1. Field of the Invention
The present invention relates to a negative voltage detection circuit, and more particularly to a negative voltage detection circuit for a synchronous rectifier MOSFET.
2. Description of the Related Art
During a positive half-cycle of an AC input signal D, an input synchronous signal C of the synchronous control circuit 100 is at a logic high level, and a current flows from the drain to the source of the power transistor 120, as indicated by an arrow I. The synchronous control circuit 100 determines the logic level of the output control signal A according to the input synchronous signal C and a feedback voltage signal B. Due to an extremely high input impedance of the synchronous control circuit 100, the current passing through a resistor 107 for noise reduction is small enough to be ignored. Therefore, the feedback voltage signal B can be considered the same as an output voltage VP of the source 120c of the power transistor 120.
For the discontinuous current mode (DCM), only a small current flows through the power transistor 120 near the end of the positive half-cycle of the input signal D, which results in a small output negative voltage value of the source 120c of the power transistor 120 due to the small turn-on resistance of the power transistor 120. On one hand, if the synchronous control circuit 100 fails to turn off the power transistor 120 by the output control signal A in time, a reverse current flowing from the source to the drain of the power transistor 120 will occur and discharge the capacitor. This will adversely result in a voltage drop of the output voltage VP of the synchronous rectifier MOSFET circuit 10. On the other hand, if the power transistor 120 is turned off before the current I induced by the input signal D during the positive-half cycle completely flows out, the power-saving efficiency will be greatly degraded.
In view of the above, it is crucial to turn off the power transistor 120 in time to maintain normal operation of the synchronous rectifier MOSFET circuit 10 and to obtain a high power saving efficiency. This will be achieved upon a precise detection mechanism for the value of the output voltage VP.
An objective of the present invention is to provide a negative voltage detection circuit for a synchronous rectifier MOSFET, capable of precisely detecting a small negative voltage.
The negative voltage detection circuit provided by the present invention comprises a reference current source, a first circuit, a second circuit, and a comparator, wherein the reference current source, the first circuit, and the second circuit form a current mirror structure.
The reference current source comprises a first transistor, a second transistor, an operational amplifier and a resistor. The input voltage of an input end of the operational amplifier is a bandgap type reference voltage, and a stable voltage source is provided to so that the reference current source provides a stable reference current. The output end of the operational amplifier is connected to the gate of the first transistor. The first transistor, the second transistor, and the resistor are connected in series, wherein the resistor together with the operational amplifier generates the reference current.
The first circuit and the second circuit respectively comprise an operational transistor and a load transistor circuit connected in series. The load transistor circuits of the first and the second circuit are both formed by a plurality of transistors connected in parallel. The number of the load transistors of the first circuit connected in parallel is different from that of the second circuit, such that the first circuit and the second circuit respectively mirror a first current and a second current different from each other based on the reference current source.
The gate of the operational transistor of the first circuit is connected to a voltage detection point through a resistor for blocking noises, the drain of which is grounded, and the source is electrically connected to an input end of the comparator, wherein a first voltage is supplied by the input end of the comparator. The gate and drain of the operational transistor of the second circuit are grounded, and the source thereof is electrically connected to another input end of the comparator, wherein a second voltage is supplied by the input end of the comparator.
When the detection voltage of the voltage detection point approaches from a negative value to 0 V and is equal to a predetermined value, the logic level of the output end of the comparator changes.
The invention will be described according to the appended drawings in which:
The reference current source 210 comprises a first transistor 211 (an NMOS type operational transistor), a second transistor 213 (a PMOS type load transistor), an operational amplifier 215, and a resistor 217. The first transistor 211, the second transistor 213, and the resistor 217 are connected in series. The input voltage at the positive input end of the operational amplifier 215 is a bandgap type reference voltage Vref which does not change along with the external environmental temperature so that the reference current source 210 generates a stable reference current Iref. The negative input end of the operational amplifier 215, the source of the first transistor 211, and the resistor 217 are electrically connected together. The value of the current passing through the resistor 217 equals the reference voltage Vref divided by the resistance value of the resistor 217. The reference current Iref can be regarded as the current passing through the resistor 217.
The first circuit 220 comprises an operational transistor 221 (PMOS type) and a load transistor circuit 223 which are connected in series. The load transistor circuit 223 is formed by M load transistors 225 (PMOS type) connected in parallel. Each of the load transistors 225 is a PMOS transistor and the same as the second transistor 213 of the reference current source 210. Therefore, the first circuit 220 mirrors a first current I1 based upon the reference current source 210, and the first current I1 is approximately M times the reference current Iref.
The second circuit 230 comprises an operational transistor 231 (PMOS type) and a load transistor circuit 233 which are connected in series. The load transistor circuit 233 is formed by N load transistors 225 connected in parallel. Each of the load transistors 225 is a PMOS transistor and the same as the second transistor 213 of the reference current source 210. Therefore, the second circuit 230 mirrors a second current I2 from the reference current source 210, and the second current I2 is approximately N times the reference current Iref, wherein M is larger than N so that the first current I1 is larger than the second current I2.
The gate of the operational transistor 221 is connected to receive the output voltage VP of the source 120c of the power transistor 120 shown in
The gate and drain of the operational transistor 231 are grounded, and the source thereof is electrically connected to the positive input end of the comparator 240. The voltage V2 at the positive input end of the comparator 240 is the voltage |Vgs2| between the gate and the source of the operational transistor 231.
First Current I1=M×Iref=K1(Vgs1−Vt1)2 (1)
Second Current I2=N×Iref=K2(Vgs2−Vt2)2 (2)
where K1 and K2 are conductance parameters of the operational transistors 221 and 231, and Vt1 and Vt2 are pinch-off voltages thereof. As the operational transistors 221 and 231 are of the same transistor type, K1=K2, Vt1=Vt2. It can be deduced from formulas (1) and (2) that
Therefore, the value Q can be selected according to the difference between the first current I1 and the second current I2, even if Q is a negative value close to 0. Moreover, the value Q is independent from the resistor 250 and not sensitive to the variation of Iref.
The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.
Number | Date | Country | Kind |
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096121267 | Jun 2007 | TW | national |