7.1 The Cuk Converter

In spite of the many advantages of the Cuk converter, a couple of significant disadvantages exist which prevent its widespread use.

• The converter is difficult to stabilize. Complex compensation circuitry is often needed to make the converter operate properly. This compensation also tends to slow down the response of the converter, which inhibits the PWM dimming capability of the converter (essential for LEDs).

• An output current controlled boost-buck converter tends to have an uncontrolled and undamped resonance due to an L- C pair ( L1 and C1). The resonance of L1 and C1 leads to excessive voltages across the capacitor, which can damage the circuit.

The damping of L1 and C1 can easily be achieved by adding a damping R-C circuit across C1. However, the problem of compensating the circuit so that it is stable is more complex.

The Supertex HV9930 solves the problem of compensation and achieving a fast PWM dimming response by using hysteretic current mode control. This uses fast comparators to control a MOSFET gate by setting upper and lower limits, which ensures fast response and accurate current levels. However, a simple hysteretic current mode control would not work, as the converter would not be able to start up. To overcome this problem, the HV9930 has two hysteretic current mode controllers one for the input current and another for the output current.

During start-up, the input hysteretic controller dominates and...