Operating a power converter at optimal efficiency

Details Operating a power converter at optimal efficiency Operating a power converter at optimal efficiency

2000-10-04

2001-10-16

Sterrett, Jeffrey (Department: 2838)

Electric power conversion systems

Current conversion

With condition responsive means to control the output...

C363S021040, C363S021120, C323S222000, C323S282000

Reexamination Certificate

active

06304473

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains generally to the field of power conversion and more particularly to a power supply control systems.
BACKGROUND OF THE INVENTION
Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Pulse width modulated (PWM) switching power supplies offer both compactness and efficiency in a number of different topologies which can be placed in two main categories: isolated switching power supplies and non-isolated switching power supplies. In a non-isolated switching power supply, such as a buck (reducing voltage) or boost (increasing voltage) switching power supply, the power output is not isolated from the power input. Isolated power supplies, such as a flyback or forward switching power supplies, have a power output that is isolated from the power input through a transformer.
In either type of power converter, however, typical control systems use a pulse-width-modulator to control the duty cycle of the power switch(es) within the converter. Consider, for example, the flyback switching power supply of FIG.
1
. The power converter includes a power switch Q
1
(typically a field effect transistor (FET)) coupled to the primary of a power transformer T
1
and a diode D
1
and capacitor C
1
coupled to the secondary of the power transformer T
1
. The control system for controlling the power converter includes a PWM controller
105
to provide the signal to turn on switch Q
1
and a feedback circuit
110
coupled to the PWM controller
105
. The feedback circuit
110
receives an output power level sense circuit that varies in time with changes in the output power level. An oscillator (not shown) included in the PWM controller
105
sets the operating frequency while a pulse-width modulator adjusts the duty cycle of the power switch Q
1
at the set operating frequency in response to sensing, for example, an output voltage, Vout. The frequency of the oscillator is relatively low, in the range of 50 KHz. The relationship between the input voltage, V
in
, and V
out
for the flyback converter illustrated in
FIG. 1
may be approximated as
V
O
=(
V
IN
*N
S
/N
P
)*
D
/(1−
D
); and
D
=(
T−toff
)/
T
; and
N
P
—number of turns on the primary winding
N
S
—number of turns on the secondary winding
where ‘D’ is duty cycle, T is the switching period, and t
off
is the off time of the power switch Q
1
.
Thus, in the flyback converter of
FIG. 1
, the off time, t
off
(and hence also the on time, t
on
) of the power switch Q
1
defines a power cycle, or power pulse, which is reflected in the value of V
out
through the above equation. Similarly, the output voltage of a forward power converter can be determined using the equation:
V
O
=(
V
IN
*N
S
/N
P
)*
D
In any case, the power pulse is thus a regulated power pulse because its characteristics have a direct relationship on the output voltage. This relationship between the characteristics of a single power cycle (or pulse) and the output voltage is generic to prior art PWM switching power supplies, regardless of whether the PWM switching power supply is direct coupled or transformer coupled. Thus, a single power cycle (or pulse) in these prior art PWM switching power supplies may be denoted as an “intelligent” power cycle or pulse because of its effect on the output voltage.
FIG. 2
provides another example of a prior art PWM based power converter control system. In this case, the power converter is illustrated generally as the power stage
205
including a switching transistor Q
1
. The control system is indicated as controller
210
including the PWM controller
105
and feedback circuit
110
. The feedback signal line is shown in this case to be a current sense on the output of power transistor Q
1
and a connection to Vout of the power converter input to a summing circuit
215
. As illustrated, this power converter PWM based control system may be used with any converter topology, whether isolated or non-isolated power converter configuration.
With this control system approach, the pulse widths of the pulses vary widely as input line voltage and output load conditions vary. Optimum system performance is achieved only at a single operating point (line and load condition), where the power pulse width and/or pulse frequency is well matched to the particular power conversion stage. Furthermore, because power pulses are closely coupled to output regulation, optimization over a wide operating range with only PWM control is difficult to achieve without degrading output regulation performance. Thus, there is a need in the art of power converters for a more versatile control system approach that can maintain optimal power converter performance and maintain high efficiency over a broad range of load and line conditions.
SUMMARY OF THE INVENTION
In accordance with a general aspect of the present invention, a power converter control system is provided, which combines a constant frequency, constant duty cycle switched mode control system, hereinafter referred to generally as pulse train regulation, described in detail in related U.S. patent application Ser. No. 09/585,928, with a preferred optimization techniques in order to control the output level of the power converter, while maintaining optimal performance for other power converter parameters. By way of non-limiting examples, a preferred power converter control system according to the present invention includes features such as quasi-resonant mode control, discontinuous mode control, and/or power factor correction.
Pulse train regulation is a control technique that makes it possible to control the output value (e.g., output voltage) of a switching power converter by controlling the rate of constant frequency, constant duty cycle switching pulses. In a preferred embodiment, the pulse train regulation is provided by a pulse rate controller, which regulates the number of pulses of power appearing over time at the output of the power converter by controlling the number of pulses of a continuous pulse train output from a pulse generator for activating a power switch. The continuous pulse train of the pulse generator preferably operates at a high frequency, for example, 1 MHz. The pulse rate controller may control the number of pulses of the pulse train that occur at the power switch by using a gating function implemented with, for example, an AND gate. The pulse rate controller determines the rate of pulses that are sent to the power switch based on output level (e.g., voltage) conditions. The number of pulses that reach the power switch will vary over line and load conditions as determined by the pulse rate controller. Unlike a PWM control technique, power pulse duty cycle and frequency as generated by the pulse generator of the pulse train control system are uncoupled from the output level conditions and are gated to maintain a desired output power level (e.g., voltage).
In accordance with the present invention, pulse train optimization provides further control techniques, which adjust, for example, the turn on time, and/or frequency of the pulses in the pulse train to optimize the performance of the power converter. In a preferred embodiment, a pulse train regulation control system includes a control system having a pulse optimizer to adjust the characteristics of the pulses output from a pulse generator. In this manner, pulse train pulses from the pulse generator are preconditioned in order to achieve optimum power supply performance, while the pulse rate controller still maintains output regulation by controlling which of the preconditioned pulses from the pulse train are sent to the power stage.
One example of a control system with pulse train optimization provides quasi-resonant operation. Resonant and quasi-resonant power converter operation improves operational efficiency, system reliability, and reduces EMI emissions of the power converter. The lower the voltage across the power switch at the time it is turned on, the better the performance of the power converter. The closer t

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