By Michael O'Loughlin
EDN
In certain applications, design requirements may call upon a system's switched-mode power supply to more promptly deliver its output than would the garden-variety power supply. Figure 1 shows such a supply's bootstrap, or start-up, circuit. In a switched-mode power supply's PFC (power-factor-corrected) preregulator, the circuit's PWM (pulse-width modulator), IC1, draws its normal operating power from auxiliary winding L1, wound on boost inductor L2’s magnetic core and diode D1.
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| Figure 1. | In a conventional switched-mode power supply’s bootstrap circuit, trickle-charge resistor RT and capacitor CH supply start-up power to the pulse-width modulator and controller, IC1. |
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Resistor RT and capacitor CH form a trickle-charge circuit that supplies power for bootstrapping into normal operation. In conventional designs, RT comprises a high resistance that delivers just enough current to overcome the standby current and supply a trickle charge to holdup capacitor CH, which stores enough energy to power the PWM circuit until the power converter begins operation. Under normal circumstances, the circuit's slow start-up response poses no problems.
When faster power-on response becomes important, you can reduce the bootstrap time by reconfiguring the start-up shunt regulator (Figure 2). Capacitor CT; shunt-regulator IC D1, a TL431; diode D3; transistor Q1; and resistors RA through RD form the bootstrap circuit. At power application, capacitor CT holds no charge, and the series-pass regulator that Q1 and D1 form determines the voltage at the PWM's power input, VAUX.
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| Figure 2. | In this augmented bootstrap circuit, transistor Q1 delivers a robust initial pulse of current to capacitor CH, ensuring faster start-up and power delivery. |
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At turn-on, the VAUX voltage reaches its peak voltage, VAUX_PEAK, which the ratio of resistors RA and RB determines. Capacitor CT and resistor RC conserve energy by setting the bootstrap circuit's turn-off time and voltage. Resistor RD supplies bias current to D1, the TL431 shunt-regulator IC, and resistor RE keeps transistor Q1 within its safe operating area by limiting its collector current.
To design the circuit, begin by selecting resistors RA and RB to establish the peak charging voltage, as the following equation shows:
where VREF represents the TL431's internal reference voltage. Next, select resistor RC to reduce the shunt-regulated voltage below the nominal VAUX voltage, VAUX_NOM, which the auxiliary winding supplies:
Choose capacitor CT’s value to set the bootstrap time, TBOOT, as follows:
As in Figure 1, diode D2 and auxiliary winding L2 provide normal operating power to IC1.
