Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific voltage responsive fault sensor
Reexamination Certificate
1999-06-01
2001-02-06
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific voltage responsive fault sensor
C361S091100, C361S093100, C361S093700, C361S079000, C361S083000, C361S018000, C323S285000
Reexamination Certificate
active
06185082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to boost power converters (“boost converters”), and more particularly to a protection circuit of a boost converter which provides under-voltage, over-voltage, and over-current protection.
2. Background of the Invention
A boost converter is typically used to produce a higher regulated voltage from a lower unregulated voltage, including power factor correction (PFC) and DC-to-DC boost regulation. Several publications explain the operation of boost converters, such as: (a) Keith H. Billings, “Switchmode Power Supply Handbook,” McGraw-Hill Book Co., p2.162-p2.166; and (b) Abraham I. Pressman, “Switching Power Supply Design,” McGraw-Hill Book Co., p24-p35.
An example of a conventional boost converter
10
is shown in FIG.
1
. The boost converter
10
includes a transistor Q
1
, inductor L
1
, diode D
1
, capacitor C
1
and a PWM controller
12
. The inductor L
1
is connected in series with VIN and the transistor Q
1
. When the transistor Q
1
is on for a time Ton, diode D
1
is reverse biased and an energy (0.5*L
1
*Ion
2
) is stored in L
1
, where Ion=VIN*Ton/L
1
. During the transistor Q
1
off time, the stored energy of L
1
feeds the capacitor C
1
through diode D
1
. Thus, controlling the Ton in the PWM controller
12
regulates the output voltage Vo.
Most power supply specifications require protection against the following common occurrences: (1) shorts to ground or overload currents, which can destroy the switching element and series-pass element; (2) output over-voltage, which can destroy voltage-sensitive loads; and (3) input under-voltage, which can not deliver sufficient power to the output and potentially will overheat the switching element. For the boost converter
10
shown in
FIG. 1
, when the input voltage VIN is higher than the specified output voltage Vo, the PWM controller
12
and the transistor Q
1
will stop the boost switching due to feedback, but this high input voltage may unrestrainedly go into the output. Further, if the output of the boost converter
10
is shorted to ground, an unlimited current might flow from input to the output through the diode D
1
.
To address these concerns, protection switches, such as the conventional protection switch shown in
FIG. 2
, have been implemented. In the configuration shown in
FIG. 2
, a MOSFET Qp serves as a protection switch. The drain of the MOSFET Qp is connected to the positive output of the boost converter
10
. The gate of the MOSFET Qp is connected to a gate driver
14
for driving the MOSFET Qp on, and the source of the MOSFET Qp is coupled to the load through a current sense resistor Rs. The output current IO flowing through the resistor Rs will produce a voltage drop Vrs. The resistors Ra, Rb, Rc and Rd form a voltage divider network for the voltage drop Vrs for providing an over-current signal to a control circuit
16
. When the input voltage is higher than a specific level and/or the output is shorted to ground, the control circuit
16
will shut off the MOSFET Qp through the gate driver
14
to protect the boost converter
10
and the load.
Since a typical N-channel MOSFET produces lower loss, as compared to a P-channel device, the MOSFET Qp is generally an N-channel device. However, the drawback of using an N-channel device is that a specific gate driver must be applied to ensure that the MOSFET is fully turned-on. To turn-on the MOSFET, the gate-to-source voltage Vgs must be higher than a threshold voltage. If a lower on-state resistance (drain-to-source), Rds-on, of the MOSFET is needed, more Vgs voltage should be applied to the MOSFET. Although many methods can be used to drive the MOSFET, such as level-shift, charge-pump and floating source, the utilization of such methods increases the complexity of the circuit.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a protection circuit in a boost converter for input under-voltage protection and output over-voltage and over-current protection. An advantage of the present invention is the arrangement of a circuit, which drives a protection switch of an N-channel MOSFET without requiring a specific gate driver, and, furthermore, provides a soft-start to the load. Another advantage of the present invention is a current sensing design that senses the current by using the Rds-on of the MOSFET, thereby improving the efficiency of the circuit.
In accordance with an embodiment of the present invention, the protection circuit includes a MOSFET that is connected in series between the ground of the boost converter and the ground of the load. Associated with a comparator, the MOSFET can be shut off when the input of the boost converter is in an under-voltage or over-voltage state. When the MOSFET is on, the current sensing circuit detects the output current of the boost converter by sensing the voltage drop across the MOSFET. Current limiting is achieved by restricting the PWM switching of the boost converter when a specified limit is reached. Nevertheless, if the output current is outside of a control range, the MOSFET can be shut off to stop the output whenever the absolute limit is reached. Since the Rds-on of the MOSFET is a function of its operating temperature, temperature compensation circuitry is employed to offset the variation of the Rds-on for the current limiting. A time delay circuit is applied to delimitate the duty cycle of overload which prevents the boost converter, the MOSFET, and a transient voltage suppressor from over-stress damage. The transient voltage suppressor is used to protect the load from power line fluctuations, spark discharge, and lighting surge.
Further scope and applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
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patent: 5422593 (1995-06-01), Fujihira
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patent: 5754419 (1998-05-01), Ho
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patent: 5844440 (1998-12-01), Lenk et al.
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“Switchmode Power Supply Handbook”, McGraw-Hill Book Co., Keith H. Billings, pp. 162-166 (No Date).
“Switching Power Supply Design,” McGraw-Hill Cook Co., Abraham I. Pressman, pp. 24-35 (No Date).
Sherry Michael J.
System General Corporation
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