Switching circuits using reactive components, such as inductors and capacitors, are increasingly used for converting power. These converters may be characterized by those that use a PWM driven inductor. This inductor has one terminal coupled to a rectified voltage supply line, and another terminal coupled to a PWM drive switch. One or more switches alternate between conduction phases and cut-off phases causing the inductance to absorb energy from the supply during the conduction phase, and release it to a load circuit during the successive cut-off phase.
One of the problems that is encountered in forming these circuits is that of precisely determining the instant in which the switch must be switched on. Often, this switching takes places when the current in the inductor becomes null in order to charge again the inductor after it has delivered to the load all of its stored energy.
This situation is encountered, for example, in power supplies using converters functioning at a high frequency. These converters are used for reasons of space and costs, and are often preferred because they do not require the use of transformers for the mains voltage. Because of their widespread use and importance, in the following description reference will be made to power supplies using DC-DC switching converters for highlighting the addressed technical problem. However, the considerations that will be made hold for any circuit using a PWM driven inductor, that is, the current in the inductor becomes null between two consecutive switchings toward the voltage source.
Many of the current electronic devices are powered by power supplies that are directly connected to the mains voltage, as depicted in FIG. 1. The mains voltage is rectified by a rectifying bridge, and then filtered with a relatively large capacitor CBULK. As may be noticed in FIG. 2, the current absorbed from the mains voltage is discontinuous because it is absorbed only when charging the capacitor CBULK, thus raising the voltage VCbulk.
These power supplies cause a strong harmonic content of the mains current, which reduces the power factor. Enforced “power quality” specifications impose the use of circuits that correct the non-linearity of the current absorbed from the mains voltage by making it almost sinusoidal for remaining below a certain maximum tolerable limit having a harmonic content.
There are many techniques for improving the power factor. Some techniques contemplate the use of passive networks using capacitors and inductors. Other techniques contemplate the use of active circuits for correcting the power factor. In the latter case, which is the case of interest, the power supply has one or more switches that are switched for charging the bulk capacitor during the whole half-wave of the AC network voltage by absorbing current at high frequency and at a level proportional to the instantaneous value of the mains voltage.
As is well known to those skilled in the art, there are different topologies of power factor correction circuits, and each of them has advantages and drawbacks depending on the output power. One of the most used topologies for circuits having an output power less than 80 W is the so-called “Boost” topology with TM (Transition Mode) control, with a fixed turn on time TON and variable frequency.
The power supply of FIG. 3 is formed by a rectifying bridge that rectifies the AC mains voltage, a power factor correction circuit PFC, a bulk capacitor CBULK, and a converter CONVERTER which may be a DC-DC or a DC-AC. The power factor correction circuit PFC, which in the considered case has a boost topology, has an inductor L driven in a PWM mode by a switch SW turned on or off by a driving circuit DRIVER. A clamping diode D prevents an inversion of the current that flows from the voltage source toward the converter.
FIG. 4 depicts the waveform of the current flowing in the inductor L. The switch SW is turned on only after the current in the inductor has become null, and is successively turned off to let the inductor deliver current toward the load. In this way the mean value of the absorbed current is sinusoidal.
According to a transition mode TM control, the switch SW remains in a conduction state for a constant time TON, the value of which depends on the load. The value of the current circulating in the inductor L when this time interval expires is   I  =                    V        IN            L        ·          T      ON      The inductor is turned off and the stored energy: E=½·L·I2 is transferred through the clamping diode D to the bulk capacitor CBULK in the form of a charging current (discharging current of the inductance). The switch is turned on again when energy transfer is completed, that is, after the current in the inductor has become null.
Generally, two alternative techniques are used for detecting the null current condition in the inductor. One technique is connecting in series to the inductor a sensing resistance and monitoring the voltage drop on it. The other technique forms an auxiliary winding magnetically coupled to the inductor L and uses the induced signal on the auxiliary winding for determining the turn on instant of the switch.
Both techniques have the following drawbacks. The first technique implies a power dissipation on the current sensing resistor, and as a consequence, a reduction of the efficiency of the system. The second technique requires the use of a transformer, which consequently increases cost. None of these techniques are amenable to a complete integration on silicon.