1. Field of the Invention
The present invention relates generally to PWM power converters, those having high efficiency in spite of the high operating frequency. More specifically, the present invention relates to high frequency PWM power converters having ultra-low voltage output.
2. Description of the Prior Art
Achieving fast response time and small size by increasing the frequency of operation generally leads to increased switching losses and lower efficiency. Prior art does not provide a satisfactory PWM converter of fixed frequency operation, above 1 MHz, and exhibits poor efficiency under high line light load conditions.
Accordingly, the object of the invention is to provide a PWM converter that is free from the defects encountered in the prior art PWM power converters. Another object of the invention is to provide regulation by the PWM signal in response to fluctuation of operating voltage and variation of load conditions. A further object of the invention is to provide a fixed frequency power converter operating above 1 MHz with substantially reduced switching losses. Other objects, features and advantages will be apparent from the following description taken in conjunction with the accompanying drawings in which like references designate the same elements.
In accordance with the present invention a PTTM power converter is provided which includes a signal input circuit for supplying a pulse width modulated signal, a controlled capacitance proportional to the pulse width of said PWM signal, a PWM to PTTM (Pulse Transition Time Modulation) converting logic, a pulse amplifier connected to the primary of a power transformer having a predetermined leakage and magnetizing inductance, and a first resonant capacitor connected in series with the primary of said power transformer. Synchronous rectification is provided by a pair of transistors driven by a gate drive transformer having a predetermined leakage and magnetizing inductance wherein said magnetizing inductance forms a parallel resonant tank with the gate to source capacitance of said synchronous rectifiers. A second resonant capacitor is connected in series with the primary winding of the gate drive transformer to form a series resonant tank with the leakage inductance of said gate drive transformer.
In order to better understand the present invention a prior art ultra low voltage isolated PWM power converter will be described with reference to
The losses associated with driving the high capacitance of the synchronous rectifiers are also significant in limiting the frequency of operation. It can be shown that a capacitance driven by a reactive energy exchange means will dissipate power on its equivalent series resistance that is directly proportional to the slope of the driving waveform such as is the case with synchronous MOSFET rectifiers that exhibit series equivalent parasitic resistances in the range of 0.5 to 2 ohm. Accordingly,
where P is power dissipation, I is the current in the primary during transition, Rg is the equivalent series gate resistance, t0 is the transition time, and T is the period.
Even if the driving waveform is maintained at the ideal, trapezoidal, the losses associated with the gate capacitances become significant for any reasonable slope selected for a given design above 1 MHz. Slow rise and fall times would introduce yet another loss component associated with synchronous rectification, namely the intrinsic diode conduction.
Next, an example of the PTTM power converter according to the present invention, which is free from the above defects, will be described with reference to
The operation of the PTTM converter is best explained with reference to the waveforms shown in
The transition at the junction of transistors 3a and 3b is purely reactive meaning that the energy available from the magnetizing and leakage inductances of the power transformer is charging controlled capacitance 14 to the DC supply rail. The value of controlled capacitance 14 is proportional to the off time or pulse transition time τ. The corresponding voltage waveform is shown in
The leakage inductance of the power transformer forms a resonant tank with first resonant capacitor 5 which results in a sinusoidal current in the primary winding 4a of the power transformer as shown in
Second resonant capacitor 13 is selected to form a series resonant tank with the leakage inductance of the gate drive transformer. The gate drive voltage amplitude is set up by the equilibrium of the energy delivered in each cycle via the series resonant tank and energy used up by the intrinsic gate resistances of synchronous rectifiers 8a and 8b. Since the current flowing through the synchronous rectifiers 8a and 8b is sinusoidal and in phase with the gate drive signal therefore it is sufficient to turn on the synchronous rectifiers 8a and 8b without loss of the effectiveness of said rectifiers. A suitable high value output capacitor 10 across the load 11 averages the current flowing in said load. The peak value of the fundamental component of a square wave under low line full load condition appearing across the junction of transistors 3a and 3b is
This waveform will become nearly sinusoidal at high line light load having a peak amplitude of VSUPPLY. Therefore, the maximum theoretical ratio of the output voltages of the PTTM converter, from low line high load to high line light load, cannot exceed
Rectifiers 16a and 16b are used to limit the voltage across first resonant capacitor 5 so as to limit the maximum available current to load 11.
According to the present invention a power converter is presented that is operational above 1 MHz at high efficiency using synchronous rectification with substantial gate to source capacitance.
It will be apparent that many modifications and variation could be effected by one skilled in the art without departing from the spirit or scope of the novel concepts of the present invention, so that the spirit and scope of the invention should be determined by the appended claims only.
Number | Date | Country | Kind |
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2481277 | Oct 2004 | CA | national |
Number | Name | Date | Kind |
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4382275 | Glennon | May 1983 | A |
5539630 | Pietkiewicz et al. | Jul 1996 | A |
5625542 | Stemmler et al. | Apr 1997 | A |
Number | Date | Country | |
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20060077694 A1 | Apr 2006 | US |