Asymmetric inductors in multi-phase DCDC converters

The present document relates to DC-to-DC converters. In particular, the present document relates to using asymmetric inductors in multi-phase, multi-operation mode-converters.

It is common to use same type of inductors for each phase of multi-phase buck converters; it is also common to use a same type of inductors for each phase to maximize the merit of multi-phases, such as better transient response, smaller ripple and less harmonic noise. However, this may not be an optimum solution in terms of efficiency, if one needs higher efficiency over a wide range from light load to heavy load.

It is commonly known that smaller value inductors give faster transient response, and that larger value inductors have low AC losses, which gives high efficiency at light load condition. Mixing different inductance values, sizes and types is not common, and would not give a benefit unless the buck architecture is adapted to using different inductance values.

Multi-operation modes of DC-to-DC converters include sleep mode often pulse-frequency modulation (PFM) mode and sync mode often pulse-width modulation mode (PWM).

A principal object of the present disclosure is to achieve both high efficiency and fast transient response of a DC-to-DC converter and to optimize the efficiency over the load range of a converter, both in sleep (PFM) and in sync (PWM) mode.

Another principal object of the present disclosure is to optimize DC-to-DC converter efficiency and transient response performance by combining types of inductors appropriately.

A further object of the disclosure is to optimize DC-to-DC converter efficiency and transient response performance for use in multi-phase converters.

A further object of the disclosure is to optimize DC-to-DC converter efficiency and transient response performance for use in in DC-to-DC converters where not all phases are used in sleep mode (for low load currents) or in multi-phase converters which apply phase shedding operation, wherein phase shedding changes the active number of phases according to the output current.

A further object of the disclosure is to optimize DC-to-DC converter efficiency and transient response performance for use of inductors having two different inductances.

Moreover an object of the disclosure is to apply the methods and circuits disclosed for a buck converter.

A further object of the disclosure is to optimize buck converter efficiency and transient response performance for use of inductors having more than two different inductances.

In accordance with the objects of this disclosure a method to achieve both high efficiency and fast transient response of a multi-phase DC-to-DC converter and to optimize the efficiency over the load range of the buck converter has been achieved. The method disclosed comprises the steps of: providing the multi-phase converter, deploying at least two circuit branches to the buck converter, wherein each branch comprises a pulse generator and inductive means, wherein all branches are jointly connected to an output node of the buck converter, wherein all branches are connected in parallel and wherein one or more branches activated together correspond to an operation phase of the buck converter, wherein activation of one or more branches is assigned to a dedicated operation phase which is dependent upon a load current, and configuring an inductance of the inductive means of each branch in order to optimize the efficiency for a range of load current in which the branch is assigned to, wherein the inductance of a branch used in a sleep mode is configured to have a relatively high inductance with low AC losses and the higher the range of the load current assigned to a branch is the lower are the configured inductance of the inductive means and the configured DC loss.

In accordance with the objects of this disclosure a multi-phase DC-DC converter configured both to achieve fast transient response and to optimize efficiency over the load range of a DC-to-DC converter has been disclosed. The buck converter firstly comprises a node for an output voltage and controlling means. Furthermore the buck converter comprises a number of circuit branches connected in parallel, wherein a first terminal of each branch is connected to an output of the error amplifier and a second terminal of each branch is connected to the node of the output voltage, wherein each branch comprises means of generating control signal, switching means and inductive means, wherein the inductive means of each branch are configured to optimize the efficiency of each branch for a range of load currents in which the branch is used, wherein activation of one or more branches is assigned to a dedicated operation phase of the converter which is dependent upon a load current.

The invention is explained below in an exemplary manner with reference to the accompanying drawings, wherein

FIG. 1 shows an example buck converter according the present disclosure.

FIG. 2 shows the measured efficiency of a buck converter with all high inductance inductors, with all low inductance inductors, and then the efficiency of the proposed disclosure.

FIG. 3 shows the simulated load transient response for the three cases shown in FIG. 2 .

FIG. 4 shows a flowchart of a method to achieve both high efficiency and fast transient response of a buck converter and to optimize the efficiency over the load range of the buck converter.

Disclosed are methods and circuits to achieve both high efficiency and fast transient response and to optimize the efficiency over the load range of a buck converter, both in sleep (PFM) and in sync (PWM) mode by combining types of inductors appropriately. In a preferred embodiment the methods and circuits are applied to a multi-phase buck converter. It should be noted that the disclosure can be applied also to multi-phase buck-boost or to multi-phase boost converters.

The buck converter of the preferred embodiment should be designed such that not all phases are used in sleep mode. The phases used in sleep mode are then populated with high inductances and/or low AC loss at light load, i.e. inductors that have very high efficiency at light load condition. This ensures the losses in sleep mode are low, and keeps overall sleep mode efficiency high.

The remaining phases are activated only in sync mode, when a load is present. The lower inductances of the inductors of these phases allow the current to ramp up quickly to meet the load, and then respond quickly to changes in the load. The higher losses in these inductors at light load condition are not significant because these phases are not used in light load. Also, as relative low inductance inductors tends to have lower direct current resistance (DCR) when compared to the higher inductance inductors, the losses at high loads are reduced, so overall efficiency at high loads is actually higher than with the higher inductance inductors.

FIG. 1 shows an example buck converter according the present disclosure operating in PWM mode. In this example the buck converter has three phases (marked 1 , 2 and 3 ). Each phase has a pair of switching devices 10 , 11 . Each pair of switching devices 10 , 11 is connected to a specific inductor 12 , 13 , and 14 . The output devices are typically controlled by a feedback control loop FB that compares the output voltage VOUT with a reference voltage. In a preferred embodiment the reference voltage 15 is an output of a digital-to analog converter (DAC). Ramp signals from a ramp signal generator 16 are used to create a pulse-width modulated control signal to drive the switching devices 10 , 11 . There are different operation modes:

Preferred types of inductors are low-loss inductors as e.g. wire wound inductors, powdered metal-alloy core inductors, low-profile power inductors, in the range of 0.22 uH to 4.7 uH generally.

Operation Mode 1:

In an exemplary operation mode only phase 1 would be operated in sleep. This may be controlled using the feedback loop FB shown or by some other method (such as a low power hysteretic mode). The remaining phases ( 2 and 3 ) would be activated when the load is sufficiently high (sync mode).

The inductor 12 of phase 1 is configured to have higher inductance or to have very low AC loss. This gives very high efficiency in sleep mode at light load condition. The phases 2 and 3 are only activated at higher load current conditions, so the inductors 13 and 14 are also used.

Operation Mode 2

In this exemplary operation mode, the 3-phase buck converter of FIG. 1 can work with phase-shedding. Phase shedding means that when load demands goes down, one or more phases are shut down to improve regulator efficiency. On the other hand when load demand increases, additional phases are activated. Only phase 1 is working when the load is light. At medium load conditions only phases 1 and 2 are working together and all phases 1 - 3 work together only at heavy load condition. One of the main differences between operation mode 1 and operation mode 2 here is that usage of phase 1 state is not limited to sleep mode. Operation of phase 1 either in sync or sleep mode is fine in operation mode 2 .

In case of load conditions between medium load and heavy load only phase 3 may be active or, with further increasing load, phases 3 and 2 may work together.

Similar to operation mode 1 in operation mode 2 , the inductor 12 of phase 1 has been configured to have higher inductance and/or low AC loss. This gives better efficiency at light load current where AC loss is dominant loss.

The inductor 14 of phase 3 could also be configured to have low DC losses and low inductance, regardless of AC loss

A configuration of an optimum inductor 13 for phase 2 depends on the application. If the application needs higher efficiency at heavy load, then inductor 13 for phase 2 would also be similarly configured as the inductor 14 of phase 3 . This gives higher efficiency at heavy load condition, and better transient response.

If a higher efficiency at medium load is important in the application, then phase 2 inductor 13 is configured to have moderate AC loss and DC loss. Thus a higher efficiency peak at medium load condition is achieved, compared to the configuration described above targeting for heavy load conditions.

The plot of FIG. 2 shows the measured efficiency of a buck converter with all high inductance inductors, i.e. 1 μH-curve 21 , with low inductance inductors, i.e. 0.47 μH-curve 22 , and then the efficiency for the inductances of a preferred embodiment of the disclosure, curve 20 . Curve 20 is based on a configuration wherein the sleep mode inductor 12 of phase 1 has an inductance of 1 μH and the inductors 13 and 14 have each an inductance of 0.47 μH. In curve 21 all coils have 1 μH inductance and in curve 22 all coils have 0.47 μH inductance. Curves 20 and 21 are based on a sleeping current limit of 600 mA while curve 22 is based on a sleeping current limit of 900 mA. It is obvious that Curve 20 offers benefits compared with either alternative. It should be noted that the inductance values outlined above are non-limiting examples. Other inductance values are feasible as well, as long as higher inductance values and/or low AC loss inductors are used on the phases that are used in sleep mode. The lower inductance value inductors and/or low DC loss inductors are used on the phases that are only used in higher current operations.

The captures of FIG. 3 below shows the simulated load transient response for the three cases shown in FIG. 2 . The top panel, curve 30 , shows the applied load. The second panel shows the inductor currents, and the third panel shows the transient voltage response at the output. The middle set of curves on the third panel is for the invention detailed here.

Curve 31 represents using all small value inductors (0.47 uH). Curve 33 represents using all large inductors (1 uH) and curve 32 represents a preferred setup (0.47 uH and 1 uH).

Curve 36 represents using all small value inductors (0.47 uH). Curve 34 represents using all large inductors (1 uH) and curve 35 represents the preferred setup (1 uH for sleeping mode and 0.47 uH for the other modes).

FIG. 4 shows a flowchart of a method to achieve both high efficiency and fast transient response of a buck converter and to optimize the efficiency over the load range of the buck converter. A first step 40 depicts a provision of a multi-phase buck converter The next step 41 shows deploying at least two circuit branches to the buck converter, wherein each branch comprises a pulse generator and inductive means, wherein all branches are jointly connected to an output node of the buck converter, wherein all branches are connected in parallel and wherein one or more branches activated together to corresponds to an operation phase of the buck converter, wherein activation of one or more branches is assigned to a dedicated operation phase which is dependent upon a load current. The following step 42 illustrates configuring an inductance of the inductive means of each branch in order to optimize the efficiency for a range of load current in which the branch is assigned to, wherein the inductance of a branch used in a sleep mode is configured to have a relatively high inductance with low AC losses and the higher the range of the load current assigned to a branch is the lower are the configured inductance of the inductive means and the configured DC loss.

In summary, the disclosure teaches efficiently using multi-phase buck converters, i.e. buck converters wherein not all phases are used in sleep mode (for low load currents) or multi-phase buck converters which apply phase shedding operation. Phase shedding changes the active number of phases according to the output current.

From 2 to n inductors having inductance values may be used with each inductor configured to be optimum for the operating conditions of the phase it is attached to.

Higher inductance and/or low AC loss inductors are used with the phases that are used in sleep mode or 1-phase sync operation. The lower inductance inductors and/or low DC loss inductors are used with the phases that are only used in high current operation conditions.

In an embodiment more than two inductors in a multi-phase converter having phase shedding function are deployed. The converter may be configured as follows:

Each phase in the multi-phase converter has an inductor configured to optimize the efficiency for the range of load currents in which that phase is used. For example a buck converter may have 4 phases, the first phase (circuit branch) used only in sleep mode and having a relative high inductance inductor with low AC losses, the second phase used in sync mode at low currents and having a lower inductance inductor with low AC losses, the third and fourth phases used in sync mode at high currents and having a low inductance inductor with low DC losses.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.