CN102280860A - PWM (Pulse Width Modulation) chip-based input overvoltage protection circuit of auto-excitation type converter - Google Patents

Abstract

The invention discloses an input overvoltage protection circuit of a self-excited converter based on a PWM chip, which includes a PWM chip, a self-feed power supply winding, a filter, a rectifier bridge, and an auxiliary device with one end connected between the filter and the rectifier bridge. The control power supply circuit, the comparison protection circuit connected to the other end of the auxiliary control power supply circuit and the output terminal of the rectifier bridge, the AND gate circuit connected to the comparison protection circuit, and the output terminal of the AND gate circuit is connected to the PWM chip. The circuit utilizes auxiliary transformer and adjustable integrated voltage stabilizer combined with wide-range power supply voltage input comparator and AND gate circuit, and utilizes the overvoltage protection function of V _DD to realize the input voltage overvoltage protection of the converter (power supply). The circuit is simple and reliable, and the power consumption is small, which meets the requirements of some application occasions, improves the reliability of the power supply (converter), and has good application value and promotion prospect.

Description

Input overvoltage protection circuit of self-excited converter based on PWM chip

technical field

The invention relates to an input overvoltage protection circuit of a self-excited converter based on a PWM control chip of On-bright Company, belonging to the technical field of circuits.

Background technique

Pulse Width Modulation (PWM, or Pulse Width Modulation) is abbreviated as Pulse Width Modulation. It is a very effective technology that uses the digital output of a microprocessor to control analog circuits. It is widely used in measurement, communication, power control and conversion. in many fields. The PWM control chip is a control chip specially used for AC/DC switching converters. As shown in Figure 1, take the PWM control chip launched by On-bright Company as an example. It is characterized by low offline power consumption and high efficiency. The power supply voltage under-voltage lockout and over-voltage protection, over-temperature protection and overload protection and other functions. However, when this type of PWM control chip is applied to a converter or power supply with a self-excited feed winding such as flyback or forward, it cannot directly lead the input voltage of the converter to the VDD of the chip after detection. To form a voltage protection circuit, a protection circuit for the input voltage of the converter must be designed in addition. The current protection circuit for the input voltage of the converter is relatively complicated.

Contents of the invention

The technical problem to be solved by the present invention is to provide a simple converter input voltage protection circuit, so that when the PWM control chip is applied to a self-excited forward or flyback converter, it can provide an effective converter input voltage protection, and has the function of automatic protection release.

In order to solve the above technical problems, the present invention provides a self-excited converter input overvoltage protection circuit based on a PWM chip, including a PWM chip, a self-feed power supply winding, a filter connected to the AC voltage input terminal, and a filter connected to the filter. rectifier bridge, characterized in that it also includes

one end is connected to the auxiliary control power supply circuit between the filter and the rectifier bridge,

A comparative protection circuit connected to the other end of the auxiliary control power supply circuit and the output end of the rectifier bridge,

An AND gate circuit connected to the comparison protection circuit, the output terminal of the AND gate circuit is connected to the PWM chip.

The auxiliary control power supply circuit includes an auxiliary control transformer and an adjustable three-terminal integrated voltage regulator.

The primary winding of the auxiliary control transformer is connected across the output bus after the filter, and the secondary winding of the auxiliary control transformer is connected to the adjustable three-terminal integrated voltage regulator through a rectifier diode.

The anode of the rectifier diode is connected to the secondary winding of the auxiliary control transformer, and the cathode is connected to the input end of the adjustable three-terminal integrated voltage regulator.

The adjustable three-terminal integrated voltage regulator includes an input terminal, an adjustment terminal and an output terminal, the adjustment terminal is connected to the ground through a resistor, and the output terminal is connected to the adjustment terminal through another resistor.

The output terminal of the adjustable three-terminal integrated voltage regulator outputs the auxiliary power supply voltage.

The comparison protection circuit includes a comparator, the DC bus voltage output by the rectifier bridge is divided by a group of voltage resistances and then connected to the non-inverting terminal of the comparator, and the output terminal of the adjustable three-terminal integrated voltage regulator The output auxiliary power supply voltage is divided by another group of voltage resistors and then connected to the inverting terminal of the comparator.

The output end of the comparator is connected with the AND gate circuit.

The AND gate circuit includes two diodes connected with a common cathode, wherein the anode of one diode is connected with the output terminal of the comparator, and the anode of the other diode is connected with the self-feed power supply winding.

The common cathode end of the diode is connected to the voltage pin of the PWM control chip.

The beneficial effects achieved by the present invention: the present invention designs a set of simple and feasible overvoltage protection circuit for the input voltage of the converter (power supply) based on the PWM chip. Voltage source, combined with a wide-range power supply voltage input comparator and "AND gate" circuit, using the overvoltage protection function of the PWM control chip V _DD , through reasonable adjustment of the auxiliary feed voltage and the voltage reference of the comparator, to establish overvoltage and undervoltage The protection adjustment point, so as to achieve the purpose of protecting the input overvoltage of the converter (power supply). The circuit is simple and reliable, with low power consumption, and effectively utilizes the functions of the control chip itself to provide reliable input overvoltage protection for the converter (power supply), which meets the needs of certain applications and improves the performance of the converter (power supply). Reliability, good application value and promotion prospect. At the same time, when the input voltage is lower than the input voltage protection point again, the overvoltage protection is automatically released without affecting the normal operation of the converter (power supply).

Description of drawings

Figure 1 is a schematic diagram of a PWM control chip applied to a flyback converter;

Fig. 2 is a schematic circuit diagram of an embodiment of the present invention;

Fig. 3 is a schematic diagram of an embodiment of the auxiliary control power supply circuit in Fig. 2;

FIG. 4 is a schematic diagram of an embodiment of the comparison protection circuit and the “AND gate” circuit in FIG. 2 .

In the figure,

1. Auxiliary control power circuit;

2. Comparison protection circuit;

3. AND gate circuit;

AC, AC voltage input;

DC, DC voltage output;

T. Auxiliary control transformer;

N1, the primary winding of the auxiliary control transformer;

N2, the secondary winding of the auxiliary control transformer;

D1, rectifier diode;

C1, capacitance;

R1, resistance;

R2, resistance;

C2, capacitance;

U1, PWM chip;

OC, comparator;

V _DC , DC bus voltage;

R3, voltage divider resistor;

R4, voltage divider resistor;

V _O , auxiliary power supply voltage;

R5, voltage divider resistor;

R6, voltage divider resistor;

R7, comparator output pull-up resistor;

C3, self-feed winding output voltage filter capacitor;

D2, diode;

D3, diode.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings. The following examples are only used to illustrate the technical solution of the present invention more clearly, but not to limit the protection scope of the present invention.

As shown in FIG. 2 , the circuit of the present invention mainly includes an auxiliary control power supply circuit 1 , a comparison protection circuit 2 and an AND gate circuit 3 . The auxiliary control power supply circuit 1 is arranged between the filter EMI and the rectifier bridge circuit. A low-power auxiliary control transformer T is connected to the AC voltage input terminal AC in parallel. The primary side of the auxiliary control transformer T is connected to the AC voltage input terminal AC, and the output voltage of the secondary side of the transformer is rectified into a DC voltage;

The rectified voltage on the secondary side of the auxiliary control transformer T is used as an input to the three-terminal linear voltage regulator to obtain a stable DC voltage source, which can be adjusted by changing the ratio of the auxiliary control transformer T and the three-terminal linear voltage regulator. The external resistance of the regulator is used for setting;

The input voltage overvoltage protection compares the output through the comparator OC, and forms an AND gate circuit with the self-feed power supply winding of the PWM chip;

The reference voltage of the input voltage protection point and the supply voltage of the comparator are provided by the manufactured DC voltage source.

The connection configuration of the circuit of the present invention will be described in detail below.

1. Auxiliary control power supply circuit 1

As shown in Figure 2 and Figure 3, the auxiliary control power supply circuit 1 is mainly composed of an auxiliary control transformer T and an adjustable three-terminal integrated voltage regulator. The primary winding _N1 of the auxiliary control transformer is connected across the output bus after the filter EMI of the AC voltage input terminal AC; the secondary winding _N2 of the auxiliary control transformer is connected to the anode of the rectifier diode D1 to form a half-wave rectification circuit. The cathode of the rectifier diode D1 is connected to the input terminal of the voltage regulator. Resistor R1 is connected between the regulator terminal and the ground, resistor R2 is connected between the regulator output terminal and the regulator terminal; capacitor C2 is connected between the regulator output terminal and the ground, and filters Function; Capacitor C1 is connected between the input terminal of the voltage regulator and the ground, and acts as a filter. The output terminal of the voltage regulator outputs the auxiliary power supply voltage V _O .

The power of the auxiliary control transformer T and the voltage regulator can be selected according to the supply current of the pin V _DD of the PWM chip U1. The optional integrated voltage regulators include adjustable three-terminal integrated voltage regulators CW117, TL431, etc.

If it is CW117, the output voltage of the regulator is determined by the following formula:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1.25</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>$

If it is TL431, the output voltage of the regulator is:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2.5</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>$

2. Compare protection circuit 2 and AND gate circuit 3

As shown in Figure 2 and Figure 4, after the AC voltage input by the AC voltage input terminal AC is rectified by the rectifier bridge, the output DC bus voltage V _DC is divided by the voltage dividing resistor R ₃ and the voltage dividing resistor R ₄ and then connected to The non-inverting terminal of comparator OC. The comparator OC can be a comparator powered by a wide range of power supply voltage, such as LM339; the auxiliary power supply voltage V _O output from the output terminal of the voltage regulator in the auxiliary control power supply circuit 1 is passed through the voltage dividing resistor R ₅ and the voltage dividing resistor R ₆ After the voltage is divided, it is connected to the inverting terminal of the comparator OC to form the input voltage protection point. The voltage value of the protection point is expressed by the following formula:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{<span\; class="notranslate">\; -</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 6</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 6</span>}}}$

The voltage value input to the non-inverting terminal of the comparator OC is determined by the following formula:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{<span\; class="notranslate">\; +</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; DC</span>}}\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="4"\; label="R"\; filenames="110519160347.png"\; state="\{\{state\}\}">R</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}}$

Connect the comparator output pull-up resistor R _{7 between} the auxiliary supply voltage V _O and _the output terminal of the comparator OC _; The common cathodes are connected, and the common cathode end is connected to the pin V _DD end of the PWM chip U1 to form an AND gate circuit 3 for the VDD pin of the PWM control chip.

Taking the PWM chip OB2268 as an example, when the input voltage is too high, the voltage signal V ₊ of the non-inverting terminal of the comparator OC is greater than the voltage signal V _- of the inverting terminal, so the output of the comparator OC is high, and its level voltage is the comparator output Voltage V _OC , the voltage value is slightly lower than the auxiliary power supply voltage V _O , this is due to the existence of the output pull-up resistor R7, the actual voltage value of the comparator output voltage V _OC is V _OC =V _O -I _O R ₇ , I _O is the output current of the comparator, which is determined by the load of the comparator. The comparator output voltage V _OC is set to be higher than the output voltage of the feedback winding and higher than the overvoltage protection point voltage of the V _DD terminal (for example, the V _DD overvoltage protection point of OB2268 is 23.5V, then V _OC can be set to 24V), As long as this condition is met. Therefore, when the input voltage of the converter (power supply) is higher than the overvoltage protection point voltage of the pin V _DD of the PWM chip, the PWM chip enters into protection and turns off the output of the PWM chip.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the technical principle of the present invention, some improvements and modifications can also be made. It should also be regarded as the protection scope of the present invention.

Claims (10)

1. A self-excited converter input overvoltage protection circuit based on a PWM chip, comprising a PWM chip, a self-feed power supply winding, a filter connected to an AC voltage input terminal, and a rectifier bridge connected to the filter, characterized in that, Also includes one end is connected to the auxiliary control power supply circuit between the filter and the rectifier bridge, A comparative protection circuit connected to the other end of the auxiliary control power supply circuit and the output end of the rectifier bridge, An AND gate circuit connected to the comparison protection circuit, the output terminal of the AND gate circuit is connected to the PWM chip. 2. The self-excited converter input overvoltage protection circuit based on a PWM chip according to claim 1, wherein the auxiliary control power supply circuit includes an auxiliary control transformer and an adjustable three-terminal integrated voltage regulator. 3. the self-excited converter input overvoltage protection circuit based on PWM chip according to claim 2, is characterized in that, on the output bus after the primary side winding of described auxiliary control transformer bridges the described filter, so The secondary winding of the auxiliary control transformer is connected to the adjustable three-terminal integrated voltage regulator through a rectifier diode. 4. The self-excited converter input overvoltage protection circuit based on a PWM chip according to claim 3, wherein the anode of the rectifier diode is connected to the secondary winding of the auxiliary control transformer, and the cathode is connected to the secondary winding of the auxiliary control transformer. Input connection of the adjustable three-terminal integrated voltage regulator. 5. the self-excited converter input overvoltage protection circuit based on PWM chip according to claim 2, is characterized in that, described adjustable three-terminal integrated voltage stabilizer comprises input terminal, adjustment terminal and output terminal, and described The adjustment end is connected to the ground through a resistor, and the output end is connected to the adjustment end through another resistor. 6 . The self-excited converter input overvoltage protection circuit based on a PWM chip according to claim 2 , wherein the output of the adjustable three-terminal integrated voltage regulator is an auxiliary power supply voltage. 7. The self-excited converter input overvoltage protection circuit based on the PWM chip according to claim 6, wherein the comparison protection circuit includes a comparator, and the DC bus voltage output by the rectifier bridge is passed through a component connected to the non-inverting terminal of the comparator after voltage division by piezoresistors, and the auxiliary power supply voltage output from the output terminal of the adjustable three-terminal integrated voltage regulator is connected to the non-inverting terminal of the comparator after being divided by another group of voltage resistances Inverting terminal. 8. The self-excited converter input overvoltage protection circuit based on a PWM chip according to claim 7, wherein the output terminal of the comparator is connected to the AND gate circuit. 9. The self-excited converter input overvoltage protection circuit based on the PWM chip according to claim 8, wherein the AND gate circuit comprises 2 diodes connected with common cathodes, wherein the anode of one diode is connected to the The output terminal of the comparator is connected, and the anode of the other diode is connected with the self-feed power supply winding. 10. The self-excited converter input overvoltage protection circuit based on a PWM chip according to claim 9, wherein the common cathode end of the diode is connected to the voltage pin of the PWM control chip.

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