JP2002218745A - Power supply unit with current-limiting function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide cost reduction, without the need for a high-wattage resistor of detecting the primary current for preventing overcurrent, and to prevent magnetic saturation of a transformer by controlling the maximum on duty ratio at switching on a load with large power consumption after starting a device, in a power supply unit of obtaining boosting output on the secondary side by performing PWM control of the primary current of the transformer, using a switching device, where power is supplied from a DC power supply, such as a battery, for supplying power to the load. SOLUTION: When power supply to a CRT 8, connected to the secondary output, is switched on during the operation of the power supply unit, a regulation circuit 9 lower a potential of a DTC terminal 5a for a certain time for controlling the maximum on-duty of the PWM control of a FET 4. It is thus possible to prevent the magnetic saturation of the transformer 3, thereby preventing the FET 4 or the like from break down.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to a television set with a video deck, etc., and limits the current when power supply to a high load such as a CRT monitor is turned on to protect circuit components. The present invention relates to a power supply device having a function of performing

[0002]

2. Description of the Related Art Conventionally, a switching element is connected to a primary winding of a high-frequency transformer (hereinafter referred to as a transformer) to which DC power is supplied, and the switching element is connected to a PWM.
There is known a switching power supply device that obtains a boosted output on the secondary side by performing (Pulse Width Modulation) control, smoothes this, and supplies DC power to a load. In this type of power supply device, the on / off duty of the switching element is controlled by detecting the output voltage state on the secondary side so that the output voltage is stabilized even with a load change.
Also, in order to prevent an excessive current from flowing through the switching element and destroy the element, a resistor is provided in the primary circuit of the transformer to detect the current flowing in the primary side, thereby setting the maximum on-duty. Restrictions are applied (for example, JP-A-11-215820, JP-A-10-25
No. 7764).

A general circuit configuration of the above-described conventional switching power supply device will be described with reference to FIG. The power supply device 20 includes a transformer 3 to which power is supplied from a DC power supply (here, a car battery) B to the primary side via the input terminal 2.
And an FET (Field Effect Transformer) that switches on and off the current flowing through the primary winding 3a of the transformer 3.
istor) 4 and a control IC 5 for PWM-controlling the FET 4 for inducing a boosted output voltage in the secondary winding 3b. The boosted output obtained from the secondary winding 3b is rectified and smoothed by the diode D and the capacitor C2, and a DC voltage is output to the output terminal 6. The secondary output voltage is detected by the resistors R2 and R3, and the control I
The signal is fed back to the feedback terminal 5b of C5. A load 7 and a CRT 8 as a monitor device are connected to the output terminal 6 via a switch S.

In this power supply device 20, the current flows through the primary winding 3a of the transformer 3 at the time of startup in order to prevent destruction of circuit components due to overcurrent caused by magnetic saturation of the transformer 3 at the time of startup of the device. A soft start function to limit the current is provided. For this purpose, the control IC 5 has a DTC (Dead Time Control) terminal 5a, to which a soft start circuit including a resistor R1 and a capacitor C1 is connected. Transformer 3
The current flowing through the primary side winding 3a, that is, the FET 4 is converted into a voltage by the resistor R8 and detected, thereby detecting the transistor Tr3 connected in parallel with the soft start circuit.
To control the potential of the DTC terminal 5a.
An excessive current is suppressed from flowing through the secondary winding 3a, and magnetic saturation of the transformer 3 is prevented.

[0005]

By the way, in the above-mentioned power supply device, a car battery (13.2V DC) is used.
Thus, when a television device with a video deck of several tens of watts is operated (117 V is required for driving the CRT), a relatively large current flows through the primary winding 3a, that is, the FET 4, so that current detection is performed. Resistor R8 for
Requires special resistance of low resistance and high wattage, which is expensive. The soft start function operates when the power supply device is started. However, when an excessive current flows during operation, for example, when a load requiring relatively large power such as a CRT monitor is switched on, the soft start function is activated. The function does not operate, a current flows at the maximum on-duty, the transformer 3 may be magnetically saturated, and the circuit components such as the FET 4 may be destroyed. If the transformer 3 is designed in response to a transient time, for example, by increasing the air gap of the transformer to prevent the magnetic saturation of the transformer 3, the heat loss of the current of the transformer, the switching element, etc. during the steady operation is large. Become.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and a switching element controls a primary side current of a transformer supplied from a low-voltage power supply by a switching element.
By performing the M control, in a power supply device that obtains a boosted output on the secondary side, a special high-wattage resistor is not required to detect a current flowing in the primary winding and to prevent an overcurrent,
When the power supply is turned on to a load that consumes relatively large power after starting the power supply device, the maximum on-duty is limited to prevent the transformer from causing magnetic saturation, and the switching element may be destroyed. It is an object of the present invention to provide a power supply device that eliminates the above problem and suppresses current heat loss during steady operation.

[0007]

In order to achieve the above object, the invention of claim 1 has a primary winding and a secondary winding, obtains a boosted output on a secondary side, and supplies a power to a load. Transformer that supplies DC and a FET (Field Effect T) that switches the DC power supply voltage supplied to the primary side of the step-up transformer.
ransistor) and this FET by PWM (Pulse Width Mod
and a control circuit for controlling the secondary winding of the step-up transformer and feeding back the rectified and smoothed output to maintain the secondary output at a constant level. A power supply device having a control terminal to which a signal for controlling the on-duty of the PWM control is inputted in order to suppress an excessive current flowing through the power supply, the control terminal has a soft start operation when the power supply of the device is turned on. Is connected, a DC power supply supplied to the primary side of the step-up transformer is a battery power supply, and one of the loads supplied by the power supply is a CR power supply.
A T (Cathode Ray Tube) monitor device, comprising an adjustment circuit for supplying a control signal to the control terminal so as to limit the maximum on-duty of PWM control for a certain period of time by inputting a power supply ON signal to the monitor device. The adjustment circuit is electrically connected to a power supply on input to the monitor, and connects the control terminal to a ground (GND).
, An RC time constant circuit that operates in response to the power supply ON input, and a second transistor that is controlled to be conductive by the charging voltage of the RC time constant circuit and controls the first transistor. Things.

In this configuration, when DC power is supplied from the battery power supply to the primary side of the step-up transformer, and the power supply is turned on, the control circuit performs a soft start by the RC circuit connected to the control terminal. It operates and feedbacks the rectified and smoothed output on the secondary side of the step-up transformer to perform PWM control on the FET.
Control is performed to maintain the secondary output at a constant voltage. In a steady state after the soft start operation, the capacitor of the RC circuit is charged by the control circuit, and the potential of the control terminal is rising.

When the power supply to the CRT monitor (load) connected to the secondary output is switched on during the operation of the power supply device, the load power is large (the secondary output voltage temporarily drops. ), An excessive current tends to flow at a maximum on-duty from a relatively low-voltage battery power supply to the primary side of the transformer by PWM control based on normal feedback. Is reduced by a certain period of time to limit the maximum on-duty of the PWM control of the FET in the same manner as when the soft start operation is started. That is, in the adjustment circuit, the first transistor is turned on by the power supply ON signal input (H) of the monitor device, and the potential of the control terminal drops to the GND level,
This limits the maximum on-duty of the PWM control, thereby limiting the current flowing through the primary winding, preventing magnetic saturation of the transformer, and preventing the switching element and the like from being destroyed.
In addition, the RC time constant circuit is also operated by the power supply ON signal input (H), so that the second transistor is turned on after a lapse of a predetermined time from the signal input, thereby turning off the first transistor. Because of conduction, the potential of the control terminal rises. Therefore, the control circuit releases the limitation of the maximum on-duty and returns to the steady-state feedback control operation. Further, since such an operation is performed, a resistor for detecting a primary current is not required unlike the related art.

A second aspect of the present invention provides a step-up transformer having a primary winding and a secondary winding, obtaining a boosted output on the secondary side and supplying power to a load, and a booster for the booster. FET that switches the DC power supply voltage supplied to the primary side
(Field Effect Transistor) and this FET as PWM
(Pulse Width Modulation) and a control circuit for controlling the secondary side of the step-up transformer to feed back a rectified and smoothed output so as to maintain the secondary side output at a constant level. In a power supply device having a control terminal to which a signal for controlling an on-duty of PWM control is input in order to suppress an excessive current flowing through the side winding, a power supply to a load device to which power is supplied by the power supply device By the signal input of supply ON, P
An adjustment circuit for providing a control signal to the control terminal so as to limit the maximum on-duty of the WM control is provided.

In this configuration, DC power is supplied to the primary side of the step-up transformer, and the control circuit feeds back the rectified and smoothed output of the secondary side of the step-up transformer to set the FET to the P-side.
By performing the WM control, the secondary side output is controlled to be maintained at a constant voltage regardless of the load state. When the power supply to the load to which the power is supplied is switched on during the operation of the power supply device, at that moment, the PWM control by the feedback will cause the current to flow to the primary side of the transformer with the maximum on-duty. However, the adjustment circuit that receives the ON signal input limits the maximum ON duty of the PWM control for a fixed time. This limits the current flowing to the primary side, prevents magnetic saturation of the transformer, and prevents destruction of switching elements and the like. Further, since the operation is performed in this manner, a resistor for detecting the primary current is not required unlike the related art.

According to a third aspect of the present invention, in the power supply device having a current limiting function according to the second aspect, an RC circuit for performing a soft start operation when the power of the device is turned on is further connected to the control terminal. That is, the DC power supplied to the primary side of the step-up transformer is a battery power. In this configuration, in addition to the operation of the invention of claim 2, when the power is turned on to the device, the control circuit performs a soft start operation, and starts supplying power to the load during the operation of the device. When the switch-on signal is received, an effect of limiting the maximum on-duty of the PWM control of the FET is obtained as in the case of starting the soft-start operation.

According to a fourth aspect of the present invention, in the power supply device having a current limiting function according to the second aspect, the DC power supply supplied to the primary side of the step-up transformer is a battery power supply, and the load device is a CRT ( Cathode Ray Tube) monitor device. In this configuration, when the power supply to the CRT monitor device is switched on, the load power is large. Therefore, the PWM control based on the normal feedback causes the maximum on-duty from the relatively low-voltage battery power supply to the primary side of the transformer. However, the adjustment circuit having received the ON signal reduces the potential of the control terminal for a certain period of time to limit the maximum ON duty of the PWM control of the FET.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A power supply device having a current limiting function according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a circuit configuration of a power supply device according to the present embodiment, which boosts a vehicle-mounted battery power supply and supplies power to a television device with a video deck. The power supply device 1 includes a step-up transformer 3 having a primary winding 3a and a secondary winding 3b, and an FET (Fiel) that switches a DC power supply voltage supplied to the primary side of the step-up transformer 3.
d Effect Transistor) 4 and this FET 4 by PWM (P
control IC (control circuit) for controlling the secondary side output of the step-up transformer 3 and controlling the secondary side output to be constant by controlling the secondary side output.
5 and an adjustment circuit 9 for limiting the maximum on-duty of the PWM control for a certain period of time by inputting a signal to turn on power supply to a load having relatively large power consumption.

The step-up transformer 3 is supplied with a DC power supply voltage from the car battery B via the input terminal 2 and outputs a step-up voltage to the secondary side. Output DC voltage,
Power is supplied to a load 7 such as a circuit board of a video deck.
A CRT (Cathode Ray Tube) 8 which is a monitor device for displaying an image as one of the loads is connected to the output terminal 6 via a switch S. The control IC 5 includes a DTC (Dead Time Control) terminal 5a (control terminal) to which a signal for controlling the on-duty of the PWM control is input so as to prevent the current flowing through the primary winding 3a from becoming excessive. It has a feedback terminal 5b. The DTC terminal 5a has a soft start circuit including a resistor R1 and a capacitor C1 for limiting the current flowing through the primary winding 3a of the transformer 3 at the time of startup and performing a soft start operation, between the ground and the ground (GND). Connected to the transistor Tr1 so that the control signal of the adjustment circuit 9 is supplied.
Are connected via a resistor R4. The secondary output voltage detected by the resistors R2 and R3 is fed back to the feedback terminal 5b.

The adjusting circuit 9 turns on the switch S to turn on the CR.
The signal is turned on by the signal (H) that turns on the power supply to T8, and the DTC terminal 5a is connected to ground (GN) through the resistor R4.
D), a first transistor Tr1 connected to D), an RC time constant circuit including a resistor R7 and a capacitor C3 that operate in response to a power supply ON signal (H), and conduction is controlled by the charging voltage of the RC time constant circuit. First transistor Tr
1 and a second transistor Tr2 for controlling
A control signal is provided to the DTC terminal 5a so as to limit the maximum on-duty of the PWM control for a fixed time by the input of the power supply on signal (H). The first transistor Tr1 has a function similar to that of the conventional transistor Tr3 (FIG. 2) for preventing overcurrent by detecting a secondary-side current of a transformer. There is no need to perform side current detection, and a special resistor for that purpose is not required.

Next, the power supply device 1 configured as described above
Will be described. Power is turned on and battery B
When the DC power is supplied to the primary side of the step-up transformer 3, the control IC 5 causes the power supply device to perform a soft-start operation by a soft start circuit composed of an RC circuit connected to the DTC terminal 5a. Of the secondary side is detected by resistors R2 and R3,
The voltage is fed back to the feedback terminal 5b and the FET 4
Is controlled so that the secondary output is maintained at a constant voltage regardless of the load state. In the steady state after the soft start operation, R
The capacitor C1 of the C circuit is charged and the DTC terminal 5
The potential of a is rising. When the potential of the DTC terminal 5a is low, the maximum on-duty of the PWM control of the FET 4 is reduced to limit the current flowing through the primary winding 3a of the step-up transformer 3. The duty is increased, and the primary winding 3a of the step-up transformer 3 is increased.
Releases the restriction on the current flowing through. DTC terminal 5a
The time required for the rise of the potential is determined by the charging current from the control IC 5 and the CR circuit, and the power supply device 1 can be soft-started by appropriately setting these.

When the switch S is turned on during the operation of the power supply device 1 and the power supply to the CRT 8 (load) connected to the secondary output is turned on, since the load power is large, the secondary output voltage Is temporarily lowered, and the control IC 5 tries to flow an excessive current from the battery B having a relatively low voltage to the primary side of the transformer 3 at the maximum on-duty by the PWM control based on the normal feedback. H) Upon receiving the input, the adjustment circuit 9 lowers the potential of the DTC terminal 5a for a certain period of time, and limits the maximum on-duty of the PWM control of the FET 4 as in the start of the soft start operation. That is, in the adjustment circuit 9, the monitor
The first transistor Tr1 is turned on by the ON signal (H), and the potential of the DTC terminal 5a is set to the GND level (resistance R
4, the maximum on-duty of the PWM control is limited, the current flowing through the primary winding 3a is limited, magnetic saturation of the transformer 3 is prevented, and F
Prevent destruction of ET4 etc. Further, the RC time constant circuit of the adjusting circuit 9 also operates according to the monitor-on signal (H), and after a lapse of a predetermined time from the input of the signal, the second transistor Tr2 is turned on, whereby the first transistor Tr1 is turned on. Is turned off, the potential of the DTC terminal 5a rises. Therefore, the control IC 5 releases the limitation of the maximum on-duty and returns to the steady state feedback control operation.

Thus, even after the CRT 8 is turned on during the operation after the power supply device 1 is started, the adjustment circuit 9 is turned on.
By lowering the potential of the DTC terminal 5a for a certain period of time by the above operation and limiting the maximum on-duty of the PWM control, the current flowing through the primary winding 3a can be limited to prevent magnetic saturation of the transformer 3. . In particular, when a power supply using a car battery B of a relatively low voltage as a power supply is operated and a power supply is turned on to a load having a high power consumption at a high voltage, an overcurrent transiently flows to the primary side of the power supply. The effect of preventing the flow of water is remarkably obtained.

The present invention can be variously modified without being limited to the configuration of the above-described embodiment. For example, an element for switching a DC voltage applied to a primary winding is limited to an FET. Instead, a switching element such as a transistor may be used. The elements used in the adjustment circuit 9 are not limited to transistors, and other switching elements may be used. The DC power supply is not limited to a car battery, and may be a power supply obtained by rectifying an alternating current. The use of the power supply device is not limited to the vehicle-mounted use, and may be applied to a general-purpose power supply device. Also, the load supplied by the power supply is not limited to the CRT,
A plurality of load devices that start simultaneously may be used.

[0021]

As described above, according to the first aspect of the present invention,
When the power supply to the CRT monitor device (load) connected to the secondary output is turned on during the operation of the power supply device, the adjustment circuit which receives the ON signal inputs the potential of the control terminal for a fixed time. Lowering the maximum on-duty of the PWM control of the FET for a certain period of time in the same manner as at the start of the soft start operation, so that magnetic saturation of the transformer is prevented,
Destruction of the switching element and the like can be prevented. Moreover, unlike the conventional case, an expensive low-resistance and high-wattage resistor is not required to detect the primary current, and the cost of the device can be reduced.

According to the second aspect of the present invention, when the power supply to the load to which the power is supplied is switched on during the operation of the power supply device, the adjustment circuit which receives the input of the on signal is fixed. Since the maximum on-duty of the PWM control is limited only by the time, it is possible to limit the current flowing to the primary side, prevent magnetic saturation of the transformer, and prevent the switching element and the like from being destroyed. In addition, since expensive low-resistance and high-wattage resistors are not required to detect the primary current as in the related art, the cost of the device can be reduced.

According to the invention of claim 3, in addition to the effect of the invention of claim 2, when the power is turned on to the device, a soft start operation is performed, and the power is supplied to the load during the operation of the device. When the supply is started, the maximum on-duty of the PWM control of the FET is limited similarly to the start of the soft start operation, so that the effect of preventing the magnetic saturation of the transformer and the destruction of the switching element and the like is enhanced.

According to the invention of claim 4, in addition to the effect of the invention of claim 2, when the power supply to the CRT monitor device is switched on, the maximum on duty of the PWM control of the FET is limited. Prevents transformer magnetic saturation,
The effect of preventing the destruction of the switching element and the like is enhanced.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a power supply device according to an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram of a conventional power supply device.

[Explanation of symbols]

 Reference Signs List 1 power supply device 3 step-up transformer 3a primary winding 3b secondary winding 4 FET (switching element) 5 control IC (control circuit) 5a DTC terminal (control terminal) 5b feedback terminal 8 CRT monitor (load) Tr1, Tr2 Transistor (switching element) C1 Capacitor (soft start circuit) R1 Resistance (soft start circuit) R7 Resistance (RC time constant circuit) C3 Capacitor (RC time constant circuit)

Claims (4)

[Claims] 1. A step-up transformer having a primary winding and a secondary winding, obtaining a boosted output on a secondary side and supplying power to a load, and supplied to a primary side of the step-up transformer. FET (Field Effect Transisto) that switches DC power supply voltage
r) and this FET is connected to PWM (Pulse Width Modulatio).
n) a control circuit for controlling and feeding back the rectified and smoothed output on the secondary side of the step-up transformer so as to control the secondary side output to be constant.
In a power supply device having a control terminal to which a signal for controlling an on-duty of PWM control is input in order to suppress an excessive current flowing through the primary winding, power supply of the device is supplied to the control terminal. An RC circuit for performing a soft start operation is sometimes connected, a DC power supply supplied to the primary side of the step-up transformer is a battery power supply, and one of the loads supplied by the power supply device is a CRT.
(Cathode Ray Tube) is a monitor device, and a power supply ON signal input to this monitor device causes P
An adjustment circuit for applying a control signal to the control terminal so as to limit a maximum on-duty of the WM control, the adjustment circuit being turned on by a power supply on input to the monitor, and setting the control terminal to ground (GND) A first transistor to be connected, an RC time constant circuit that operates in response to the power supply ON input, and a second transistor that is controlled to be conductive by the charging voltage of the RC time constant circuit and controls the first transistor. And a power supply device having a current limiting function. 2. A step-up transformer having a primary winding and a secondary winding, obtaining a boosted output on the secondary side and supplying power to a load, and supplied to a primary side of the step-up transformer. FET (Field Effect Transisto) that switches DC power supply voltage
r) and this FET is connected to PWM (Pulse Width Modulatio).
n) a control circuit for controlling and feeding back the rectified and smoothed output on the secondary side of the step-up transformer so as to control the secondary side output to be constant.
In a power supply device having a control terminal to which a signal for controlling an on-duty of PWM control is input in order to suppress an excessive current flowing through the primary winding, a load device supplied with power by the power supply device A power supply device having a current limiting function, comprising: an adjustment circuit that supplies a control signal to the control terminal so as to limit a maximum on-duty of PWM control for a certain period of time by inputting a power supply ON signal to the power supply. 3. The control terminal is further connected to an RC circuit for performing a soft start operation when the power of the apparatus is turned on, and a DC power supply supplied to a primary side of the step-up transformer is a battery power supply. The power supply device having a current limiting function according to claim 2, characterized in that: 4. The current according to claim 2, wherein the DC power supplied to the primary side of the step-up transformer is a battery power, and the load device is a CRT (Cathode Ray Tube) monitor device. Power supply device with limiting function.