Shunt regulator speeds up power supply startup

Some application designs have higher requirements on the speed of the system's switching power supply to provide voltage output. Figure 1 is a bootstrap circuit, also known as a startup circuit, for such a power supply system. In the power factor correction (PFC) pre-regulator of the switching power supply, the circuit's pulse width modulator (PWM) IC1 draws the power required for normal operation from the auxiliary coil L1 wound around the magnetic core of the boost inductor L2 and the diode D1.

Figure 1: In a traditional switching power supply bootstrap circuit, trickle charge resistor RT and capacitor CH provide startup power to the pulse width modulator and controller IC1

The resistor RT and the capacitor CH together form a trickle-charge circuit, which is responsible for providing power to the bootstrap boost component IC1 for its normal operation. The RT resistor value in the traditional design is usually large, so that after providing the required standby current, the circuit can continue to charge the potential holding capacitor CH with a small current, thereby ensuring that the capacitor can supply enough power to the PWM circuit before the power converter starts operating. Although the startup response speed of this type of circuit is slow, it is not a problem under normal operating conditions.

If the system's startup response speed needs to be accelerated, designers can rearrange the shunt regulator of the startup circuit to speed up the bootstrap boost time (Figure 2). The bootstrap boost circuit is composed of capacitor CT, shunt regulator component D1 (TL431), diode D3, transistor Q1, and resistors from RA to RD. When the power is just turned on, capacitor CT has no charge, and the voltage VAUX at the PWM power input is determined by the series regulator composed of Q1 and D1.

Figure 2: In this improved bootstrap boost circuit, transistor Q1 provides a reliable initial current pulse to capacitor CH, enabling the circuit to start up and provide power output faster.

At power-up, the VAUX voltage rises to a peak voltage VAUX_PEAK, which is determined by the ratio of resistors RA to RB. Capacitor CT and resistor RC set the cutoff time and voltage of the bootstrap boost circuit to conserve power. Resistor RD provides bias current to shunt regulator D1 (TL431), while resistor RE limits the collector current of transistor Q1, ensuring it stays in the safe operating area.

When designing the circuit, first select the resistors RA and RB according to the following formula so that the peak charging voltage meets the requirements:

VREF is the internal reference voltage of TL431. Then select the resistor RC so that the parallel voltage value is lower than the rated VAUX voltage (VAUX_NOMINAL) provided by the auxiliary coil:

Finally, select the capacitor CT and set the bootstrap boost time TBOOT to:

This circuit is shown in Figure 1, where diode D2 and auxiliary coil L2 provide normal operating power to IC1.