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PSUs 101: A Detailed Look Into Power Supplies

Switching Controllers And Isolators

The main purpose of the regulator, or switching controller, is to maintain a regulated output voltage and control the amount of energy delivered to the system. This is accomplished by adjusting the duty cycle of the main switchers when PWM is used. The duty cycle can be adjusted from 0 to 100 percent, but its range is usually smaller. With great approximation, we could say that output voltage is the product of input voltage and duty cycle (Vout = Vin × duty cycle).

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The PWM controller uses a voltage reference as the PSU's "ideal" reference to which the output voltage is constantly compared. In the PWM IC, there is a voltage error amplifier that performs a high-gain voltage comparison between the output voltage and the aforementioned reference. According to this comparison, an error voltage-to-pulse width converter sets the duty cycle in response to the level of the error voltage from the voltage error amplifier.

In addition to determining the duty cycle of the main switchers, the PWM controllers usually incorporate other functionality, such as a soft-start circuit that triggers the PSU to start smoothly reducing large inrush currents, an over-current amplifier that protects the PSU from overloading and undervoltage lockout that prevents the PSU from starting when the voltage within the control IC is too small to drive the main switchers.  

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In PSUs that use an LLC resonant converter and a Champion CM6901 controller, the main switchers operate in PWM mode only at light loads, while at higher loads, FM mode is used. With Infineon solutions (ICE2HS01G), the resonant controller operates in PFM, in which the repetition rate (or frequency) of fixed-duration pulses varies. That differs from PWM, where the width of square pulses varies at a constant frequency. In order to avoid high-frequency switching, the aforementioned controller employs Missing Cycle Mode with light loads and Burst Mode with no load.

In order for the voltage feedback from the DC outputs to reach the voltage error amplifier of the switching controller, isolated feedback is needed. There are two methods of electrical isolation: optical (optocoupler or optoisolator) and magnetic (transformer). In modern PSUs, optoisolators are commonly used, and the voltage error amplifier is placed in the secondary side of the optoisolator.