JP2012125084A - Insulated type dc power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable accurate overvoltage detection to be performed on the primary side in an insulated type DC power supply and also enable overvoltage detection to be performed without increasing the number of terminals of a power supply control IC.SOLUTION: An insulated type DC power supply is provided with a first resistor for detecting an electric current flowing in the primary winding of a transformer, a second resistor connected between the terminal of the first resistor and a current detection terminal of a power supply control IC, and a third resistor connected between a terminal of an auxiliary winding and the current detection terminal. The power supply control IC includes a current detection circuit which detects an electric current flowing in the primary winding based on voltage converted from current to voltage by the first resistor and an overvoltage detection circuit having a comparator which, based on voltage sampled during a period while the potential of a sampling circuit connected to the current detection terminal and that of the current detection terminal remain flat, determines whether there is an overvoltage state which has occurred, so that when the overvoltage detection circuit detects an overvoltage state of output, a switching element is halted from being driven.

Description

  The present invention relates to an insulation type DC power supply device provided with a voltage conversion transformer, and more particularly to a technique effective for use in overvoltage detection in an insulation type DC power supply device.

  The DC power supply device includes an insulated AC circuit composed of a diode bridge circuit that rectifies an AC power supply, and a DC-DC converter that steps down the DC voltage rectified by the circuit and converts it into a DC voltage having a desired potential. -There is a DC converter. As an insulation type AC-DC converter, for example, a switching element connected in series with a primary side winding of a voltage conversion transformer is turned on and off by a PWM (pulse width modulation) control method, and flows to the primary side winding. There is known a switching power supply device that controls a current and controls a voltage induced in a secondary winding.

  By the way, in an insulated DC power supply device, if an overvoltage condition in which the output voltage on the secondary side becomes abnormally high may occur, the load, etc. may be damaged, so the overvoltage detection function and overvoltage are detected in the primary side control circuit. In such a case, a protective function is often provided to stop the control operation. As a method of detecting the overvoltage state of the output on the secondary side, an overvoltage detection circuit is provided on the secondary side and an overvoltage detection circuit is fed back to the primary side with a photocoupler, etc., and an overvoltage detection circuit is provided on the primary side There is.

  Furthermore, as a secondary side overvoltage detection method on the primary side, a rectifier circuit comprising a rectifier diode and a smoothing capacitor that rectifies the alternating current induced in the auxiliary winding to generate the power supply voltage of the control circuit on the primary side There is a technique of detecting an overvoltage state by monitoring the rectified and smoothed voltage (see, for example, Patent Document 1). There has also been proposed an invention in which an overvoltage detection circuit detects a voltage obtained by resistance-dividing a voltage before rectifying an alternating current induced in an auxiliary winding (see, for example, Patent Document 2).

JP 2009-225499 A JP 2009-165316 A

However, since the overvoltage detection method disclosed in Patent Document 2 is provided with a dedicated terminal for inputting an overvoltage detection voltage to the primary side control circuit, the primary side control circuit is formed as a semiconductor integrated circuit. There is a problem that the number of terminals increases, leading to an increase in chip size and an increase in cost.
On the other hand, in a switching power supply device that controls on and off of the current flowing through the primary winding by the PWM control method, the AC voltage induced in the auxiliary winding is a rising portion as shown in FIG. The waveform has ringing. The magnitude of this ringing varies greatly depending on load conditions and manufacturing variations of the transformer.

Therefore, the voltage rectified and smoothed by the rectifier circuit that rectifies the alternating current induced in the auxiliary winding subject to overvoltage detection has a magnitude including ringing with variation, and is an overvoltage as disclosed in Patent Document 1. The detection circuit has a problem that accurate overvoltage detection cannot be performed.
In view of this, the present inventors conducted various studies on how to enable accurate overvoltage detection on the primary side in a PWM control type switching power supply, and as a result, as shown in FIG. The voltage in the flat period (T2) after the ringing converges among the induced voltages of the auxiliary winding shown in A) is proportional to the output voltage on the secondary side, and an accurate overvoltage can be obtained by monitoring the voltage in this period. It was found that detection can be performed.

The present invention has been made under the background as described above, and an object of the present invention is to provide an insulation type that includes a transformer for voltage conversion and controls on and off of a current flowing in the primary winding by a PWM control method. It is an object of the present invention to provide a technology capable of performing accurate overvoltage detection on the primary side in a DC power supply device.
Another object of the present invention is to provide a semiconductor integrated circuit as a primary side control circuit that constitutes an insulation type DC power supply device that includes a voltage conversion transformer and controls on and off of a current flowing through the primary side winding by a PWM control method. The present invention provides a technique capable of detecting overvoltage without increasing the number of terminals.

In order to achieve the above object, the present invention
A voltage conversion transformer having an auxiliary winding, a switching element for intermittently passing a current through the primary winding of the transformer, a first terminal for current detection, and a second terminal for feedback; An insulation type having a power supply control semiconductor integrated circuit that controls on / off of the switching element with a PWM control pulse by feedback of an output detection voltage from the secondary side of the transformer to the second terminal via a photocoupler In the DC power supply,
A first resistor connected in series with the switching element for detecting a current flowing in a primary winding of the transformer;
A second resistor connected between the terminal of the first resistor and the first terminal;
A third resistor connected between the terminal of the auxiliary winding and the first terminal,
The semiconductor integrated circuit for power control is
A current detection circuit that detects a current flowing in the primary winding of the transformer based on a voltage that is current-voltage converted by the first resistor;
A sampling circuit connected to the first terminal and a voltage sampled by the sampling circuit during a period in which the potential of the first terminal is flat are compared with a predetermined reference voltage to determine whether an overvoltage state has occurred. An overvoltage detection circuit having a comparator,
When the overvoltage detection circuit detects an output overvoltage state based on the voltage on the primary side of the transformer, the drive control of the switching element is stopped.

  According to the configuration described above, since the overvoltage is determined in a flat portion after the ringing of the terminal voltage waveform of the auxiliary winding similar to the terminal voltage waveform of the secondary winding converges, the voltage on the primary side is accurate. Overvoltage detection can be performed.

Preferably, the second resistor is connected in series with the first resistor between the switching element and the first terminal, and a voltage corresponding to an input voltage of the first terminal is It is inputted to a current detection circuit, and the first terminal is configured to serve as both the terminal for detecting the overvoltage and the terminal for detecting the current.
Thereby, the number of terminals of the power supply control semiconductor integrated circuit can be reduced, and the chip size can be reduced.

Preferably, the power supply control semiconductor integrated circuit detects whether an output overcurrent state has occurred based on the potential of the first terminal, and detects the overcurrent state when the overcurrent state is detected. When the overcurrent protection circuit that limits the flowing current is provided, the overcurrent when the input AC voltage is different by setting the resistance ratio of the second resistor and the third resistor to a predetermined ratio. The difference between the output current values at the time of detection is configured to be small.
As a result, even if the magnitude of the input AC voltage AC changes, the overcurrent protection circuit can operate and limit the current when the output current reaches substantially the same current value (for example, 7 A).

More preferably, the resistance value of the second resistor is several hundred to several tens of kΩ, and the resistance ratio of the second resistor to the third resistor is the primary peak current during the overcurrent protection operation. Ipocp, the first terminal overcurrent detection threshold voltage Vthocp, the first terminal overvoltage detection threshold voltage Vthovp, the primary side switch off voltage of the auxiliary winding Vboff, overvoltage detection In this case, when K is the margin, the resistance value of the first resistor is RA, the resistance value of the second resistor is RB, and the resistance value of the third resistor is RC, RA << RB, R A << R C, R A is set according to the following formula: RA = Vthocp / Ipocp, and RB, RC is set according to the following formula: RC / RB = (Vboff / Vthovp × K) −1.
As a result, the resistance value of the voltage dividing resistor for overvoltage detection can be easily set, and the output current reaches almost the same current value (for example, 7 A) even when the magnitude of the input AC voltage AC changes. When the overcurrent protection circuit is activated, the current can be limited.

Desirably, the allowance “K” in the equation that gives the resistance ratio between the second resistor and the third resistor is in the range of 0.7 to 0.9.
Accordingly, it is possible to avoid applying a voltage higher than the allowable voltage to the load when there is a variation in the allowable voltage of the load by giving a margin to the overvoltage determination level.

Preferably, the sampling circuit captures and holds a first sampling capacitor that captures and holds a predetermined reference voltage and a second sampling that captures and holds a voltage lower than the reference voltage by a voltage corresponding to an overvoltage determination level. A first sampling capacitor and a second sampling capacitor at a first timing to sample the reference voltage and a voltage lower than the reference voltage by a voltage corresponding to an overvoltage determination level at a second timing. Then, the voltage held in the first sampling capacitor and the voltage obtained by adding the voltage of the current detection terminal to the voltage held in the second sampling capacitor are compared by the comparator. The potential of the first terminal is taken into the second sampling capacitor, and the difference voltage from the voltage corresponding to the overvoltage determination level lower than the reference voltage is supplied to the comparator for comparison. To do.
Thereby, the overvoltage determination level can be generated by the resistance voltage dividing circuit, and the determination level can be easily changed by changing the resistance ratio of the resistance voltage dividing circuit.

  According to the present invention, an overvoltage detection can be accurately performed on the primary side in an insulation type DC power supply device that includes a voltage conversion transformer and controls on and off of the current flowing in the primary side winding by the PWM control method. . In addition, when a primary-side control circuit that constitutes an insulation type DC power supply device that includes a transformer for voltage conversion and performs on-off control of current flowing in the primary-side winding by the PWM control method is formed into a semiconductor integrated circuit, There is an effect that overvoltage detection can be performed without increasing the number.

It is a circuit block diagram which shows one Embodiment of the insulation type AC-DC converter as a DC power supply device which concerns on this invention. FIG. 2 is a circuit configuration diagram illustrating a configuration example of a primary side control circuit (power supply control IC) of a transformer in the insulated AC-DC converter of FIG. 1. It is a timing chart which shows the mode of the change of the voltage of each part in the primary side control circuit (power supply control IC) of an Example, or a signal. (A) is the waveform of the terminal voltage of the auxiliary winding in the isolated AC-DC converter, (B) is the waveform of the current flowing through the primary winding when operating in the continuous mode, and (C) is the discontinuous mode. It is a wave form diagram which shows the waveform of the electric current which flows into a primary side coil | winding when operate | moving by.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.
FIG. 1 is a circuit configuration diagram showing an embodiment of an insulated AC-DC converter as a DC power supply device using a switching power supply device to which the present invention is applied.

  The DC power supply device of this embodiment includes a noise blocking filter 11 made up of a common mode coil, a diode bridge circuit 12 that rectifies an AC voltage (AC) and converts it into a DC voltage, and smoothes the rectified voltage. A voltage conversion transformer T1 having a smoothing capacitor C1, a primary side winding Np, a secondary side winding Ns and an auxiliary winding Nb, and an N connected in series with the primary side winding Np of the transformer T1 It has a switching transistor SW made of a channel MOSFET and a power supply control circuit 13 for driving the switching transistor SW. In this embodiment, the power supply control circuit 13 is formed as a semiconductor integrated circuit (power supply control IC) on one semiconductor chip such as single crystal silicon.

  The secondary side of the transformer T1 is connected between the rectifying diode D2 connected in series with the secondary side winding Ns and between the cathode terminal of the diode D2 and the other terminal of the secondary side winding Ns. Smoothing capacitor C2 is provided, and the primary side winding Np is rectified and smoothed by rectifying and smoothing the AC voltage induced in the secondary side winding Ns by passing a current intermittently through the primary side winding Np. And a DC voltage Vout corresponding to the winding ratio of the secondary winding Ns.

  Further, the secondary side of the transformer T1 is provided with a coil L3 and a capacitor C3 constituting a filter for cutting off switching noise or the like generated in the switching operation on the primary side, and for detecting the output voltage Vout. And a photodiode 15a as a light-emitting side element of a photocoupler that is connected to the detection circuit 14 and transmits a signal corresponding to the detection voltage to the power supply control circuit 13 on the primary side. On the primary side, a photocoupler connected to the feedback terminal FB of the power supply control IC 13 and driven by the detection circuit 14 to transmit a signal to the feedback terminal FB is provided.

  Further, on the primary side of the DC power supply device of this embodiment, a rectifying diode D0 connected in series with the auxiliary winding Nb, and a smoothing connected between the cathode terminal of the diode D0 and the ground point GND. A rectifier circuit CMT including a capacitor C0 is provided, and a voltage rectified and smoothed by the rectifier circuit CMT is applied to the power supply voltage terminal VCC1 of the power supply control IC 13. At the same time, the voltage before being rectified by the diode bridge circuit 12 is applied to the high voltage starting terminal VCC0 of the power supply control IC 13 via the diode D1 and the resistor R0, and a voltage is induced in the auxiliary winding Nb at the time of starting the power supply. The power supply control IC 13 can be operated before being operated. The auxiliary winding Nb is set in the same manner as the secondary winding Ns so that a voltage having a waveform similar to the voltage induced on the secondary side is induced.

Further, in the present embodiment, a current detection resistor Ra and a voltage dividing resistor Rb are connected between the source terminal of the switching transistor SW and the current detection terminal CS of the power supply control IC 13, and the rectification A voltage dividing resistor Rc for input voltage correction connected between the terminal of the auxiliary winding Nb to which the anode of the diode D0 is connected and the current detection terminal CS of the power supply control IC 13 is provided. Further, a capacitor C4 is connected between the current detection terminal CS of the power supply control IC 13 and the ground point, and a low-pass filter is configured by the resistor Rb and the capacitor C4.
In this embodiment, the potential of the connection node C between the resistor Rb and the resistor Rc is input to the current detection terminal CS of the power supply control IC 13. However, the overcurrent protection operating point is not corrected for the input voltage fluctuation. In a good case, the power control IC 13 is provided with an overvoltage detection terminal separately from the current detection terminal CS, the terminal of the current detection resistor Ra is connected to the current detection terminal CS, and the resistor Rb and the resistor Rc are connected to the overvoltage detection terminal. Node C may be connected.

FIG. 2 shows a specific configuration example of the power supply control IC 13.
As shown in FIG. 2, the power supply control IC 13 has a built-in oscillator and generates a clock signal 31 (hereinafter simply referred to as a clock) necessary for turning on and off the primary side switching transistor SW, and a current detection. A sampling circuit 32 for taking in the voltage applied to the terminal CS and the reference voltage Vref1, an overvoltage detection circuit 33 for monitoring the voltage applied to the current detection terminal CS and detecting an output overvoltage state, and current detection Based on the voltage Vcs applied to the terminal CS, the current detection circuit 34 for detecting the current flowing through the primary winding Np, the voltage of the feedback terminal FB to which the light receiving transistor 15b of the photocoupler is connected, and the current detection terminal PWM pulse generation circuit 35 for generating a PWM control pulse based on the voltage applied to CS, An overcurrent detection circuit 36 that detects an overcurrent based on the voltage Vcs applied to the current detection terminal CS and interrupts the current flowing through the primary winding Np is provided.

  The sampling circuit 32 takes switches S2 and S3 that take in a difference between a switch S1 and a sampling capacitor Cs2 that take in a reference voltage Vref1, and a reference voltage Vref1 and a predetermined voltage (voltage at an overvoltage determination level) Vj. And a sampling capacitor Cs1, and a switch S4 for supplying a voltage obtained by taking a difference between the voltage Vcs applied to the current detection terminal CS and a predetermined voltage (voltage of an overvoltage determination level) Vj to the overvoltage detection circuit 33. Prepare. Of the switches S1 to S4, S1 to S3 are simultaneously turned on and off by the same clock CK, and the switch S4 is turned on and off complementarily to S1 to S3 by a clock / CK having a phase opposite to that of the clock CK. The resistors R1 and R2 that divide the reference voltage Vref1 to generate a predetermined voltage have a resistance ratio set so that the voltage drop amount of the resistor R1 is the same as the voltage Vj at the overvoltage determination level. As a result, the potential of the connection node N2 between the resistors R1 and R2 becomes Vref1-Vj.

The overvoltage detection circuit 33 includes an overvoltage detection comparator CMP1 for comparing the voltages held in the sampling capacitors Cs1 and Cs2, a mask gate circuit G1 for cutting off the output of the comparator CMP1, and a comparator using the gate circuit G1. A timer circuit TMR that sets a time for masking the output of CMP1 and a latch circuit LAT that latches the output of the comparator CMP1 that has passed through the gate circuit G1 when the output changes to a high level.
The output of the latch circuit LAT is configured to be supplied to a clock generation circuit 31 or an internal power supply circuit (not shown). The comparator CMP1 detects an overvoltage state of the output, and the output of the latch circuit LAT changes to a high level. Then, the operation of the internal circuit of the control IC 13 is stopped and an overvoltage protection state is entered. This overvoltage protection state is maintained until the power supply control IC 13 is restarted and the latch circuit LAT is reset.

  The current detection circuit 34 inverts the voltage corresponding to the input voltage of the current detection terminal CS captured by the sampling capacitor Cs1, the output of the inverting amplification circuit AMP1, and the voltage Vref2 of the variable voltage source VS. The PWM pulse generation circuit 35 includes an RS flip-flop FF1 that is set (ON) by the clock CK from the clock generation circuit 31 and reset (OFF) by the output of the comparator CMP2. . The output of the RS flip-flop FF1 is supplied as a PWM pulse to a driver (not shown), and the ON / OFF drive signal ON / OFF of the switching transistor SW is generated and output. The voltage Vref2 of the variable voltage source VS is a voltage that is changed according to the feedback voltage received by the light receiving transistor 15b of the photocoupler. The variable voltage source VS is, for example, the voltage VFB of the feedback terminal FB and a predetermined reference voltage. An error amplifier that outputs a voltage corresponding to the potential difference can be used.

  The overcurrent detection circuit 36 includes a comparator CMP3 that compares the output of the inverting amplifier circuit AMP1 with a predetermined reference voltage Vref3 that gives a determination level of overcurrent, and the output of the inverting amplifier circuit AMP1 is lower than the reference voltage Vref3. Then, the output changes to a high level, and the RS flip-flop FF1 of the PWM pulse generation circuit 35 is reset. As a result, the PWM pulse is forcibly changed to a low level to turn off the switching transistor SW, the current flowing through the primary winding Np is cut off, and the overcurrent protection function is activated.

As described above, in the present embodiment, the power supply control IC 13 performs the three functions of the PWM control, the overvoltage protection function, and the overcurrent protection function based on the voltage of one terminal (current detection terminal CS). Therefore, the number of external terminals required for the power supply control IC 13 can be reduced as compared with the conventional method, and the chip size can be reduced.
As will be described later, when the load is relatively light, the DC power supply device of this embodiment operates in a discontinuous mode in which the pulse width of the PWM pulse is changed according to the load to maintain the output voltage constant. When the load is relatively heavy, the power supply control IC 13 is configured to operate in a discontinuous mode in which the current peak value is changed according to the load and the output voltage is kept constant.

Next, the operation of the power supply control IC 13 will be described with reference to the timing chart of FIG.
3A shows a change in the voltage Vcs input to the current detection terminal CS. The voltage Vcs is obtained by dividing the voltage of the terminal (node B) of the auxiliary winding Nb by the resistors Rb and Rc and the voltage Vd (the potential of the node N1) which is current-voltage converted by the current sense resistor Ra. The voltage waveform at the node B is similar to the voltage waveform at the terminal (node A) of the secondary winding Ns.

  The power supply control IC 13 of this embodiment includes the comparator CMP2 of the PWM pulse generation circuit 35 in a period T1 in which a current is passed through the primary winding Np and the auxiliary winding Nb and the voltage Vcs is lower than the ground potential GND. Thus, current detection for PWM control is performed, and overvoltage detection is performed by the comparator CMP1 of the overvoltage detection circuit 33 in a flat portion (period T2) after the ringing of the voltage Vcs converges.

  Specifically, at timing t1, the switches S1 to S3 are turned on by the clock CK, and the switch S4 is turned off by the clock / CK. At this timing, the flip-flop FF1 of the PWM pulse generation circuit 35 is set, and the PWM pulse changes to a high level, so that the switching transistor SW is turned on, a current is passed through the primary winding Np, and the voltage Vcs is Start to change.

  While the switches S1 to S3 are turned on, Vref1-Vj that is the potential of the connection node N2 of the resistors R1 and R2 is taken into the sampling capacitor Cs1, and the reference voltage Vref1 is taken into the sampling capacitor Cs2. After that, when the switches S1 to S3 are turned off at timing t2 and the switch S4 is turned on, the charging voltage of the sampling capacitor Cs1 changes by the input voltage Vcs of the current detection terminal CS. That is, the voltage Vcs ′ obtained by level shifting the voltage Vcs by (Vref1−Vj) is input to the non-inverting input terminal of the comparator CMP1.

  In the period T1, the level-shifted voltage Vcs' is inverted and amplified by the inverting amplifier circuit AMP1 of the PWM pulse generation circuit 35, and compared with the reference voltage Vref2 by the comparator CMP2. When Vcs ′ reaches Vref2, the output of the comparator CMP2 changes to high level, the flip-flop FF1 is reset, and the PWM pulse changes to low level, so that the switching transistor SW is turned off, and the primary winding Np The current is cut off. At this time, since the reference voltage Vref2 is changed by the voltage VFB of the feedback terminal FB, the pulse width of the PWM pulse is controlled according to the feedback voltage from the secondary side, and the current on the primary side so that the output voltage becomes constant. Control will be performed.

  In this embodiment, since the voltage Vcs ′ obtained by level-shifting the voltage Vcs is input to the inverting amplifier circuit AMP1, the reference voltage Vref2 is also referred to when a voltage that is not level-shifted is input in consideration thereof. Set to a value shifted from the voltage. Vref2 'in FIG. 3A corresponds to Vref2 and indicates a virtual reference voltage of the PWM comparator CMP2 with respect to the input voltage Vcs of the current detection terminal CS. Further, when the input voltage Vcs of the current detection terminal CS falls below the absolute level Vref3 ′ in the period T1, it is detected by the comparator CMP3 of the overcurrent detection circuit 36, and the PWM pulse is forcibly changed to the low level. Then, the switching transistor SW is turned off. Vref3 'in FIG. 3A is a virtual potential corresponding to Vref3 of the comparator CMP3 in consideration of input of a voltage Vcs' obtained by level shifting the voltage Vcs.

  In this embodiment, when the input voltage Vcs of the current detection terminal CS falls to Vref2 'and the output of the PWM comparator CMP2 changes to a high level, the timer circuit TMR is started (timing t3). When a predetermined time elapses, the timer expires, the output of the timer circuit TMR changes to a high level (timing t4), the AND gate G1 as the mask circuit is opened, and the output of the comparator CMP1 is allowed to pass. . The timer circuit TMR is reset by the clock CK. That is, the output of the comparator CMP1 can pass through the AND gate G1 during the period T2 up to the timing t5 when the CK rises.

  This period T2 is a flat period of the input voltage Vcs of the current detection terminal CS as shown in FIG. In the period T2, the overvoltage detection circuit 33 of this embodiment compares the reference voltage Vref1 with the potential obtained by shifting the input voltage Vcs of the current detection terminal CS by (Vref1-Vj) by the comparator CMP1, that is, Vcs and Vj are compared. Compare. When Vcs becomes larger than the preset determination voltage Vj, the output of the comparator CMP1 changes to high level, which is latched by the latch circuit LAT, and the internal circuit (clock generation) of the power supply control IC 13 (Including the circuit) is stopped.

  As described above, in the power supply control IC 13 of the present embodiment, three functions of PWM control, overvoltage protection function, and overcurrent protection function can be realized based on the voltage of one terminal (current detection terminal CS). Compared to the conventional method, the configuration has an advantage that the number of external terminals required for the power supply control IC 13 is reduced and the chip size can be reduced.

  Note that the waveform of the voltage Vcs of the current detection terminal CS shown in FIG. 3A is a continuous mode waveform. In the power supply control IC 13 of this embodiment, when the load is relatively light, the waveform of the current flowing through the primary winding is a triangular wave as shown by the broken line in FIG. 4C, and the PWM pulse has a pulse width corresponding to the load. Changes to operate in a discontinuous mode where the output voltage remains constant. On the other hand, when the load is relatively heavy, the waveform of the current flowing through the primary winding becomes a trapezoidal wave with a constant pulse width as shown by the solid line in FIG. 4B, and the current peak value changes according to the load. Operates in a discontinuous mode that keeps the output voltage constant.

  By the way, the AC-DC converter of the embodiment shown in FIG. 1 can be configured such that the input AC voltage AC can operate with respect to a relatively wide range of voltages such as 80V to 260V. However, like the power supply control IC 13 of the above-described embodiment, based on the voltage of one terminal (current detection terminal CS), it is possible to realize three functions of PWM control, overvoltage protection function, and overcurrent protection function. In this case, unless any countermeasure is taken, the voltage at the terminal (node A) of the secondary winding Ns and the voltage at the terminal (node B) of the auxiliary winding Nb greatly vary depending on the magnitude of the input AC voltage AC. Therefore, for example, if the overcurrent protection circuit is designed to limit the current when the output current reaches 7 A when the input AC voltage is 100 V, the output current reaches 10 A when the input AC voltage is 230 V. When the overcurrent protection function works, the output current value when the overcurrent protection function works sometimes changes depending on the magnitude of the input AC voltage AC. .

  Therefore, in the AC-DC converter of this embodiment, the resistor Rb is provided between the current sense resistor Ra and the current detection terminal CS, and the resistance value of Rb, the terminal (node B) of the auxiliary winding Nb, and the current detection terminal are provided. By setting the resistance value of the resistor Rc to CS as follows, the output current reaches almost the same current value (for example, 7 A) even if the magnitude of the input AC voltage AC changes. The overcurrent protection circuit works to limit the current. Hereinafter, how to set the resistance values of the resistors Rb and Rc that enable such control will be described.

As a prerequisite, the necessary factors are defined as follows.
Vinac: AC input voltage, Vo: secondary output voltage, Io: secondary rated output current,
Ioocp: Secondary side output current during overcurrent protection operation
Vf: VF of secondary side diode D2, η: efficiency,
fosc: switching frequency (= frequency of clock CK),
Vthocp: Overcurrent protection threshold voltage of power supply control IC,
Vthovp: overvoltage detection threshold voltage of the current detection terminal CS,
Ipr: transformer primary peak current at rated load,
Ipocp: Primary peak current during overcurrent protection operation
Vboff: voltage at the time of switch-off of the primary side of the transformer auxiliary winding,
Lp: transformer primary winding inductance,
Np: number of primary turns of transformer,
Ns: the number of secondary turns of the transformer,
RA: resistance value of current sense resistor Ra, RB: resistance value of resistor Rb in series with resistor Ra, RC: resistance value of resistor Rc between terminal (node B) of auxiliary winding Nb and current detection terminal CS.

なお、連続モードにおける１次側巻線に流れる電流の波形を示すと図４（Ｂ）のようになる。式（１）における第１項すなわち左側の項は図４（Ｂ）の台形状の電流波形の平均電流（傾斜部の中点の電流）Ｉ１、式（１）における第２項すなわち右側の項は図４（Ｂ）の台形状の傾斜部の中点からピークまでの電流Ｉ２である。また、式（１）において、「Ｄ」は連続モードでのＰＷＭパルスのデューティ比（一定）であり、次式（２）で表わされる。

In determining the resistance values of the resistors Rb and Rc, first, the peak current Ipocp on the primary side of the transformer at the overcurrent protection operating point is obtained using the following equation (1).

FIG. 4B shows the waveform of the current flowing through the primary winding in the continuous mode. The first term in equation (1), that is, the left term is the average current I1 of the trapezoidal current waveform in FIG. 4B (current at the midpoint of the inclined portion) I1, and the second term in equation (1), that is, the right term. Is the current I2 from the midpoint to the peak of the trapezoidal slope in FIG. In Expression (1), “D” is the duty ratio (constant) of the PWM pulse in the continuous mode, and is expressed by the following Expression (2).

次に、抵抗ＲｂとＲｃの抵抗値ＲBとＲCを決定する。ここで、ＲB値は任意であるが、低過ぎると消費電力が増え、高過ぎると外来ノイズの影響を受け易くなる。そこで、ＲBの値は数百〜１０ｋΩ程度に設定することとした。そして、ＲBの値を決定したら、抵抗Ｒｃの抵抗値ＲCを、抵抗比ＲC／ＲBを表す次式（４）を用いて設定する。

Next, the resistance value RA of the current sense resistor Ra is obtained. RA can be obtained by the following equation (3) from the overcurrent protection threshold voltage Vthocp of the power supply control IC and the primary peak current Ipocp during the overcurrent protection operation represented by equation (1). .

Next, resistance values RB and RC of the resistors Rb and Rc are determined. Here, the RB value is arbitrary, but if it is too low, the power consumption increases, and if it is too high, it is easily affected by external noise. Therefore, the value of RB is set to about several hundred to 10 kΩ. When the value of RB is determined, the resistance value RC of the resistor Rc is set using the following equation (4) representing the resistance ratio RC / RB.

In the present example, this equation (4) was derived as follows.
First, when the voltage of the current detection terminal CS of the power supply control IC during normal operation and during the off period of the primary side switching transistor SW is set to Vcsoff, no current flows through SW during the off period of SW. The current flowing through Ra is zero. Therefore, if the terminal voltage (voltage at node B) of the transformer auxiliary winding in the SW off period is Vboff, the voltage Vcsoff of the current detection terminal CS in the SW off period is expressed by the following equation (5).

Here, the resistance values RA, RB, RC of the resistors Ra, Rb, Rc are in a relationship of RA << RB, RA << RC, that is, the resistance value RA is sufficiently smaller than the resistance values RB, RC. If set, the above equation (5) becomes the following equation (5) ′.

On the other hand, when the setting rate (margin) of the current detection terminal CS with respect to the overvoltage detection threshold voltage Vthovp is K, the voltage Vcsoff of the current detection terminal CS during the SW off period is expressed by the following equation (6).
Vcsoff = Vthovp × K (6)
Therefore, if the left-hand side of equation (5) ′ is replaced by equation (6), the following equation (7) is obtained.

本実施形態のＡＣ−ＤＣコンバータにおいては、上記のようにして、抵抗Ｒａ〜Ｒｃの抵抗値ＲA，ＲB，ＲCを決定するようにしたことによって、入力交流電圧ＡＣの大きさが変わっても、過電流保護機能が働く際の出力電流値をほぼ一定（例えば７Ａ）することができるようになる。 And if the said Formula (7) is deform | transformed, the said Formula (4) will be obtained.

In the AC-DC converter of the present embodiment, the resistance values RA, RB, RC of the resistors Ra to Rc are determined as described above, so that even if the magnitude of the input AC voltage AC changes, The output current value when the overcurrent protection function works can be made substantially constant (for example, 7 A).

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the embodiment. For example, in the above embodiment, a feedback signal proportional to the output voltage is transmitted from the secondary detection circuit to the primary power supply control IC 13 via the photocoupler. In such a case, a circuit for detecting the output current may be provided on the secondary side, and a signal proportional to the output current may be transmitted to the power control IC 13 on the primary side via the photocoupler. .

  In the above embodiment, the switching transistor SW that allows current to flow intermittently through the primary winding of the transformer is a separate element from the power supply control IC 13. However, the switching transistor SW is incorporated into the power supply control IC 13. You may comprise as one semiconductor integrated circuit.

12 Diode bridge circuit (rectifier circuit)
13 Power control circuit (Power control IC)
14 Secondary side detection circuit (IC for detection)
15a Photocoupler light emitting side diode 15b Photocoupler light receiving side transistor 31 Clock generation circuit 32 Sampling circuit 33 Overvoltage detection circuit 34 Current detection circuit 35 PWM pulse generation circuit 36 Overcurrent detection circuit

Claims (6)

A voltage conversion transformer having an auxiliary winding, a switching element for intermittently passing a current through the primary winding of the transformer, a first terminal for current detection, and a second terminal for feedback; An insulation type having a power supply control semiconductor integrated circuit that controls on / off of the switching element with a PWM control pulse by feedback of an output detection voltage from the secondary side of the transformer to the second terminal via a photocoupler A DC power supply,
A first resistor connected in series with the switching element for detecting a current flowing in a primary winding of the transformer;
A second resistor connected between the terminal of the first resistor and the first terminal;
A third resistor connected between the terminal of the auxiliary winding and the first terminal;
With
The semiconductor integrated circuit for power control is
A current detection circuit that detects a current flowing in the primary winding of the transformer based on a voltage that is current-voltage converted by the first resistor;
A sampling circuit connected to the first terminal and a voltage sampled by the sampling circuit during a period in which the potential of the first terminal is flat are compared with a predetermined reference voltage to determine whether an overvoltage state has occurred. An overvoltage detection circuit having a comparator,
And the overvoltage detection circuit is configured to stop driving control of the switching element when detecting an overvoltage state of an output based on a voltage on a primary side of the transformer. Type DC power supply.   The second resistor is connected in series with the first resistor between the switching element and the first terminal, and a voltage corresponding to the input voltage of the first terminal is input to the current detection circuit. 2. The insulated DC power supply device according to claim 1, wherein the first terminal is configured to serve as the terminal for detecting the overvoltage and the terminal for detecting the current.   The power supply control semiconductor integrated circuit detects whether an output overcurrent state has occurred based on the potential of the first terminal, and restricts the current flowing through the primary winding of the transformer when the overcurrent state is detected. When an overcurrent protection circuit is provided, the output current when the overcurrent is detected when the input AC voltage is different by setting the resistance ratio of the second resistor and the third resistor to a predetermined ratio. 3. The insulated DC power supply device according to claim 2, wherein a difference between the values is reduced. The resistance value of the second resistor is several hundreds to several tens of kΩ, and the resistance ratio of the second resistor to the third resistor is the primary peak current during the overcurrent protection operation, Ipocp, The terminal overcurrent detection threshold voltage is Vthocp, the first terminal overvoltage detection threshold voltage is Vthovp, the primary side switch-off voltage of the auxiliary winding is Vboff, and the margin for overvoltage detection. Is K, the resistance value of the first resistor is RA, the resistance value of the second resistor is RB, and the resistance value of the third resistor is RC, the relationship of RA << RB, RA << RC RA has the following formula: RA = Vthocp / Ipocp
RB and RC are given by the following formula: RC / RB = (Vboff / Vthovp × K) −1
The insulated DC power supply device according to claim 3, which is set according to   5. The insulation according to claim 4, wherein a margin “K” in the equation that gives a resistance ratio between the second resistance and the third resistance is in a range of 0.7 to 0.9. Type DC power supply. The sampling circuit is
A first sampling capacitor that captures and holds a predetermined reference voltage, and a second sampling capacitor that captures and holds a voltage lower than the reference voltage by a voltage corresponding to an overvoltage determination level;
Sampling the reference voltage and a voltage lower than the reference voltage by a voltage corresponding to an overvoltage determination level in the first sampling capacitor and the second sampling capacitor at a first timing;
The voltage held in the first sampling capacitor at the second timing and the voltage obtained by adding the voltage of the current detection terminal to the voltage held in the second sampling capacitor in the previous period are compared by the comparator. The insulation type DC power supply device according to any one of claims 1 to 5.

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