METHOD FOR SERIES RESONANT CONVERTER CONTROL WITH SYNCHRONOUS RECTIFIER

Technical Field

The present invention relates to a series resonant converter. More particularly, the present invention relates to a series resonant converter and a method for operating the same in a step-up mode so that all switching devices can perform zero-voltage switching.

Background Art

As generally known in the art, rapid development in electronic, computer, and semiconductor technologies has changed our lives drastically. For example, although we had to rely on maps for geographic information, we can now employ a navigation system, which enables its users to easily locate destinations and, when equipped on transportation means, guides them to the destinations. In addition, extensive use of Internet has enabled us to enjoy indoor shopping and communicate with remote people in the cyber space. Furthermore, automatic vacuum cleaners have recently appeared to clean our houses on their own.

Electronic products, which are extensively used in our

daily lives as mentioned above, generally have a number of devices coupled to a single body. Power is supplied to respective devices from the outside so as to realize desired functions . However, power supplied to the body from the outside does not always match the level of operating power needed by respective devices inside an electronic product. Therefore, each device has a DC/DC converter positioned on its front end so as to convert the external power into a necessary level of operating power. Since each device is equipped with its own DC/DC converter, the higher the conversion efficiency is, the more power saving can be expected from the power supply.

FIG. 1 shows a two-stage insulated power supply, which is currently mass-produced. The power supply is provided with AC power, which is rectified by rectifiers D1-D4 (composed of bridge diodes) and converted into DC voltage V _dc by a conventional boost converter.

There are two types of converters: boost converters and buck converters. The boost converters have an input voltage smaller than the output voltage, while the buck converters have an input voltage larger than the output voltage. Referring to FIG. 1, as the AC power passes through the rectifiers and the boost converter, the output voltage increases to V _άCf which is converted into V ₀ through an insulated DC/DC converter.

There are various insulated DC/DC converters, in terms of circuitry type, including flyback converters, forward converters, push-pull converters, half-bridge converters, series resonant converters, and parallel resonant converters. FIG. 2 is a circuit diagram of a series resonant converter power supply adapted for synchronous rectification. The control power in a power conversion system must maintain a constant output voltage regardless of fluctuating input voltage and load. In addition, the output side must be electrically insulated from the input side. A conventional MOSFET (Metal-Oxide Semiconductor Field Effect Transistor) is used as the switching device in FIG. 2. By turning on/off the MOSFET on the output side of a transformer in accordance with the polarity of output current, the conduction loss resulting from the forward voltage drop across a diode is reduced drastically. This is particularly advantageous in the case of a low-voltage large-current power supply.

FIG. 3 is a circuit diagram of a series resonant boost converter power supply, which is a modification of a series resonant buck converter power supply when a boost converter is connected to the circuitry of a series resonant converter power supply adapted for synchronous rectification. Series resonant converter power supplies are generally operated in a step-down mode and, in order to operate them in a step-up mode, boost converts must be connected thereto.

However, when a boost converter and a DC/DC converter are connected to each other in series, the conversion efficiency is lower than in the case of a single converter. In addition, the series connection increases the cost.

Disclosure of the Invention

Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a series resonant converter and a method for operating the same in a step-up mode so that all switching devices can be subjected to zero-voltage switching.

According to an aspect of the present invention, there is provided a method for controlling a series resonant converter adapted for synchronous rectification in a step-up mode so that an output DC voltage is converted into a predetermined level of voltage, the converter having an input-side switching unit for converting a DC input voltage into an AC voltage by using an input-side switching device, an LC resonant circuitry connected to the input-side switching unit so as to store energy in an L and a C by using an LC resonance phenomenon, the energy being transmitted as output, an insulation conversion unit having a primary-side wiring connected to the LC resonant circuitry so as to convert resonant current into secondary current in

accordance with a wiring ratio, the resonant current being transmitted to a secondary-side wiring, and an output-side switching unit constituting a full-bridge circuit by using switches QIl, Q12, Q13, and Q14, a (+) terminal being connected between the switches QlI and Q12, a (-) terminal being connected between the switches Q13 and Q14, the method including the steps of (a) short-circuiting an output-side circuit by using the output-side switching unit of the converter so that a resonant inductor of the LC resonant circuitry is charged with (+) resonant current from the LC resonant circuitry when the resonant current maintains (+) during a process of resonating an input voltage from the input-side switching unit by the LC resonant circuitry; (b) controlling the output-side switching unit so as to release a short circuit of the output-side circuit, inputting the (+) resonant current into the insulation conversion unit from the LC resonant circuitry, subjecting the (+) resonant current to synchronous rectification through the output-side switching unit, and outputting the (+) resonant current as an output voltage; (c) short-circuiting the output-side circuit by using the output-side switching unit of the converter so that the resonant inductor of the LC resonant circuitry is charged with (-) resonant current from the LC resonant circuitry when the resonant current maintains (-) during a process of resonating the input voltage from the input-side switching unit by the LC resonant circuitry; and

(d) controlling the output-side switching unit so as to release a short circuit of the output-side circuit, inputting the (-) resonant current into the insulation conversion unit from the LC resonant circuitry, subjecting the (-) resonant current to synchronous rectification through the output-side switching unit, and outputting the

(-) resonant current as the output voltage.

Brief Description of the Drawings

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 shows a two-stage insulated power supply, which is currently mass-produced;

FIG. 2 is a circuit diagram of a series resonant converter power supply adapted for synchronous rectification; FIG. 3 is a circuit diagram of a series resonant boost converter power supply, which is a modification of a series resonant buck converter power supply when a boost converter is connected to the circuitry of a series resonant converter power supply adapted for synchronous rectification; FIG. 4 is a circuit diagram of a high-efficiency series resonant boost converter adapted for synchronous

rectification according to a preferred embodiment of the present invention;

FIG. 5 is an equivalent circuit diagram of a high- efficiency series resonant boost converter adapted for synchronous rectification according to a preferred embodiment of the present invention; and

FIG. 6 shows simple operation waveforms for understanding of the control of a high-efficiency series resonant boost converter adapted for synchronous rectification according to a preferred embodiment of the present invention.

Best Mode for Carrying Out the Invention

Reference will now be made in detail to the preferred embodiments of the present invention.

FIG. 4 is a circuit diagram of a high-efficiency series resonant boost converter adapted for synchronous rectification according to a preferred embodiment of the present invention.

The inventive series resonant boost converter receives power from an AC power source via a rectification diode and, in order to constitute a full-bridge circuitry, uses switching devices on the input and output sides of a transformer so that the input and output circuits are electrically insulated from each other.

An input power supply 400 supplies the entire circuitry with power (AC or DC power) .

A rectification unit 410 is used when the input power supply 400 supplies AC power. The rectification unit 410 employs bridge diodes Dl, D2, D3, and D4 so as to convert AC power into DC power.

An input-side switching unit Ql, Q2, Q3, and Q4; 420 is adapted to switch in accordance with the polarity of resonant current, which is inputted to an LC resonant circuitry 430 via the input-side switching unit 420 in the case of ZVS (zero-voltage switching) necessary to reduce power loss at an output-side switching unit QIl, Q12, Q13, and Q14; 450.

An input-side capacitor 412 is positioned between the rectification unit 410 and the input-side switching unit 420 and acts as a filter, which reduces high-frequency current of power inputted to the input-side switching unit 420 via the rectification unit 410, for the sake of flattening.

The LC resonant circuitry 430 stores electric energy from the input-side switching unit 420 as magnetic energy in a resonant inductor and as electrostatic energy in a resonant capacitor.

An insulation conversion unit 440 is positioned between the input-side switching unit 420 and the output- side switching unit 450 so as to electrically insulate them from each other. The insulation conversion unit 440

converts power and current inputted from the input-side switching unit 420.

The output-side switching unit 450 is adapted to perform ZVS in order to avoid power loss during switching. In contrast to conventional step-down control methods, the present invention proposes that switches of the output-side switch unit 450 are operated so as to short-circuit the output-side circuit and charge the resonant inductor with resonant current supplied via the input-side switching unit 420 while the output-side circuit is short-circuited. When switches of the output-side switching unit 450 are operated so as to restore the short-circuited output-side circuit, the resonant current, which has accumulated in the LC resonant circuitry 430 during the short circuit, is transmitted to the output power via the insulation conversion unit 440 and the output-side switching unit 450.

FIG. 5 is an equivalent circuit diagram of a series resonant boost converter adapted for synchronous rectification according to a preferred embodiment of the present invention.

In the circuit diagram shown in FIG. 5, the switches of the input-side and output-side switching units shown in FIG. 4 are replaced with MOSFETs, which can be used as synchronous rectifiers because they have low switching loss and exhibit the characteristics of resistors during conduction. Each MOSFET incorporates a body diode, which

controls the slope of voltage increase, and a body capacitor. FIG. 6 shows simple operation waveforms for understanding of the control of a series resonant boost converter adapted for synchronous rectification according to a preferred embodiment of the present invention. The operation waveforms shown in FIG. 6 exhibit different types of operation in 18 sections from tO to tl8, which will now be described respectively. Section 1 (to-tl) Resonant current Ir has maintained (+) before tO, which corresponds to the moment Q12 is turned off. After tO, drain-source voltage Vdsl2 of Q12 increases, while that of Q14 decreases. In section 1, input-side switches Ql and Q4 are turned on so that input-side current flows through them. Section 2 (tl-t2)

The Vdsl2 and Vdsl4 become Vo and 0, respectively, at tl. Then, current begins to flow through DQ14. Although forward voltage drop over a diode occurs in Vdsl4, it is sufficiently smaller than the output voltage to be regarded as OV. Therefore, it can be said that, when Q14 is turned on in section 2, ZVS has occurred with no turn-on switching loss. The Q14 is turned on at t2. In section 2, input-side switches Ql and Q4 are turned on so that input-side current flows through them. Section 3 (t2-t3)

The Q14 is turned on at t2. As a result, energy,

which has been stored in the resonant circuit, is transmitted to the output side. In section 3, the resonant current is (+) , i.e. it flows in the forward direction of DIl and D14. However, current also flows through QIl and Q14, which have been turned on. When the voltage drop across QIl and Q14 is sufficiently smaller than the forward direction voltage drop over the diode, most current flows through QlI and Q14 as synchronous current. The synchronous current continues until Ql and Q4 are turned off at t3, which corresponds to the moment just before the resonant current becomes 0.

Section 4 (t3-t4)

The Ql and Q4 are turned off at t3. Then, Vds3 begins to decrease, while Vds4 begins to increase. The Vds3 and Vds4 become 0 and |Vac|, respectively, at t4. Section 5 (t4-t5)

The Vds3 and Vds4 become 0 and |Vac|, respectively, at t4. Then, the resonant current flows through DQ2 and DQ3 on the primary side. When Q2 and Q3 are turned on in section 5, it can be said that ZVS has occurred. Since the resonant current is (+) , the output-side current flows towards DQIl and DQ14 through QIl and Q12. In the waveform, Q2 and Q3 are turned on at t5.

Section 6 (t5-tβ) The Q2 and Q3 are turned on at t5. Although the resonant current is initially larger than 0, it soon becomes

0 and then goes below 0. When the resonant current is smaller than 0, input-side current flows in the forward direction of DQ2 and DQ3, then in the forward direction of Q2 and Q3. In addition, output-side current flows in the forward direction of DQIl and OQlA, then in the forward direction of QlI and Q14. Considering the fact that energy begins to flow inversely from the output side to the resonant circuit when the resonant current is smaller than 0, QIl must be turned off as soon as possible after the resonant current drops below 0 (at t6) . Section 7 (tβ-t7)

The QIl is turned off at tβ. Then, Vdsll begins to increase, while Vdsl3 begins to decrease. The Vdsll and Vdsl3 become Vo and 0, respectively, at t7. Section 8 (t7-t8)

The Vdsll and Vdsl3 become Vo and 0, respectively, at t7. Then, the resonant current begins to flow through DQ13 and DQ14 on the output side. When Q13 is turned on in section 8, it can be said that ZVS has occurred. In the operation waveform, Q13 is turned on at t8. Section 9 (t8-t9)

The Q13 is turned on at t8. When the voltage across both ends of Q13 is sufficiently smaller than the forward voltage drop across the diode, most resonant current begins to flow towards DQ13 through Q13 and Q14. In section 9, energy flows into the resonant circuit from the input side

and increases the resonant current. The flow energy is directed from the input-side power source to the resonant inductor, as well as from the resonant capacitor to the resonant inductor. Section 9 continues until Q14 is turned off at t9. The sections from tO to t9 correspond to half the entire switching cycle. In sections from t9 to tlδ, the polarity of voltage and current is opposite to that in the sections from tO to t9, and the switching devices are replaced with devices, which face each other in the vertical direction at a pole.

Section 10 (t9-tlθ)

The resonant current has maintained (-) before t9, at which Q14 is turned off. Then, Vdsl4 of Q14 begins to increase, while Vdsl2 of Q12 begins to decrease. In section 10, input-side switches Q2 and Q3 are turned on so that input-side current flows through Q2 and Q3. Section 11 (tlO-tll)

The Vdsl4 and Vdsl2 become Vo and 0, respectively, at tlO. Then, current begins to flow through DQ12. When Q12 is turned on section 11, it can be said that ZVS has occurred with no turn-on switching loss. In the operation waveform, Q12 is turned on at til. In section 11, input- side switches Q2 and Q3 are turned on so that input-side current flows through Q2 and Q3. Section 12 (tll-tl2)

The Q12 is turned on at 11. Then, energy, which has

been stored in the resonant circuit, is transmitted to the output side. In section 12, the current flows in the forward direction of DQ12 and DQ13, because the resonant current is (-) . However, current also flows through Q12 and Q13, which have been turned on. When the voltage drop across Q12 and Q13 is sufficiently smaller than the forward voltage drop across the diode, most current flows through

Q12 and Q13 as synchronous current. The synchronous current continues until Q2 and Q3 are turned off atl2 just before the resonant current becomes 0.

Section 13 (t!2-tl3)

The Q2 and Q3 are turned off at 12. Then, Vds4 begins to decrease, while Vds3 begins to increase. The Vds4 and Vds3 become 0 and |Vac|, respectively, at tl3. Section 14 (t!3-tl4)

The Vds4 and Vds3 become 0 and |Vac|, respectively, at tl3. Then, the resonant current flows through DQl and DQ4 on the input side. When Ql and Q4 are turned on in section 14, it can be said that ZVS has occurred. The output-side current flows towards DQ12 and DQ13 through Q12 and Q13, because the resonant current is (-) . In the waveform, Ql and Q4 are turned on.

Section 15 (t!4-tl5)

The Ql and Q4 are turned on at tl4. Although the resonant current is initially smaller than 0, it soon becomes 0 and then increases above 0. When the resonant

current is larger than 0, input-side current flows in the forward direction of DQl and DQ4, then in the forward direction of Ql and Q4. On the secondary side, current flows in the forward direction of DQ12 and DQ13, then in the forward direction of Q12 and Q13. In addition, energy begins to flows inversely from the output side to the resonant circuit when the resonant current is larger than 0. Therefore, Q13 must be turned off as soon as possible after the resonant current increases above 0 (at tl5) . Section 16 (t!5-tl6)

The Q13 is turned off at tl5. Then, Vdsl3 begins to increase, while Vdsll begins to decrease. The Vdsl3 and Vdsll become Vo and 0, respectively, at tlδ. Section 17 (t!6-tl7) The Vdsl3 and Vdsll become Vo and 0, respectively, at tlβ. Then, the resonant current begins to flow through DQIl and DQ12 on the output side. When QIl is turned on in section 17, it can be said that ZVS has occurred. In the operation waveform, QIl is turned on at tl7. Section 18 (t!7-tl8)

The QIl is turned on at tl7. When the voltage across both ends of QlI is sufficiently smaller than the forward voltage drop across the diode, most resonant current begins to towards DQIl flow through QIl and Q12. In section 18, energy flows into the resonant circuit from the input side and increases the resonant current. The flow energy is

directed from the input-side power source to the resonant inductor, as well as from the resonant capacitor to the resonant inductor. Section 18 continues until Q12 is turned off at tl8. Turning off Q12 at tl8 is exactly timed with turning off Q12 at tθ. This completes a full cycle.

The above-mentioned sections from tO to tl8 are repeated for each switching cycle of the operation waveforms according to the present invention. It can be said, from another point of view, that in section 9 (t8-t9) and section 18 (tl7-tl8) the boost converter switches on. As the proportion of switching-on section to half switching cycle

(tθ-t9 or t9-tl8) increases, the proportion of output voltage to input voltage increases accordingly. In other words, the output voltage increases in proportion to the switching-on section while the input voltage remains the same.

The construction of gate driving circuits for driving the input-side and output-side switching units 420 and 450 can be easily understood by those skilled in the art, and detailed description thereof will be omitted herein.

Industrial Applicability

As can be seen from the foregoing, according to the present invention, the synchronous rectifier of a series resonant converter adapted for synchronous rectification is

utilized as a boost converter so that it can be operated as a series resonant boost converter. This reduces the manufacturing cost of the insulated power supply while improving the conversion ratio. In addition, the fact that all switching devices can perform ZVS increases the power supply's efficiency.

While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment and the drawings, but, on the contrary, it is intended to cover various modifications and variations within the spirit and scope of the appended claims.