How does current mode control work? Referring to Fig. 1, the block SW represents the power switching stage of a PWM converter (see previously posted article.) HC is the inner current control loop which converts the sensed current into a voltage signal that represents the magnitude of each current pulse generated by the switching stage. In this way we can achieve pulse-by-pulse current control. Pulse-by-pulse control allows very rapid response to over-current or fault conditions. The current sensed is usually the current ramp that enters and energizes the inductor. This energy will later be dumped into the filter capacitor C which filters output voltage ripple. The LC filter generates two poles in the transfer function which need to be dealt with by the feedback control and compensation system. One of the advantages of the current control method is that it tends to nullify one of the inductor poles so that the outside voltage control loop can have simplified compensation for the error amplifier A (see the URL listed below for more information on operational amplifiers and feedback.)
The outer voltage control loop senses a ratio of the output voltage, using the voltage divider R1 and R2. This voltage is compared to the voltage reference applied as one of the inputs to the error amplifier. The error amplifier may have compensation elements Z1 and Z2 in Fig. 1, but with the current mode control inner loop, the compensation can usually be simplified to a simple RC net for Z2 and just a resistor for Z1. The output of the voltage error amplifier is summed together with the current sense signal to control the operation of the converter. The two control loops provide excellent control of the system when properly designed.
One caution is that for higher duty cycles, a sub-harmonic or low frequency oscillation may occur. Fortunately there is a cure for this problem called slope compensation that adds some slope to the current sense ramp to prevent the instability. Excellent articles on this subject are provided by Venable ™, Ridley Engineering ™, and Texas Instruments ™.
There are also good current mode control integrated circuits that combine circuits for both the current mode control and voltage mode control loops. These IC’s are available from Texas Instruments ™, Linear Technology ™, and other companies. You may want to look-up the datasheet for an IC made by Texas Instruments ™ by the part number UC1842. This family of IC’s is the grand-daddy of current mode control IC’s and is still available. It also available from a number of manufacturers and distributors.
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