Electronic Design

Setup Bypasses The Problems Of Boot-Strap/Bias-Supply Circuits

In some power-supply applications, the pulse-width-modulator (PWM) controller is powered up from an auxiliary winding tapped off the power stages transformer (Fig. 1). This technique reduces power loss and keeps overall efficiency high.

The auxiliary winding, D1 and C1, provides power and hold-up energy for the PWM. Resistor R1 is used to trickle charge C1 off the input voltage. C1 must be sized to hold up the supply voltage (VCC) long enough for the PWM to start switching the power MOSFET. As a result, energy is stored in the power transformer and delivered to VCC of the PWM through the auxiliary winding. This technique is known as boot strapping. But some problems with this circuit at light-load conditions will make it problematic for the power-supply designer.

Under light-load conditions, C1 must supply all of the energy. Generally, C1 has to be large enough to hold up VCC for at least 10 switching cycles. However, under light- and no-load conditions, the current into the PWM (ICC) will discharge VCC to the point where the PWM goes into under-voltage lockout. This makes the output unstable.

Designers might think that they can reduce the size of R1 or increase the size of C1. But shrinking R1 only creates more losses and lowers the overall efficiency. Also, increasing C1 only decreases the startup time of the PWM. Slight modifications to the circuit in Figure 1 can reduce the size of the holdup capacitor and provide power under no-load conditions while maintaining high efficiencies at the full output power of the converter (Fig. 2).

Electrical components C2, D2, R3, R4, R5, and T1 form a series pass regulator that provides power to VCC of the PWM under light- to no-load conditions. Resistor R1 supplies current limiting to protect the series pass transistor (T1). All of these components together form a boot-strap/bias-supply circuit.

The series pass regulator is designed to supply a bias voltage of less than the bias voltage developed by the auxiliary winding. When the auxiliary winding starts to supply voltage to VCC, it produces a voltage that's large enough to back bias the base emitter junction of T1, causing the series pass regulator to turn off. This circuit lets designers exploit the lower losses of powering the PWM with an auxiliary winding, and it supplies energy to the control circuitry at light loads.

To set up the circuit, first set up the shunt regulator voltage. The following equation can be used to calculate the shunt voltage (Vshunt) by knowing what the voltage is from the auxiliary supply (Vaux):

Vshunt = Vaux - 1 V

It's important to note that Vshunt minus Vaux should not exceed the maximum reverse voltage of the base to emitter junction of T1. This information can be found in the transistor's data sheet.

Once you decide on the shunt voltage, you can easily set up the series pass regulator. R3 is typically sized to allow just enough current to bias the shunt regulator to keep your losses at a minimum. Experience has shown that it's good to set this for twice the minimal shunt regulator's bias current (ID2):

R3 = (Vin - Vshunt)/(2 × ID2)

R4 and R5 program the shunt voltage. R4 can be selected by choosing a resistor for R5 and plugging the reference voltage (Vref) into the following equation:

R4 = R5 × (Vshunt - Vref)/Vref

R1 limits the current through T1, protecting it from overcurrent conditions. It can be selected based on the maximum current rating (Imax) and Ohm's Law:

R1 = (Vshunt - Vbe)/Imax

C1 is typically sized for holdup time (Tholdup) and can be sized by knowing Vbias, the under-voltage lockout (UVLO) of the PWM, and the operating current of the PWM (ICC):

C1 = (ICC × Tholdup)/(Vbias - UVLO)

C2 is a holdup and filtering capacitor for most applications. A 1-µF ceramic capacitor will work fine for this electrical component.

Due to efficiency requirements in some applications, power-supply designers may want the bias voltage to run lower than the PWM IC's turn-on threshold. Accomplishing this task would require an extra resistor and diode (Fig. 3). R2 is a trickle-charge resistor implemented to boot strap the PWM. It lets C1 charge up to the PWM's turn-on threshold voltage. Diode D3 is required to ensure that T1's maximum reverse base-to-emitter voltage isn't exceeded. It also is worth mentioning that the shunt voltage must be adjusted to accommodate D3's forward voltage drop.

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