The present disclosure relates to feedback control of a power supply. More particularly, the present disclosure relates to feedback control of power supplies operating in continuous current mode (CCM) and in discontinuous current mode (DCM).
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Most switch-mode power supplies are designed so that their magnetic energy storage elements (e.g. an inductor or transformer) operate in CCM from a full load down to the lowest allowed loading condition. Cost and size limitations of the magnetics often require the inductor or transformer to operate in DCM at lower levels of load. The cut-off level below which the power supply will operate in DCM is typically at 30% load. The feedback compensation for the power circuit operation can be optimized for CCM. Optimizing for CCM means that the power supply will have the fastest dynamic load response when it is operating in CCM. When the power supply enters DCM, the dynamic load response becomes slower. This limitation of optimizing for one mode is inherent when using known feedback control schemes employing ordinary op-amp circuits or other circuits with fixed feedback compensation.
It is becoming a requirement that power supplies provide a fast dynamic load response down to about 5% load. An example of a fast dynamic load response may be maintaining the output voltage over/undershoots within +/−5% of regulation for +/−50% load steps. This means that the feedback compensation of the power supply in DCM must equal or exceed the speed of the compensation in CCM. This is very difficult to do when using fixed feedback compensation because the open loop gain of the power stage is higher when it is in CCM and lower when it is in DCM. In other words, fixed feedback compensation means that the dynamic load response in CCM will be faster than in DCM.
To meet the requirement for dynamic load response under both continuous and discontinuous conduction mode, the feedback compensation must dynamically adapt with the operating mode of the power circuit. In the prior art, this dynamic adaptation was accomplished by first detecting the operating mode and then changing the feedback compensation.
A common way of detecting the operating mode is by monitoring the load current. This requires a current sensing network connected to the load or the magnetic component. Using a current sensing network presents some drawbacks. One drawback is the power loss in the sensing element, since the current sensing element is usually a resistor. Another drawback is the speed limitation of the sensing network. If the sensing network is looking at the current of a magnetic element (an output inductor for instance), the sensed signal needs to be filtered to remove high frequency components. The delay caused by filtering may be significant enough to render the current sense signal unusable to properly control the feedback compensation.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The present disclosure teaches an advantageous feedback control using sensed duty cycle to provide an effective way of timely detecting the operating mode to dynamically adjust the feedback compensation properly.
An example of the present disclosure is a feedback control circuit for use with a power supply. A feedback controller is for providing feedback signals to the power supply in both continuous current mode and discontinuous current mode to maintain output regulation. A duty cycle sensor controller connected to the feedback controller senses a duty cycle of the power supply. The feedback controller optimizes the feedback signal provided to the power supply for continuous current mode or discontinuous current mode based on the sensed duty cycle.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.
The present disclosure provides an effective way of detecting the operating mode of the magnetic element of the power supply for dynamically and timely providing feedback control to a power supply. The present disclosure uses readily obtainable duty cycle information as the basis for detecting the operating mode.
The following discloses an example of the feedback control provided by the present disclosure using a regulated buck converter with an input of 16V and an output of 12V. The maximum load requirement is 100 A. A typical design of the inductor will usually result in a critically discontinuous load of 30% or 30 A. As those skilled in the art will appreciate, the level at which DCM begins is generally dictated by the cost and size tradeoffs for the inductor design. The power supply is required to deliver a dynamic load (step load) of 20 A starting from 2 A. A typical duty cycle v load characteristic is shown in
When the load is decreased and the converter enters DCM, the duty cycle becomes proportional to the square root of the load current. If the load current is normalized to the critical DCM, the equation for the duty cycle becomes:
Dnorm=√{square root over (Inorm)}
If a fixed feedback compensation were used that is optimized for CCM, the output voltage overshoot and undershoot requirements can be met if the dynamic load is in the region of 30 A to 100 A. However, if the load condition is in DCM (for example 2 A to 22 A dynamic load), the fixed feedback compensation results in a slower response because the gain of the power circuit is lower in DCM. Because of the slower response the output voltage of the power circuit likely will exceed the overshoot and undershoot limits.
Therefore, it can be seen that faster feedback compensation is required for DCM. If the fixed feedback compensation is optimized for DCM, the dynamic load response in DCM can be improved; but the feedback loop will tend to become unstable in CCM. This problem may be overcome by switching to the appropriate compensation for each of the two operating modes. If the detection of the operating mode is not fast enough, the problem will not be solved when the dynamic load is crossing the boundary between the continuous and discontinuous modes.
For example, assume the load is at 40 A and then the load is required to step down to 20 A. In such a condition, the power supply will transition from CCM to DCM when passing through a mode boundary at about the 30 A level. The current compensation starts out being optimized for CCM and the power supply is achieving a fast response at the duty cycle for 40 A to 30 A load. As the power supply tries to correct at the duty cycles from a 30 A to 20 A load, the circuit will be too slow unless its compensation is change almost instantly.
To change the feedback compensation almost instantaneously to match the operating mode, the present disclosure uses sensed duty cycle information. From the example above, the initial load was at 40 A and the initial duty cycle is about 75% (from
The equations above show that if the duty cycle is lower than the duty cycle required in the continuous conduction mode (75% for our example), then the operating mode is discontinuous. This information is predetermined and is preferably pre-loaded into the controller 14. This is applicable under steady state conditions and is also true under dynamic load conditions with certain limitations. The dynamic load limitations require that feedback control remains closed loop and does not saturate and that the magnetic components of circuit 12 do not saturate. It will be appreciated that the duty cycle information may be used to determine the mode of operation almost instantaneously by sensing the duty cycle and then adapting the feedback compensation based on the sensed duty cycle information.
When operating in discontinuous mode, it is desirable to have a continuously variable gain or feedback compensation as the duty cycle changes. However, to minimize the computing resource requirements when using digital control (as shown in
Preferably, the feedback controller 14 includes a memory (not shown) for storing a minimum duty cycle for CCM. The controller 14 optimizes the feedback signal for CCM if the sensed duty cycle is equal to or greater than the stored minimum duty cycle. Conversely, the controller 14 optimizes the feedback signal for DCM if the sensed duty cycle is less than the stored minimum duty cycle.
The present disclosure provides a feedback control circuit that has many benefits over the prior art. The circuit 10 improves the load transient response of power supplies operating in both continuous and discontinuous conduction modes. The circuit 10 operates without any current information; thus, eliminating the need for a current sense network and reducing cost and parts count. The feedback control circuit can be implemented in either analog and digital control. For digital control, software implementation allows for an easier debugging process; thus, leading to a shorter design cycle and faster time to market.
Any Buck derived topology that operates in both continuous and discontinuous conduction mode which is required to meet tight dynamic load response may benefit from the present disclosure. The present disclosure can also be applied to Boost and Buck-Boost Topologies or other topologies derived from these two.
The description of the present disclosure is merely exemplary and those skilled in the art will appreciate that variations other than those described will fall within the scope of the present disclosure.