Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
1999-11-01
2001-06-19
Ton, My-Trang Nu (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C323S282000
Reexamination Certificate
active
06249156
ABSTRACT:
The present invention relates to an electric circuit, and particularly though not exclusively to a synchronous regulator circuit.
Synchronous regulator circuits are used to provide regulate DC power for laptop computers, distributed power supplies, portable instruments, global positioning systems, etc. Synchronous regulator circuits may be used in a “bucks” arrangement, where an unregulated voltage is stepped down to a lower regulated voltage, or a “boost” arrangement where an unregulated voltage is stepped up to a higher regulated voltage.
In a known conventional synchronous regulator circuit a field effect transistor (MOSFET) is switched on and off by a pulse width modulated signal to provide a regulated step down function of an input voltage, for example an input of 5V is converted to a regulated 3.3V output, This is achieved by applying an appropriate pulse width to a low pass inductor and capacitor which provide the DC output. During a recovery cycle (i.e. when the modulated signal has a low value) the MOSFET is switched off, and current from the inductor flows to earth via a diode. The diode is usually a Shottky diode, to keep the forward voltage drop as low as possible and thus achieve good efficiency.
There is inevitably some loss associated with the use of a diode in this manner, and even a Shottky diode will have a forward drop across it of around 400 mV. This drop of voltage constitutes a loss of power in the circuit, and is particularly significant when the circuit is used to provide high load currents, for example 4A or higher.
The diode may be replaced by a MOSFET working in its reverse direction. Where a MOSFET is used the voltage drop can by substantially reduced if the MOSFET has a low enough resistance between drain and source when it is turned on. The efficiency of the circuit is thus improved, even taking into account the gate charge of the MOSFET. For example, if a MOSFET having an effective drain to source resistance of 20 mOhm (when turned on) is used as a synchronous rectifier, then at 4A it will only have a forward conduction drop of 80 mV as opposed to 400 mV of the Shottky diode. Efficiency is thus substantially improved, which is particularly important for portable and battery operated equipment.
A disadvantage of the use of a second MOSFET in this way is that operation of the second MOSFET must be synchronised with operation of the first MOSFET. If the MOSFETS are not synchronised and are both turned on together, a potentially destructive current will be allowed to flow though the circuit (so called conduction overlap).
Many companies provide dedicated integrated circuits which have a main output for driving the first MOSFET and a synchronous outputs for driving the second MOSFET of a synchronous regulator circuit. It is essential that the timing between the main output and the synchronous output is exactly synchronised, so that it does not allow any conduction overlap between devices (as described above). In the case of integrated circuits, appropriate sequencing is used to inhibit the cross coupling of gates, or a suitable dead time delay is introduced. This ensures that cross conduction cannot happen.
The provision of an integrated circuit having a main output and a synchronous output, is expensive. In order to overcome this problem drive for the second MOSFET must be derived externally, and yet still provide the correct timing sequences, to avoid cross conduction. A circuit of this type is shown in U.S. Pat. No. 5,592,071, issued Jan. 7, 1997, and assigned to Dell Corporation of Austin Tex., USA (the circuit is shown in FIG.
1
).
The circuit comprises a first MOSFET Q
1
and a second MOSFET Q
2
, the source of the first MOSFET Q
1
being connected to a primary inductor of a transformer T
1
, and the gate of the second MOSFET Q
2
being connected to a secondary inductor of the transformer T
1
, via a diode D
1
and resistor
106
. The circuit has a third MOSFET Q
3
, the source of the third MOSFET Q
3
being connected to ground, and the gate of the third MOSFET Q
3
being connected, together with the gate of the first MOSFET Q
1
, to a timer circuit U
1
which provides a pulse width modulated signal. When the modulated signal is high the first Q
1
and third Q
3
MOSFETS are turned on, and current flows through the first MOSFET Q
1
and into the transformer T
1
, charging a tank circuit formed by the primary inductor of the transformer T
1
and a capacitor
104
connected in parallel across an output of the circuit. The third MOSFET Q
3
clamps the gate of three second MOSFET to ground, thereby shutting it off and ensuring that current cannot flow through the second MOSFET Q
2
when the first MOSFET Q
1
is turned on. When the pulse width modulated signal is low the first Q
1
and third Q
3
MOSPETS turned off, causing a flux reversal of the primary and secondary inductors of the transformer T
1
. When this occurs the secondary inductor of the transformer T
1
forward biases the diode D
1
and drives the gate of the second MOSFET Q
2
high. The second MOSFET Q
2
turns on and conducts, providing a free-wheeling circuit through the diode D
1
to ground. Thus, the circuit uses flux reversal of the transformer T
1
, via the secondary inductor of the transformer, to turn on the second MOSFET Q
2
. A feedback circuit having a shunt regulator U
1
is used to regulate operation of the timer circuit U
2
such that the voltage output from the circuit is properly regulated.
The circuit is advantageous because it removes the requirement for a second synchronous pulse width modulated signal to be provided by the timer circuit U
1
.
In addition to the buck regulator circuit shown, U.S. Pat. No. 5,592,071 also describes a boost regulator circuit which operates in the same manner.
The circuits described in U.S. Pat. No. 5,592,071 suffer from the disadvantage that they require three MOSFETS (or other equivalent switches). It is an object of the invention to provide an electric circuit which overcomes this disadvantage.
According to the invention there is provided an electric circuit comprising:
a transformer having first and second magnetically coupled inductors;
a first switch which is connected to the first inductor;
a second switch which is connected to the second inductor so as to be turned on and off by the second inductor; and
a timing circuit for connection to the first switch for turning the first switch on and off;
the second inductor being magnetically couple to the first inductor such that flux reversal in the first inductor which occurs as the first Switch is turned off will induce flux reversal in the second inductor which will thereby turn on the second switch,
wherein the circuit further includes a damping element arranged to damp the operation of the second switch.
The damping element acts to inhibit current flow through the second switch as the first switch is turned on, thereby allowing flux reversal to occur in the first and second inductors. The flux reversal in the second inductor turns off the second switch. The damping element thus safeguards against shoot through current and cross conduction, and also damps any ringing present in the circuit.
Preferably, the damping element is connected to an output of the second switch.
Preferably, the damping element is connected between the output of the second switch and an output of the circuit.
The electric circuit may comprise part of a synchronous regulator circuit, which may be a boost circuit, a buck circuit or a buck-boost circuit.
In a conventional buck circuit, the first switch has an input for connection to a DC voltage source and an output connected to the first inductor of the transformer, and the second switch has an output connected to an output of the first switch. The electric circuit described according to the invention may comprise part of a buck circuit. Where this is the case, the damping element is preferably connected between the output of the second switch and the first inductor.
In a conventional boost circuit, the first switch has an input connected to an output of the firs
Nu Ton My-Trang
Zetex PLC
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