Modulators – Pulse or interrupted continuous wave modulator – Pulse width modulator
Reexamination Certificate
1998-07-06
2001-06-12
Kinkead, Arnold (Department: 2817)
Modulators
Pulse or interrupted continuous wave modulator
Pulse width modulator
C332S110000, C327S291000, C327S589000, C326S088000, C326S092000
Reexamination Certificate
active
06246296
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an improved pulse width modulated (PWM) power stage gate drive using a bootstrap capacitor such as can be used in motor controllers.
BACKGROUND OF THE INVENTION
In PWM amplifier power stages utilizing IGBT (insulated gate bipolar transistor) or FET (field effect transistor) power switching devices, the switching transistors are usually connected in pairs in totem pole fashion between the rails of the power source. The output load terminal can be connected to the upper rail through the upper switching transistor or can be connected to the lower rail through the lower switching transistor. The power supplied to the load is controlled according to the pulse width determined by the ON time during each operating cycle. Most motor controllers are of a three phase design and therefore include three pairs of power switching transistors.
Such power stages generally require a floating power supply to bias the upper switching devices into the conductive or ON state. A “bootstrap” capacitor can be employed for this purpose. The bootstrap capacitor is charged while the lower switching transistor is conductive and connects the capacitor to the lower rail. When the lower transistor is OFF and the upper switching transistor is being rendered conductive, the capacitor is level shifted to the upper rail and drives the upper switching transistor into the fully conductive state. With this arrangement a portion of each operating cycle must be reserved for recharging the capacitor and, therefor, the lower switching device must be ON for a minimum portion of each operating cycle regardless of the instantaneous power needs. As a result the duty cycle is limited to about 85% and only about 70% of the available power can be supplied to the load.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved PWM type amplifier with a higher duty cycle than could be supplied by prior conventional designs.
Another object of the invention is to provide an improved PWM amplifier of smaller size and reduced complexity.
With the PWM amplifier according to the invention the amount of time allotted for charging the bootstrap capacitor is substantially reduced and therefore permits a higher maximum duty cycle than could be achieved with conventional designs. The bootstrap capacitor is refreshed only when needed as a function of time or existing storage level. For example, if the operating cycle is at the 50 kHz rate, the refresh rate could be set at 1 kHz with a short refresh time on the order of 3 microseconds. With this arrangement the effective maximum duty cycle would be increased to about 99.7%. Alternatively, the system could measure the charge state of the bootstrap capacitor and refresh the capacitor only when the charge state falls below a predetermined level.
REFERENCES:
patent: 4633106 (1986-12-01), Backes et al.
patent: 4992749 (1991-02-01), Tokumo et al.
Kinkead Arnold
Kollmorgen Corporation
Morgan & Finnegan , LLP
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