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How Switchmode Controllers Work

Ever wondered how switchmode regulator ICs work? Here's everything you need to know but were afraid to ask.

by Nicholas Vinen

ELSEWHERE IN THIS ISSUE, we present the LED Dazzler, a 10W LED driver that uses switchmode regulation to control its output current. During the course of its design, we initially spent quite some time working on circuits based on a common switchmode controller IC, the UC3843 (or TL3843).

There are significant advantages in using a controller IC such as the UC3843. You can use virtually any switching topology such as buck, boost, buck/boost, boost-buck, Cuk, SEPIC etc. The switching frequency, frequency response, current limit and other parameters can be customised to suit the application.

Because you choose the switching devices and their configuration, it is possible to build a regulator that will deliver a lot of current (10A or more) or one which can handle high voltages, rather than being restricted to the specifications of a particular integrated regulator.

But while the UC3843-series datasheet contains all the information necessary to understand its inner workings and thus build a circuit around it, the authors assume that the reader is fully familiar with the operation of switchmode regulators.

Switchmode basics

The main point to consider for any switching regulator is that the output voltage is typically controlled by the duty cycle of a Mosfet. The Mosfet is turned on and off rapidly and its duty cycle varies the output voltage because it determines the ratio of switch on-time to off-time.

Click for larger image
Fig.1: block diagram of a typical switchmode controller IC. It is shown here controlling a boost regulator circuit based on L1, D1 and an N-channel Mosfet.

The majority of switchmode regulators use a fixed frequency pulse width modulation (PWM) scheme. Others use a scheme where either the on-time or off-time is fixed and the other varies. Both methods allow control of the duty cycle but with the latter type, the frequency also varies.

Fig.1 shows the functional block diagram of a typical switchmode controller IC (modelled on the UC3843), used here as part of a boost regulator.

For boost regulators, with the switch off (ie, 0% duty cycle), the output voltage (Vout) is one diode drop below the input voltage (Vin). As the duty cycle increases, so does the output voltage. The practical upper limit depends on the load impedance but is generally around three to four times the input voltage.

In short, when the switch is on, current flows from the input supply through the inductor and Mosfet and then to ground, and this stores energy in the inductor’s magnetic field. During this time, the diode (D1) is reverse biased, so load current is supplied by the output capacitor (C2).

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