DC to DC converter method and circuitry

Details DC to DC converter method and circuitry DC to DC converter method and circuitry

2002-03-07

2003-09-16

Berhane, Adolf D. (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S283000

Reexamination Certificate

active

06621256

ABSTRACT:

BACKGROUND
Power supplies for computers, personal digital assistants, cellular phones and other hand held mobile electronic devices and systems have exacting demands. Two types of converters are used to meet these demands. One type is a pulse width modulated (PWM) converter and the other is a hysteretic (ripple) converter. A typical single mode DC-to-DC converter
8
with either a PWM or hysteretic controller is shown in
FIG. 1
a
. The converter
8
has a PWM controller
12
or a hysteretic controller
14
. The output of the controller drives the gates of output switches, typically upper and lower mosfet power transistors
16
,
18
. The mosfets are connected together at a switching node
20
. At high output current levels, the PWM controller with a synchronous rectifier provides efficient and controllable output regulation. For low output currents, the efficiency of the DC-to-DC converter operating in a fixed frequency PWM mode gets lower because PWM switching losses become dominant. One the other hand, the hysteretic converter is efficient for low output currents but is not efficient for high output currents.
Demands on a system can change from tens of amps to milliamps in as short a time as a few microseconds. In order to address the variable and frequently inconsistent current requirements, many DC-to-DC converters, especially those used in mobile systems, include both a pulse width modulation (PWM) converter and a hysteretic converter. Such dual mode controllers provide a high efficiency over a wide range of load current levels.
A typical dual mode converter
10
is shown in
FIG. 1
b
. The converter
10
has a PWM controller
12
and a hysteretic controller
14
. The output of the controller drives the gates of upper and lower mosfet power transistors
16
,
18
. The mosfets are connected together at a switching node
20
. The switching node
20
is connected to an inductor
22
that is connected to a parallel network comprising an output capacitor
23
and a load as represented by resistor
24
. A sense resistor
26
is connected in series with the inductor
22
. The voltage across the sense resistor is coupled to the controller
10
to provide data on the load current. A comparator
13
in the controller receives the voltage signal from the sense resistor
26
, compares it to a reference value indicative of a critical current, and operates a switch to switch the controller between the PWM modulator
12
and the hysteretic controller
14
when the sensed current falls below the threshold value of the reference input to the comparator.
At high output current levels, the PWM controller with a synchronous rectifier provides efficient and controllable output regulation. As the load current gets lower, the efficiency of the DC-to-DC converter operating in a fixed frequency PWM mode gets lower because PWM switching losses become dominant. A simple hysteretic (ripple) controller improves the converter efficiency for light loads. The integrated circuit senses the load current and, when load current falls below a minimum threshold, it invokes the hysteretic (ripple) controller and disables the PWM controller. When the load current increases above the minimum threshold, the PWM controller resumes control. In this way high efficiency is maintained over a wide range of load currents.
The optimal transition point for the threshold current usually lies at the current level where the inductor current becomes “critical.” Critical current is a value of load current for which the total energy stored in the inductor
22
is delivered to the load each cycle. At load currents below the critical value, the inductor current must go through zero and reverse direction at some point in the cycle. When the inductor current changes direction, energy is taken from the output filter capacitor
23
due to the bidirectional conductivity of the synchronous rectifier, lower fet
18
. To maintain the output in regulation more energy will be delivered to the filter capacitor
23
at the next operating cycle. Unless the controller is switched to the hysteretic controller
14
, the converter efficiency dramatically degrades. Power and energy are wasted. In mobile systems that rely on battery power, the overall life of the system is likewise reduced.
To prevent the energy losses when operating at sub-critical currents, diode-like conduction is required of the lower mosfet
18
. This assures discontinuous inductor current operation. Operating the converter
10
in the discontinuous conduction mode under fixed PWM mode control creates its own challenges because the small-signal loop becomes broken, closed-loop gain increases and the converter easily becomes unstable. This leads to the conclusion that hysteretic mode is preferred for safe, stable and efficient operation at sub-critical current.
In order to select the PWM or the hysteretic mode of operation, the controller
10
senses the load current or any current in the circuit proportional to the load current and compares the sensed load current to a reference. If the load current is higher than the reference, the PWM mode of operation is activated. Otherwise, the converter
10
operates in the hysteretic mode. This widely used approach depends on the tolerance of the currently sensing circuitry. As the output voltages of DC-to-DC converters for modern computer applications are getting lower and the output currents are getting higher and vary widely over short periods of time, it is becoming very difficult to measure the current precisely and efficiently. This leads to increased uncertainty of the switch over point and, therefore, to unpredictable operation of the whole converter.
SUMMARY
The invention is a new DC-to-DC converter circuit and a method for DC to DC conversion. The circuit includes a pulse width modulator controller and a hysteretic controller. Both controllers convert an input first voltage into an output second voltage during a series of repeated switching cycles. The circuit has a mode selection circuit for selecting one of the two controllers in accordance with the current demand of a load coupled through an inductor to the second voltage. The mode selection switch has a comparator for comparing the second voltage to a reference voltage (ground) for sensing the polarity of the second voltage at the end of each switching cycle. The polarity of the output voltage at the end of the switching cycle is a measure of the state of the inductor. If the inductor is in continuous operation and the load current is above the critical current, then the polarity of the switch node will be positive. If the inductor is in discontinuous operation, then the polarity of the switch node will be negative.
One or more counters are coupled to the comparator and to the mode selection switch. The counters record the polarity of the second voltage at the end of each switching cycle and keep that data for a given number of cycles. If the polarity of the switch node does not change, then the controller remains in whichever mode (PWM or hysteretic) that it has been operating in. By using counters, the invention avoids premature switching for a single change in polarity. Such changes may occur for spurious reasons that are not related to enduring load conditions. As such, the counters maintain the mode selection switch in its current mode so long as the polarity of the second voltage at the end of each cycle does not change. However, when the polarity changes and the changes endure for more than n switching cycles, then the counters operate the mode selection switch to change the mode of operation to the other mode. If the converter was operating in the PWM mode, then it is switched to the hysteretic mode and vice versa.
The DC-to-DC converter circuit operates to switch the mode selection switch to the hysteretic mode controller when the polarity of the second voltage for n number of cycles is positive and to switch the mode selection switch to the pulse width modulator controller when the polarity of the second voltage for n number of cycles is negative. There are sep

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