Many electrically powered devices and components thereof require power of a particular substantially constant voltage, referred to as a direct current (DC) voltage. Even if the device uses a battery for providing power, a power converter is often required to supply the particular voltage(s) required. Similarly, a power converter is required for charging of rechargeable batteries. Accordingly, power converters of many designs and topologies have been developed for a wide variety of applications.
Among known designs of power converters, resonant switching power converters have become popular due to their ability to limit switching losses and electrical stresses during operation as well as providing very high efficiency and reduced electromagnetic interference (EMI) noise compared with other types of power converters. Among resonant power converters, so-called LLC resonant converters are becoming increasingly attractive because of their flexibility of application, simplicity, efficiency, the simplicity of their control and their ability to provide over-current protection and deliver a range of voltages that may be well above the input voltage.
Typically, an LLC resonant converter will comprise a pair of switching transistors operated in a complementary fashion and a resonant circuit comprising a capacitor and two inductors; one of which generally comprises the magnetizing inductance of a winding of a transformer through which power is output to a rectifier, filter and/or regulator to provide power to an electrically powered device. An LLC resonant converter typically operates at a switching frequency near the resonant frequency, f0, of the LLC circuit for highest efficiency. As an electrical load is increased and more power must be delivered, simple sensing and feedback of the output voltage to a voltage controlled oscillator (VCO) can be arranged to reduce the switching frequency and increase the voltage gain to automatically compensate for the increased required power and thus provide good voltage regulation over a wide range of current. By the same token, particular conditions of voltage, current or switching frequency can be sensed and the VCO can be controlled to increase the switching frequency to reduce gain of the power converter and thus provide a degree of over-current protection in a very simple and robust manner.
However, resonant converters inherently have a transient response at start-up when power is applied to them, either initially or after an interruption of operation. Such transient behavior generally involves substantial electrical stress that can damage the switches or other components and may continue for a substantial number of switching cycles, possibly in an oscillatory manner, until the resonant behavior assumes a steady state. The term “soft start-up” is generally applied to any power converter start-up process where the electrical stress is limited such that the power converter is not damaged by the start-up transients. Since generally unpredictable start-up transients are a characteristic of resonant converters, some soft start-up provisions are required.
To reduce the electrical stress during start-up, an LLC must be initially operated at a switching frequency above the resonant frequency but a suitably high frequency is difficult to predict. If the start-up frequency is too low, even if above the resonant frequency, large currents and voltages appear in the resonant circuit. If the frequency is too high, the switching frequency will generally decrease quickly during the soft start-up process and will merely delay the onset of the electrical stress. Further, as the switching frequency rapidly decreases, the current stress will generally trigger the over-current protection (OCP) and the switching frequency will be made to increase to limit the stress before decreasing smoothly to a steady state operating point to complete the start-up process. If the switching frequency is controlled to decrease slowly, the soft start-up process will be similarly prolonged.