JP2002199707A - Dc-dc conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a deboosting noninsulated dc-dc conversion circuit which operates stably with low loss, even when a low voltage is inputted, with merely adding a small number of circuit components to the conventional circuit. SOLUTION: The dc-dc conversion circuit 1 of this invention is a deboosting noninsulated circuit which on/off-controls a power transistor Q1 by a constant- current circuit (drive circuit 12). The constant-current circuit is composed by comprising a control transistor Q2, whose collector terminal (one of the input or the output terminals) is connected to the base terminal of the power transistor, a constant-voltage generating resistor R1, whose one terminal is connected to the emitter terminal (the other terminal) of the control transistor Q2 and the other terminal is grounded, and a constant-voltage dropping circuit 121, whose one terminal is connected to the base terminal of the control transistor and the other terminal is grounded. When a control pulse P is inputted to the base terminal of the control transistor Q2, the control transistor Q2 operates, so that its ON-current becomes constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-down / non-isolated DC / DC conversion circuit which operates stably even at a low voltage input and has small loss.

[0002]

2. Description of the Related Art Conventionally, a circuit shown in FIG. 3 is known as a step-down / non-insulated DC / DC conversion circuit. DC of FIG.
The / DC conversion circuit 9 is used for a printer, for example, and includes a main circuit 91, a drive circuit 92, an output voltage detection circuit 93, and a control pulse generation circuit 94.

The main circuit 91 includes an input capacitor C ₁ ′,
PNP-type power transistor Q ₁ ′, reactor L ₁ ′, diode D ₁ ′, and output capacitor C ₂ ′
Consists of The input capacitor C ₁ ′ is connected to the input terminal a ₁ ′,
a ₂ ′ (a ₂ ′ is ground), the emitter terminal E of the PNP type power transistor Q ₁ ′ is connected to the input terminal a
₁ ′, the collector terminal C of the power transistor Q ₁ ′ is connected to _one terminal of the reactor L ₁ ′. Output capacitor _{C 2} 'is the output terminal _b 1', _{b 2} 'between _{(b 2'} are ground) is connected to, 'the other terminal of the output terminal _{b 1'} reactor _{L 1} is connected to. The cathode terminal K of the diode D ₁ ′ is connected between the power transistor Q ₁ ′ and the reactor L ₁ ′, and the anode terminal A is grounded.

The driving circuit 92 includes a resistor R₁'And PNP type
Control transistor Q₂'. Resistance R₁'One
The other terminal is a control transistor Q₂′ Collector terminal C
And the control transistor Q₂′ Emitter terminal
E is grounded. The output voltage detection circuit 93
Force terminal b₁', B₂'And a resistor R₂'When
R ₃'In series. The control pulse generation circuit 94
The input terminal is a resistor R₂', R₃′, Connected to the output
Is a control transistor Q₂′ Base terminal C
Have been.

[0005] The DC / DC conversion circuit 9 generates a control pulse.
The resistor R₂'And R₃′ (The output voltage
Force voltage V_O'Partial pressure) V_S'Has been entered. Control pad
Loose generation circuit 94 receives input voltage V_S'Will be the set value
The control pulse P having such a duty is output. Control
Transistor Q₂', A control pulse P is applied to the base terminal B.
Then, the control transistor Q₂'Is turned on.
Control transistor Q ₂′ On current is power transistor
TA Q₁'Base current I_B′, So the power
Star Q₁'Also turns on. Therefore, the input terminal a₁′
From the power transistor Q₁', Reactor L₁′
Output terminal b₁′ Flows through the main circuit current I ′. This and
Come, reactor L₁′ Has the input voltage V_iOutput voltage with
V_O'(Output capacitor C₂'Voltage)
The resulting electromotive force is induced.

[0006]

[SUMMARY OF THE INVENTION In DC / DC converter 9 of FIG. 3, the values of _'the base current I _B of _the' power transistor Q ₁ is, depending on the magnitude of the input voltage V _{i ',} substantially V _i '/ R ₁ '. Startup or power failure, or may be input voltage becomes low when the load is large (in Fig. _{4, V i '= V Low} ). The value of the time base current I _{B 'becomes} smaller. As a result, the main circuit current I 'also decreases (I' = I _{Low in} FIG. 4), and the power supply voltage of the electric circuit system decreases.

In order to alleviate such inconvenience, the DC / DC conversion circuit 9 does not reduce the input voltage as shown in FIG. 4 (in FIG. 4, V _i ′ = V
The _{nom al),} there was set a 'to operate in the saturation region (in FIG. _{4, I'} power transistor _{Q 1 = I Nomal)} 'value of (≒ _V i' resistor _{R 1} / I ') .

However, when the power transistor Q ₁ ′ is operated in the saturation region (when the base current of Q ₁ ′ is increased), the turn-off of the control transistor Q ₂ ′ is delayed, and the Q ₁ ′ and Q ₂ ′ are turned off. There is a problem that power loss increases.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a step-down / non-insulation type DC / DC converter which does not reduce the output voltage even at the time of low-voltage input and has a small loss only by adding a small number of circuit components to the conventional circuit.
That is, to realize a DC conversion circuit.

[0010]

SUMMARY OF THE INVENTION The present invention is applied to a step-down / non-insulated DC / DC conversion circuit in which a power transistor is turned on / off by a constant current circuit. And a resistor for generating a constant current,
It is configured to include a constant voltage drop circuit.

The control transistor has one input / output terminal connected to the base terminal B of the power transistor. The constant current generating resistor has one terminal connected to the other terminal of the control transistor and the other terminal grounded. The constant voltage drop circuit has one terminal connected to the base terminal of the control transistor and the other terminal grounded.

According to the present invention, when a control pulse is input to the base terminal of the control transistor, the control transistor operates so that the ON current is constant. Alternatively, the control transistor operates so that the base current of the power transistor becomes constant.

A plurality of (typically 2) constant voltage drop circuits are provided.
) Series-connected diodes.
When the control transistor is turned on by a control pulse input to the base of the control transistor, the series-connected diode is turned on in the forward direction.

According to the DC / DC conversion circuit of the present invention, when the control transistor is turned on, the input / output current (for example, the collector current) of the control transistor causes the power transistor to turn on in an unsaturated region. Thus, the value of the constant current generating resistor and the voltage drop value of the constant voltage drop circuit can be set.

The main circuit can be composed of an input capacitor, an output capacitor, a reactor, a diode, and a power transistor. In this case, the input capacitor has a non-grounded terminal connected to the input main circuit terminal (for example, an emitter) of the power transistor, and the reactor has one terminal connected to the output main circuit terminal (for example, the collector) of the power transistor. Connected to the diode
One terminal of the cathode terminal is connected between the connection terminal of the power transistor and the reactor, and the anode terminal is grounded. The output capacitor has a non-grounded terminal connected to the other terminal of the reactor.

The DC / DC conversion circuit of the present invention can include an output voltage detection circuit connected between the terminals of the output capacitor, and a control pulse generation circuit connected to the output voltage detection circuit. The control pulse generation circuit inputs a detection signal generated by the output voltage detection circuit, and can output a control pulse to the base terminal of the control transistor and the constant voltage drop circuit according to the value of the detection signal.

[0017]

FIG. 1 is an explanatory view showing an embodiment of the present invention. The DC / DC conversion circuit 1 shown in FIG. 1 is used for a printer, and includes a main circuit 11, a drive circuit (constant current circuit) 12, an output voltage detection circuit 13, and a control pulse generation circuit 14. ing. DC of this embodiment
The / DC conversion circuit 1 is used to reduce a DC voltage of, for example, 42 volts to 5 volts for driving an electronic circuit.

The main circuit 11 includes an input capacitor (electrolytic capacitor) _{C 1,} a power transistor to _{Q 1} PNP type,
A reactor _{L 1,} consisting of a diode _{D 1,} the output capacitor _{C 2} (the electrolytic capacitor). Input capacitor C
₁ is connected between the input terminal _a 1, _{a 2 (a 2} a ground terminal), the emitter terminal of the PNP power transistor _{Q 1} is connected to the input terminal _{a 1,} a collector terminal, one of the reactor _{L 1} Connected to terminal. Between the output capacitor _{C 2} is output terminal _b 1, _{b 2 (b 2} a ground terminal)
Is connected to the other terminal of the reactor L ₁ is an output terminal b
₁ connected. The cathode terminal K of the diode _{D 1}
Is connected between the power transistor Q ₁ and the reactor L ₁ , and the anode terminal A is grounded.

The drive circuit 12 includes a PNP-type control transformer.
Jista Q₂And a resistor R for generating a constant voltage ₁And constant voltage drop circuit
121. Control transistor Q₂Collector
Terminal is power transistor Q₁Connected to the base terminal of
Control transistor Q₂Of the resistor R₁One
Connected to the other terminal, and a resistor R₁Other terminal is ground
Have been. The control transistor Q₂Base end of
Is connected to one terminal of the constant voltage drop circuit 121,
The other terminal is grounded. Constant voltage drop circuit 12
1 is a diode D in this embodiment.₂, D₃Series connection
Consisting of a circuit, D₂Of the control transistor
Star Q₂Is connected to the base terminal of₃Cathode terminal
K is grounded.

The output voltage detection circuit 13 has output terminals b ₁ ,
connected between b _2, it comprises a serial connection of a resistor _{R 2} and _{R 3.} The input terminal of the control pulse generation circuit 14 is connected to a resistor R ₂ ,
R ₃ and the output terminal is connected to the control transistor Q ₂
Is connected to the base terminal.

[0021] In DC / DC converter circuit 11, _{V S} (the partial pressure of the output voltage _{V O)} voltage at the connection point between the resistor _{R 2} and _{R 3} in the control pulse generating circuit 14 is input. Control pulse generating circuit 14 outputs the control pulse P of Diyuti as the input voltage V _S is set Setpoint.

[0022] to the base terminal of the control transistor _{Q 2,}
When the control pulse P is given, the constant voltage drop circuit 121 turns on (the diodes D ₂ and D ₃ turn on). When the control transistor Q ₂ is turned on, control transistor Q ₂
Since the on-current is the base current I _B of the power transistor Q _1, is also turned on the power transistor Q _1. From the input terminal _{a 1,} the power transistor _{Q 1,} flows through the main circuit current I to the output terminal _{b 1} through the reactor _{L 1.} At this time, the reactor _{L 1,} the input voltage _{V i} and the output voltage _{V O}
Electromotive force having a difference value of (output voltage of the capacitor C ₂₎ is induced.

In the DC / DC conversion circuit 1 shown in FIG.
When the pulse generation circuit 14 outputs the control pulse P,
That is, the control transistor Q₂Is on,
Anti-R ₁Of the voltage drop V_R1Is the control transistor Q
₂The emitter-base voltage of V_EB, Constant voltage drop circuit
The value of the voltage drop of 121 is V_DThen, V_R1= V_D-V_EB= Const.

[0024] Therefore, the value of the base current _{I B} of the power transistor _{Q 1} is, the _V R1 _/ R 1 = const.

[0025] As apparent from the above, during start-up or power failure, or load even if such a low input voltage is greater (in FIG. 2, reference V _Low), the value of the base current I _B is constant In this case, the main circuit current I does not greatly decrease, and the power supply voltage of the electric circuit system does not decrease.

In the DC / DC conversion circuit 1 shown in FIG.
As shown in the characteristic curve of FIG. 2, even when the input voltage is not reduced (V _Normal = 42 [V] in FIG. 2), the input voltage is reduced (V in FIG. 2).
_{Even if Low} <42 [V]), the power transistor Q
₁ to operate in the unsaturated region (I is not so saturated) is set the value of the resistor R _1.

[0027] Thus, the turn-off of the control transistor Q ₂ becomes faster, the power loss of the power transistors Q ₁ and the control transistor Q ₂ is not increased.

Further, as described above, a step-down / non-insulated DC / DC conversion circuit which does not reduce the output voltage even when a low voltage is input and has a small loss only by adding a small number of circuit components to the conventional circuit. Can be realized.

[0029]

According to the DC / DC converter of the present invention, the control current of the power transistor is made constant irrespective of the input voltage by using the constant voltage drop circuit and the constant voltage generating resistor. A stable DC can be output even when a low voltage is input.

By appropriately setting the magnitude of the voltage drop by the constant voltage drop circuit and the resistance of the constant voltage generating resistor, the power transistor can operate in the unsaturated region.

Further, the DC / DC conversion circuit of the present invention can be realized only by adding a small number of circuit components to the conventional circuit.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing current characteristics of the power transistor of FIG.

FIG. 3 is a circuit diagram showing an example of a conventional DC / DC conversion circuit.

FIG. 4 is an explanatory diagram showing current characteristics of the power transistor of FIG.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 DC / DC conversion circuit 11 main circuit 12 drive circuit 13 output voltage detection circuit 14 control pulse generation circuit 121 constant voltage drop circuit C ₁ input capacitor C ₂ output capacitor D ₁ , D ₂ , D ₃ diode L ₁ reactor Q ₁ power transistor Q ₂ control transistor R _{1, R} 2, _{R 3} resistors

Claims (6)

[Claims] 1. A step-down / non-insulation type DC / DC conversion circuit for controlling on / off of a power transistor by a constant current circuit, wherein the constant current circuit has one input / output terminal connected to a base terminal of the power transistor. A control transistor connected to the control transistor, one terminal is connected to the other terminal of the control transistor, the other terminal is connected to a constant voltage generating resistor grounded, and one terminal is connected to a base terminal of the control transistor. And a constant voltage drop circuit having the other terminal grounded.When a control pulse is input to the base terminal of the control transistor, the control transistor has a constant on-current. Operate DC
/ DC conversion circuit. 2. The DC / DC conversion circuit according to claim 1, wherein the control transistor operates so that a base current of the power transistor becomes constant. 3. The constant voltage drop circuit comprises a series-connected diode, and when the control transistor turns on the control transistor, the series-connected diode is turned on in a forward direction. Item 3. The DC / DC conversion circuit according to item 1 or 2. 4. The resistance value and the constant voltage so that the input / output current of the control transistor turns on the power transistor in an unsaturated region when the control transistor is turned on. 4. The DC / DC conversion circuit according to claim 1, wherein a value of a voltage drop of the drop circuit is selected. 5. A main circuit comprising an input capacitor, an output capacitor, a reactor, a diode, and the power transistor, wherein the input capacitor has a non-grounded terminal on an input side of the power transistor. One terminal of the reactor is connected to an output side main circuit terminal of the power transistor, and the diode has a cathode terminal connected between the connection terminal of the power transistor and the reactor and an anode terminal. The DC / DC conversion circuit according to any one of claims 1 to 4, wherein the output capacitor has a non-grounded terminal connected to the other terminal of the reactor. 6. An output voltage detection circuit connected between terminals of the output capacitor, and a control pulse generation circuit connected to the output voltage detection circuit, wherein the control pulse generation circuit is configured to detect the output voltage. The detection signal generated by the circuit is input, and according to the value of the detection signal,
6. The DC / DC conversion circuit according to claim 1, wherein a control pulse is output to the base terminal of the control transistor and the constant voltage drop circuit.