JP2004147452A - Gate drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gate drive circuit that contributes to an improvement in conversion efficiency in a chopper power supply of a discontinuous operation. <P>SOLUTION: The gate drive circuit comprises a p-channel MOSFET Q11, a switching element Q12 with a control terminal such as a MOSFET and a bipolar transistor, and three resistors R11, R12 and R13 of an NPN transistor Q13. A source electrode of the Q11, a collector electrode of the Q13 and the R12 are connected, the other end of the R12, a base electrode of the Q13 and the R13 are connected, the other end of the R13, a drain electrode of the Q12 or a collector electrode of a collector and the R11 are connected, and the other end of the R11, an emitter electrode of the Q13 and a gate electrode of the Q11 are connected. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a P-channel MOSFET gate drive circuit used in various electronic devices. In particular, it is effective as a gate drive circuit for a MOSFET used as a main switching element of a discontinuous operation chopper power supply.
[0002]
[Prior art]
The MOSFET as a switching element has advantages in that its switching speed is high and that almost no drive current is required statically. However, since the MOSFET has a gate capacitance, it is necessary to rapidly charge and discharge the gate capacitance in order to perform high-speed switching.
[0003]
FIG. 6 shows the simplest P-channel MOSFET gate drive circuit.
[0004]
FIG. 7 is a circuit diagram of a discontinuous operation step-down chopper power supply using the gate drive circuit of FIG.
[0005]
In FIG. 7, Q21 is a P-channel MOSFET as a main switching element, L21 is a choke coil, D22 is a regenerative Schottky diode, ZD21 is a Zener diode as a reference power supply, and 201 is an open collector output comparator. The open collector transistor Q22 in FIG. 4 is built in the comparator 201.
[0006]
8A and 8B show operation waveforms of the circuit of FIG. 7, wherein FIG. 8A shows the drain current of the MOSFET, FIG. 8B shows the drain-source voltage, FIG. 8C shows the gate-source voltage, and FIG. Output voltage.
[0007]
When the output of the comparator 201 turns on, the gate voltage of the P-channel MOSFET of Q21 becomes a voltage obtained by dividing 24 V by R21 and R22, and exceeds the threshold value of the voltage between the gate and the source, so that the P-channel MOSFET of Q21 turns on.
[0008]
Further, when the output of the comparator 201 is turned off, the gate voltage of the P-channel MOSFET of Q21 becomes 24 V, and when the voltage between the gate and source becomes 0 V, the P-channel MOSFET of Q21 is turned off.
[0009]
When the P-channel MOSFET of Q21 is turned off, the charge stored in the gate capacitance of the P-channel MOSFET of Q21 is discharged through R22. The voltage dividing ratio between R22 and R21 is determined by the threshold value of the gate-source voltage of the P-channel MOSFET of Q21. Further, when the sum of these two resistance values is reduced, the output current of 201 becomes large, so there is a limit in reducing the resistance value. Therefore, the resistance value of R22 cannot be made too small.
[0010]
Therefore, the discharge speed of the electric charge accumulated in the gate capacitance cannot be so high. For this reason, the output voltage of the comparator 201 in FIG. 8 (D) cannot be increased to a large extent. This part has a large drain current and a large drain-source voltage, resulting in a large loss.
[0011]
In order to solve this problem, the output of 201 can be set to a push-pull state, or a method disclosed in Japanese Patent Application Laid-Open No. 5-153773 can be used.
[0012]
[Problems to be solved by the invention]
However, the use of a push-pull output causes problems such as a complicated gate drive circuit and inexpensiveness.
[0013]
Further, although the circuit disclosed in Japanese Patent Application Laid-Open No. 5-153773 is considerably simplified, it still has many component constants and is affected by the switching speed of the added diode.
[0014]
[Means for Solving the Problems]
In order to solve the above problem, the present invention proposes a gate drive circuit having the following configuration.
[0015]
It comprises a P-channel MOSFET Q11, a switching element Q12 having a control terminal such as a MOSFET or a bipolar transistor, an NPN transistor Q13, and three resistors R11, R12, R13. A source electrode of Q11, a collector electrode of Q13 and R12 are connected. Is connected to the base electrode of Q13 and R13, the other end of R13 is connected to the drain electrode of Q12 or the collector / collector electrode and R11, and the other end of R11 is connected to the emitter electrode of Q13 and the gate electrode of Q11. Is done.
[0016]
When controlling the speed when the P-channel MOSFET is off, a resistor can be added to the collector of Q13 or the gate of Q11.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
(First embodiment)
FIG. 1 is a circuit diagram of a gate drive circuit according to a first embodiment of the present invention.
[0018]
In FIG. 1, Q11 is a P-channel MOSFET, and Q12 and Q13 are NPN transistors.
[0019]
Q12 operates as a switching element. When Q12 is on, Q11 is on, and when Q12 is off, Q11 is also off.
[0020]
The resistors R12 and R13 are voltage dividing resistors for determining the base voltage of Q13. Assuming that the threshold voltage between the gate and the source of Q11 is Vth, the ratio between the resistance values of R12 and R13 is such that when Q12 is turned on, the voltage across R12 becomes Vth or more and R13 × Vth / (R13 + R12) becomes 0.7V or more. Is set.
[0021]
When Q12 is on, the current flowing in R12 is divided into the current flowing in R13 and the base current of Q13. The base current of Q13 is 1 / hfe of the collector current of Q13. The collector current of Q13 is limited by R11. If the resistance value of R11 is appropriately selected and a transistor having a sufficiently large hfe is selected, the base current of Q13 is negligible compared to the current flowing through R13. Since the emitter voltage of Q13 is a voltage lower by about 0.7V from the base voltage, the gate-source voltage of Q11 is at least Vth + 0.7V, and Q11 is turned on.
[0022]
When Q12 is turned off, the collector voltage of Q12 increases, but the gate-source voltage of Q11 does not become 0V. Since there is a charge accumulated in the gate capacitance of Q11, it becomes Vth or more. Therefore, Q13 does not turn off, and the charge accumulated in the gate capacitance of Q11 is rapidly discharged by the emitter current of Q13 to 0V, and Q11 turns off.
[0023]
When Q12 turns on, the gate capacitance of Q11 is charged via R11, and Q11 turns on.
[0024]
In a series of operations, the operation of Q13 is fast because Q13 performs an emitter follower operation.
[0025]
FIG. 2 shows a circuit diagram of a discontinuous operation step-down chopper power supply to which the present gate drive circuit is applied.
[0026]
In FIG. 2, Q11 is a P-channel MOSFET as a main switching element, L11 is a choke coil, D12 is a regenerative Schottky diode, ZD11 is a Zener diode as a reference power supply, and 101 is an open collector output comparator. The open collector transistor Q12 in FIG. 1 is built in the comparator 101.
[0027]
FIG. 3 shows operation waveforms of the circuit of FIG. (A) is the drain current of the MOSFET, (B) is the drain-source voltage, (C) is the gate-source voltage, and (D) is the output voltage of the comparator 201.
[0028]
Compared with the waveform of FIG. 8, it can be seen that the off speed of the P-channel MOSFET, which is the main switching element, is much faster.
[0029]
In a discontinuous operation chopper power supply, the turn-off speed of the main switching element has a great effect on efficiency. By using the gate drive circuit of the present invention, the efficiency of a chopper power supply that operates discontinuously can be easily improved.
[0030]
(Second embodiment)
FIG. 4 is a circuit diagram of a gate drive circuit according to a second embodiment of the present invention.
[0031]
This circuit is obtained by adding a gate current limiting resistor R14 to the collector of Q13 in the circuit of FIG.
[0032]
In the first embodiment, the P-channel MOSFET of Q11 can operate as fast as possible in this circuit configuration, but there is no current limiting resistor at the time of discharging the gate capacitance, and the switching speed may be too high. Faster switching reduces losses, but may cause adverse effects such as noise emission and circuit malfunction. In this case, the switching speed can be controlled by applying a resistance as in the present embodiment.
[0033]
The same effect can be obtained by inserting a gate current limiting resistor R15 into the gate of Q11 as shown in FIG.
[0034]
【The invention's effect】
As described above, the use of this patent can improve the turn-off speed of the P-channel MOSFET by adding several components.
[Brief description of the drawings]
FIG. 1 is a gate drive circuit diagram of a P-channel MOSFET showing a first embodiment of the present invention.
FIG. 2 is a circuit diagram of a discontinuous step-down chopper power supply circuit provided with a gate drive circuit according to a first embodiment of the present invention.
FIG. 3 is an operation waveform of the circuit of FIG. 2;
FIG. 4 is a diagram showing a gate drive circuit of a P-channel MOSFET according to a second embodiment of the present invention;
FIG. 5 is a P-channel MOSFET gate drive circuit diagram 2 showing a second embodiment of the present invention.
FIG. 6 is a gate drive circuit diagram of a conventional P-channel MOSFET.
FIG. 7 is a circuit diagram of a discontinuous step-down chopper power supply circuit provided with a conventional gate drive circuit.
FIG. 8 is an operation waveform of the circuit of FIG. 7;
[Explanation of symbols]
101, 201-open collector output comparator Q11, Q21-P-channel MOSFET
Q12, Q13, Q21, Q22-NPN transistor D11, D21-Schottky diode D12, D22-Small signal diode C11, C12, C21, C22-Electrolytic capacitor L11, L12-Choke coil ZD11, 21-Zener diode

Claims (3)

A P-channel MOSFET Q11,
A switching element Q12 with a control terminal such as a MOSFET or a bipolar transistor;
An NPN transistor Q13,
It has three resistors R11, R12, R13,
The source electrode of Q11, the collector electrode of Q13 and R12 are connected,
The other end of R12 is connected to the base electrode of Q13 and R13,
The other end of R13 is connected to the drain electrode of Q12, or the collector and collector electrode and R11,
The other end of R11 is connected to the emitter electrode of Q13 and the gate electrode of Q11,
A gate drive circuit for controlling conduction and interruption between a source and a drain of Q11 by a base current of Q12 or a source-gate voltage. The circuit configuration according to claim 1,
A gate drive circuit comprising a resistor R14 having one end inserted into the collector electrode of Q13 and the other end inserted into a connection point between R12 and the source electrode of Q11. The circuit configuration according to claim 1,
A gate drive circuit characterized in that one end of the resistor R15 is inserted into the connection point between the emitter electrode of Q13 and R11 and the other end is inserted into the gate electrode of Q11.

Images