Electricity: power supply or regulation systems – Input level responsive
Reexamination Certificate
2001-04-19
2003-06-17
Han, Jessica (Department: 2838)
Electricity: power supply or regulation systems
Input level responsive
C363S041000
Reexamination Certificate
active
06580260
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems for controlling the power applied to a load, and in particular to a method to improve the performance of pulse width modulation (PWM) amplification.
2. Description of Related Art
For certain load control applications, it is desirable to have a high degree of precision in terms of linearity and power transfer efficiency. For instance, photolithographic systems require high resolution when in the scanning mode. Therefore, power transfer must be highly linear for controlling the positioning stages in the photolithographic systems, requiring that a discrete change in the position control signal to result in a proportional discrete change in the output signal for the positioning stages. At other times, photolithographic systems operating in a stepping mode require rapid changes in the positioning of the stages, which demand efficient power transfer to generate large acceleration and deceleration forces.
One of the highly effective methods of power delivery control is the use of pulse modulation amplifiers. They are used to supply drive current to inductive loads, such as linear, voice-coil, and DC motors. A pulse modulation amplifier, such as a PWM amplifier, receives an analog waveform and outputs a series of square wave pulses. The square wave pulse has an amplitude and duration such that the integrated energy of the pulses is equivalent to the energy of the sampled input analog waveform multiplied by a gain factor created by the amplifier. The resulting PWM waveform may be filtered to produce an analog waveform replicating the original input waveform multiplied by the gain factor. The frequency of the desired sine wave is called the system frequency, while the frequency at which the switch operates is called the modulation frequency.
a. Prior Art Open-loop PWM
FIG. 1
is an example of a basic prior art PWM control circuit. The PWM controller
100
converts an analog input level Vs
110
into a variable duty cycle switch drive signal. As higher voltage output Vo
140
is required, the switch
120
is held on for a longer period. The switch
120
is usually both on and off once during each cycle of the switching frequency. As less voltage output is required, the duty cycle or percent of on time is reduced. A transistor operating as a switch dissipates no power when it is off and dissipates very little heat when it is on because of its low on-resistance. Other losses include heat generated in the flyback diode
150
, which is small because the diode conducts only a very small portion of the time.
The inductor
130
stores energy during the on portion of the cycle for filtering. As a result, the load sees little of the modulating frequency but responds to the system frequency, which is significantly below the modulation frequency. With the PWM, the direct unfiltered amplifier output is either near the supply voltage or near zero. Continuously varying filtered output levels are achieved by changing only the duty cycle. For example, a high voltage requires the switch to be on longer than 50% of the time as shown in
FIG. 2
a
; a medium voltage requires the switch to be on around 50% of the time as shown in
FIG. 2
b
; and a low voltage requires the switch to be on less than 50% of the time as shown in
FIG. 2
c.
As the duty cycle or the modulation frequency is increased and the polarity of the second half of the period is switched, the output square wave becomes more reflective of the sinusoidal input as shown in FIG.
3
. The increase in modulation frequency also results in efficiency being quite constant as output power varies. However, there is a practical limit to higher frequency switching because of the limitation of the switching speed of the power switching devices, typically transistors.
b. Design Considerations of PWM
The challenge of designing a pulse width modulator is to get enough dynamic range to deliver the specified output while variables such as output current, input voltage, and temperature fluctuate over wide ranges. If output current remains constant, the average energy into the filter inductor must remain constant. As input voltage rises, the energy delivered to the inductor in a given time must be increased. If the input voltage is constant but output current decreases, less energy must be delivered to the inductor. The only variable the controller has to work with is the pulse width, which must be increased or decreased depending upon the load requirement. Therefore, PWM switching control is highly critical in determining the waveform output.
Furthermore, the design of the PWM has to take into considerations the following desired parameters: low internal losses to provide high operating efficiency, leading to small size and low cost equipment; high signal-to-noise ratio to provide quality power to the load; high modulation frequency to produce a variable frequency sine wave with minimum harmonics to minimize motor heating; and high surge ratings to protect against overcurrent and overvoltage conditions, thus improving reliability.
The main problem to resolve for all high-power amplifier and oscillator equipment is the removal of excess thermal energy produced in active devices, which can include switching resistance, diode forward drops, copper losses, and core losses. The temperature rise of the PWM circuit must be within the allowable limit as prescribed by the manufacturer of each component. The PWM circuit therefore must be designed to withstand worst-case internal power dissipation for considerable lengths of time in relationship to the thermal time constants of the heat sinking hardware. Consequently, the PWM circuit has to have the necessary heat dissipation device to cool itself under worst-case conditions, which include highest supply voltage, lowest load impedance, maximum ambient temperature, and lowest efficiency output level. In the case of reactive loads, maximum voltage-to-current phase angle or lowest power factor must also be addressed. The available cooling methods to remove the thermal generation include natural convection, forced convection, and conduction. If the excess thermal energy is not removed properly, the temperature rise can create circuit failure and/or reduce power delivery efficiency.
The other problem to be resolved is noise, or interference, which can be defined as undesirable electrical signals that distort or interfere with the original or desired signal. Examples of noise sources include thermal noise due to electron movement within the electrical circuits, electromagnetic interference due to electric and magnetic fluxes, and other transients that are often unpredictable. The main techniques used to reduce noise consist of applying shielding around signal wires, increasing the distance between the noise source and signal, decreasing the length that the desired signal must travel, and proper grounding of the entire system.
The ratio of the signal voltage to the noise voltage determines the strength of the signal in relation to the noise. This is called signal-to-noise ratio (SNR) and is important in assessing how well power is being delivered. The higher the SNR, the better the delivery of desired power. PWM amplification system with low SNR may not be suitable for photolithography motor drives and other high performance applications, which may require noise free power.
Further, conventional PWM amplifier systems do not provide drive current in a linear fashion and typically have poor total harmonic distortion (THD) characteristics. The THD and switching transients, which are associated with very high speed rising and falling edges, can cause noise and generate excessive undershoot and overshoot ringing effects. If these voltage spikes were allowed to exist they could cause high stress and possibly destruction of both amplifier and power supply components. To resolve the ringing effects, amplifier must use fast surge suppression to prevent ringing in the output signals.
The design challenges are compounded at higher frequency. As
Han Jessica
Liu & Liu
Nikon Corporation
LandOfFree
PWM feedback control by using dynamic pulse-to-pulse error... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with PWM feedback control by using dynamic pulse-to-pulse error..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and PWM feedback control by using dynamic pulse-to-pulse error... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3125279