JP2011125074A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To fix a starting time regardless of an input voltage and lessen a power loss after start by enabling start, even if the input voltage is a low voltage in the vicinity of the starting voltage of a control circuit. <P>SOLUTION: A switching power supply includes a transformer which transmits a power supplied to a primary winding to a secondary winding and a tertiary winding, a switching element which supplies an input voltage intermittently to the primary winding of the transformer, a control circuit which starts to control the switching action of the switching element by the starting voltage applied to a power terminal, a capacitor which is connected to the power terminal of the control circuit, a rectifying device which leads a power conducted to the tertiary winding to the power terminal of the control circuit, a constant current circuit which generates a constant current based on the input voltage and outputs it to the capacitor, and a constant current control circuit which stops the action of the constant current circuit after start of the control circuit. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a switching power supply, and more particularly to a switching power supply that can be started even when an input voltage is low, can have a constant start-up time regardless of the input voltage, and can reduce power loss.

  In order to operate the switching power supply, it is necessary to first start the control circuit IC by supplying power to the control circuit IC. A conventional configuration related to the starting circuit of the switching power supply will be described with reference to the circuit diagram of FIG.

  When the input voltage Va is applied to the switching power supply, a current flows into the capacitor C1 connected via the starting resistor R1, charges are accumulated, and the terminal voltage increases. This terminal voltage is applied to the power supply terminal Vcc of the control circuit IC (U1). When the starting voltage of the control circuit IC (U1) is reached, the control circuit IC (U1) starts operating, and the transistor Qs The switching power supply is started by controlling on / off.

  After the control circuit IC (U1) operates and the switching power supply is activated, current flows into the capacitor C1 via the tertiary winding of the transformer T1 and the diode D1, and power is supplied. Also at this time, due to the difference between the input voltage Va and the terminal voltage of the capacitor C1, a current flows into the capacitor C1 via the starting resistor R1. For this reason, when the switching power supply operates, power loss occurs at the starting resistor R1.

  In order to reduce the power loss in the starting resistor R1, there is a case where a circuit configuration as shown in FIG. In the example of this figure, a field effect transistor Q1 and a trigger circuit TRG and a resistor R4 are added as a circuit for turning on / off the field effect transistor Q1 between the starting resistor R1 and the capacitor C1.

  At startup, the TRG circuit is kept in a high impedance state. In this state, when the input voltage Va is applied, the voltage rises to the gate of the field effect transistor Q1 via the resistor R4, so that the field effect transistor Q1 is turned on. Then, current flows into the capacitor C1 via the starting resistor R1 and the field effect transistor Q1, and the terminal voltage increases. When this terminal voltage reaches the starting voltage of the control circuit IC (U1), the control circuit IC (U1) starts operating, and the switching power supply is started by controlling on / off of the transistor Qs.

  After the control circuit IC (U1) operates and the switching power supply is activated, the TRG circuit is switched to the low impedance state. Then, the field effect transistor Q1 is turned off and the power supply path from the starting resistor R1 to the capacitor C1 is cut, so that no power loss occurs in the starting resistor R1.

JP 2005-198408 A

  The time until the switching power supply starts is from when the input voltage Va is applied until the terminal voltage of the capacitor C1 reaches the starting voltage of the control circuit IC (U1) due to the current flowing from the starting resistor R1 into the capacitor C1. It will be time.

  Here, when the value of the input voltage Va is not constant and the voltage range is wide, it flows into the capacitor C1 when the low voltage input voltage Va is applied and when the high voltage input voltage Va is applied. Since a large difference occurs in the current, a large difference also occurs in the starting time of the switching power supply.

  In the configuration for reducing the loss in the starting resistor R1 shown in FIG. 3B, the input voltage is applied to the gate terminal of the field effect transistor Q1 in order to turn on the field effect transistor Q1 when the switching power supply is started. Va is applied, but this voltage value must be higher than the source terminal voltage value of the field effect transistor Q1, that is, the total value of the terminal voltage of the capacitor C1 and the gate threshold voltage of the field effect transistor Q1.

  If the gate threshold voltage of the field effect transistor Q1 is 5V and the starting voltage of the control circuit IC (U1) is 18V, the input voltage Va is required to be 23V or higher. Will not start.

  Therefore, the present invention can be started even when the input voltage is a low voltage near the start-up voltage of the control circuit section, and the start-up time can be made constant regardless of the input voltage, and the power loss after start-up can be reduced. An object of the present invention is to provide a switching power supply capable of

  In order to solve the above-described problems, a switching power supply according to the present invention includes a transformer that transmits power supplied to a primary winding to a secondary winding and a tertiary winding, and an input voltage that is intermittently applied to the primary of the transformer. A switching element to be supplied to the winding; a control circuit unit that starts controlling the switching operation of the switching element by applying a starting voltage to the power supply terminal; a capacitor connected to the power supply terminal of the control circuit part; A rectifying element that guides power transmitted to the tertiary winding to a power supply terminal of the control circuit unit; a constant current circuit unit that generates a constant current based on the input voltage and outputs the constant current to the capacitor; and the control circuit And a constant current control circuit unit for stopping the operation of the constant current circuit unit after the unit is activated.

  In the present invention, since the capacitor connected to the power supply terminal of the control circuit unit is charged by the constant current from the constant current circuit unit, the startup time of the switching power supply can be made constant regardless of the input voltage. After the switching power supply is activated, the constant current control circuit unit stops the operation of the constant current circuit unit, so that power loss in the constant current circuit unit can be reduced.

  More specifically, the constant current circuit unit includes a starting resistor R1, a resistor R2, a resistor R3, a pnp transistor Q2, and a pnp transistor Q3, and the starting resistor R1 is connected to an input terminal of the input voltage. The resistor R2 and the emitter of the pnp transistor Q3 are connected. The other end of the starting resistor R1 is connected to the emitter of the pnp transistor Q2 and the base of the pnp transistor Q3. The base of the pnp transistor Q2, the collector of the pnp transistor Q3, and the resistor R3 are connected to the ends, and the collector of the pnp transistor Q2 can be connected to the power supply terminal of the control circuit unit. .

  With this configuration, the switching power supply can be activated if the input voltage satisfies the starting voltage of the control circuit section + the base-emitter voltage of the pnp transistor Q3 + the collector-emitter saturation voltage of the pnp transistor Q2. That is, it is possible to start even when the input voltage is a low voltage near the starting voltage of the control circuit unit.

  According to the present invention, the input voltage can be started even at a low voltage near the startup voltage of the control circuit unit, the startup time can be made constant regardless of the input voltage, and power loss after startup can be reduced. A switching power supply is provided.

It is a circuit diagram which shows the structure relevant to the starting circuit of the switching power supply of this embodiment. It is a circuit diagram which shows another example of the structure relevant to the starting circuit of the switching power supply of this embodiment. It is a circuit diagram which shows the conventional structure regarding the starting circuit of a switching power supply.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a configuration related to the startup circuit of the switching power supply according to the present embodiment. As shown in the figure, the switching power supply includes pnp transistors Q2 and Q3, a starting resistor R1, a resistor R2, and a resistor R3, a starting constant current circuit 10, a field effect transistor Q1, an npn transistor Q4, and a resistor R4. , A constant current control circuit 20 including a resistor R5 and a resistor R6. About another structure, it can be made the same as that of the past.

  That is, an electric field that is a transformer T1 that transmits power supplied to the primary winding to the secondary winding and the tertiary winding, and a switching element that intermittently supplies the input voltage Va to the primary winding of the transformer T1. The control circuit IC (U1) that starts control of the switching operation of the field effect transistor Qs by applying a starting voltage to the effect transistor Qs and the power supply terminal Vcc, and one end of the power supply terminal Vcc of the control circuit IC (U1) The capacitor C1 is connected and the other end is grounded, and the diode D1 is a rectifier that guides the power transmitted to the tertiary winding to the power supply terminal Vcc of the control circuit IC (U1).

  Note that the control circuit IC (U1) includes a Vref terminal that outputs a reference voltage of, for example, 5 V after activation.

  The starting constant current circuit 10 generates a constant current based on the input voltage Va and performs an operation of supplying the constant current to the capacitor C1, and the constant current control circuit 20 stops the circuit operation of the starting constant current circuit 10. The operation to be performed is performed.

  That is, when the switching power supply is started, a constant current is supplied to the capacitor C1 by the starting constant current circuit 10, so that the starting time is constant regardless of the value of the input voltage Va. The control circuit 20 stops the circuit operation of the starting constant current circuit 10 to reduce power loss.

  In the example of this figure, the starting constant current circuit 10 has a starting resistor R1, a resistor R2, and an emitter of a pnp transistor Q3 connected to the input terminal of the input voltage Va. The other end of the starting resistor R1 is connected to the emitter of the pnp transistor Q2 and the base of the pnp transistor Q3. The other end of the resistor R2 is connected to the base of the pnp transistor Q2, the collector of the pnp transistor Q3, and the resistor R3. Is connected. The collector of the pnp transistor Q2 is connected to the power supply terminal Vcc of the control circuit IC (U1). However, the starting constant current circuit 10 may be constituted by other circuits.

  In the constant current control circuit 20, a resistor R4 is connected to the input terminal of the input voltage Va. Unlike the starting resistor R1, the resistor R4 can be made sufficiently large. The field effect transistor Q1 functions as a switch for controlling the current flowing through the resistor R3 of the constant current control circuit 20, and the other end of the resistor R4 is connected to the gate.

  The gate of field effect transistor Q1 is further connected to the collector of npn transistor Q4. The base of the npn transistor Q4 is connected to the Vref terminal of the control circuit IC (U1) via the resistor R5, is connected to the emitter via the resistor R6, and is grounded. However, the constant current control circuit 20 may be configured by other circuits.

  Next, the operation of the switching power supply according to this embodiment will be described. When the input voltage Va is applied, the gate voltage of the field effect transistor Q1 rises through the resistor R4, and the field effect transistor Q1 is turned on, whereby a current starts to flow through the resistors R2 and R3.

  When the emitter-base voltage of the pnp transistor Q2 becomes 0.6 to 0.7 V due to the voltage drop of the resistor R2, the pnp transistor Q2 becomes conductive. Then, current flows into the capacitor C1 via the starting resistor R1 and the pnp transistor Q2, and the terminal voltage of the capacitor C1 increases.

  On the other hand, when the current flowing through the starting resistor R1 increases and the emitter-base voltage of the pnp transistor Q3 becomes 0.6 to 0.7 V due to the voltage drop of the starting resistor R1, the pnp transistor Q3 becomes conductive. As a result, the voltage drop of the resistor R2 decreases, the base current of the pnp transistor Q2 decreases, and the current flowing into the capacitor C1 via the starting resistor R1 and the pnp transistor Q2 is suppressed.

  Then, the voltage drop of the starting resistor R1 decreases, and the base current of the pnp transistor Q3 decreases. As a result, the voltage drop of the resistor R2 becomes large again, so that the base current of the pnp transistor Q2 increases and the current flowing into the capacitor C1 via the starting resistor R1 and the pnp transistor Q2 increases.

By repeating such an operation, the current flowing through the starting resistor R1 and the pnp transistor Q2 is
Icnt = (base-emitter voltage of Q3) / starting resistor R1
The constant current Icnt is expressed as follows.

  This value is independent of the input voltage Va and is constant. Since the capacitor C1 is charged with the constant current Icnt when the switching power supply is activated, the time until the terminal voltage reaches the activation voltage of the control circuit IC (U1) is constant. That is, the startup time of the switching power supply can be made constant regardless of the input voltage Va.

  When the control circuit IC (U1) starts operating, a voltage is output from the Vref terminal, and the npn transistor Q4 is turned on. As a result, the gate voltage of the field effect transistor Q1 is lowered, so that the field effect transistor Q1 is turned off. Therefore, the current flowing through the resistors R2 and R3 is cut off, and the pnp transistor Q2 is turned off. As a result, no current flows into the capacitor C1 via the starting resistor R1 and the pnp transistor Q2, so that power loss in the starting resistor R1 can be eliminated.

In the starting constant current circuit 10 of the switching power supply, assuming that the collector-emitter voltage of the pnp transistor Q2 and the voltage applied to the starting resistor R1 are Vb, if the field effect transistor Q1 is on, the voltage Vb is
Vb> (base-emitter voltage of pnp transistor Q3) + (collector-emitter saturation voltage of pnp transistor Q2)
Is satisfied, the starting constant current circuit 10 can perform a constant current operation.

That is, if the value of the input voltage Va is equal to or higher than Vth shown below, the starting constant current circuit 10 can perform a constant current operation, and the switching power supply can start the operation.
Vth = (start-up voltage of control circuit IC (U1)) + (base-emitter voltage of pnp transistor Q3) + (collector-emitter saturation voltage of pnp transistor Q2)
For example, if the starting voltage of the control circuit IC (U1) is 18 V, the base-emitter voltage of the pnp transistor Q3 is 0.6 V, and the collector-emitter saturation voltage of the pnp transistor Q2 is 0.4 V, the input voltage Va If the voltage is 19 V or higher, a constant current operation is performed, and the switching power supply can start operation.

  As described above, according to the switching power supply of the present embodiment, the input voltage can be started even at a low voltage near the start-up voltage of the control circuit unit, and the start-up time can be made constant regardless of the input voltage. Later power loss can be reduced.

  Next, another configuration of the switching power supply according to the present embodiment will be described with reference to the circuit diagram of FIG. This example can be effectively applied when the control circuit IC (U1) does not include the Vref terminal that outputs the reference voltage.

  In this example, a constant current control circuit 30 including a Zener diode ZD1 is used instead of the constant current control circuit 20.

  That is, a diode D2 and a capacitor C2 are added to the output of the tertiary winding, and the cathode of the diode D2 is connected to the cathode of the Zener diode ZD1. Then, the anode of the Zener diode ZD1 is connected to the base of the npn transistor Q4 via the resistor R5. Other configurations are the same as those in the above embodiment.

  With this configuration, even when the control circuit IC (U1) does not include the Vref terminal for outputting the reference voltage, the operation of the starting constant current circuit 10 can be stopped after the switching power supply is started. .

  That is, when the switching power supply is activated and power is transmitted to the tertiary winding, the capacitor C2 is charged. When the terminal voltage becomes equal to or higher than the Zener voltage of the Zener diode ZD1, the npn transistor Q4 is turned on.

  As a result, the gate voltage of the field effect transistor Q1 is lowered, so that the field effect transistor Q1 is turned off. Therefore, the current flowing through the resistors R2 and R3 is cut off, and the pnp transistor Q2 is turned off.

  As a result, no current flows into the capacitor C1 via the starting resistor R1 and the pnp transistor Q2, so that power loss in the starting resistor R1 can be eliminated.

  In this example, the operation of the constant current circuit for start 10 is not stopped immediately after the switching power supply is started, but the start constant is increased after the terminal voltage of the capacitor C2 rises to the Zener voltage of the Zener diode ZD1. The operation of the current circuit 10 is stopped. For this reason, the stop timing of the operation of the starting constant current circuit 10 can be adjusted.

DESCRIPTION OF SYMBOLS 10 ... Constant current circuit for starting, 20 ... Constant current control circuit, 30 ... Constant current control circuit, C1 ... Capacitor, C2 ... Capacitor, D1 ... Diode, D2 ... Diode, U1 ... Control circuit IC, Q1 ... Field effect transistor, Q2 ... pnp transistor, Q3 ... pnp transistor, Q4 ... npn transistor, Qs ... field effect transistor, R1 ... starting resistor, R2 ... resistor, R3 ... resistor, R4 ... resistor, R5 ... resistor, R6 ... resistor, T1 ... transformer , TRG: trigger circuit, ZD1: Zener diode

Claims (2)

A transformer for transmitting power supplied to the primary winding to the secondary winding and the tertiary winding;
A switching element for intermittently supplying an input voltage to the primary winding of the transformer;
A control circuit unit that starts controlling the switching operation of the switching element by applying a starting voltage to the power supply terminal;
A capacitor connected to a power supply terminal of the control circuit unit;
A rectifying element for guiding the power transmitted to the tertiary winding to the power supply terminal of the control circuit unit;
A constant current circuit unit that generates a constant current based on the input voltage and outputs the constant current to the capacitor;
A switching power supply comprising: a constant current control circuit unit that stops the operation of the constant current circuit unit after the control circuit unit is activated. The constant current circuit unit includes a starting resistor R1, a resistor R2, a resistor R3, a pnp transistor Q2, and a pnp transistor Q3.
The starting resistor R1, the resistor R2, and the emitter of the pnp transistor Q3 are connected to the input terminal of the input voltage,
The other end of the starting resistor R1 is connected to the emitter of the pnp transistor Q2 and the base of the pnp transistor Q3.
The other end of the resistor R2 is connected to the base of the pnp transistor Q2, the collector of the pnp transistor Q3, and the resistor R3.
The switching power supply according to claim 1, wherein a collector of the pnp transistor Q2 is connected to a power supply terminal of the control circuit section.