JP3283591B2 - Overcurrent protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit in a DC / DC converter.

[0002]

2. Description of the Related Art Conventionally, in a DC / DC converter,
For some reason, an excessive output current may flow due to a short circuit on the output side of the power supply. An overcurrent protection circuit is provided to prevent damage to components due to such overcurrent.

As this type of circuit, for example, "SILI"
CONIX Power Products data
book Application notes A
N88-3, pp. 66-67 1991 ".

FIG. 4 is a block diagram showing the configuration of a conventional forward converter based on current feedback control. The operation of overcurrent protection in the converter will be described.

In the figure, reference numeral 21 denotes an OP amplifier, 26, 2
7 is a comparator, 22, 25 and 28 are reference power supplies, 2
9 and 32 are OR circuits, 30 is a flip-flop, 31 is a clock, 33 is an inverter, 34 is a main switch, 36
Is a main transformer, 37 is a rectifying / smoothing circuit, 23, 24, 3
5 and 38 are resistors.

First, an outline of the control in the case where the converter is performing a steady operation near the rated load will be described with reference to the waveform diagrams of each part of the block diagram of FIG. 4 shown in FIG. 5 and FIG.

The symbols a to h in the waveform diagram of FIG. 5 indicate signal values of the portion shown in the block diagram of FIG.

The voltage generated at the resistor 35 inserted in the source of the main switch 34 is as shown in FIG.
This is similar to the current flowing in the main switch. The period during which the main switch current is flowing is forcibly limited to 50% or less, thereby preventing problems such as destruction of the main switch at the time of starting the converter and saturation of the main transformer. The limitation of the period during which the main switch current is flowing is performed by setting the on-duty ratio of the clock 31 for controlling the main switch 34 to, for example, 50%.

The comparator 27 receives the voltage a generated at the resistor 35 and the voltage of the reference power supply 28, and inputs the output to an OR circuit 29. Since the resistance value of the resistor 35 is selected so that the peak voltage of the voltage a generated in the resistor 35 does not exceed the value of the reference voltage 28 at the time of rating, the output of the comparator 27 is in the “low” state at the time of rating. And does not affect the subsequent circuit operation.

Here, assuming that the converter output has risen as shown by the waveform b → b ′ in FIG. 5, the negative input of the comparator 26 falls as shown by the waveform c ′, so that the waveform d ′ As shown, the comparator output is generated earlier than the waveform d.

As a result, the on-duty ratio of the main switch gate signal decreases as shown by the waveform h ', and both the on-duty ratio and the peak value of the main switch current decrease. Control is performed.

Next, the case where the output current of the converter exceeds the rating will be described with reference to the waveform diagrams of the respective parts of the block diagram of FIG. 4 shown in FIG.

If for some reason the output current exceeds the rating, the main switch current increases, and the peak value of the voltage generated at the resistor 35 exceeds the value i of the reference voltage 28, the output of the comparator 27 changes to a "high" state. Become. This "hi
The on-duty ratio of the main switch gate signal decreases as shown by the waveform h due to the "gh" state. Therefore, the output current is limited by the drooping characteristic, and the overcurrent protection circuit is driven. It shows the drooping characteristic determined by the value of 28.

When the output current enters the drooping characteristic, the converter output voltage b drops and falls below the standard to become equal to or lower than the voltage of the reference power supply 22, and the output of the amplifier 21 becomes a "high" state. As a result, the output of the comparator 26 is always in the "low" state, does not contribute to the circuit operation, and even if the converter output voltage b subsequently changes, the output current changes according to the drooping characteristic.

[0015]

However, the above-described conventional apparatus has the following problems.

Even in applications where the average current of the output current is small and the peak current of short duration is repeatedly generated, (1) the droop start current is 5% to 5% of the maximum rating in consideration of the variation of the reference voltage and the resistance. It needs to be set to 10% increase.

That is, if the value of the droop start current at which the droop characteristic actually starts operating is set to a low value due to variations in the reference voltage, resistance, and the like, excessive output current occurs before the maximum rating is reached. The function of the current protection circuit operates and the output current is suppressed low. (2) Also, when the output voltage decreases during the droop, the output current increases as the output voltage decreases,
The output current when the output terminal is completely short-circuited often exceeds the droop start current by about 20%, so that the function of the overcurrent protection circuit is not sufficiently exhibited. (3) Therefore, converter loss increases when the output is short-circuited.

For example, when an output current that exceeds the droop start current by about 20% flows when the output is short-circuited, the converter loss when the output is short-circuited is reduced by 20% from the rated converter loss.
% Or more. (4) For this reason, heat generation of the main switch and the secondary side rectifier diode increases, and their current peak values also become extremely large. Then, it is necessary to have a large margin as compared with obtaining a rated output for these elements. (5) In particular, in the case where the average current of the output current is small and the peak current having a short duration is repeatedly generated, the average current of the main switch, the diode, and the radiator is small despite the small average output. A converter with a large capacity is required, which is an obstacle to cost reduction and miniaturization.

Therefore, the present invention eliminates the above-mentioned conventional problems and reduces the converter loss when the load is short-circuited in the overcurrent protection circuit, thereby reducing the cost of the main switch, rectifier diode, and radiator used in the converter. It is an object of the present invention to provide an excellent device having a reduced size.

[0020]

In order to achieve the above object, the present invention provides an overcurrent protection circuit of a current feedback control type converter for detecting that a current value and a duration of a main switch current exceed a set value. Detecting means for changing a DC voltage to a current signal based on an output of the detecting means and a droop start value of an output current;
And a current feedback signal control means for controlling a current feedback signal of the converter whose signal level is increased by superimposing the signal, and the time from the detection of the overcurrent to the limitation of the load current can be set by selecting the set value. In addition, a first integrator for integrating a voltage signal due to a main switch current in an overcurrent protection circuit of a current feedback control type converter, and an integrated value of the first integrator being equal to or more than a certain value And a second integrator for detecting that the output of the first comparator has been connected for a certain period of time. The output signal of the second integrator is converted to a current signal. DC voltage
By changing the droop start value of the output current, the signal
This is used as a bias signal for the current feedback signal in the current feedback control with the bell increased .

[0021]

An overcurrent protection circuit of a current feedback control type converter according to the present invention integrates a voltage signal generated in a resistor by a main switch current, inputs the voltage signal to a comparator, and outputs an output when the integrated value exceeds a predetermined value. Then, by generating an output when the output is further connected for a certain period of time, a biased overcurrent protection circuit is formed as a current feedback signal in the current feedback control by applying a bias to the voltage signal. The cost and size of the main switch, rectifier diode, and radiator used in the converter can be reduced by reducing the loss when the load is short-circuited.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an overcurrent protection circuit according to the present invention.

In FIG. 1, 1 is a main transformer, 2 is a main switch, 4 and 7 are integrators, 5, 8, 13, and 15 are comparators, 6, 9, and 14 are reference power supplies, 3, 10, and 12 are resistance,
Reference numeral 11 denotes a diode, reference numeral 17 denotes an OR circuit, and other reference numerals are the same as those in the block diagram of FIG.

FIG. 2 is a waveform diagram of each part in a block diagram showing an embodiment of the overcurrent protection circuit of the present invention shown in FIG.

In FIG. 1, one end of the primary winding of the main transformer 1 is connected to the power supply + E, and the other end is connected to the drain of the main switch 2. The other end of the resistor 3 whose one end is connected to the source of the main switch 2 is connected to the ground.

1 is an embodiment of the overcurrent protection circuit of the present invention. FIG. 1 shows the overcurrent protection circuit according to the conventional current feedback control shown in FIG. This is applied to a forward converter.

The overcurrent protection circuit includes a connection between the connection point between the resistor 3 of the conventional forward converter and the source of the main switch 2 and the positive inputs of the comparators 13 and 15 shown in FIG. It is provided between them.

Next, an overcurrent protection circuit in a portion surrounded by a broken line in FIG. 1 will be described.

A resistor 1 is connected between the connection point of the resistor 3 and the source of the main switch 2 and the positive input of the comparators 13 and 15.
2 and a series circuit of integrators 4 and 7 and comparators 5 and 8 are connected in parallel.

The input of the integrator 4 includes a resistor 3 and a main switch 2
The output of the integrator 4 is connected to the positive input of the comparator 5, and the reference power supply 6 is connected between the negative input of the comparator 5 and the ground.

The output of the comparator 5 is connected to the input of the integrator 7, and the output of the integrator 7 is connected to the positive input of the comparator 8. A reference power supply 9 is connected between the negative input of the comparator 8 and the ground, and the output of the comparator 8 is connected to the positive inputs of the comparators 13 and 15 via a resistor 10.

The other end of the resistor 10 is connected to the anode of a diode 11, and the cathode of the diode 11 is connected to the positive inputs of the comparators 13 and 15, respectively. A resistor 12 is connected between the cathode of the diode 11 and the source of the main switch 2.

Further, a reference power supply 14 is connected between the negative input of the comparator 13 and the ground, and its output is connected to an OR circuit 17.
Connected to the input. An error amplifier output signal is provided to the negative input of the comparator 15, and the output is connected to another input of the OR circuit 17. (A) Operation under rated load; First, operation under rated load will be described with reference to FIGS.

Voltage A generated at resistor 3 by load current
Is integrated by the integrator 4 and compared with the reference voltage C of the reference power supply 6 in the comparator 5. Further, the output voltage of the comparator 5 is integrated by the integrator 7 and compared with the reference voltage F of the reference power supply 9 in the comparator 8.

Here, by selecting the integration constant of the integrator 4 to be long, the output of the integrator 4 can be treated almost as a direct current. The integration constant of the integrator 4 is suitably about 20 to 2000 times the operation cycle of the converter.

Therefore, by selecting the reference voltage value of the reference power supply 6, both the outputs D and G of the comparators 5 and 8 can be set to the "low" state at the rated load or less.

When the output of the comparator 8 is in a "low" state, the diode 11 is in a reverse bias state and the input impedance of the integrator 4 is high. 5, 8, the reference power sources 6, 9, the resistors 3, 10, 12, and the diode 11 do not affect the converter operation at all, and the operation of the overcurrent protection function of the forward converter by the conventional current feedback control shown in FIG. Become equal. (B) Operation when a load current exceeding the rating flows; Next, an operation when a load current exceeding the rating flows will be described.

When a load current exceeding the rating flows, the voltage A generated in the resistor 3 by the load current is integrated by the integrator 4 and compared with the reference voltage C of the reference power supply 6 by the comparator 5 to obtain D As shown in FIG.
Further, the "high" state of D is integrated by the integrator 7, compared with the reference voltage F of the reference power supply 9 in the comparator 8, and becomes the "high" state as indicated by G. Here, the time constant of the integrator 7 is about 10 times or more the time constant of the integrator 4 and acts only on the rising portion. When the voltage of the A continues a predetermined value for a predetermined time or more, , The output of the comparator 8 is obtained.

That is, the output of the comparator 8 in the "high" state is to time after the load current exceeding the rated value starts to flow due to the delay caused by the integrators 4 and 7.

When the voltage of G becomes "high", the comparator 8 → the resistor 10 → the diode 11 → the resistor 12 → the resistor 3
→ A current flows through the ground route, and a DC bias voltage is generated in the resistor 12 as a signal voltage for current control.

The voltage of H becomes a voltage value obtained by adding the bias voltage to the voltage of A, and is compared with the reference voltage I of the reference power supply 14 in the comparator 13 to output the voltage J. And the voltage K is set to a “high” state.

As a result, the output of the OR circuit 17 becomes "hi".
gh "state, the main switch 2 is controlled to reduce the output voltage of the converter, and the output of A also decreases.

When the output voltage of the converter further decreases, the output B of the integrator 4 becomes equal to or lower than the reference voltage C, and the voltage of G becomes "low". As a result, the voltage J is in a “low” state.

Here, when the integration constants of the integrators 4 and 7 are τ ₁ and τ ₂ , respectively, and τ ₂ >> τ ₁ is selected, the time delay to can be substantially set to the integration constant of the integrator 7. it can.

Therefore, within the to time period after the load current exceeding the rating starts to flow, the drooping characteristic determined by the values of the resistor 3 and the reference power supply 14 is exhibited as described in the circuit shown in FIG. The current is given by the following equation (a). Note that losses due to conversion in the converter are ignored.

I _s1 = n (V _REF / R ₃ −EDT / 2L) (a) where I _s1 : a droop start value of the output current (from time 0 to to) n: the number of turns on the secondary side of the transformer Primary side turns ratio when set to 1 E: Converter input voltage L: Transformer primary side inductance T: Converter operating cycle D: Converter on-duty ratio (dimensionless) V _REF : Voltage of reference power supply 14 Value.

On the other hand, since the output G of the comparator 8 becomes "high" after the elapse of the to time from the start of the flow of the load current exceeding the rating, the comparator 8 → the resistor 10 → the diode 11 →
A current flows through the route of the resistor 12 → the resistor 3 → the ground, and the DC bias voltage is applied to the signal voltage for the current control.
Occurs. For this reason, the drooping start value of the output current is expressed by equation (b).

I _s2 = n {(V _REF −V _B ) / R ₃ −EDT / 2L} (b) where V _B = R ₁₂ (V _cc −V _f ) / (R ₁₀ + R ₁₂ ) R ₁₀ , wherein _{R 12 >> R 3, I s2} : droop starting value at time to after the output current V _B: bias voltage V _cc generated across the resistor 12: the power supply voltage V _f of the comparator 8: forward voltage of the diode 11 Rn is the value of the resistor n.

[0050] Here, a comparison of droop starting value I _s1 and I _s2 of output current represented by the formula (a) and Formula (b).

[0051] First, comparing the first item of the drooping start value I _s1 and I _s2, the first item of the droop starting value I _s1 is nV _REF /
The first item of the droop starting value I _s2 whereas a R ₃ is nV
_REF / R ₃ are -nV _B / R _3, clearly towards the hanging start value I _s2 is smaller by the amount of nV _B / R _3.

Further, looking at the second term of the droop start values I _s1 and I _s2 , it is nEDT / 2L, but the on-duty ratio decreases when the to time elapses after the load current exceeding the rating starts to flow. ED of droop start value _Is1
As a term for which T / 2L is smaller and the droop start value is smaller, EDT / 2L of the droop start value _Is1 is larger.

However, since the value of the bias voltage V _B generated in the resistor 12 can be easily changed by the resistance value, by increasing the value of the bias voltage V _B , the droop start value I _s2 of the equation (b) is obtained. Is sufficiently small, and the droop start value can be set to be I _s1 > I _s2 as a whole.

[0054] Thus, by selecting the droop starting value I _s2 of the output current to a fraction of I _s1, the peak current value is repeated guarantees output current connection time is less than said to h I _s1 or less, In addition, when a load short circuit or overload occurs, the circuit operates as a pullback protection circuit, so that the output current can be reduced to a fraction of the peak current. This situation is shown in the drooping characteristic diagram of the overcurrent protection circuit of the present invention in FIG.

[0055] That is, in the conventional overcurrent protection function, shows a drooping characteristic shown by the solid line in FIG. 3, from the point P ₁ and the output current is increased with changes in the output voltage. In this respect P ₁ is the current flowing through the main switch and the rectifier diode will occur heat generation problems associated with the thus power loss and it too.

[0056] In contrast, drooping characteristic indicated by a broken line in FIG. 3, the drooping characteristics of the overcurrent protection circuit of the present invention shows a case where the elapsed to time, the drooping from drooping start value I _s2 start to reach the point P _2. In this respect P _2, the current flowing through the main switch and the rectifier diode becomes small compared to that of the prior art, it is possible to solve the problem of heat generation due to the power loss and it.

[0057] Further, when the peak current value is connected time I _s1 following the t ₀ or more output current, dashed line - according to the output current value when the connection time becomes the t ₀
, The drooping characteristic indicated by the broken line is obtained.

[0058] Further, when the peak current value of the connection time is the t ₀ less output current I _s1 below may be repeatedly compensated traverse the portion of the constant voltage drooping characteristic indicated by a solid line . If a load short circuit or overload occurs at this point, the circuit operates as a pullback protection circuit according to the trajectory indicated by the dashed line.

In order to improve the response characteristics, the integrator 7
Must operate only on the rising edge of the signal and have no delay in the falling edge.

The present invention is not limited to the above-described embodiment, but can be variously modified based on the gist of the present invention, and these are not excluded from the scope of the present invention.

[0061]

As described above in detail, according to the present invention, in a converter using a current feedback circuit, a voltage signal generated in a resistor by a main switch current is integrated and input to a comparator so that the integrated value is constant. When the output reaches a value or more, an output is obtained, and when the output continues for a certain period of time, a voltage obtained from a circuit generated at the output is used to apply a bias to the voltage signal as a current feedback signal in current feedback control. Therefore, even when applied to a converter that has a small average output current and guarantees a peak output current in a short connection time, (1) the output current at the time of load short circuit and overload is greatly reduced, and the main switch, Since the maximum current of the rectifier diode is suppressed and the loss is reduced, the element rated capacity and the radiator capacity are reduced, and the converter can be reduced in size and cost. (2) Since the time from when an overcurrent is detected to when the current is drawn can be set arbitrarily from several milliseconds to several tens of seconds, a good starting characteristic is compensated even for a load requiring a large current to flow at the time of starting. It is possible to expect such effects as possible.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an overcurrent protection circuit according to the present invention.

FIG. 2 is a waveform diagram of each part in a block diagram showing an embodiment of the overcurrent protection circuit of the present invention.

FIG. 3 is a drooping characteristic diagram of the overcurrent protection circuit of the present invention.

FIG. 4 is a block diagram showing a configuration of a conventional forward converter based on current feedback control.

FIG. 5 is a waveform chart of each part of the block diagram of FIG. 4;

FIG. 6 is a waveform chart of each part in the block diagram of FIG. 4;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transformer 2 Main switch 3,10,12,23,24,35,38 Resistance 4,7 Integrator 5,8 Comparator 6,9,14,22,25 Reference power supply 11 Diode 13,15 Comparator 17 OR circuit 21 OP amplifier 30 Flip-flop 31 Clock 32 OR circuit 33 Inverter 37 Rectifier / smoothing circuit

Claims (4)

(57) [Claims] 1. An overcurrent protection circuit for a current feedback control type converter, comprising: (a) detecting means for detecting that a current value and a duration of a main switch current have exceeded a set value; DC signal to current signal based on output
Signal by superimposing the output voltage by changing the droop start value of the output current.
Current feedback signal control means for controlling a current feedback signal of a converter having an increased signal level , and (c) a time from detection of an overcurrent to limitation of a load current can be set by selecting the set value. Overcurrent protection circuit. 2. An overcurrent protection circuit for a current feedback control type converter, comprising: (a) a first integrator for integrating a voltage signal due to a main switch current; and (b) an integrated value of the first integrator is constant. (C) a second integrator for detecting that the output of the first comparator has been connected for a predetermined time, and (d) a second integrator for detecting that the output of the first comparator has been connected for a predetermined time. Output signal of the integrator 2 to a DC signal
Voltage by changing the droop start value of the output current.
An overcurrent protection circuit characterized in that it is used as a bias signal for a current feedback signal in current feedback control with a high signal level . 3. The overcurrent protection circuit according to claim 2, wherein a time constant of said first integrator is larger than a converter operation cycle. 4. The time constant of said second integrator is equal to said first integrator.
4. The overcurrent protection circuit according to claim 2, wherein the overcurrent protection circuit is larger than a time constant of the integrator.