JPH0984338A - Bipolar forward converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To alleviate the DC component in an exciting current. SOLUTION: On the side of a primary winding Np of a transformer Te, a main switch Qa, a clamp capacitor Cc, a MOS FET Qb as a secondary switch and a DC power supply Vi are provided. On the secondary side of the transformer Te, the series circuit of diodes Da and Db is connected in parallel. The series circuit of choke coils La and Lb is connected in parallel with a secondary winding Ns. A smoothing capacitor Co is connected to the connecting central point of the diodes Da and Db and the connecting central point of the choke coils La and Lb. In this constitution, a resistor Rb is connected in series with the choke coil Lb between the connecting point of the choke coil La and the smoothing capacitor Co and the choke coil Lb.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bipolar forward type converter.

[0002]

2. Description of the Related Art An example of a bipolar forward type DC-DC converter is shown in FIG. In this converter, the DC power supply V _i, the series circuit of the MOS · FETs Q _a is connected in parallel a primary winding N _P and the main switch of the transformer T _e. Further, a series circuit of a clamp capacitor C _c forming an active clamp circuit and a MOSFET Q _{b serving} as a sub switch is connected in parallel with the primary winding N _P of the transformer T _e . Secondary winding N _s of transformer T _e
Is connected in parallel with a series circuit of two diodes D _a and D _b having cathodes connected to each other, and a series circuit of a choke coil L _a and a choke coil L _b is connected in parallel to the secondary winding N _s. And between the midpoint of connection between the diodes D _a and D _{b and} the midpoint of connection between the choke coils L _a and L _b ,
A parallel circuit of a smoothing capacitor _Cb and a load R is connected. This circuit does not require a center tap in the transformer and the current ripple of each smoothing circuit has an opposite phase,
It has the feature that the ripple current of the output capacitor is small.

According to the analysis by the inventors of the present application, this converter
The data circuit has leakage inductance and
Depending on the size of the transformer excitation inductor and the weight of the load
Therefore, there are multiple operation modes. Below is the weight of the load
The operation mode in the case of no will be described. Waveform of each part
It is shown in FIG. In this operation mode, one switching cycle T
_sIt is divided into 8 states.

In FIG. 4, V_gsQ_a, V_gsQ_bIs M
OS / FETQ_aAnd Q_bGate-source voltage of V_ds
Q_aIs MOS ・ FETQ_aDrain-source voltage of
Pressure, V _LPIs the primary winding voltage of the transformer T, i_LPIs tran
Excitation current of T, i_lgIs the leakage inductor current,
i_La, I_LbIs the choke coil L_a, L_bWith the current flowing through
is there. Note that FIG. 3 is an equivalent circuit of the circuit shown in FIG.
In FIG. 3, T is an ideal transformer and lg is a leakage inductor.
Ta, L_pIs an exciting inductor, r_a, R_bIs a chalk carp
Le L_a, L_bInternal resistance of D_Qb, D_QaIs MOSFETQ
_a, Q_bDiode part, C_a, C_bIs MOS FET
Of the condenser.

<State S ₁ (t ₀ −t ₁ )> This state S ₁
In the earlier state, the switch Q _b is turned on to supply electric power to the secondary side of the transformer T with the energy of the excitation inductor L _p and the clamp capacitor C _c. At time t ₀ , the switch Q _b is turned off, and the currents i _lg flowing through the leakage inductor l _g charge and discharge the capacitors C _a and C _b , respectively.
The drain-source voltage V _ds Q _{a of the} switch Q _a decreases. The energy stored in the exciting inductor L _p and the clamp capacitor C _c of the transformer is transmitted to the secondary side through the transformer T, the choke coil L _b , and the diode D _b . The current i _La of the choke coil L _a continues to flow back through the diode D _b and the capacitor C ₀ .

<State S ₂ (t ₁ -t ₂ )> The voltage of the transformer T becomes zero, and the transformer T tries to transfer power by changing the polarity, but the transfer current is limited to the leakage inductor lg and the primary side is limited. Power cannot be transmitted from the secondary side to the secondary side through the transformer. As a result, the transformer excitation inductor L _p
Voltage V _Lp becomes zero and the exciting current i _Lp becomes constant. A voltage is generated in the leakage inductor lg in the direction of blocking the current flowing through it. Current i of the leakage inductor lg
Before _lg becomes zero, the capacitors C _b and C _a continue to be charged and discharged by the current, and the drain-source voltage V _ds Q _a of the switch Q _a becomes zero at time t ₂ . During this time, the current to the load side flows back through the choke coil L _a and the diode D _a and the choke coil L _b and the diode D _b , respectively. Excitation current i _LP from the current i _lg of leakage inductance lg
Since the secondary conversion current of the current except for flows through the diodes D _a and D _b , the diode current smoothly transitions from D _b to D _a .

<State S ₃ (t ₂ −t ₃ )> At time t ₂ ,
The diode D _Qa is turned on, and ZVS of the switch Q _a can be realized. Until the current i _lg flowing through the leakage inductance lg reaches the sum of the primary converted value of the current i _La of the excitation current i _Lp and the choke coil L _a, the voltage of the excitation inductor L _p of the transformer remains zero, leakage inductance Input voltage is applied to lg. Current to the load is refluxed choke coil L _a, from L _b diode D _a, through D _b.

<State S ₄ (t ₃ −t ₄ )> At time t ₃ ,
Current i _lg of leakage inductance lg is reached to the sum of the primary translation current transformer magnetizing current i _Lp and the choke coil L _a current i _La, the current of the diode D _b at zero current and off. The transformer transfers power from the primary side to the secondary side by a diode D
_It is transmitted through _a and the choke coil L _a . The current i _Lb of the choke coil L _{b flows} back through the diode D _a and the load side.

<State S_Five(T_Four-T_Five)〉 Time t_Fourso,
Switch Q_aTurns off, and the exciting current i of the transformer_LpWhen
Choke coil L_aCurrent i_LaThe sum of the primary conversion values of
Indexer C_a, C_bThe voltage of the transformer decreases
You. The power transfer to the load is in the previous state S₂Is the same as. <State S₆(T_Five-T₆)〉 Time t_FiveAnd the transformer power
Pressure V_LpBecomes zero. The transformer changes the polarity and the power is 2
It tries to transfer to the next side, but it transfers to the leakage inductor lg.
The current is limited and the power cannot be transmitted to the secondary side.
You. As a result, the exciting inductor L of the transformer _pVoltage V
_LpRemains zero and choke coil L_a, L_bCurrent
i_La, I_LbIs the diode D_a, D_bReturn to the load through
Shed. The current of the diode is D_aTo D_bSmoothly
Transition. Current i of the leakage inductor lg_lgBy switch
Chi Q_bVoltage continues to decrease to the leakage inductor lg
Generates a voltage in the direction of blocking the current.

<State S₇(T₆-T₇)〉 Time t₆so,
Switch Q_bDrain-source voltage V_dsQ_bIs zero
Realizes zero voltage switching. Leakage inductor l
current i of g_lgIs the choke coil L _bCurrent i_LbPrimary conversion of
Calculated value and exciting current i_LpUntil the excitation index is equal to
Kuta L_pVoltage V_LpRemains zero. Power transfer to load
And diode D_a, D_bCurrent is the previous state S_bSame as
The same.

<State S ₈ (t ₇ −t ₈ )> At time t ₇ ,
The transformer reaches a level at which power can be transferred to the secondary side, and transfers the energy of the exciting inductor L _p and the clamp capacitor C _{c to} the secondary side through the diode D _b and the choke coil L _b . Since the power cannot be supplied to the secondary side with the energy of only the reset voltage of the clamp capacitor C _c , the exciting current i _Lp has a DC component. Diode D _a is off at zero current, the current of the choke coil L _a is refluxed through the diode D _b.

Current i in each of the above states_LP, I_La, I_Lb
The theoretical equations such as can be derived as follows. In addition, in the following
Is the power non-transmission period due to the leakage inductor lg of the transformer
The dead time and dead time should be small enough to ignore.
Let switch parasitic capacitance C_a, C _bAnd leakage inductor l
Ignore g. In this case, one switching cycle T_sSmell
There are two states, DT_sThe above state S_FourAnd then
Between (1-D) T_sState S ₈As the extended state averaging method
adopt.

It is assumed that the state variable is X, the high frequency variable is Y, and the coefficient matrix is G.

[0014]

[Equation 1]

The equations for each state are as follows:

[0016]

[Equation 2]

Next, i _Lp in equation (8) is represented by a state variable.
Since the change in the magnetic flux of L _p within one switching cycle is zero, the following equation holds.

[0018]

(Equation 3)

## EQU1 ## From the above equation, the average value of the high frequency component Y is obtained, and the following basic method is obtained.

[0020]

[Equation 4]

With the equation (11) set to zero, the following characteristic equation in the steady state is obtained.

[0022]

(Equation 5)

[0023]

(Equation 6)

Here, when n ² is sufficiently large, r _p / n
² can be ignored, and equations (14)-(17) are as follows.

[0025]

(Equation 7)

[0026]

In the above bipolar forward converter, from equations (13) and (18),

[0027]

(Equation 8)

The following relationship is obtained. From equation (23),
Excitation current i_LpIs the choke coil L_bCurrent i_Lb
Proportional to. Therefore, the exciting current i of the transformer_LpIs straight
The flow reference component shifts the reference level of operation.
As a result, the utilization rate of the transformer core is reduced. One
Marie and become magnetically saturated. Therefore, the load current increases,
Choke coil L _bCurrent i_LbAs the number of transformers increases
The DC component of the exciting current increases accordingly and the transformer
There is a problem that the maximum magnetic flux density of the core is exceeded and it will be destroyed.
You. Therefore, choke coil L_bInternal resistance r_bLarge
It's good if you do it, but the internal resistance of the circuit is pattern resistance,
It is determined by the winding resistance of the core and the resistance of the soldered part.
Control lance, r_bOnly difficult to grow.
Also, since these internal resistances are very small values,
If the variation in construction is converted to the resistance value ratio, there will be a large variation.
There is a problem that becomes.

The present invention has been made by paying attention to the above problems, and it is not necessary to adjust the internal resistance of the choke coil, the DC component of the exciting current of the transformer can be reduced, and the core can be effectively used. An object is to provide a suitable bipolar forward converter.

[0030]

The bipolar forward converter according to claim 1 of the present invention is a bipolar forward converter which is capable of supplying energy to the secondary side through the transformer even if the polarity of the transformer is changed. The primary winding of the transformer and the first switching means (main switch) are connected in series, the capacitor for clamping and the second switching means (sub switch) are connected in series, and are coupled to the transformer. In parallel with the secondary winding of the transformer, connect a series circuit of two diodes with the cathodes commonly connected, and
A series circuit of a series circuit of a first choke coil and a resistor that stores energy when the second switching means is on, and a second choke coil that stores energy when the first switching means is on. It is connected in parallel to the secondary winding of the transformer, and an output smoothing capacitor is provided between the connection midpoint of the two diodes and the connection point of the series circuit of the first choke coil and the resistor and the second choke coil. The first switching means and the second switching means are alternately connected to each other with an off period provided.
I try to turn it off.

The bipolar forward converter according to a second aspect of the present invention is the bipolar forward converter according to the first aspect, in which two diodes are connected to each other as cathodes but are connected in common to two anodes. It is a thing. In either case, since a resistor is connected in series to the first choke coil, it is possible to use a resistor having a large resistance value, thereby reducing the current flowing in the first choke coil. Therefore, the direct current component of the exciting current of the transformer can be reduced, and the problem due to saturation of the transformer can be eliminated.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail with reference to embodiments. FIG. 1 is a circuit diagram of a bipolar forward converter showing an embodiment of the present invention. This converter includes a DC power supply V _i and a transformer T _e.
The primary winding N _p and the series circuit of the MOS • FET Q _a that is the main switch (first switching means) are connected in parallel. In addition, in parallel with the primary winding N _p of the transformer T _e , a series circuit of a clamp capacitor C _c that constitutes an active clamp circuit and a MOS switch FET Q _b that is a sub switch (second switching means) is connected. . The secondary winding N _s of the transformer T _e, 2 diodes D _a cathode each other are connected, the series circuit of the D _b are connected in parallel, the further secondary winding N _s, the choke coil L _a And a choke coil L _b are connected in parallel, and a parallel circuit of a smoothing capacitor C _b and a load R is provided between the midpoint of connection between the diodes D _a and D _{b and} the midpoint of connection between the choke coils L _a and L _b. Are connected. The above configuration is similar to that of the basic circuit shown in FIG. The feature of the converter of this embodiment is that a resistor R _b is connected in series to the choke coil L _b between the connection point of the choke coil L _a and the smoothing capacitor C _o and the choke coil L _b .

By connecting this resistor R _b ,
The resistance that enters in series with the choke coil L _b is the internal resistor r
and _b, the resistor R _b, and the by the resistance value of the resistor R _b and large, it is possible to reduce the current i _Lb that flows through the choke coil L _b. As shown in FIG. 5, r _a = r _b
Then, the average value of i _La = the average value of i _Lb = I _o / 2, and the average value of i _Lb cannot be reduced. Therefore, as shown in FIG. 6, the average value of i _Lp is also large. , And by connecting the resistor R _b in series, r _a << r _b + R _b , and as shown in FIG. 5 and FIG. 6, the average value of i _{Lb and} the average value of i _Lp are small, and the saturation characteristic is significantly increased. To be improved.

FIG. 7 is a circuit diagram of a bipolar forward converter showing another embodiment of the present invention. This converter is different from that shown in FIG. 2 in that the anodes of two diodes D _a and D _b are connected in series and the series circuit of the diodes D _a and D _b is connected to the secondary of the transformer T. connected in parallel to the winding N _s, it is connected to the connection point of the choke coil L _b and a smoothing converter C _o, in series with the choke coil L _a between the choke coil L _a resistor R _a. The function and effect are similar to those shown in FIG.

FIG. 8 is a circuit diagram showing a bipolar forward converter according to still another embodiment of the present invention. The converter differs from that shown in FIG. 2, the active clamp circuit comprising a series circuit of the sub-switch Q _b and the clamp capacitor C _c, are connected in parallel with the main switch Q _a. Other configurations and the point of improving the saturation characteristic of the transformer are the same as those of FIG.

FIG. 9 is a circuit diagram showing a bipolar forward converter according to still another embodiment of the present invention. This converter is different from that shown in FIG. 2 in that an active clamp circuit composed of a clamp capacitor C _c and a series circuit of a MOS • FET which is a sub switch is connected in parallel to a secondary winding N _s of a transformer T. That is what I did. The other circuit configuration is the same as that of FIG. 2, and there is no change in the effect of improving the saturation characteristic of the transformer.

As shown in FIGS. 2, 8 and 9, in the present invention, the active clamp circuit may be coupled to the transformer T. FIG. 10 is a circuit diagram showing a bipolar forward converter according to still another embodiment of the present invention. This converter uses MOSFETs Q _c and Q _d instead of the diodes D _a and D _b shown in FIG. The other circuit configuration is similar to that shown in FIG. Thus, instead of a diode, a MOS
-By using the FET, the on resistance becomes smaller than that of the diode, and the power can be efficiently transmitted by that amount.

Incidentally, FIG. 2, FIG. 7, FIG. 8, FIG. 9 and FIG.
In the embodiment shown by, the main switch Q _a and the sub switch Q a
_{In each of b} , an example using MOS • FET is shown, but a bipolar transistor may be used instead of these MOS • FET, or a parallel circuit of a contact switch and a diode may be used.

[0039]

According to the present invention, a resistor is positively connected in series with a choke coil to reduce the DC component of the exciting current of the transformer, so that the magnetic core of the transformer can be effectively utilized, Can be miniaturized. A large load current can be taken. Since the ripple component of one choke coil remains, the ripple component of the output capacitor is canceled by the sum with the ripple component of the other choke coil, and a capacitor with a small capacitance can be used.
The size can be reduced. Since the current of one choke coil is small, the choke coil can use a small core and winding. Therefore, the other choke coil through which a large current flows can be made larger. And so on.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a basic circuit of a bipolar forward converter.

FIG. 2 is a circuit diagram showing a bipolar forward converter according to an embodiment of the present invention.

FIG. 3 is a circuit diagram showing an equivalent circuit of the bipolar forward converter shown in FIG.

FIG. 4 is a time chart of operation waveforms of respective parts of the bipolar forward converter shown in FIG.

5 is a waveform diagram for explaining the degree of improvement of the current flowing through the choke coil of the bipolar forward converter of the embodiment shown in FIG.

FIG. 6 is a waveform diagram for explaining the degree of improvement of the excitation current of the transformer of the bipolar forward converter according to the embodiment.

FIG. 7 is a circuit diagram showing a bipolar forward converter according to another embodiment of the present invention.

FIG. 8 is a circuit diagram showing a bipolar forward converter according to still another embodiment of the present invention.

FIG. 9 is a circuit diagram showing a bipolar forward converter according to still another embodiment of the present invention.

FIG. 10 is a circuit diagram showing a bipolar forward converter according to still another embodiment of the present invention.

[Explanation of symbols]

Vi DC power supply Q _a Main switch MOS / FET Q _b Sub switch MOS / FET C _c Clamp capacitor _Te Transformer D _a , D _b Diode L _a , L _b Choke coil C _o Smoothing capacitor R _b Resistor

Claims (3)

[Claims] 1. A bipolar forward converter capable of supplying energy to a secondary side through a transformer even if the polarity of the transformer is changed. A direct current source, a primary winding of the transformer, and a first switching means are provided. Connected in series, a clamp capacitor and a second switching means are connected in series, coupled to the transformer, and connected in parallel to the secondary winding of the transformer, a series circuit of two diodes having their cathodes commonly connected A series circuit of a first choke coil and a resistor that are connected and that store energy when the second switching means is on; and a second choke coil that stores energy when the first switching means is on. Is connected in parallel to the secondary winding of the transformer, the midpoint of connection of the two diodes, the first choke coil and the resistor. An output smoothing capacitor is connected between the connection points of the series circuit and the second choke coil, and the first switching means and the second switching means are alternately turned on and off with an off period provided therebetween. A bipolar forward converter characterized in that 2. A bipolar forward converter capable of supplying energy to the secondary side through the transformer even if the polarity of the transformer is changed. A direct current source, a primary winding of the transformer, and a first switching means are provided. Two diodes connected in series, a clamp capacitor and a second switching means are connected in series, coupled to the transformer, and connected in parallel to the secondary winding of the transformer, with the anodes commonly connected in series. A series circuit of a first choke coil and a resistor for connecting a circuit and for storing energy when the second switching means is on, and a second choke for storing energy when the first switching means is on. A series circuit with a coil is connected in parallel to the secondary winding of the transformer, a connection midpoint of the two diodes, and the first choke coil. An output smoothing capacitor is connected between the connection point between the anti-series circuit and the second choke coil, and the first switching means and the second switching means are alternately turned on and off with an off period provided therebetween. Bipolar forward converter characterized by 3. The diode according to claim 2 is replaced by two Ms.
The bipolar forward converter according to claim 2, wherein an OSFET is used.