JP2996014B2 - DC-DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter, and more particularly to an improvement in a power loss characteristic of a main switch of a voltage resonance type DC-DC forward converter.

[0002]

2. Description of the Related Art As is well known, the turn-on loss of a DC-DC converter is reduced by zero volt switching (ZVS).
The method has been significantly improved by applying the method to a voltage resonance type or current resonance type DC-DC converter. Especially, the voltage resonance type converter is
Since the output capacitance of the SFET can be used as a part of the resonance capacitor, it is suitable for increasing the switching frequency. Therefore, many studies have been conducted to obtain high efficiency at a high frequency using a voltage resonance type DC-DC converter.

FIG. 6 is a circuit diagram of a conventional example of a single-type voltage resonance DC-DC forward converter. The circuit includes a DC power supply 1, a main transformer 5, a main switch 3, a capacitor 4 connected in parallel to the main switch 3, a voltage clamper 70, a rectifier diode 11, a current snubber circuit 71, an input capacitor 2, an output capacitor 12, and a main switch. 3 comprises a loop control circuit 14 for controlling the switching. In the figure, the coil 15 represents the leakage inductance of the main transformer and, if necessary, the inductance of an externally attached coil.

A main switch 3 and a capacitor 4 are connected in series to a main transformer 5, and the main switch 3 is connected to a loop control circuit 1
4 to control the output of the converter. The capacitor 4 has an inductance L of the main transformer 5.
_P forms a series resonance circuit, which turns on the main switch 3 with a voltage of zero volts when the main switch 3 is turned on.
Is applied. In this way, the power loss at turn-on,
That is, the turn-on loss can be reduced.

The voltage clamper 70 resets the magnetic energy stored in the core of the main transformer 5 during the ON period of the main switch 3 to prevent the core from being magnetically saturated. The voltage clamper 70 includes a diode, a resistor and a capacitor connected in parallel with each other, and is connected in parallel to the primary winding 6 of the main transformer 5. The magnetic energy stored in the core is dissipated by the resistance in the voltage clamper 70 during the off period of the main switch 3, and the surge voltage at the time of turning off is absorbed by the capacitor. The current snubber circuit 71 protects the diode 11 by shunting the surge current induced by the coil 15 when the secondary current of the main transformer 5 is turned off. This circuit consists of a resistor and a capacitor connected in series, the resistor dissipating the energy of the current and the capacitor smoothing the current.

This voltage resonance type DC-DC forward converter operates as follows. First, when the main switch 3 is turned off, beginning the resonant current in the primary winding 6 flows, the voltage V _DS of the main switch 3 rises from 0 volts. When the voltage _VDS returns to 0 after a half cycle, the main switch 3 is turned on under the control of the loop control circuit 14. In this way, turn-on losses are prevented. The length of the ON period is proportional to the set value of the output voltage V _O of the converter, and if there is a deviation from the set value, the length of the ON period is adjusted so as to compensate for the deviation.

FIG. 7 is a diagram showing a multi-output single-pole voltage resonance type DC-DC.
FIG. 3 is a configuration diagram of a conventional example of a forward converter. For simplicity, only the main output circuit 72 and one auxiliary output circuit 73 are shown. Usually, the highest output is selected as the main output, and the selected main output is controlled by the voltage resonance switching of the main switch 3. The auxiliary output circuit 73 has its own output adjusting circuit so that the control of the main output does not affect the auxiliary output. The converter composed of the voltage resonance switching circuit and the main output circuit shown in FIG. 7 is the most usual converter (for example, see IEICE 90 Vol. 439, PE 90-69), and the auxiliary output circuit 7
3 rectifies the output current of the auxiliary secondary winding 77 of the main transformer by the rectifier diode 11 and outputs the rectified output current. Output voltage V
_AO is independent of the main output circuit 72,
Adjusted by 6.

[0008]

In the conventional voltage-resonant DC-DC converter shown in FIG. 6, in order to apply a sinusoidal resonance voltage to the main switch, a peak voltage is necessarily applied to the switch element. As a result, the high peak voltage causes voltage stress on the switching element. Although it is necessary to use a high-voltage switch to prevent this voltage stress, the high-voltage switch usually has a high on-resistance, causing an on-resistance loss, which causes a decrease in the efficiency of the converter. Further, when the voltage clamper 70 and the current snubber circuit 71 are used to prevent the adverse effects caused by the application of a high voltage, not only power loss occurs in these circuits but also if these circuits are not used carefully, The voltage waveform itself is deformed, which also causes a turn-on loss of the main switch.

The conventional multi-output type DC- shown in FIG.
In the DC converter, there is a problem that a power loss is inevitably generated when the fluctuation of the input voltage of the auxiliary output circuit is compensated using the series dropper. The change in the input voltage of the auxiliary output circuit usually occurs when the load current of the main output circuit changes, or when the input voltage to the main output circuit changes, the on-period of the main switch changes in response to the change. Occurs when the frequency changes.

An object of the present invention is to provide a voltage-resonant DC-DC forward converter with a snubber circuit that can use a low-voltage, low-on-resistance main switch and reduce the power loss of a rectifier diode. Is to do.

[0011]

To achieve the above object, a voltage resonance type DC-DC forward converter according to the present invention comprises a capacitor and a first diode connected in series, and a secondary transformer of a main transformer. A series circuit that is connected to both ends of a winding and forms a resonance circuit that conducts a resonance current in the forward direction of the first diode together with the secondary winding, and charges the electric charge stored in the capacitor with the first diode And a discharging means for discharging during the off period of the first diode. The forward direction of the first diode corresponds to a period in which the secondary current is supplied from the secondary winding to the smoothing circuit via the second diode for rectification. Is
Snubber means is provided so as to prevent conduction of the resonance current.

[0012]

The operation of the voltage resonance type DC-DC forward converter according to the present invention can be described in three periods. First, after the main switch is turned off,
During the period (period I) until the forward voltage of the diode becomes 0 and the supply of the secondary current to the load stops, the voltage applied to the first diode becomes forward and the resonance circuit becomes conductive. , The capacitor is charged by the forward resonance current of the first diode, the forward voltage of the first diode decreases as the charging voltage of the capacitor increases, and the period until the first diode is finally turned off ( Period II),
The charge stored in the capacitor is discharged through the discharging means during this period until the end of the off period of the main switch after the first diode is turned off (period III).

In the following description, the resonance circuits formed on the primary side and the secondary side of the main transformer will be referred to as a primary resonance circuit and a secondary resonance circuit, respectively, and the resonance current flowing through each resonance circuit will be referred to as the primary resonance circuit. It is referred to as a resonance current or a secondary resonance current. Since in the period I, the resonance in the secondary resonant circuit is prevented, the voltage applied to the main switch, rises with a time constant corresponding to the resonance period T ₁ of the first resonance circuit. In the period II, the secondary resonance circuit conducts and resonance occurs, which acts on the resonance of the primary resonance circuit via the main transformer. Therefore, the resonance of the primary resonance circuit is converted to the primary side. capacitance and inductance of the next resonant circuit is performed at the resonance period T ₂ of the equivalent resonance circuit connected in parallel to the self-induction of the primary winding. Accordingly, in the period II, the voltage applied to the main switch, change slowly with a large time constant T ₂ compared to T _1. In the period III, the conduction of the secondary resonance circuit is stopped again, so that the voltage applied to the main switch is changed to the period T ₁ again.
And then drop to 0 volts. Thus, the voltage applied to the main switch is equal to the period I and III.
Rises and falls with a sinusoidal waveform, but slowly changes in period II. Therefore, the applied voltage of the main switch is
The peak portion becomes a truncated sine wave, that is, a truncated sine wave, and a high peak voltage is prevented from being applied to the main switch.

As described above, the primary resonance current flows through the primary circuit of the main transformer such that the voltage applied to the main switch becomes a truncated sine wave in the period II. As a result, a secondary voltage that changes slowly in the period II is generated in the secondary winding. As is well known, after the second diode (rectifying diode) is turned off, the secondary voltage of the main transformer is applied to the second diode as a reverse voltage. Therefore, the period II during which the reverse recovery current flows through the second diode II
If the change in the secondary voltage (the rising of the secondary voltage in the reverse direction) is small in the initial stage, the power loss due to the secondary voltage of the reverse recovery current decreases. The power loss is reduced because the snubber circuit of the present invention changes the secondary voltage slowly during period II.

[0015]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a voltage resonance DC- equipped with a snubber circuit of the present invention.
FIG. 3 is a circuit diagram of a DC forward converter. Since the main parts of the converter have already been described with reference to FIG. 6 described above, the description thereof will be omitted. The snubber circuit 18 includes a series connection of a capacitor 8 and a diode 9 and a coil 10. Both ends of the series connection are connected to the main transformer 5
Are connected to both ends of the secondary winding 7, and the secondary winding 7, the diode 9, and the capacitor 8 constitute a series resonance circuit that conducts only in the forward direction of the diode 9. Hereinafter, this resonance circuit is referred to as a secondary resonance circuit, and the resonance current generated in the secondary resonance circuit is referred to as a secondary resonance current. The forward direction of the diode 9 is such that the secondary winding 7 passes through the rectifier diode 11
While the secondary current is being supplied to 2, the diode 9 is directed in a direction to prevent the generation of the secondary resonance current. The coil 10 is connected between the connection between the capacitor 8 and the diode 9 and the cathode of the rectifier diode 11, and forms a discharge path for the electric charge accumulated in the capacitor 8. The discharge is performed when the diode 9 is off. The coil 15 represents the leakage inductance of the main transformer 5 and the inductance of an externally attached coil if necessary. When the main switch 3 is turned off, the capacitance of the capacitor 8 slows the rise of the resonance voltage applied to the main switch 3 and lowers the peak, and does not make the off period of the main switch 3 longer than a predetermined value. It is determined as follows. Coil 1
The inductance of 0 is made large enough to prevent ripple from occurring at the output of the converter, but small enough to reset the charge on capacitor 8 before the next cycle of voltage resonant switching begins. Therefore, the inductance of the coil 10 is determined so that the time constant of the capacitance of the capacitor 8 and the inductance of the coil 10 becomes substantially equal to the average period of the voltage resonance switching.

Next, the operation of the snubber circuit will be described. FIG. 2 is a time chart of signals of respective parts for explaining the operation of the snubber circuit according to the present invention. FIG. 3 is a diagram illustrating the operation from the secondary side of the voltage resonance DC-DC forward converter during the period when the secondary resonance circuit is conducting. The equivalent circuit seen is shown. When the main switch 3 is turned on at time t ₀ , the primary current is
Through, rising at a constant gradient, whereby induced electromotive force E ₂ is generated in a certain secondary winding (see Figure 2 (b)). Induced electromotive force E ₂ generates a secondary current rises with a constant gradient (see FIG. 2 (c)). Then the gradient is (E ₂ −V _O
−V _F ) / L _e . Here, E ₂ is an equivalent electromotive force as viewed from the secondary side, where E is the electromotive force of the DC power source 1 and n ₂ / n ₁
Is the winding ratio of the main transformer, E ₂ = (n ₂ / n ₁ ) E. V _O and V _F are the output voltage of the converter and the forward voltage applied to the diode 11, respectively.
_Le is the inductance of the coil 15.

When the main switch 3 is turned off at time t ₁ , the voltage V _DS applied to the main switch 3 by the resonance current (hereinafter, referred to as the primary resonance current) generated in the primary winding 6 becomes It rises with a sine function waveform having a period T ₁ = 2π (L _P C _P ) ^1/2 . Here the capacity of the L _P, C _P each primary winding of the inductance and the resonant capacitor 4. At the same time, 2
Induced electromotive force of the same cycle T ₁ is induced in the secondary winding 7 in the opposite direction (the direction of the induced electromotive force generated in the secondary winding 7 when the main switch 3 is in the on state is positive). as a result,
The voltage V ₇ across the secondary winding 7 decreases as shown in FIG. Reduces the forward current I _D of the diode 11 by the voltage drop V _7, current I _D at time t ₂ becomes zero.

When the diode 11 is turned off at time t ₂ , the secondary resonance circuit is cut off from the smoothing circuit 12 except for the electric circuit passing through the coil 10. In this state, when the electromotive force generated in the secondary winding 7 in the reverse direction exceeds the reverse bias applied to the diode 9, the diode 9 is turned on, and the secondary resonance current flows in the forward direction. Conduct. FIG. 3 shows an equivalent circuit of the voltage resonance DC-DC forward converter when secondary resonance occurs. Resonant inductance L _P, capacitance C _P and C _S (Indakudansu L _e
), And the resonance period T ₂ is 2π [L _P {C _P
+ (N ₂ / n ₁ ) ² C _S }] ^1/2 . Thus, the V _{DS of} FIG.
And gradual rise and lower peak value as shown in the time chart of V ₇ and is achieved. When the voltage of the capacitor 8 reaches the maximum value, the secondary resonance stops because the reverse resonance current is blocked by the diode 9.
The electric charge charged in the capacitor 8 is the coil 10
And is input to the smoothing circuit 12.

As is clear from the above description, in the period (time period I) from time t ₁ to time t ₂ when the secondary resonance is blocked, the curves of the voltages V _DS and V ₇ have the short period T _1. rising sinusoidal waveform, in a period from the time t ₂ the secondary resonance is occurred until the time t ₃ (period II), the curve of the voltage V _DS and V _7, the long period T except transitional period changes in _second sinusoidal waveform, the period from the time t ₃ when the secondary resonance is prevented again until time t ₄ (the end time of the off period of the main switch 3) (period III) in the voltage V _DS and V The curve ₇ falls with a short-period sinusoidal waveform. Period steep rise of the voltage V _DS in I and III, it is possible to shorten the off period of the main switch 3 by falling (off period can ripple in the output of the too long converter). Further, it is possible to lower the peak value of the voltage V _DS by slow behavior of the voltage V _DS in the period II, as a result, it is possible to use low voltage, low on-resistance of the switching element as a main switch.

The snubber circuit of the present invention has another advantage. Beginning of period II, when the diode 11 is Tsu switched from the ON state to the OFF state at time t _2, the reverse recovery current flows through the diode 11. This current is shown in FIG.
This is shown as a reverse current in (c). Reverse recovery current flows during the period from time t ₂ to time t ₅ (period IV). As described above, since the diode 9 is in the ON state during the period IV, the voltage V ₇ is applied to the diode 11 via the diode 9 and the coil 10 in the reverse direction. As a result, the reverse recovery current and the voltage V ₇ to make a positive power factor, whereby the power loss. The power loss, the slope of the voltage V ₇ becomes less extent is slow in the period IV. Since the voltage V ₇ rises slowly (in the reverse direction) in the period IV by the snubber circuit of the present invention, when the snubber circuit of the present invention is not used (in this case, it rises with a sinusoidal waveform having a period T ₁ ). Power loss can be reduced as compared with

The operation of the snubber circuit of the present invention can be summarized as follows from the viewpoint of energy conservation. When the main switch is turned off, a primary resonance current flows due to the residual magnetic energy of the main transformer. If the snubber circuit of the present invention is not provided, the energy of the primary resonance current is determined by the charging voltage of the resonance capacitor 4 as shown in FIG.
Large enough to produce the peak represented by the dotted line. However, when the snubber circuit is provided, a part of the residual magnetic energy is stored in the capacitor 8 as electrostatic energy through the secondary resonance current. Therefore, the energy of the primary resonance current decreases by the amount of the electrostatic energy of the capacitor 8, and as a result, the voltage of the resonance capacitor 4 (= V _DS )
Cannot be ascended and has a waveform with a peak portion cut off. On the other hand, the magnetic energy stored as electrostatic energy in the capacitor 8 is supplied to the load via the coil 10 and is effectively used there.

The snubber circuit of the present invention can be used in half-wave and full-wave voltage resonant DC-DC forward converters as shown in FIGS. 4 and 5, respectively.

[0023]

As described above, the DC-DC of the present invention is
The converter has the following effects. First DC-D of the present invention
Voltage resonant DC-DC with snubber circuit as C converter
The forward converter accumulates the energy of the induced current flowing through the secondary winding of the main transformer as electrostatic energy in the capacitor of the snubber circuit during the period when the supply of the secondary current to the load is stopped. The waveform of the voltage applied to the main switch can be made into a truncated sine waveform, whereby a low-voltage, low-on-resistance switch element can be used as the main switch. , The rising slope of the reverse voltage applied to the diode can be made gentler, whereby the power loss due to the reverse recovery current can be reduced. (C) Furthermore, the capacitor of the snubber circuit By discharging the accumulated charge to the load through the coil, the electrostatic energy of the capacitor can be effectively used.

[Brief description of the drawings]

FIG. 1 shows a voltage resonance DC-D having a snubber circuit according to the present invention.
It is a circuit diagram of a C forward converter.

FIG. 2 is a time chart of signals of each section for explaining the operation of the snubber circuit of the present invention.

FIG. 3 is an equivalent circuit viewed from the secondary side of the voltage resonance DC-DC forward converter during a period when the secondary resonance circuit is conducting.

FIG. 4 shows half-wave voltage resonance D using the snubber circuit of the present invention.
It is a C-DC forward converter.

FIG. 5 is a full waveform voltage resonance D using the snubber circuit of the present invention.
It is a C-DC forward converter.

FIG. 6 is a circuit diagram of a conventional example of a single-type voltage resonance DC-DC forward converter.

FIG. 7 is a configuration diagram of a conventional example of a multi-output single-pole voltage resonance DC-DC forward converter.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 DC power supply 2 input capacitor 3 main switch 4 resonance capacitor 5 main transformer 6 primary winding 7 secondary winding 8 capacitor 9 diode 10 coil 11 rectifying diode 12 output capacitor 13 load 14 loop control circuit 15 leakage inductance

Claims (1)

(57) [Claims] A capacitor and a first diode are connected in series, and are connected to both ends of a secondary winding of a main transformer to generate a resonance current in the forward direction of the first diode together with the secondary winding. A series circuit forming a conducting resonance circuit; and discharging means for discharging the electric charge accumulated in the capacitor during an off period of the first diode. During the period in which the secondary current is supplied from the winding to the smoothing circuit via the second diode for rectification, the conduction of the resonance current is determined to be prevented . Daioh
Inductive element connected between the connection point of the gate and the smoothing circuit
And a voltage resonance type having a snubber means DC-D
C forward converter.