Power Electronics

Bootstrap Circuit for Green-Mode Standards

To conserve energy, there has been a big push to lower the standby power in off-line power supplies. Many countries around the world are adopting green-mode standards. A good example of these standards is the European Union (EU) Code of Conduct with its stringent standby power-consumption specifications. In 2001, the input standby power for an off-line converter in Europe was limited to roughly 1 W. In 2007, the standby power requirement will be less than 0.5 W in most applications.

In off-line power supplies, designers have been meeting these specifications with quasi-resonant converters and frequency fold-back techniques to reduce switching losses. In these designs, every possible trick is used to eliminate losses when the supply is in standby mode, because even the power dissipated by the bootstrap circuit could cause the power supply to fail these stringent standby input power requirements. This article reviews a technique to bootstrap a power supply that only dissipates power during startup and makes it easier to meet these new green-mode specifications.

Traditional Bootstrap Circuit

Fig. 1 shows an off-line flyback converter being controlled by a pulse-width modulator (PWM). The input to this converter is a universal input of 85 Vrms (VIN(MIN)) to 265 Vrms (VIN(MAX)). The schematic shows the bias (VAUX) to the PWM controller is supplied by an auxiliary winding off the flyback transformer. Resistor RT and CHOLD form the bootstrap circuit.

The PWM controller will dissipate roughly 0.25 W once the device is active, driving the switching FET. The holdup capacitance (CHOLD) is generally sized to supply energy to the IC during supply startup. In most applications CHOLD is between 50 µF and 100 µF. This example uses the worst-case holdup capacitance of 100 µF.

In this example, the PWM has a turn-on threshold of roughly 9 V and approximately 500 µA of startup current. A designer might use an 82-kΩ resistor as a trickle charge resistor. This allows roughly 1.5 mA to 4.5 mA of trickle charge current to charge up the holdup capacitor. The RT, CHOLD and the PWM controller's startup threshold determine the minimum amount of time needed for startup. The bootstrap circuit in this configuration will turn on in roughly 200 ms to 640 ms. The only problem with this is that the 82-kΩ resistor would dissipate roughly 1.6 W (PRT) at maximum input voltage (VIN(MAX)) and would not pass the EU Code of Conduct in standby mode.

The next approach a designer might take is to allocate a power budget (PLIM) for the trickle charge resistor to pass EU specifications. For this design example, the trickle charge resistor will be selected to dissipate a maximum of 0.1 W. This requires a trickle charge resistance (RT) of roughly 1.4 MΩ. The only problem with this approach is the power supply would take roughly 3 s to 11 s for the power converter to turn on and this is too much time for most applications.

The circuit in Fig. 2 can be configured to provide a fast startup for the off-line power converter while dissipating little to no energy after power up. This makes it easier for the designer to meet the standby power requirements. The circuit requires a fast turn on to operate correctly. Most off-line power supplies have a power switch (S1) that will add input power to the circuit quickly when the switch is turned on.

The circuit forms a timed series pass regulator. Components R1 and C1 form the timing of the bootstrap circuit. Resistors R2, R3 and the shunt regulator D2 set the VAUX voltage during startup. Once the circuit has timed out, it will be turned off, therefore dissipating no power. Diode D1 protects the other electrical components in the circuit from a negative voltage when C1 discharges. Resistor R4 limits the current into Q4, keeping the transistor within its safe operating area. The circuit is not difficult to set up and can be set up with the following equations.

VSHUNT is sized to be just below the auxiliary voltage (VAUX) supplied by the auxiliary winding of the flyback transformer. This will turn off transistor Q1 as soon as the auxiliary winding is powered up, conserving energy.

R2 is calculated by selecting R3 and knowing the internal reference (VREF) of the shunt regulator D2.

Resistor R1 is sized to provide a bias current (IBIAS(D2)) to the shunt regulator. The resistor should be sized to allow at least three to four times the minimum bias current recommended in the shunt regulator's data sheet. This resistor may consist of several resistors in series to meet the high input-voltage requirement of 375 V.

Capacitor C1 is sized on the amount of bootstrap time (tBOOT) the circuit requires for startup. The bootstrap circuit will time out in roughly five RC time constants.

The bootstrap circuit was constructed and tested in an off-line flyback circuit with the components in Fig. 2. The holdup capacitance (CHOLD) was two 47 µF capacitors in parallel. The circuit was constructed in the lab and tested with 375 Vdc applied to the input (Fig. 3). This dc input represents the peak input line voltage of the design.

The test showed that bootstrap circuit applied power to the IC for roughly 50 ms to 100 ms. This is roughly 6 to 10 times faster than the trickle charge resistor technique present in Fig. 1. The bootstrap circuit timed out in roughly 400 ms. This can be observed by trace V1 in the scope plots.

Meeting Stringent Requirements

In off-line power-supply designs, the standby power requirements are getting more stringent, thus tougher to meet. By 2007, the EU Code of Conduct for standby power will drop to less than 0.5 W. Today, frequency fold-back, zero current and zero voltage switching techniques are being used to meet these specifications. Designers need to remove losses anywhere possible to meets these requirements. The bootstrap circuit presented here is faster than traditional methods and also turns itself off after startup. The latter feature removes unwanted power dissipation, making it easier for designers to meet their green-mode specifications.

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