Resonant switching power converter with adaptive dead time control

Resonant switching power converter with adaptive dead time control

The present invention generally relates to switching power regulator circuits, and more specifically, the present invention relates to a resonant switching power converter in which the dead time between pulses is adaptively controlled.

This application claims the priority of US Provisional Patent Application No. 61/083,717 filed on July 25, 2008 in accordance with 35 USC §119(e).

In a resonant switching power converter, in order to avoid introducing loss and stress in the switching circuit, it is desirable to switch the transistor in the switching circuit when the voltage difference across the source and drain is at a minimum. Since any potential that exists across a switching transistor and any drain-source connection capacitor will cause a waste of energy when the transistor is activated, zero voltage switching (ZVS) control is desired. If the switching circuit switches too early, the transistors and capacitors will be discharged through the switching circuit and waste energy. However, if the switching circuit is switched too late, energy can be wasted by conducting current through the body diode of the transistor back to the power supply rail. ZVS control increases the efficiency of the power supply and also reduces the pressure experienced by the switching transistor, enhancing reliability. In addition, because the switching frequency of the resonant converter is usually within the audio range, transients generated at the switching frequency can cause electromagnetic interference (EMI) and audible noise.

In order to provide ZVS control, the dead time between pulses can be set to a duration that causes the input of the resonant tank to have shifted from close to a power supply rail to another power supply rail (for bipolar pulses), or a completion has been completed. Full cycle (for unipolar pulses). However, because the frequency of the resonant converter is changed to control the inductor current and thus the energy is supplied to the load, the duration of the dead time will not be corrected for all operating conditions. Usually the dead time is set to facilitate the higher power operating conditions (ie, the higher frequency operating conditions) because the loss due to the non-optimal switching time is greater for higher inductor current levels.

Therefore, it is desirable to provide a resonant switching power converter with improved efficiency, reduced pressure, and audio/EMI noise.

The above objective of providing a resonant switching power converter with improved efficiency, reduced pressure and reduced EMI and audible noise is accomplished in a resonant switching power converter and its operating method.

The resonant switching power converter includes a resonant tank circuit, a switching circuit for transferring energy from an input voltage source to the resonant tank circuit, and a transformer for coupling the resonant tank circuit to an output of the switching power converter . A dead time between pulses generated by the switching circuit is adaptively set by a control circuit according to the voltage of the input voltage and an indication of the inductor current passing through the sensing element of the resonance tank circuit.

The indication of the inductor current can be the switching frequency, the current directly or indirectly measured through one of the sensing elements, or other related values such as output load current or input line current of the power supply. The control circuit can calculate the dead time from the indication of the current on a cycle-by-cycle basis, or use a look-up table to set the dead time according to the input voltage and the inductor current value.

The foregoing and other objects, features, and advantages of the present invention will become apparent from the following. More specifically, the description of the preferred embodiments of the present invention is illustrated in the accompanying drawings.

The present invention covers a circuit and method for adaptively controlling the dead time between pulses of a resonant switching power converter to increase the efficiency of the converter and reduce pressure and audible/EMI noise. The dead time is controlled to follow an indication of a value of the input voltage of the converter and an indication of the magnitude of the current flowing through an inductance of the resonant tank used in the resonant switching power converter.

Referring now to FIG. 1, it illustrates a resonant switching power converter circuit according to an embodiment of the present invention. A switching control circuit 10 controls the implementation of a switching circuit by transistors N1 and N2. A series resonant tank circuit is formed by an inductor and a capacitor and is supplied with energy by the switching action of transistors N1 and N2. A transformer T1 couples energy from the resonant tank circuit to a rectifier bridge BR1, which provides a rectified current to charge the output capacitor C3. Output voltage V_OUTA feedback circuit 12 can be maintained at a predetermined voltage by providing a feedback signal to the switching control circuit 10 during operation. Alternatively, the operation can be relative to the output voltage V_OUTFor an open loop, if the load impedance across the output is not expected to change substantially, the output voltage is a particularly suitable design. The switching control circuit 10 may be an integrated circuit integrated in a single die, and may include other elements described in the circuit of FIG. 1, for example, switching transistors N1 and N2 and/or a feedback circuit 12.

In Figure 1, the inductance of the resonant tank circuit is illustrated as an inductor L1 plus the leakage inductance of the main winding of any transformer T1. However, it should be understood that the inductance of the resonant tank circuit can be fully provided by the leakage inductance of the transformer T1 and therefore the inductor L1 will not exist in some embodiments of the present invention. The capacitance of the resonant tank circuit is provided by the total series capacitance of the tank. The total series capacitance includes the parallel combination of capacitors C1 and C2, and the parasitic winding capacitance C as described._TConnect in parallel with any reflective capacitance in the main winding of transformer T1, and the parasitic capacitance of transistors N1 and N2 when transistors N1 and N2 are not activated (C_pN1And C_pN2) Of the parallel combination.

The switching control circuit 10 adaptively controls a dead time between the pulses generated by turning on the transistors N1 and N2. The transistors are activated alternately to provide alternate pulses of opposite polarity. The corresponding power supply voltage +V of the resonant tank circuit formed by L1 and capacitors C1 to C2_INOr -V_INone. If the dead time is not provided between these pulses, when one of the transistors N1 and N2 is activated, the corresponding parasitic capacitance C_pN1And C_pN2The charge of one will be discharged. Similarly, the parasitic capacitance C_pN1And C_pN2The other one will be charged suddenly to about the full input voltage. Placed in the parasitic capacitance C_pN1And C_pN2And when the dead time is not provided, the charge removed from it represents wasted energy and reduces the efficiency of the resonant converter. In addition, charging and discharging parasitic capacitance C_pN1And C_pN2The required high level of the current increases the pressure level in the transistors N1 and N2 and can cause EMI and/or audible noise. By providing a dead time, the voltage V_SAfter a pulse is terminated, it turns to a voltage close to the opposite power supply rail. When the next pulse occurs and one of the transistors N1 or N2 is activated, the voltage drop from the corresponding parasitic capacitor discharges is smaller, increasing the resonance The efficiency of the power converter. This operation is generally known as zero voltage switching (ZVS), and as described above, the dead time is generally set to provide ZVS operation close to the highest frequency of operation of the resonant switching power converter, while the loss is at a higher frequency Bigger. (A greater number of transitions occur per unit time interval, and therefore a greater amount of energy waste will occur.)

In the present invention, the dead time between pulses is adaptively controlled. In particular, in the digital implementation of a resonant power converter according to an embodiment of the present invention, the pulse time is generally determined by a counter, and any dead time provided is also generated from a count value and is Set to comply with the input voltage value and the sensor current I flowing through the sensor L1_LAn indication of the magnitude of, causes an appropriate dead time to be maintained to provide ZVS operation, or the operation of maintaining the voltage at the parasitic capacitance of the activated transistor is significantly reduced from the complete power supply voltage range.

Inductor current I_LThe indication of the magnitude of can be obtained from several sources, including a measuring circuit that directly measures the current of the inductor, an example of which is the capacitor C in the described embodiment_SAnd resistor R_Ssupply. Because when connecting transformer T1 to capacitor C_SThe return current at the node of is connected to capacitors C1, C2 and C_SIf one of the capacitors is relatively small, it is used for capacitor C_S, A small part of the inductor current I_LWill pass through capacitor C_SAnd generate across resistor R_SAnd the inductor current I_LA voltage drop proportional to one. Trans resistor R_SThis voltage can then be used as the inductor current I_LOne directly indicates αI_L, Without significantly affecting the performance of the resonant switching power converter. Resistor R_SCan be incorporated into an integrated circuit including the switching control circuit 10 and internally connected to the power supply voltage -V_IN. In this implementation, the capacitor C_SIs connected to one of the pins of the integrated circuit, the integrated circuit thereby directly receives through the capacitor C_SPart of the inductor current I_LAnd it is used as the inductor current I_LOne of the magnitudes indicates.

In addition to or as an alternative to the sensor current measurement described above, the sensor current I_LThe indication of the magnitude of can be the operating frequency of the resonant converter, because the inductor current I_LThe system is roughly proportional to the operating frequency. For digital control converters, the operating frequency system has generally been specified in the switching control 10 in some digital form, because a splitter or other mechanism is used to generate the gate terminals that will eventually generate the control transistors N1 and N2 The switching frequency of the control signals CA and CB. Similarly, in a current mode resonant converter, an indication of the magnitude of the output current is provided by the self-feedback circuit 12 and used to control the switching frequency of the resonant switching power converter and therefore is also related to the inductor current I_LProportionally. In a voltage mode resonant converter, a current load current sensing circuit can be included to provide the indication of the magnitude of the inductor current, and can exist for other purposes, such as overcurrent protection. Finally, the inductor current can be directly sensed via a series sensing resistor, or a primary inductor winding can be provided to a circuit that obtains the integral of the voltage across the inductor winding, such as an analog low-pass filter.

Because the operating frequency is adjusted by line and load conditions to maintain a specific output voltage or current level, the line voltage is also generally known in the switching control 10 in some form. Therefore, using the frequency and voltage information available inside the switching control, the dead time can be set to provide different values of ZVS operation over the full range of frequencies and the load/line conditions under which the resonant switching power converter of Figure 1 operates.

Referring now to FIG. 2A, it illustrates an adaptive dead time control circuit that can be used in the switching control 10 of FIG. 1 according to an embodiment of the present invention. A calculation block 20A receives at least one input voltage value V_IAnd an indication of the magnitude of the current flowing through the inductor L1, which can be the switching frequency F_S, The measured inductor current indication αI_LOr both. The calculation block 20A calculates an output value dead time number from the input values. The input values are then used to set a dead time counter 22, which determines the difference between the trailing edge of a last pulse and the leading edge of the next pulse. time. The calculation block 20A can be a processing element, which starts from the input voltage value V_IAnd by frequency F_SAnd/or sensor current indication αI_LThe provided sensor current indicates the number of dead time of the calculated output value, or a dedicated digital circuit can be provided as a calculation block 20A.

Referring now to FIG. 2B, it illustrates an adaptive dead time control circuit that can be used in the switching control 10 of FIG. 1 according to another embodiment of the present invention. A look-up table 20B uses at least one input voltage value V_IAnd an indication of the magnitude of the current flowing through the inductor L1 is processed. The indication can be the switching frequency F_S, The measured inductor current indication αI_LOr both. If the look-up table 20B selects and provides an output value dead time number based on the input values, the input values are then used to set the dead time counter 22.

Referring now to FIG. 3, the operation of the resonant switching power converter of FIG. 1 is described. Provide a variable dead time t between the start (high) states of the control signals CA and CB_d, Such as following the input voltage V_IAnd the current value of the inductor is determined by the indication. In these dead time, the voltage V_SConvert to voltage V_ZLOr voltage V_ZHDepends on the inductor current I_Lofpolarity. (When the non-switching transistor starts, the inductor current I_LDischarge the capacitor across the transistor that was previously charged to the inductor. For example, during the dead time, the inductor current I_LObserved in the continuing tendency. ) In addition to the power supply rail for the sake of clarity, the voltage V is drawn_ZLOr voltage V_ZH, And as explained, about 25% of the power supply rails can lead to a waste of parasitic capacitance C_pN1And C_pN2The efficiency of charging and discharging is about 16:1 improved. However, the voltage at the end of the dead time can be accurately set to be different from the zero voltage that will be applied to a power supply rail under the resonance tank, so that parasitic capacitance C does not occur._pN1And C_pN2It discharge. In addition, because the voltage V during the dead time_SWill not exceed the power supply rails, but at some point the reversal is due to the beginning of one of the vibrations in the resonance tank, the dead time can be set by the control circuit to be close to the voltage due to the beginning of the vibration V_SA point in the reverse direction, which is either before or after.

Although the present invention has been explicitly illustrated and described with reference to its preferred embodiments, those skilled in the art should understand that the foregoing and other changes and details in form can be made without departing from the scope and spirit of the present invention.

<p>10. . . Switching control circuit</p><p>12. . . Feedback circuit</p><p>20A. . . Calculation block</p><p>20B. . . Checklist</p><p>twenty two. . . Dead time counter</p><p>BR1. . . Rectifier bridge</p><p>C1. . . Capacitor</p><p>C2. . . Capacitor</p><p>C3. . . Output capacitor</p><p>CA. . . control signal</p><p>CB. . . control signal</p><p>C_pN1. . . Parasitic capacitance</p><p>C_pN2. . . Parasitic capacitance</p><p>C_S. . . Capacitor</p><p>C_T. . . Parasitic winding capacitance</p><p>F_S. . . frequency</p><p>I_L. . . Inductor current</p><p>L1. . . sensor</p><p>N1. . . Transistor</p><p>N2. . . Transistor</p><p>R_S. . . Resistor</p><p>T1. . . transformer</p><p>t_d. . . time</p><p>V_I. . . Input voltage value</p><p>V_OUT. . . The output voltage</p><p>V_S. . . Voltage</p><p>V_ZL. . . Voltage</p><p>V_ZH. . . Voltage</p><p>+V_IN. . . Power supply voltage</p><p>-V_IN. . . Power supply voltage</p><p>αI_L. . . Sensor current indication</p>

Figure 1 is a simplified schematic diagram depicting a resonant switching power converter according to an embodiment of the present invention;

2A and 2B are block diagrams, which describe a dead time control circuit that can be used in the switching control 10 of FIG. 1 according to an embodiment of the present invention; and

FIG. 3 is a timing diagram describing the operation of the switching power converter in FIG. 1.