STCMB1 datasheet(18/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

18/59

DocID028958 Rev 4

7.4

VCC pin

This pin is the supply voltage of the IC. An electrolytic capacitor of at least 22 µF (typ. value)

must be connected between the pin and GND in order to sustain the voltage during the

start-up phase and supply the IC, before the self-supply circuitry from the half-bridge

transformer deliver enough voltage. Place also a ceramic capacitor (0.1 µF typ.) in parallel

to the electrolytic capacitor to bypass the high frequency noise to GND and supply the

STCMB1 device with a proper clean voltage.

The VCC voltage is used also to supply the MOSFET gate drivers, not having a dedicated

clamping, supplying the STCMB1 from the auxiliary (self-supply) winding from the resonant

transformer may need a small BJT voltage regulator to keep the supply voltage regulated

and avoid the aging of the gate oxide due to an excessive VCC, or limit transients that could

damage the IC as in case of dead shorts.

A typical VCC operating voltage is 12 - 13 V.

7.5

PFC section

The STCMB1 device implements a transition mode (TM) PFC section that uses a

proprietary “Constant On-Time” (COT) control methodology termed “Enhanced Constant

On-Time” (ECOT). Like the COT, this control mode does not require a sinusoidal input

reference, thereby reducing the system cost and external component count, while providing

a distortion of the input current at the same level as current mode control.

It includes also a complete set of protections: the cycle-by-cycle overcurrent (OCP), output

overvoltage (OVP), feedback failure (FFP), boost inductor saturation and inrush current

detection both at the start-up and after mains sags or missing cycles.

The output voltage (Vout) is controlled by mean of a transconductance error amplifier and

an accurate internal voltage reference; “Zero Current Detection” (ZCD) by sensing the

voltage on the auxiliary winding on the PFC inductor enables the TM operation.

The current sense on the return path allows continuous monitoring of the inductor current.

THD optimization is implemented too.

A “two-level” line voltage feedforward function helps to reduce the changes in the system

gain due to changes in input voltage.

7.6

PFC: enhanced constant on-time control with THD optimizer

TM-operated PFCs are characterized by good efficiency even if the large input current ripple

requires bigger EMI filters.

The COT is a common control technique where the turn-on of the power transistor in each

cycle is commanded when the current through the boost inductor falls to zero and the turn-

off is commanded when the duration of the on-time has reached the value programmed by

the control loop. It is well-known that if this on-time is constant or even slowly varying during

each line cycle, the converter draws an ideally sinusoidal current from the line.

The combination of the TM operation and COT control is a very popular solution used in

several PFC controllers for output power up to 300 W. The block diagram and key

waveforms of the traditional approach are shown in Figure 7 and Figure 8 respectively.

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