JP2006254657A - Power supply device with overcurrent prevention function - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power supply device with an overcurrent prevention function by which a switching frequency is not necessary to be changed and a sufficient overcurrent prevention effect can be obtained. <P>SOLUTION: In the power supply device, a detection transistor Q1 is arranged in parallel to an output transistor Q2 and a resistor R1 is disposed in a current path of the detection transistor Q1. The detection transistor Q1 is turned on prior to the turning-on of the output transistor Q2. A resistor R2 dropping a voltage by a current of the detection transistor Q1, a comparator 16 comparing a voltage generated by a voltage drop with constant voltage B1 and a latch 14 inverting a Q terminal output when an output of the comparator 16 is inverted are arranged. When overcurrent is detected by turning-on of the detection transistor Q1, the output transistor Q2 is not turned on. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a power supply device that supplies power to a load. More specifically, the present invention relates to a power supply device having an overcurrent prevention function for preventing overcurrent due to overload or short circuit.

2. Description of the Related Art Conventionally, power supply devices that have semiconductor elements that switch power supply and supply power to a load via an inductor have been used (Patent Documents 1, 2, etc.). In many power supply devices of this type, switching of semiconductor elements is operated with a pulse signal, and the output is adjusted with the pulse width. Also, this type of power supply device may be in an overcurrent state due to an overload or a short circuit. For this reason, an overcurrent protection function is generally provided. When an overcurrent is detected, the current is limited by means such as narrowing the pulse width.
JP 7-46828 A JP 2001-61272 A

  However, the conventional general power supply device described above has the following problems. In other words, overcurrent cannot be effectively prevented by simply narrowing the pulse width. The cause is the relationship between the response delay due to the internal circuit, the switching frequency, and the inductance of the inductor. Depending on this relationship, the slope of the current at the time of OFF is gentle, and the degree of current decrease is insufficient. For this reason, as shown in FIG. 6, even if the current is limited, the current eventually increases further. Thus, it is not effective prevention of overcurrent.

  On the other hand, it is conceivable to take a sufficiently long off time by lowering the switching frequency when an overcurrent is detected. This is effective in preventing overcurrent, but the switching frequency itself causes another problem. For example, the switching signal cannot be used as a clock for another circuit. A change in switching frequency also means that various circuit constants also change. For this reason, it is necessary to anticipate this in the circuit design, which is complicated. In addition, it becomes difficult to evaluate switching noise. Furthermore, a circuit for changing the switching frequency is required.

  The present invention has been made to solve the above-described problems of the prior art. That is, an object of the present invention is to provide a power supply device with an overcurrent prevention function that does not need to change the switching frequency and can obtain a sufficient overcurrent prevention effect.

  The power supply device with an overcurrent prevention function of the present invention, which has been made for the purpose of solving this problem, supplies power from an original power supply to a load via an inductor and has an overcurrent prevention function, and is intermittently connected to the load. A main output element for supplying electric power, a pre-output means for supplying electric power to the load at a current smaller than the current by the main output element, before the start of power supply to the load by the main output element, and power by the pre-output means An overcurrent detection unit that detects an overcurrent in a state where the supply is performed, and a power supply prohibition unit that prohibits power supply to the load by the main output element when the overcurrent detection unit detects an overcurrent. Yes.

  In this power supply device with an overcurrent prevention function, the pre-output means supplies power to the load before power supply to the load by the main output element starts. The power supply by the pre-output means is performed with a current smaller than that of the main output element. In this state, overcurrent detection is performed by the overcurrent detection unit. That is, it is detected whether more current flows than the current supplied by the pre-output means in the normal state. If it is not an overcurrent, the main power element supplies the original power. In the case of an overcurrent, the power supply prohibiting unit prohibits power supply by the main output element. That is, power is not supplied by the main output element. This prevents overcurrent when the main output element is turned on. In this overcurrent prevention function, it is not necessary to change the switching frequency for operating the main output element.

  In the power supply device with an overcurrent prevention function of the present invention, the pre-output means includes a detection element that is turned on prior to the start of power supply to the load by the main output element, and when the detection element is on. It is desirable that the resistance value of the current path by the detection element is larger than the resistance value of the current path by the main output element when the main output element is on. By doing so, the power supply current by the pre-output means is smaller than the power supply current by the main output element. Therefore, even if it is in an overcurrent state, an excessive current does not immediately flow at the time of overcurrent detection before the start of power supply by the main output element.

  Further, it is desirable that the overcurrent detection unit includes a detection resistor that causes a voltage drop due to the current of the detection element and a comparison unit that compares a voltage due to the voltage drop of the detection resistor with a constant voltage. In this case, a voltage drop occurs in the detection resistor due to the start of power supply by the pre-output means. The degree of the voltage drop is compared with a constant voltage by the comparison means. Thereby, an overcurrent at the stage of power supply by the pre-output means can be detected.

  In the power supply device with an overcurrent prevention function of the present invention, the time difference from when the pre-output means starts supplying power until the main output element starts supplying power is excessive after the pre-output means starts supplying power. It is desirable that it is longer than the time required until the power supply prohibition unit prohibits the power supply by detecting the current. Thereby, the power supply prohibition of the main output element is ensured when the overcurrent is detected by the overcurrent detection unit.

  The power supply device with an overcurrent prevention function of the present invention preferably includes an output control unit that starts power supply by the pre-output means and starts power supply by the main output element after a predetermined time. This ensures a period during which the pre-output means supplies power prior to the start of power supply by the main output element. Overcurrent detection is performed within this period.

  Moreover, in the power supply device with an overcurrent prevention function of the present invention, it is desirable that the power supply prohibition unit also stops power supply by the pre-output means. This ensures more reliable circuit and load protection when an overcurrent is detected by the overcurrent detector.

In the power supply device with an overcurrent prevention function of the present invention, a diode is provided between the main output element and the pre-output means and the ground, and the current supplied to the load when only the pre-output means is on. The resistance value of the path of
{TON * VINmax−T * VOUTmin−tOFF * Vf} / (IOUTmax * tON)
It is desirable to be larger than the lower limit value given by. “When only the pre-output means is on” means when the pre-output means is on and the main output element is off. This prevents excessive current rise even under the worst conditions, that is, when the difference between the input voltage and the output voltage is maximum.

Furthermore, the resistance value is T * (VOUTmin + Vf) / (IOUTmax * tON)
It is desirable to be smaller than the upper limit value given by. As a result, no current flows backward from the diode.

  The power supply device with an overcurrent prevention function of the present invention may further include a second detection element that is turned on / off together with the main output element, and a second overcurrent detection unit that detects an overcurrent of the second detection element. desirable. Thereby, even after the power supply by the main output element is started, the overcurrent detection by the second overcurrent detection unit is performed. In this way, it is not necessary to provide a detection resistor in the current path of the main output element. Then, there is little resistance loss at the time of ON. The second overcurrent detection unit includes a second detection resistor that causes a voltage drop due to the current of the second detection element, and a second comparison unit that compares the voltage due to the voltage drop of the second detection resistor with a constant voltage. can do.

  According to the present invention, there is provided a power supply device with an overcurrent prevention function that does not require a switching frequency to be changed and that provides a sufficient overcurrent prevention effect.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. The power supply device with an overcurrent prevention function of this embodiment is configured as shown in the circuit diagram of FIG. This power supply device with an overcurrent prevention function has an output transistor Q2. This power supply device with an overcurrent prevention function basically supplies current from the original power supply VIN to the load 100 via the output transistor Q2. For this purpose, the power supply device with an overcurrent prevention function includes a PWM control device 1 and an oscillator 2. The PWM control device 1 turns on / off the output transistor Q2 based on the basic pulse signal from the oscillator 2.

  In addition, the PWM control device 1 is provided with an inductor L1 for smoothing the output current IOUT. A Schottky diode D1 for regulating the output voltage VOUT is provided on the drain side of the output transistor Q2.

  The power supply device with an overcurrent prevention function is further provided with an element for the overcurrent prevention function. That is, the detection transistor Q1 is provided in parallel with the output transistor Q2. A resistor R1 is provided in series with the detection transistor Q1. Here, the sum of the on-resistance Ron1 of the detection transistor Q1 and the resistance value of the resistor R1 is larger than the on-resistance Ron2 of the output transistor Q2 (Ron1 + R1> Ron2). In the following description, Ron >> Ron1 and R1 >> Ron2, and Ron1 and Ron2 may be ignored unless particularly necessary. Further, a resistor R2 is provided on the source side of the output transistor Q2 and the detection transistor Q1 (R1 >> R2). The PWM control device 1 performs an on / off operation not only on the output transistor Q2 but also on the detection transistor Q1.

  The PWM control device 1 is configured around a drive signal generation unit 10 and a delay circuit 11. The PWM control device 1 further includes OR circuits 12 and 13, a latch 14, a release signal generation unit 15, and a comparator 16. The drive signal generation unit 10 is a block that generates drive signals to the detection transistor Q1 and the output transistor Q2 based on the basic pulse signal from the oscillator 2. The delay circuit 11 is a block that generates a delay signal obtained by delaying the output signal of the drive signal generation unit 10. The delay signal output from the delay circuit 11 is a signal obtained by delaying only the right shoulder (off → on) of the output signal of the drive signal generation unit 10, and the left shoulder (on → off) is not delayed. Such a delay signal can be easily created as a logical product of the original signal (the output signal of the drive signal generator 10) and its simple delay signal.

  The OR circuit 12 is a block that outputs a logical sum of the inversion of the output signal of the drive signal generation unit 10 and the Q terminal output of the latch 14. The output signal of the OR circuit 12 becomes the gate voltage of the detection transistor Q1. The OR circuit 13 is a block that outputs a logical sum of the inversion of the output signal of the delay circuit 11 and the Q terminal output of the latch 14. The output signal of the OR circuit 13 becomes the gate voltage of the output transistor Q2. Both the detection transistor Q1 and the output transistor Q2 are on when the gate voltage is low, and are off when the gate voltage is high.

  The comparator 16 has a role of monitoring the source voltages of the detection transistor Q1 and the output transistor Q2. Therefore, the source voltage of the detection transistor Q1 and the output transistor Q2 is input to the negative input terminal of the comparator 16, and the constant voltage B1 is input to the positive input terminal. The output terminal of the comparator 16 is connected to the S terminal of the latch 14. The output signal of the release signal generator 15 is input to the R terminal of the latch 14.

  The power supply device with an overcurrent prevention function of FIG. 1 operates as follows. The basic operation of the power supply device with an overcurrent prevention function in FIG. 1 is to adjust the current supply to the load 100 by the drive signal from the PWM control device 1. In the power supply device with an overcurrent prevention function of FIG. 1, the Q terminal output of the latch 14 is low in the normal state. Therefore, the OR circuits 12 and 13 output the inversion of the outputs of the drive signal generation unit 10 and the delay circuit 11. As a result, the detection transistor Q1 and the output transistor Q2 are periodically turned on and off. Thus, the current IOUT is supplied to the load 100 mainly by the output transistor Q2 (because R1 + Ron1 >> Ron2). That is, the output transistor Q2 is a main output element.

First, normal operation will be described with reference to the “normal” portion of the timing chart of FIG. In the timing chart of FIG. 2, the basic pulse period is indicated by T. At time t0, the gate voltage of the detection transistor Q1 (the output of the OR circuit 12) is lowered by the output of the drive signal generator 10 being increased. For this reason, the state of the detection transistor Q1 is switched from OFF to ON. Thereby, a current is supplied from the detection transistor Q1. For this reason, the output current IOUT to the load 100 turns from a decrease to an increase. The slope G1 of the increase in output current IOUT at this time is
{VIN-VOUT- (R1 + Ron1 + R2) * IOUT} / L1
Given in. Since R1 is quite large, this gradient G1 is small. This power supply by the detection transistor Q1 before time t2 is a pre-output.

At time t2 when Δt has elapsed from time t0, the gate voltage of the output transistor Q2 (output of the OR circuit 13) is lowered by the output of the delay circuit 11 being increased. For this reason, the state of the output transistor Q2 is also switched from OFF to ON. After time t2, current is also supplied from the output transistor Q2. For this reason, the slope of the increase in the output current IOUT to the load 100 increases. The output current IOUT in this state is mostly due to the contribution of the output transistor Q2 (because R1 + Ron1 >> Ron2). Therefore, the slope G2 of the increase in the output current IOUT at this time is
(VIN-VOUT-R2 * IOUT) / L1
It is represented by Since R2 is small,
(VIN-VOUT) / L1
May be considered.

At time t3 (according to the duty ratio) further after time t2, the gate voltages of the detection transistor Q1 and the output transistor Q2 are both increased. Therefore, both the detection transistor Q1 and the output transistor Q2 are turned off. As a result, the supply of current from the detection transistor Q1 and the output transistor Q2 is stopped. For this reason, the output current IOUT to the load 100 turns from increasing to decreasing. The slope G3 of the decrease in the output current IOUT at this time is
-(VOUT + Vf) / L1
Given in. Here, Vf is a forward voltage of the Schottky diode D1. Because of the inductance of the inductor L1, the output current IOUT cannot be decreased rapidly. Thereafter, the above is repeated at the period T.

  Next, the operation when an overcurrent state occurs due to some cause will be described with reference to the “overcurrent” portion of the timing chart of FIG. When the gate voltage of the detection transistor Q1 goes down at time t0, the state of the detection transistor Q1 is switched from OFF to ON. This is the same as in the normal case.

  However, in the overcurrent state, VOUT decreases, so that the slope G1 of the increase in the output current IOUT thereafter is larger than that in the normal state. Further, the voltage drop of the resistor R2 is large, and the source voltage of the detection transistor Q1 is rapidly reduced. Therefore, the output current IOUT quickly exceeds the overcurrent detection threshold due to a rapid increase. The overcurrent detection threshold is a current value at which the source voltage of the detection transistor Q1 due to the voltage drop of the resistor R2 becomes equal to the constant voltage B1. In FIG. 2, the vertical axis of “IOUT” at “overcurrent” is reduced compared to “IOUT” at “normal time”.

  Therefore, before reaching time t2, the gate voltage of the detection transistor Q1 is increased at time t1, and the state of the detection transistor Q1 is turned off. This is because the output of the comparator 16 is inverted due to a decrease in the source voltage, and the latch 14 is set. This is detection of an overcurrent state. That is, the detection transistor Q1 is a detection element that plays a role of allowing a small amount of current to flow before the output transistor Q2 is turned on. The resistor R2 is a detection resistor that causes a voltage drop in this state. The comparator 16 plays a role of detecting overcurrent due to the voltage drop. It can be seen from FIG. 2 that it takes some time from the time when the output current IOUT exceeds the overcurrent detection threshold to the time t1 when the detection transistor Q1 is turned off. This is due to an internal delay of the circuit and is an inevitable phenomenon.

For this reason, the output of the OR circuit 12 is fixed to high after time t1. Therefore, the detection transistor Q1 is turned off. As a result, the supply of current from the detection transistor Q1 is stopped. For this reason, the output current IOUT to the load 100 turns from increasing to decreasing. The slope of the decrease in the output current IOUT at this time is the same as the slope G3 after time t3 of the normal operation,
-(VOUT + Vf) / L1
Given in.

  Further, setting the latch 14 at time t1 means that not only the OR circuit 12 but also the output of the OR circuit 13 is fixed to high. For this reason, in the state where the overcurrent is detected, the output transistor Q2 is not turned on even after the time t2. That is, when an overcurrent is detected by turning on the detection transistor Q1, the off operation is performed before the output transistor Q2 is turned on. As a result, power supply by the output transistor Q2 is prohibited. For this reason, the output current IOUT continues to decrease due to the inclination until time t4, which is the start time of the next cycle. When the time t4 is reached, the release signal generator 15 resets the latch 14, and the operations after the time t0 are repeated. The above is repeated at the period T.

  In the operation in the overcurrent state, only the initial short period (time t0 to time t1) of the period T is on (however, only the detection transistor Q1), and the rest (time t1 to time t4), that is, the period Most of T is off. For this reason, the off period is long. In addition, the current increase at the time point when switching from on to off (time t1) is not so large. This is because only the detection transistor Q1 having a large resistance is turned on until that time. Therefore, even if the output current IOUT increases to some extent at time t1, it sufficiently decreases to the original level by time t4. For this reason, damage to the load 100 or the power supply device itself due to the continued overcurrent state is prevented. This is the overcurrent prevention function in the power supply device with the overcurrent prevention function of FIG.

  This overcurrent prevention function assumes an overcurrent due to a short circuit to a low voltage. Since VOUT decreases during an overcurrent, the gradient G1 increases and the gradient G3 decreases. In other words, the slope when the current increases is large and the slope when the current decreases is small. In this embodiment, in order to suppress an increase in current, it is important to increase the resistance R1 and decrease the slope G1.

  In normal times, the output current IOUT does not increase so much that the output of the comparator 16 is inverted.

In this power supply device with an overcurrent prevention function, it is necessary that the current does not increase even when the difference between the input voltage VIN and the output voltage VOUT is maximum. For this purpose, all the on-resistances RON including the resistors R1, R2 and the detection transistor Q1 must satisfy the following equation.
(VINmax−VOUTmin−RON * IOUTmax) * tON <(VOUTmin + Vf) * tOFF
VINmax: Maximum value of VIN VOUTmin: Minimum value of VOUT within the range in which overcurrent operation is guaranteed IOUTmax: Maximum value of IOUT, that is, overcurrent protection threshold tON: Moment of overcurrent detection (see “Overcurrent” in FIG. "Time", the moment tOFF from the moment IOUT reaches the overcurrent detection threshold) to time t1: Time required from time t1 to time t4 (≈ T-tON)
Therefore, RON must satisfy the following equation.
RON> {tON * VINmax−T * VOUTmin−tOFF * Vf} / (IOUTmax * tON) (1)
This equation defines the lower limit of RON.

On the other hand, if the voltage drop due to RON is too large, current flows from the Schottky diode D1. This hinders overcurrent detection. The following equation holds for the voltage drop of RON.
RON * IOUTmax> VIN + Vf
The VIN threshold VINth at which the current from the Schottky diode D1 begins to flow is
VINth = RON * IOUTmax−Vf (2)
Is required. Since the effect of the output transistor Q2 cannot be obtained when VIN is equal to or less than VINth, it is necessary to establish overcurrent protection when the detection transistor Q1 is turned on. For this reason, the following equation must be satisfied.
(VINth−VOUTmin) * tON <(VOUTmin + Vf) * tOFF
Applying the above equation (2) to this,
RON <T * (VOUTmin + Vf) / (IOUTmax * tON) (3)
Is obtained. This equation defines the upper limit of RON.

RON may be set within the upper and lower limits determined by the above equations (1) and (3). For example,
VINmax = 15V
VOUTmin = 0V
tON = 0.05μs
tOFF = 0.95μs
Vf = 0.5V
IOUTmax = 1A
Then,
5.5Ω <RON <10Ω
It becomes.

  The delay amount Δt is preferably slightly larger than the delay of the internal circuit (from the time when IOUT reaches the overcurrent detection threshold until time t1). This is because if this is too short, the off operation at the time of overcurrent detection may not be in time for the output transistor Q2 to be on. On the other hand, if the delay amount Δt is too long, the duty ratio of PWM control cannot be increased very much. This is because the ratio of the delay amount Δt to the period T cannot be ignored.

  Fig. 3 shows a graph based on actual measurement of the output current IOUT when an overcurrent occurs. A portion indicated by an arrow A in FIG. 3 is when overcurrent detection is initially turned off (corresponding to time t1 in FIG. 2). In the graph of FIG. 3, even after the time indicated by the arrow A, the current value does not increase gradually and is kept at a substantially constant level. From this, it can be understood that the overcurrent prevention function works effectively. FIG. 4 is a graph of measured values of the output voltage VOUT at that time. It can be understood that the peak value of the voltage after the time indicated by the arrow A is suppressed as compared with the previous voltage value.

  Next, a modified example will be described. FIG. 5 shows a circuit diagram of a power supply device with an overcurrent prevention function according to a modification. The difference between the power supply device with an overcurrent prevention function in FIG. 5 and the power supply device with an overcurrent prevention function in FIG. 1 is as follows. Except for the following differences, it is the same as the power supply device with an overcurrent prevention function of FIG.

  First, a detection transistor Q3 is provided along with the detection transistor Q1 and the output transistor Q2. The detection transistor Q3 is turned on and off by the output of the OR circuit 13, like the output transistor Q2. Resistors R2 and R3 are provided on the source sides of the detection transistor Q1 and the detection transistor Q3, respectively. On the other hand, the source side of the output transistor Q2 is directly connected to the original power source VIN without a resistor. The sum of the ON resistance of the detection transistor Q3 and the resistance value of the resistor R3 is larger than the ON resistance of the output transistor Q2 (R3 + Ron3 >> Ron2).

  Further, the PWM control device 3 is provided with a comparator 17 in addition to the comparator 16. The constant voltage B2 is input to the positive input terminal of the comparator 17, and the source voltage of the detection transistor Q is input to the negative input terminal. An OR circuit 18 for inputting the logical sum of the outputs of the comparators 16 and 17 to the S terminal of the latch 14 is provided.

  The power supply device with an overcurrent prevention function shown in FIG. 5 basically operates in the same manner as the power supply device with an overcurrent prevention function shown in FIG. The operational features of the power supply device with an overcurrent prevention function of FIG. 5 are as follows. First, in the ON state (from time t2 to time t3 in FIG. 2), the resistance loss is small. This is because a resistor is not included in the current path of the output transistor Q2 (source power supply VIN → output transistor Q2 → inductor L1 → load 100).

  Furthermore, it can be mentioned that overcurrent detection is also effective between time t2 and time t3 in FIG. This is because the detection transistor Q3 operated by the same control signal as the output transistor Q2 is also monitored for overcurrent by the resistor R3 and the comparator 17. That is, if the current of the detection transistor Q3 exceeds the limit value (the voltage drop corresponds to the constant voltage B2) between time t2 and time t3, the latch 14 is also set. That is, the overcurrent prevention function works. As a result, a function to prevent overcurrent acts more reliably.

  As described in detail above, in the power supply device with an overcurrent prevention function according to the present embodiment (FIG. 1), the detection transistor Q1 is provided in parallel with the output transistor Q2, and the resistance of the current path of the detection transistor Q1 is set as the output transistor. It is larger than that of Q2. Further, a delay signal is generated by the delay circuit 11 based on the output signal of the drive signal generator 10. Then, the detection transistor Q1 is operated by the output signal of the drive signal generation unit 10, and the output transistor Q2 is operated by the delay signal of the delay circuit 11. As a result, before the output transistor Q2 is turned on, the detection transistor Q1 allows a minute current to flow.

  If an overcurrent is detected at this stage, the output transistor Q2 and the detection transistor Q1 are turned off. This prevents an excessive current from being supplied from the output transistor Q2 in an overcurrent state. Here, the above-described overcurrent prevention function is exhibited without changing the frequency of the basic pulse signal from the oscillator 2. In this way, a power supply device with an overcurrent prevention function that can obtain a sufficient overcurrent prevention effect without changing the switching frequency is realized.

  In addition, in the power supply device with an overcurrent prevention function of FIG. 5, a detection transistor Q3 operated by the same signal as the output transistor Q2 is provided separately from the detection transistor Q1. Then, the detection resistor is excluded from the current path of the output transistor Q2. As a result, both overcurrent detection after the output transistor Q2 is turned on and low resistance loss when the output transistor Q2 is turned on are compatible.

  This embodiment is merely an example of the present invention and does not restrict the present invention. Therefore, the present invention can be variously modified and improved without departing from the scope of the invention. For example, in the power supply device with an overcurrent prevention function of FIG. 1, the resistor R1 is omitted by using the detection transistor Q1 whose on-resistance is larger than the on-resistance of the output transistor Q2 (Ron1 >> Ron2). May be. Even when the resistor R1 is provided, the order of the detection transistor Q1 and the resistor R1 may be any. A resistor may be disposed on both the source side and the drain side of the detection transistor Q1.

  It is also conceivable that the source side of the output transistor Q2 is directly connected to the original power source VIN so that the detection resistor R2 is connected to the source side of only the detection transistor Q1. In this way, it is possible to eliminate the loss due to the resistor R2 at the on time. However, as a compensation, overcurrent detection from time t2 to time t3 in FIG. 2 does not function effectively. On the other hand, by providing the second detection transistor Q3, the power supply device with an overcurrent prevention function in FIG. 5 achieves both low loss at the time of ON and overcurrent detection after time t2. It is also possible to arrange the detection resistors R2 and R3 on the drain side instead of the source side of the detection transistors Q1 and Q3.

  Further, the output signal of the delay circuit 11 can be a simple delay signal of the output signal of the drive signal generation unit 10. In this case, in the normal portion of FIG. 2, the gate voltage of the output transistor Q2 is delayed not only on the right shoulder but also on the left shoulder with respect to the gate voltage of the detection transistor Q1. A configuration in which the Q terminal output of the latch 14 is input only to the OR circuit 13 is also conceivable. In that case, the OR circuit 12 may be eliminated, and the inversion of the output signal of the drive signal generator 10 may be used as it is as the gate voltage of the detection transistor Q1. In this way, even when an overcurrent is detected, the power supply by the detection transistor Q1 is performed according to the “gate voltage of Q1” in the normal portion of FIG. Since the on-resistance of the current path of the detection transistor Q1 is large, this does not cause any serious trouble. Further, the Schottky diode D1 may be a single conduction means based on a potential difference, and may be replaced with a commonly used diode such as a Zener diode, a PIN type diode, or a thyristor diode.

It is a circuit diagram of a power supply device with an overcurrent prevention function concerning an embodiment. 2 is a timing chart for explaining operations of the power supply device with an overcurrent prevention function of FIG. 1 during normal time and overcurrent. It is a graph which shows transition of the electric current value at the time of overcurrent. It is a graph which shows transition of the voltage value at the time of overcurrent. It is a circuit diagram of a power supply device with an overcurrent prevention function concerning a modification of an embodiment. It is a graph which shows the change of the electric current in the state in which overcurrent detection is made | formed in the conventional power supply device with an overcurrent prevention function.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 PWM controller 10 Drive signal generation part 11 Delay circuit 14 Latch 16 Comparator 17 Comparator D1 Schottky diode L1 Inductor Q1 Detection transistor Q2 Output transistor Q3 Detection transistor R2 Resistance R3 Resistance

Claims (7)

In the power supply with overcurrent prevention function that supplies power from the main power supply to the load via the inductor and has overcurrent prevention function,
A main output element that intermittently supplies power to the load;
Prior to starting power supply to the load by the main output element, pre-output means for supplying power to the load at a current smaller than the current from the main output element;
An overcurrent detection unit for detecting an overcurrent in a state where power is supplied by the pre-output means;
A power supply device with an overcurrent prevention function, comprising: a power supply prohibition unit that prohibits power supply to the load by the main output element when the overcurrent detection unit detects an overcurrent. In the power supply device with an overcurrent prevention function according to claim 1,
The pre-output means includes a detection element that is turned on prior to the start of power supply to the load by the main output element, and a resistance of a current path by the detection element when the detection element is on A value greater than the resistance value of the current path by the main output element when the main output element is on;
In the overcurrent detection unit,
A detection resistor that causes a voltage drop due to the current of the detection element;
A power supply device with an overcurrent prevention function, comprising: comparing means for comparing a voltage caused by a voltage drop of the detection resistor with a constant voltage. In the power supply device with an overcurrent prevention function according to claim 1,
The time difference from when the pre-output means starts supplying power until the main output element starts supplying power is:
A power supply device with an overcurrent prevention function characterized by being longer than a required time from when the pre-output means starts power supply to when the power supply prohibition unit prohibits power supply due to overcurrent detection. In the power supply device with an overcurrent prevention function according to claim 1,
A power supply device with an overcurrent prevention function, comprising: an output control unit that starts power supply by the pre-output unit and starts power supply by the main output element after a predetermined time. In the power supply device with an overcurrent prevention function according to claim 1,
The power supply prohibiting unit also stops power supply by the pre-output means, and is a power supply device with an overcurrent prevention function. In the power supply device with an overcurrent prevention function according to claim 1,
A diode is provided between the main output element and the pre-output means and the ground,
When only the pre-output means is on, the resistance value of the path of the current supplied to the load is
{TON * VINmax−T * VOUTmin−tOFF * Vf} / (IOUTmax * tON)
Greater than the value given by
T * (VOUTmin + Vf) / (IOUTmax * tON)
A power supply device with an overcurrent prevention function, wherein the power supply device is in a range smaller than a value given by. tON: Time required from when the overcurrent detection unit detects overcurrent until the power supply prohibition unit prohibits power supply VINmax: Maximum voltage of the original power supply T: Period of intermittent power supply to the load VOUTmin : Minimum value tOFF of the output voltage to the load within the range where overcurrent operation is guaranteed tf: Time from when the power supply prohibition unit prohibits power supply to the start of power supply of the pre-output means next time Vf: Forward voltage IOUTmax of the diode: maximum value of output current to the load In the power supply device with an overcurrent prevention function according to claim 1,
A second detection element that is turned on and off together with the main output element;
A power supply device with an overcurrent prevention function, comprising: a second overcurrent detection unit that detects an overcurrent of the second detection element.

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