JP2577773Y2 - DC-DC converter - Google Patents

Description

[Detailed description of the invention]

[0001]

The present invention relates to a DC-DC converter using a MOS FET as a switching element, which can be used even at an input voltage lower than the operable voltage of the MOS FET.
It relates to a DC converter.

[0002]

2. Description of the Related Art MOS FETs are often used as switching elements because of their lower driving power, higher operation speed and better high-frequency characteristics than bipolar transistors. When a MOS FET is used as a switching element of a DC-DC converter, the DC-DC converter has high efficiency because the power loss in the switching element is reduced, and the capacity of the element is reduced because the switching frequency can be increased. The size of the device can be reduced.

However, the on-gate voltage V _{GS (ON) at which} the MOS FET can be turned on is generally about 4 to 5 [V], and the minimum operating voltage of the DC-DC converter is limited by the on-gate voltage V _{GS (ON).} Will be. For this reason, in a DC-DC converter required to operate at a low input voltage, a bipolar transistor having a low on-voltage (V _{BE (ON)} ≒ 0.7 [V]) is used as a switching element. However, in a DC-DC converter using a bipolar transistor as a switching element, there is a limit in increasing the frequency due to the characteristics of the bipolar transistor, and it has been difficult to reduce the size and increase the efficiency.

[0004] In view of this, attempts have been made to use a MOS FET as a switching element even at a low input voltage, and a highly efficient DC-DC converter has been provided. FIG. 4 shows an example of such a conventional DC-DC converter. The circuit configuration of the DC-DC converter of FIG. 4 is as follows. The input terminal 1 is connected to the drain of a main switching transistor Q1 composed of an N-channel type MOS FET via the primary winding N1 of the main converter transformer 3. The source of the main switching transistor Q1 is connected to ground, and the gate is connected to the POUT1 terminal (pulse output terminal) of the main converter control circuit 4. The capacitor C1 is provided between the input terminal 1 and the ground, and may be omitted depending on the circuit.

[0005] One end of the secondary winding N2 of the main converter transformer 3 is connected to the ground, and the other end is connected to the anode of the diode D1. The cathode of the diode D1 is the output terminal 2
And a smoothing capacitor C2 is provided between the cathode and the ground. The voltage dividing resistors R1 and R2 connected in series are connected in parallel with the smoothing capacitor C2, and the voltage dividing resistors R1 and R2 are connected in series.
2 at the connection point Err _{1 of the} main converter control circuit 4.
Input to the terminal. Input voltage V _IN of DC-DC converter
Is higher than the on-gate voltage V _{GS (ON)} of the MOS FET, the V _CC1 terminal (power supply input terminal) of the main converter control circuit 4 is connected to the input terminal 1 in the above circuit configuration, and the input voltage V _IN is By applying the voltage to the main converter control circuit 4, the main switching transistor Q1 can be driven.

However, when the input voltage V _IN is lower than the on-gate voltage V _{GS (ON) of the} MOS FET, simply connecting the V _CC1 terminal of the main converter control circuit 4 to the input terminal 1 causes the main switching transistor Q 1 to be turned off. It cannot be driven. Therefore, in the DC-DC converter of FIG. 4, the V _CC1 terminal of the main converter control circuit 4 and the input terminal 1
Step-up auxiliary DC having an auxiliary switching transistor Q2 formed of an NPN bipolar transistor
A DC converter 9 is provided. This auxiliary DC-DC
In the converter 9, the auxiliary switching transistor Q2 has an emitter connected to the ground, a base connected to the POUT2 terminal of the auxiliary converter control circuit 8, and a collector connected to the input terminal 1 via the primary winding N3 of the auxiliary transformer 7. I do.

One end of the secondary winding N4 of the auxiliary transformer 7 is connected to the ground, and the other end is connected to the anode of the diode D2. The cathode of the diode D2 serves as the output terminal of the auxiliary DC-DC
Connected to the _CC1 terminal, and a smoothing capacitor C3 is provided between the cathode and the ground. A voltage dividing resistor R4 connected in series,
R5 is connected in parallel with the smoothing capacitor C3, and the voltage at the connection point between the voltage dividing resistors R4 and R5 is connected to the auxiliary converter control circuit 8
To the Err ₂ terminal of Auxiliary converter control circuit 8
V _CC2 terminal is connected to input terminal 1. With the circuit configuration as described above, even when the input voltage V _IN is lower than the on-gate voltage V _{GS (ON) of the} MOS FET, the auxiliary D using a bipolar transistor as a switching element can be used.
The main converter control circuit 4 is controlled by the C-DC converter 9.
The voltage input to the V _CC1 terminal of the
Can be higher than _{GS (ON)} .

[0008]

[Problems to be Solved by the Invention] DC-DC shown in FIG.
As described above, the converter converts the voltage input to the V _CC1 terminal of the main converter control circuit 4 by the auxiliary DC-DC converter 9 into the on-gate voltage V _{GS (ON)} of the MOSFET.
With the above, low input voltage can be handled.
However, the on-gate voltage V _{GS (ON) of} this MOS FET
Since an auxiliary DC-DC converter for obtaining the above voltage is provided, the circuit configuration becomes complicated, which causes an increase in the size of the device and an increase in cost. Accordingly, an object of the present invention is to provide a DC-DC converter that can cope with a low input voltage even when a MOS FET is used as a switching element, and that can reduce the size and cost by simplifying the circuit configuration as much as possible.
It is to provide a DC converter.

[0009]

According to the present invention, a MOS FET is used as a main switching element, and a MOSFE is used.
DC to which a voltage lower than the voltage at which T can operate is input.
In a DC converter, a chop-up type booster circuit using a bipolar transistor as a switching element is provided, and the booster circuit supplies a circuit for supplying a drive signal to a main switching element with a voltage higher than a voltage at which a MOS FET can operate. Further, the switching element of the booster circuit is driven by the same switching signal as the main switching element of the DC-DC converter.

[0010]

FIG. 1 shows a DC-DC converter according to the present invention using a MOS FET as a switching element. However, the same parts as those in FIG. 4 are denoted by the same reference numerals. The circuit configuration of the DC-DC converter of FIG. 1 is as follows. The input terminal 1 is connected to the ground via the capacitor C1, and the input terminal 1 is connected to the drain of the main switching transistor Q1 via the primary winding N1 of the main converter transformer 3. The source of the main switching transistor Q1 is connected to the ground, and the gate is connected to the POUT terminal of the main converter control circuit 4. One end of the secondary winding N2 of the main converter transformer 3 is connected to the ground, and the other end is connected to the anode of the diode D1.
The cathode of the diode D1 is connected to the output terminal 2, and a smoothing capacitor C2 is provided between the cathode and the ground.

The voltage dividing resistors R1 and R2 connected in series are connected in parallel with the smoothing capacitor C2, and the voltage at the connection point between the voltage dividing resistors R1 and R2 is connected to the Err terminal of the converter control circuit 4. V _CC terminal and input terminal 1 of converter control circuit 4
Between them, a chop-up type booster circuit 5 using a bipolar transistor as a switching element is provided. In this booster circuit 5, an auxiliary switching transistor Q2 formed of an NPN-type bipolar transistor has an emitter connected to the ground, a base connected to a resistor R3 and a capacitor C3.
4 of the converter control circuit 4 via the parallel circuit of
Terminal, the collector is connected to the anode of the diode D2, and the collector is connected to the input terminal 1 via the choke coil L. The cathode of the diode D2 is connected to the _Vcc terminal of the converter control circuit 4 as an output terminal of the booster circuit 5, and is further connected to the ground via the capacitor C3.

An outline of the operation of the DC-DC converter according to the present invention having the above-described circuit configuration will be described below.
Upon application of the input voltage V _IN to the input terminal 1, an input voltage V _IN applied via a choke coil L and diode D2 to the V _CC terminal of the converter control circuit 4. At this time, the voltage V _{CC (L)} input to the V _CC terminal is substantially equal to the voltage value obtained by subtracting the forward voltage V _D of the diode D2 from the input voltage V _IN . As a result, the converter control circuit 4 starts operating, and the voltage VOUT is almost changed from the POUT terminal (pulse output terminal).
_CC amplitude equal to _(L) of the pulse signal V _{P (L)} is output.

If the voltage V _{CC (L)} input to the V _CC terminal of the converter control circuit 4 is lower than the on-gate voltage V _{GS (ON) of the} MOSFET, the pulse signal V _{P (L )} output from the POUT terminal is output. ₎ Shows the main switching transistor Q1
Cannot be turned on. On the other hand, the pulse signal _{VP (L)} is supplied to the resistor R3 and the capacitor C4
Is applied also to the base of the auxiliary switching transistor Q2 formed by the bipolar transistor through the parallel circuit of. The on-base voltage V _{BE (ON)} at which a bipolar transistor can be turned on is generally about 0.7 [V].
And can be expressed as V _{GS (ON)} >> V _{BE (ON)} . Accordingly, the auxiliary switching transistor Q2 starts the switching operation in response to the pulse signal VP _(L) .

When the auxiliary switching transistor Q2 is turned on, a current flows from the input terminal 1 to the ground via the choke coil L and the auxiliary switching transistor Q2, whereby energy is stored in the choke coil L. The energy stored in the choke coil L is released in the form of a flyback voltage when the auxiliary switching transistor Q2 is turned off. The flyback voltage generated is rectified and smoothed by the diode D2 and the capacitor C3, V _CC of converter control circuit 4
Increase the voltage input to the terminal. The voltage at this time is V
_{CC (H)} (= V _{CC (L)} + ΔV _CC ; ΔV _CC = Ldi / d
t).

If the value of each element of the booster circuit 5 is determined so that the voltage V _{CC (H)} is equal to or higher than the on-gate voltage V _{GS (ON) of} the main switching transistor Q1 using a MOS FET, the voltage V _By the pulse signal _{VP (H)} of the converter control circuit 4 when _{CC (H)} is input to the _Vcc terminal,
The main switching transistor Q1 can be turned on, and starts a switching operation. When the main switching transistor Q1 starts operating, the output voltage V _O appearing at the output terminal 2 increases, and the voltage dividing resistors R1, R2
Feeds back a voltage detection signal corresponding to the output voltage V _O to the Err terminal of the converter control circuit 4. The converter control circuit 4 generates a pulse signal _{VP (H)} according to the voltage detection signal.
Is controlled to keep the output voltage V _O constant.

As a result, the same pulse signal _{VP (H)} as that of the main switching transistor Q1 is input to the auxiliary switching transistor Q2, so that the voltage ΔV increased by the flyback voltage generated in the choke coil L
_CC is almost constant regardless of the input voltage V _IN . Therefore, the voltage V _{CC (H)} input to the V _CC terminal of the converter control circuit 4 boosted by the boost circuit 5 increases almost linearly as the input voltage V _IN increases, as shown in FIG. become. As described above, the DC shown in FIG.
In a DC converter, the input voltage V _IN is
, The voltage input to the V _CC terminal of the converter control circuit 4 can be made higher than the on-gate voltage V _{GS (ON)} by the booster circuit 5 even if the voltage is lower than the on-gate voltage V _{GS (ON) of} the main switching transistor Q1. Therefore, the DC-DC converter can operate even at a low input voltage.

In the DC-DC converter shown in FIG. 1, the present invention is applied to a system in which a pulse signal output from the POUT terminal of the converter control circuit 4 is directly applied to the gate of the main switching transistor to drive. Such a circuit configuration is used for a DC-DC converter having a relatively small capacity. In a DC-DC converter that needs to handle a large amount of power, the capacity of the main switching transistor must be increased, so that sufficient drive power is supplied to the gate of the main switching transistor with the output capacity of the POUT terminal of the converter control circuit. The drive cannot be supplied and the drive may be insufficient. Therefore, a means is provided for providing a drive circuit between the POUT terminal of the converter control circuit and the gate of the main switching transistor.

FIG. 3 shows an embodiment in which the present invention is applied to a high power DC-DC converter having this drive circuit. 1 and 4 are denoted by the same reference numerals. 3, the V _CC1 terminal (power input terminal) of the converter control circuit 4 is directly connected to the input terminal 1, and the POUT terminal is connected to the PI terminal (pulse input terminal) of the drive circuit 6. The DO terminal (drive signal output terminal) of the drive circuit 6 is connected to the gate of the main switching transistor Q1, and the V _CC3 terminal (power input terminal)
Is connected to an input terminal 1 via a chop-up type booster circuit 5 using a bipolar transistor as a switching element.
Connect to The circuit configuration of the booster circuit 5 and the D
The circuit configuration of the C-DC converter is the same as that of FIG.

In the circuit shown in FIG.
It is assumed that an input voltage V _IN lower than the on-gate voltage V _{GS (ON) of the} main switching transistor Q1 composed of an FET is applied. Converter control circuit 4 starts operation when a voltage equal to input voltage V _IN is input to V _CC1 terminal, and
The pulse signal _VP is output from the T terminal. Drive circuit 6
The the V _CC3 terminal by the input voltage V _IN to the input terminal 1 is applied, the voltage V _CC of the value obtained by subtracting the forward voltage V _D of the diode D2 from the input voltage V _{IN (L)} is input . The drive circuit 6 _applies a voltage V _{CC (L)} , PI
In response to input of the pulse signal V _P to the terminal, the amplitude is substantially equal to the voltage V _{CC (L),} the drive signal period synchronized with the pulse signal V _P V _{DR (L)} is outputted from the DO terminal. The main switching transistor Q1 has a drive signal V
_{Even if DR (L)} is input, the voltage V _{CC (L)} is
Does not operate because it is lower than the on-gate voltage V _{GS (ON) of} .

The pulse signal V _P output from the converter control circuit 4 is input connection via a parallel circuit of the auxiliary switching transistor Q2 base resistor R3 and the capacitor C4 of the boosting circuit 5, to operate the auxiliary switching transistor Q2 . By the switching operation of the auxiliary switching transistor Q2, the booster circuit 5 boosts the input voltage V _IN, and the on-gate voltage V
Drive circuit 6 with a voltage V _{CC (H)} higher than _{GS (ON)}
To the V _CC3 terminal. Drive circuit 6, V by a voltage V _{CC (H)} is input to the _CC3 terminal, amplitude outputs a substantially V _{CC (H)} equal to the drive signal V _{DR (H).} Since the voltage value of the drive signal V _{DR (H)} becomes higher than the on-gate voltage V _{GS (ON)} of the MOS FET, the main switching transistor Q1 performs a switching operation.

[0021]

[Effect of the Invention] As described above, the DC-DC of the present invention is used.
The converter uses a chop-up type booster circuit compared to a conventional DC-DC converter provided with an auxiliary DC-DC converter so that the converter can operate at a voltage lower than the on-gate voltage of the MOS FET. The device can be miniaturized without the need for. In addition, since the auxiliary switching transistor including the bipolar transistor of the booster circuit is driven by a pulse signal for operating the main switching transistor of the DC-DC converter, an independent control circuit for operating the booster circuit is unnecessary. Therefore, the circuit configuration is simplified and the device can be downsized. Since the booster circuit supplies a voltage higher than the on-gate voltage of the main switching transistor composed of the MOS FET to the circuit that supplies the drive signal to the main switching transistor, the DC-voltage is lower than the on-gate voltage of the MOSFET.
The operation of the DC converter becomes possible.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a DC-DC converter according to the present invention.

FIG. 2 shows a characteristic of a voltage V _CC input to a circuit that supplies a drive signal to a main switching transistor with respect to an input voltage V _IN of a DC-DC converter.

FIG. 3 is a circuit diagram of another embodiment of the DC-DC converter of the present invention.

FIG. 4 is a circuit diagram of a conventional DC-DC converter corresponding to a low input voltage.

[Explanation of symbols]

 Reference Signs List 1 input terminal 2 output terminal 3 converter transformer 4 converter control circuit 5 chop-up type booster circuit 6 drive circuit

Claims (3)

(57) [Scope of request for utility model registration] 1. A MOS FE as a main switching element
In a DC-DC converter using T and inputting a voltage lower than a voltage at which a MOS FET can operate, a chop-up type booster circuit using a bipolar transistor as a switching element is provided, and the booster circuit is a main switching element. A voltage that enables the main switching element to operate, and that the switching element of the booster circuit is driven by the same switching signal as the main switching element of the DC-DC converter. Characteristic DC-DC converter. 2. The booster circuit according to claim 2, wherein a pulse output terminal of a converter control circuit is connected to a gate of the main switching element, so that a switching signal output from the converter control circuit is used as a drive signal for causing the main switching element to operate. 2. The DC-DC converter according to claim 1, wherein the controller supplies a voltage at which the main switching element is operable to the converter control circuit. 3. A drive circuit for supplying a drive signal to a gate of the main switching element, wherein the drive circuit operates a switching signal from a converter control circuit and a main switching element from the booster circuit. 2. D according to claim 1, characterized in that a possible voltage is supplied.
C-DC converter.