CN104363683A - Non-electrolytic-capacitor long-service-life constant-current LED driving power supply - Google Patents

Abstract

The invention discloses a long-life constant-current LED drive power supply without an electrolytic capacitor. It includes bridge rectifier circuit, main switching tube, flyback transformer, Buck-boost auxiliary circuit, secondary rectifier circuit, output filter capacitor and LED load. The present invention has the following features and advantages: ①: most of the input power directly reaches the load after only one energy conversion of the flyback transformer, avoiding the secondary conversion of the energy of the whole machine; ②: the voltage of the energy storage capacitor C _a is designed as a DC voltage In the working form of superimposing large pulsating ripple voltage, CBB high-voltage ceramic capacitors or film capacitors with small capacitance can be used instead of electrolytic capacitors, thereby improving the life of the LED drive power supply; ③: Only two switching tubes operate in one power frequency cycle, Moreover, the switching tube _S2 realizes zero-current and zero-voltage turn-on, which reduces the switching loss of the switching tube; ④: The auxiliary circuit and the main circuit can work together to realize constant current driving of the LED load.

Description

A long-life constant current LED drive power supply without electrolytic capacitors

technical field

The invention relates to the technical field of power electronics, in particular to a long-life constant-current LED drive power supply without an electrolytic capacitor, which belongs to the field of AC/DC and DC/DC converters.

Background technique

With the increasingly serious energy crisis and environmental pollution, energy conservation and emission reduction have become the focus of attention. High-brightness light-emitting diodes (light-emitting diodes, LEDs) have the advantages of high luminous efficiency, long life, small size, energy saving and environmental protection, and easy dimming. LED lighting has become a key industry for energy saving and emission reduction. Different from traditional light-emitting devices, LEDs have unique optical-electrical-thermal characteristics. Therefore, high-quality LED driving power is the key to ensuring LED luminous quality and overall lighting performance.

However, short-life electrolytic capacitors are a technical bottleneck in the development of high-quality LED drive power. As a semiconductor light-emitting device, LED chips have a lifespan of 80-100kh, while the lifespan of electrolytic capacitors commonly used in LED drive power is only about 10khs, which is far lower than the long lifespan of LED lamp beads. Therefore, the use of electrolytic capacitors is limited. The overall life of the LED lighting source is shortened, and the development of high-performance LED drive power is restricted. Therefore, under the premise of not affecting the optical performance and thermal performance of LED, use CBB high-voltage ceramic capacitors or film capacitors with small capacitance instead of electrolytic capacitors, and develop long-life, high-efficiency electrolytic capacitor-free LED drive power to become the LED drive power industry. urgent problem to be solved. In the traditional solution, although inductive energy storage elements can be used instead of electrolytic capacitors, inductive elements are large in size, large in loss, low in power density, and the constant current accuracy of the driving current is not high, which is not suitable for the development trend of LED drive power supply.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of the LED drive power supply that affects the overall life of LED lighting due to the use of electrolytic capacitors, overcome the shortcomings of the traditional LED drive power supply such as short life, large volume, low efficiency of the whole machine, many components, and low integration, and provide a non-toxic Long-life constant current LED driver with electrolytic capacitors. It has the characteristics and advantages of no electrolytic capacitor, small size, high precision constant current, high efficiency and high reliability.

In order to achieve the above object, the present invention adopts the following technical solutions: a long-life constant current LED drive power supply without electrolytic capacitors, including: a bridge rectifier circuit, a flyback transformer, a main switching tube, an energy storage capacitor, and a secondary side rectifier circuit , Buck-boost auxiliary circuit and filter capacitor; characterized in that: the bridge rectifier circuit is sequentially connected to the flyback transformer, the main switching tube, the energy storage capacitor, the secondary side rectifier circuit, the Buck-boost auxiliary circuit and the filter capacitor.

The bridge rectifier circuit is composed of a first diode D _r1 , a second diode D _r2 , a third diode D _r3 and a fourth diode D _r4 ; the first diode D _r1 The anode is connected to the cathode of the third diode _Dr3 , the anode of the second diode _Dr2 is connected to the cathode of the fourth diode _Dr4 , and the cathode of the first diode _Dr1 is connected to the cathode of the second diode The cathode of D _r2 is connected, and the anode of the third diode D _r3 is connected to the anode of the fourth diode D _r4 ; the flyback transformer is composed of a primary winding N _p and a secondary winding N _s , and the primary The end with the same name of the side winding _Np is connected to the cathodes of the first diode _Dr1 and the second diode _Dr2 , and the end with the same name of the primary winding _Np is connected to the drain of the main switch _S1 The source of the main switching tube _S1 is connected to the cathodes of the third diode D _r3 and the fourth diode D _r4 ; the secondary side rectification circuit is composed of the fifth diode D R and the second diode D _r The switch tube _S2 is composed, the cathode of the fifth diode _DR is connected to the drain _S2 of the second switch tube, and the anode of the fifth diode _DR is connected to the terminal with the same name of the secondary winding _Ns ;

The Buck-boost auxiliary circuit is composed of a charging branch and a discharging branch of the energy storage capacitor C _a ; the charging branch is composed of a sixth diode D _a1 , and the anode of the sixth diode D _a1 is connected to the secondary winding N _s is connected to the terminal with the same name, and the cathode of the sixth _diode D _{a1 is} connected to _the positive electrode of the energy storage capacitor C _a ; , the Buck-boost circuit that the eighth diode D _a3 forms; The source of the third open tube _S3 is connected to the negative pole of the energy storage capacitor C _a , and the drain of the third open tube _S3 is connected to the eighth diode D The anode _{of a2} is connected to one end of the inductance L _a ; the other end of the inductance L _a is connected to the cathode of the seventh diode D _a3 and the cathode of the energy storage capacitor C _a , and the cathode of the eighth diode D _a3 is connected to the output filter capacitor The positive end of C _o is connected to the positive end of the LED load; the anode of the seventh diode D _a2 is connected to the negative end of the output filter capacitor C _o and the negative end of the LED load.

The bridge rectifier circuit composed of the first diode D _r1 , the second diode D _r2 , the third diode D _r3 and the fourth diode D _r4 realizes AC/DC conversion.

The flyback transformer works in a discontinuous current mode to implement input power factor correction.

The voltage of the energy storage capacitor C _a is designed to be a DC voltage superimposed with a large pulsating ripple voltage, and a CBB high-voltage ceramic capacitor or film capacitor with a small capacitance can be used instead of an electrolytic capacitor.

The Buck-boost auxiliary circuit can balance the low-frequency pulsating power of instantaneous input power and output power, and it can be realized under the condition that the input power is less than the output power (p _in <p _O ) and the input power is greater than the output power (p _in >p _O ). Constant output power regulation, auxiliary circuit and main circuit can realize constant current driving LED load.

The filter capacitor C _o can filter the output voltage ripple.

Compared with the prior art, the present invention has the following obvious outstanding substantive features and significant advantages: ①: Most of the input power directly reaches the load after only one energy conversion of the flyback transformer, avoiding the secondary conversion of the energy of the whole machine ; ②: The voltage of the energy storage capacitor C _a is designed to be a DC voltage superimposed large pulsating ripple voltage working form, and a CBB high-voltage ceramic capacitor or film capacitor with a smaller capacitance value can be used to replace the electrolytic capacitor, thereby improving the life of the LED drive power supply; ③: Only two switching tubes operate in one power frequency cycle, and the switching tube _S2 realizes zero-current and zero-voltage turn-on, which reduces the switching loss of the switching tube; ④: The auxiliary circuit and the main circuit can work together to realize constant current driving LED Load; ⑤: It has the characteristics and advantages of no electrolytic capacitor, small size, high precision constant current, high efficiency and high reliability.

Description of drawings

FIG. 1 is a schematic diagram of a long-life constant-current LED drive power supply without electrolytic capacitors according to the present invention.

Fig. 2 is the main working waveform of a long-life constant current LED drive power supply without electrolytic capacitor of the present invention.

Fig. 3 is a logic sequence and main working waveform of a switching tube of a non-electrolytic capacitor long-life constant current LED drive power supply of the present invention under the condition of _pin > p _o .

FIG. 4 is an equivalent circuit of each switch mode of a long-life constant current LED drive power supply without electrolytic capacitors under the condition of _pin >p _o according to the present invention.

Fig. 5 is a switch tube logic sequence and main working waveform of a long-life constant current LED drive power supply without electrolytic capacitors under the condition of _pin < p _o according to the present invention.

FIG. 6 is an equivalent circuit of each switch mode of a long-life constant current LED drive power supply without electrolytic capacitors under the condition of _pin < p _o according to the present invention.

Detailed ways

The present invention will be further explained below in conjunction with the accompanying drawings and preferred embodiments.

Embodiment 1: As shown in Figure 1, the long-life constant current LED drive power supply without electrolytic capacitors includes: bridge rectifier circuit (1), main switch tube (2), flyback transformer (3), Buck-boost Auxiliary circuit (4), secondary side rectifier circuit (5), output filter capacitor (6) and LED load (7); characterized in that: bridge rectifier circuit (1) is sequentially connected to flyback transformer (3), main switching tube (2), Buck-boost auxiliary circuit (4), secondary side rectification circuit (5), output filter capacitor (6) and LED load (7).

Embodiment 2: This embodiment is basically the same as Embodiment 1, and the special features are as follows: the bridge rectifier circuit (1) consists of a first diode (D _r1 ), a second diode (D _r2 ), a second diode Composed of three diodes (D _r3 ) and a fourth diode (D _r4 ); the anode of the first diode (D _r1 ) is connected to the cathode of the third diode (D _r3 ), and the second diode The anode of the tube (D _r2 ) is connected to the cathode of the fourth diode (D _r4 ), the cathode of the first diode (D _r1 ) is connected to the cathode of the second diode (D _r2 ), and the third diode ( D _r3 ) anode is connected to the anode of the fourth diode (D _r4 ); the flyback transformer (2) is composed of a primary winding (N _p ) and a secondary winding (N _s ), and the primary winding ( The terminal with the same name of N _p ) is connected to the cathodes of the first diode (D _r1 ) and the second diode (D _r2 ), and the terminal of the same name of the primary winding (N _p ) is connected to the main switch The drain of the tube (2) (S ₁ ) is connected, the source of the main switching tube (2) (S ₁ ) is connected to the third diode (D _r3 ) and the fourth diode (D _r4 ) The cathode of the secondary side rectification circuit (5) is formed by connecting the cathode of the fifth diode (D _R ) to the drain of the second switching tube (S ₂ ), and the anode of the fifth diode (D _R ) It is connected with the end of the same name of the secondary winding (N _s ); the Buck-boost auxiliary circuit (4) is composed of a charging branch and a discharging branch of the energy storage capacitor (C _a ); the charging branch is composed of a sixth diode (D _a1 ), the anode of the sixth diode (D _a1 ) is connected to the terminal with the same name of the secondary winding (N _s ), the cathode of the sixth diode (D _a1 ) is connected to the positive pole of the energy storage capacitor (C _a ) connected; the discharge branch is a Buck-boost circuit composed of a third open tube (S ₃ ), an inductor (L _a ), a seventh diode (D _a2 ), and an eighth diode (D _a3 ); The source of the third open transistor (S ₃ ) is connected to the negative pole of the energy storage capacitor (C _a ), the drain of the third open transistor (S ₃ ) is connected to the anode of the eighth diode (D _a2 ), the inductor (L _a ) is connected to one end; the other end of the inductor (L _a ) is connected to the cathode of the seventh diode (D _a3 ) and the cathode of the energy storage capacitor (C _a ), and the cathode of the eighth diode (D _a3 ) is connected to The positive terminal of the output filter capacitor (C _o ) is connected to the positive terminal of the LED load (7); the anode of the seventh diode (D _a2 ) is connected to the negative terminal of the output filter capacitor (6) (C _o ) , and the negative terminal of the LED load (7) is connected.

Describe the concrete operating principle, design principle of the present embodiment below in conjunction with accompanying drawing 2-6:

Figure 2 is the main working waveform of the long-life constant current LED drive power supply without electrolytic capacitors.

It can be seen from the circuit topology in Figure 1 that the circuit topology is based on a three-port converter integrated with a Flyback circuit and a Buck-boost circuit. The energy storage capacitor C _a at the third port can balance the pulsating power between input power and output power. The Flyback secondary winding N _s and D _a1 form the charging branch of the energy storage capacitor C _a ; the Buck-boost converter composed of the switch S ₃ , inductor L _a , diodes D _a2 , D _a3 , and the load is the discharge of C _a branch road.

It can be seen from the main working waveforms in Figure 2 that the circuit working principles are different under different power conditions. When _pin > p _o , the excess energy will charge C _a , and the voltage v _Ca of C _a will rise, at this time S ₃ is in a constant off state, and S ₂ is controlled to provide a constant current for the LED; when _pin < p _o , the insufficient energy is provided by C _a , the voltage v _Ca of C _a drops, at this time _S2 is in a constant open state, and _S3 is controlled to provide a constant current for the LED. S ₁ controls the average voltage of the energy storage capacitor C _a , and makes the Flyback converter work in the current discontinuous mode to realize the PFC function. Therefore, when the circuit works stably, the average voltage of C _a is constant, and the duty cycle of S ₁ is basically constant.

1. Analysis of circuit working principle

1.1 Switching modal analysis when p _in >p _o

Figure 3 shows the main working waveform when p _in >p _o . Under this power condition, the circuit has four switching modes, and the corresponding equivalent circuit is shown in Figure 4.

1) Switching mode 1[t ₀ ,t ₁ ]: The equivalent circuit is shown in Figure 4(a). Before time t ₀ , the excitation current im of the Flyback converter _is zero, and the filter capacitor C _o supplies power to the LED load; at time t ₀ , the switches S ₁ and S ₂ are turned on, diodes D _r1 and D _r4 (or diodes D _r2 and D _r3 ) is turned on, and S ₃ is in a constant off state when _pin >p _o . Since the secondary diode _DR is under the back pressure and does not conduct during the conduction period of _S1 , no current flows through _S2 , and _S2 is invalidly turned on at this stage. Assuming that the input voltage _v remains constant for one switching cycle, _i rises linearly from zero:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Where L _m is the excitation inductance.

At time t ₁ , switch tube S ₁ is turned off, and the size _{of im} at this time is:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

2) Switching mode 2[t ₁ ,t ₂ ]: The equivalent circuit is shown in Figure 4(b). At time _t1 , the switch tube _S1 is turned off, and _S2 is still turned on. The energy stored in the transformer is released to the load through the secondary coil, and i _m decreases linearly. The secondary coil current during [t ₁ ,t ₂ ] can be expressed as:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\frac{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

In the formula, D ₁ is the duty ratio of S ₁ , and T _s is the switching period of S ₁ , S ₂ , and S ₃ .

By controlling _S2 so that the energy released to the LED in a switching cycle is constant, the LED can be driven with a constant current. Therefore, S ₂ is turned off at time t ₂ , according to formula (3), the transformer secondary current at time t ₂ is:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\frac{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Where _D2 is the effective duty ratio of switch _S2 .

In this switching mode, in order to ensure that the transformer energy is released to the LED instead of charging the energy storage capacitor C a through the diode D _a1 , the voltage of _{C a must} meet the following conditions:

v _Ca (t) > V _o (5)

3) Switching mode 3[t ₂ ,t ₃ ]: The equivalent circuit is shown in Fig. 4(c). After S ₂ is turned off, the remaining energy in the transformer charges the energy storage capacitor C _a through the diode D _a1 , and the _im continues to decline linearly. Assuming that _the Ca voltage v _Ca remains constant during a switching cycle, during [t ₂ ,t ₃ ] there are:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; Ca</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

At time _t3 , i _m drops to zero, and the time interval from _t2 to _t3 is:

${\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; Ca</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; Ca</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

4) Switching mode 4[t ₃ ,t ₄ ]: The equivalent circuit is shown in Figure 4(d). In this switching mode, the filter capacitor C _o supplies power to the LED load. The primary and secondary coil currents of the transformer are zero, and the transformer is magnetically reset.

In order to realize the PFC function, the Flyback converter needs to be designed to work in the current discontinuous mode, so when p _in > p _o , the conditions need to be met:

T _pin>po ＝D ₁ T _s +D ₂ T _s +ΔT ₁ <T _s (8)

Based on the above analysis, it can be seen that when p _in > p _o , the duty cycle of switch tube S ₁ is basically unchanged, S ₃ is in a constant off state, and S ₂ is controlled to provide a constant driving current for the LED; S ₂ and S ₁ are simultaneously turned on to achieve S ₂ is turned on with zero voltage and zero current, and S ₃ has no switching action, which reduces the switching loss; the excess energy _of the input power pin in one switching cycle is absorbed by C _a of the third port.

1.2 Switching modal analysis when p _in < p _o

Figure 5 shows the main working waveform when _pin < p _o . Under this power condition, the switching mode of the circuit is different due to the working mode of the Buck-boost circuit, and the switching mode, main working waveform and equivalent circuit of the circuit will also be different. In this paper, the following five switching modes are taken as examples for analysis, and the corresponding equivalent circuits are shown in Figure 6.

1) Switching mode 1[t ₀ ,t ₁ ]: The equivalent circuit is shown in Figure 6(a). At time t ₀ , the switch tubes S ₁ and S ₃ are turned on. Although S ₂ is always on when _pin < p _o , the secondary diode _DR is subjected to back pressure during the conduction period of S ₁ and does not conduct, so S ₂ does not current flows. Assuming that the input voltage v remains constant _within one switching cycle, the Flyback excitation current _im increases linearly from zero:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 9</span>}<span\; class="notranslate">\; )</span>$

Where L _m is the excitation inductance.

At time t ₁ , switch tube S ₁ is turned off, and the size _{of im} at this time is:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>$

Since the energy provided by _pin to the load is insufficient during the period of _pin < p _o , the switch tube _S3 is turned on at time t ₀ , C _a replenishes energy through the Buck-boost converter, and the current i _La of the inductor L _a increases linearly. Assuming that the voltage v _ca of C _a remains constant in one switching cycle, there are:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; La</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; ca</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 11</span>}<span\; class="notranslate">\; )</span>$

2) Switching mode 2[t ₁ ,t ₂ ]: The equivalent circuit is shown in Figure 6(b). At time _t1 , switch _S1 is turned off, _S3 is still on, the energy stored in the transformer is released to the load through the secondary coil, C _a continues to provide energy to the load, and i _La continues to rise linearly. During [t ₁ ,t ₂ ], the secondary coil current i _S2 decreases linearly:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\frac{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 12</span>}<span\; class="notranslate">\; )</span>$

3) Switching mode 3[t ₂ ,t ₃ ]: The equivalent circuit is shown in Fig. 6(c). Switching tube _S3 is turned off at time _t2 , inductance L _a freewheels through diode D _a2 , and i _S2 continues to decrease linearly. The L _a current at time _t2 can be expressed as:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; La</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; ca</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; ca</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 13</span>}<span\; class="notranslate">\; )</span>$

Where D ₃ is the duty cycle of the switch tube S ₃ .

After the switch tube _S3 is turned off, the inductor L _a current i _La can be expressed as:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; La</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; La</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 14</span>}<span\; class="notranslate">\; )</span>$

t ₃ moment i _S2 drops to zero, that is, the excitation current i _m drops to zero. The time interval between t ₁ and t ₃ is:

${\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 15</span>}<span\; class="notranslate">\; )</span>$

4) Switching mode 4[t ₃ ,t ₄ ]: The equivalent circuit is shown in Figure 6(d), and i _La continues to decrease linearly during [t ₃ ,t ₄ ]. At time _t4 , i _La drops to zero, and the time interval between _t2 and _t4 is:

${\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; La</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 16</span>}<span\; class="notranslate">\; )</span>$

5) Switching mode 5[t ₄ ,t ₅ ]: The equivalent circuit is shown in Fig. 6(e). In this switching mode, the filter capacitor C _o supplies power to the LED load. The primary and secondary coil currents of the transformer are zero, and the transformer is magnetically reset.

In order to realize the PFC function, the Flyback converter needs to be designed to work in the current discontinuous mode, so the conditions need to be met when p _in < p _o :

T _pin<po ＝D ₁ T _s +ΔT ₂ <T _s (17)

Based on the above analysis, it can be seen that when p _in < p _o , the duty cycle of switch tube S ₁ is basically unchanged, S ₂ is in a constant on state, and S ₃ is controlled to provide a constant driving current for the LED; S ₂ has no switching action, reducing the _Switching loss; the insufficient energy of the input power pin in one switching cycle is supplemented by C _a of the third port.

The present invention is not limited to the above specific implementation manners, and those skilled in the art can carry out various implementation manners according to the content disclosed in the present invention. It should be understood that the above-mentioned embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. After reading the present invention, those skilled in the art all fall within the appended claims of the present application to the amendments of various equivalent forms of the present invention limited range.

Claims (5)

1. a long-life constant current LED driving power source for no electrolytic capacitor, comprising: bridge rectifier (1), main switch (2), flyback transformer (3), Buck-boost auxiliary circuit (4), secondary side rectification circuit (5), output filter capacitor (6) and LED load (7); It is characterized in that: bridge rectifier (1) connects flyback transformer (3), main switch (2), Buck-boost auxiliary circuit (4), secondary side rectification circuit (5), output filter capacitor (6) and LED load (7) successively.

2. the long-life constant current LED driving power source of a kind of no electrolytic capacitor according to claim 1, is characterized in that: described bridge rectifier (1) is by the first diode (D _r1), the second diode (D _r2), the 3rd diode (D _r3) and the 4th diode (D _r4) composition; Described first diode (D _r1) anode connect the 3rd diode (D _r3) negative electrode, the second diode (D _r2) anode connect the 4th diode (D _r4) negative electrode, the first diode (D _r1) negative electrode and the second diode (D _r2) negative electrode docking, the 3rd diode (D _r3) anode and the 4th diode (D _r4) anode docking; Described flyback transformer (2) is by former limit winding (N _p) and vice-side winding (N _s) composition, described former limit winding (N _p) Same Name of Ends and described first diode (D _r1) and the second diode (D _r2) negative electrode connect, described former limit winding (N _p) different name end and described main switch (2) i.e. (S ₁) drain electrode connect, main switch (2) i.e. (S ₁) source electrode and described 3rd diode (D _r3) and the 4th diode (D _r4) negative electrode connect; Described secondary side rectification circuit (5) is by the 5th diode (D _r) negative electrode and second switch pipe (S ₂) the connected formation of drain electrode, the 5th diode (D _r) anode and vice-side winding (N _s) Same Name of Ends be connected; Described Buck-boost auxiliary circuit (4) is by storage capacitor (C _a) charging paths and discharge paths composition; Described charging paths is by the 6th diode (D _a1) composition, the 6th diode (D _a1) anode and vice-side winding (N _s) Same Name of Ends be connected, the 6th diode (D _a1) negative electrode and storage capacitor (C _a) positive pole be connected; Described discharge paths is by the 3rd open pipe (S ₃), inductance ( l _a ), the 7th diode (D _a2), the 8th diode (D _a3) the Buck-boost circuit that forms; Described 3rd open pipe (S ₃) source electrode and storage capacitor (C _a) negative pole be connected, the 3rd open pipe (S ₃) drain and the 8th diode (D _a2) anode, inductance ( l _a ) one end be connected; Described inductance ( l _a ) the other end and the 7th diode (D _a3) negative electrode, storage capacitor (C _a) negative pole be connected, the 8th diode (D _a3) negative electrode and output filter capacitor (C _o) anode, LED load (7) anode be connected; Described 7th diode (D _a2) anode and described output filter capacitor (6) i.e. (C _o) negative terminal, described LED load (7) negative terminal be connected.

3. the long-life constant current LED driving power source of a kind of no electrolytic capacitor according to claim 1, it is characterized in that: most of input power directly arrives load through the conversion of flyback transformer (2) primary energy, avoids the quadratic transformation of complete machine energy.

4. the long-life constant current LED driving power source of a kind of no electrolytic capacitor according to claim 1, is characterized in that: described storage capacitor (C _a) voltage design be the working forms of direct voltage superposition large pulsation ripple voltage, capacitance can be used less.

5. the long-life constant current LED driving power source of a kind of no electrolytic capacitor according to claim 1, is characterized in that: only have two switching tube actions in a power frequency period, and second switch pipe (S ₂) to achieve zero current no-voltage open-minded, reduces the switching loss of switching tube.