary inductance of the transformer 1 (Fig. 5 A and D). Then, on the secondary and tertiary sides, due to the energy accumulated in the primary inductance Lp during the on period of the switching element Q, a current is flows during the off period of the switching element Q (Fig. 5 B and D), and further transforms the voltage. The current seen from the iron core of vessel 1,
The equivalent inductor current can be considered to be as shown in FIG. 5C. The first rise of the primary current is because the primary side turns on before the secondary current becomes 0, and 2
The fall rate of the secondary current is dis/dt=-Vo/Ls where Vo: Output voltage (DC) [Ls: Secondary inductance Ls=[ns/np] ²・Lp ns: Number of turns of secondary winding 1b np: Number of turns of primary winding 1a]

[Prior art and its problems]

In the above-described flyback converter using current mode control, a snubber circuit is provided to absorb the surge voltage of the switching element. An example of this snubber circuit is shown as 9 in FIG. Snubber circuit 9 is a suppression resistor between the drain of switching element Q and circuit ground.
Inserting Rs and capacitor Cs in series,
A diode Ds is inserted in the forward direction between the terminals of the resistor Rs, forming a so-called discharge blocking type, so that when the switching element Q is turned off, the current flows through the diode.
When the capacitor Cs is charged through Ds and the switching element Q is turned on, the discharge current of the capacitor Cs flows through the resistor Rs, so that overcurrent is prevented from flowing into the switching element Q. However, in this case, when the switching element Q is turned on, the electric charge charged in the capacitor Cs causes a current to flow in the path Cs → Rs → Q → Re → earth → Cs, so it flows to the current detection resistor Re. As shown in FIG. 6, the current ip has a waveform in which the discharge current of the capacitor Cs is superimposed, and there is a problem that false detection occurs when the level of the output voltage Vc of the error amplifier circuit 7 is low (Vc'). Therefore, the operating range of current detection (that is, the detection range of peak current Ip) must be narrowed for use, and there is a problem that it is not possible to follow a wide range of changes in the load. In order to improve this, some devices have added a dummy load to perform current detection, but this has the problem of wasting power. On the other hand, in order to absorb the surge voltage, the capacitor Cs and the resistor Rs need to be made large.If the capacitor Cs is made large, the discharge of the capacitor Cs will end during the ON period of the switching element Q. In order to achieve this, it is necessary to reduce the time constant of Cs and Rs, so it is necessary to reduce the resistance Rs. In addition, with the recent trend toward higher frequencies, there are also factors that make it impossible to further increase the time constant. Therefore, the discharge peak current (Vdmax/Rs, Vdmax is approximately twice Vs) of the capacitor Cs cannot be ignored. For this reason, attempts have been made to protect the switching element Q by reducing the capacitor Cs or increasing the resistor Rs, but the surge voltage becomes high and there is a problem that sufficient protection cannot be achieved. In addition, as shown in Fig. 7 as a snubber circuit,
A so-called CR snubber circuit formed by inserting a suppressing resistor Rs and a capacitor Cs in series between the drain of the switching element Q and the circuit ground can also be considered.
When switching element Q is turned off, the current is Lp(1
When the current is commutated from the path of (inductance) → Q → Re → earth to the path of Lp → Rs → Cs → earth, a voltage drop occurs at the resistor Rs, and the voltage between the drain and source of the switching element Q becomes the voltage of the capacitor Cs. There is a problem in that the snubber effect cannot be sufficiently exerted as the snubber becomes higher.

[Purpose of invention]

The present invention was developed in view of the above points, and its purpose is to provide a device that can accurately detect current and fully exhibit the surge voltage absorption effect. There is a particular thing.

[Summary of the idea]

In order to achieve the above object, the present invention aims to
A capacitor and a diode with an anode connected to the capacitor are inserted in series between one pole of the switching element connected to the next winding and the negative terminal of the DC power supply. The feature is that a suppressing resistor is inserted between the switching element and the other pole to form a snubber circuit.

[Example of idea]

Embodiments of the present invention will be described below with reference to FIGS. 1 to 3. Since FIG. 1 is formed in substantially the same manner as FIG. 4, the same members will be given the same reference numerals, different structures will be explained, and redundant explanations will be omitted as much as possible. In Figure 1,
10 is a snubber circuit, which connects a capacitor between one pole (drain) of the switching element Q connected to one end (positive pole) of the primary winding 1a of the transformer 1 and the circuit ground.
Cs and a diode Ds inserted in the forward direction are inserted in series, and a suppression resistor Rs is inserted between the connection point of the anode of the capacitor Cs and the diode Ds and the other pole (source) of the switching element Q. Then, when switching element Q is turned off, the current is 2
→1a→Cs→Ds→Flows through the path of 2 and capacitor Cs
When charged and switching element Q turns on,
The discharge current is' of the capacitor Cs is made to flow through the path Cs→Q→Rs→Cs, and is prevented from flowing through the current detection resistor Re. 11 is a low-pass filter circuit, which connects the other pole (source) of the switching element Q and the connection point between the resistor Re and the circuit ground.
Insert a resistor Rf and a capacitor Cf in series, and
Connect the connection point of Rf and capacitor Cf to the non-inverting input terminal of operational amplifier A ₂ forming the comparator circuit,
It absorbs the discharge current of the spike-like stray capacitance superimposed on the current flowing through the primary winding 1a, and outputs a voltage Vp obtained by converting the current flowing through the resistor Re to the operational amplifier _A2 . . In this way, since neither the charging current nor the discharging current of the capacitor Cs flows through the current detection resistor Re, the peak current Ip is accurately detected and the switching element is controlled to be turned on or off.

[Effect of the idea]

According to the present invention, the snubber circuit that absorbs the surge voltage of the switching element prevents charging and discharging current from flowing through the current detection resistor that detects the peak current, making it possible to accurately detect the peak current. It can accurately follow a wide range of changes in load,
Moreover, stable control operations can be performed. Moreover, since there is no need to provide a dummy load, a current mode control type DC-DC converter can be configured without wasting power. Further, since the circuit constants of the snubber circuit capacitor and suppressing resistor can be set with emphasis on only the surge voltage absorption effect, it is possible to provide sufficient surge voltage countermeasures for the switching element.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is an explanatory diagram of the snubber circuit of Fig. 1, and Fig. 3 is a block diagram showing an embodiment of the present invention.
Figure 4 is a block diagram showing the conventional example, Figure 5 is an explanatory diagram of the operation of Figure 4, Figure 6 is the primary current waveform diagram of Figure 4, Figure 7 is another example. FIG. 2 is an explanatory diagram of a snubber circuit showing a conventional example. 1; Transformer, 1a; Primary winding, 1b; Secondary winding, 2; DC power supply, 9, 10; Snubber circuit, 1
1; Low-pass filter circuit, Q: Switching element, Re: Current detection resistor, Cs: Capacitor,
Rs: Suppression resistor, Ds: Diode.

Claims (1)

[Scope of utility model registration request] A transformer has the other end of its primary winding connected to the positive terminal of the DC power supply, a switching element has one pole connected to one end of the primary winding, and the other pole of the switching element and the other end of the DC power supply. Equipped with a current detection resistor inserted between the negative terminal and a snubber circuit that absorbs the surge voltage of the switching element,
In a DC-DC converter that outputs a DC voltage by controlling the primary current of the transformer by turning on and off the switching element, the snubber circuit connects one pole of the switching element and the negative terminal of the DC power supply. A capacitor and a diode inserted in the forward direction are inserted in series between the side terminal and the connection point between the capacitor and the diode is connected to the other pole of the switching element via a suppressing resistor, A DC-DC converter characterized in that the DC-DC converter is configured to prevent charging and discharging current of the capacitor from flowing to the current detection resistor.