Electronic Design

Derive Auxiliary Supplies From A Positive Buck DC-DC Converter

Many systems require a low-power supply in addition to the main supply. A typical example is when an analog front-end amplifier needs ±5 V, while the main digital circuitry requires +5 V only. For reasons of cost, inventory management, electromagnetic compatibility, and so on, a separate —5-V converter may not be appropriate. So, some means must be found to provide extra power rails from the main supply.

Implementing a step-down IC converter's switching action may derive one or more outputs, isolated or non-isolated, quasi-regulated or unregulated. Auxiliary output currents of 10% to 30% of the main output are quite possible.

Step-Down Waveforms
A review of the waveforms found in a working step-down converter will identify the voltage and currents that may be used to generate additional outputs (Fig. 1).

At the LX pin, there's a switching-voltage waveform of amplitude:

Voltage across the main inductor, L1, during the power cycle (LX connected to VIN), is:

Continuous Inductor Current Operation
When the power switch is off, the voltage at the LX connection goes negative, turning on diode D1 to ensure that the inductor current continues to circulate. Continuous operation takes place when the power cycle begins before the circulating current in D1 falls to zero. Relevant waveforms are shown in Figure 2. By knowing the various rms currents and voltages associated with the key components, power dissipation may be calculated as follows:

1. Internal LX switch power dissipation:

2. IC quiescent power dissipation:

3. Schottky diode (D1) power dissipation:

4. Load power dissipation:

where:

RON_SW = data-sheet on-resistance of the internal power switch (VIN to LX)

RLOAD = effective resistance connected at the power-supply output

IQUIESCENT = quiescent current of the control IC with no switching action

IDIODE_RMS = Schottky-diode (D1) forward rms current

VDIODE_FORWARD = forward voltage drop across Schottky-diode D1 at rated current

ILOAD_RMS = load rms current

When additional supplies are derived from the main step down, it's essential that the main output is under load at all times and the main inductor is in continuous conduction throughout the main step-down load range.

Inductor Selection
To ensure adequate energy storage in the inductor, the voltage across the inductor, the operating frequency, and the inductor current ripple are needed to set the value of the main inductor. The maximum duty cycle and minimum input voltage determine the minimum value, as given by:

where:

and:

for continuous operation.

Ripple current is chosen as a percentage of output current and is selected to be 30% for the MAX5035. Note that the ripple current sets the minimum load current before the onset of discontinuous operation. An additional auxiliary supply will increase the peak current requirements of the power switch, requiring care to limit the auxiliary power drawn.

For many applications, the standard setup of 100-µH and 68-µF output-filter values from the evaluation kit will be suitable, and these values are retained for the additional supplies. The MAX5035 features fixed internal type 3 compensation, which imposes certain limitations on the choice of output capacitor. Choose the equivalent series resistance (ESR) so the "zero" frequency occurs between 20 and 40 kHz. See the application section of the MAX5035 data sheet.

Main-Inductor Transformer-Derived Auxiliary Output
Because the primary Schottky-diode voltage drop is relatively constant (300 to 500 mV typically depending on current), and because the controller regulates the output voltage, the inductor voltage drop is also relatively constant during the OFF time of the power switch. By connecting a secondary rectifier and capacitor so that conduction occurs during the flyback period (diode ON), some energy can be tapped off the main inductor. Figure 3 shows two versions of this arrangement. Isolating the auxiliary winding from the main step-down permits flexible connection arrangements. Figure 3a shows the auxiliary output referred to zero volts, and Figure 3b shows the auxiliary output referred to the main positive output. The output voltage of the auxiliary output is given by:

where N1 = primary turns and N2 = secondary turns.

This output is independent of input-voltage changes, as D2 is ON when the internal LX power switch is OFF. Capacitor C7 should be chosen to support the output during the maximum on time of the power switch. The secondary output suffers a 2% to 3% output variation as the forward voltage drop of D1 varies with temperature and load current. Because N1 and N2 of the transformer are dc-isolated from one another, the extra output may be referenced to any dc voltage.

For a given inductor value, the onset of discontinuous current in the main primary loop limits secondary power at the auxiliary output. In other words, D1 must remain in conduction at the end of the flyback period. At the onset of discontinuous operation, conduction through D1 becomes zero and the voltage at LX becomes unclamped, potentially damaging the main step-down IC.

Secondary loading causes a change of primary current at the point of transition between the internal LX switch on to off. This current step, shown in Figure 4, is:

where D = duty cycle, PSEC = secondary power, and VLX = peak voltage excursion at LX.

In principle, there's lots of flexibility when choosing turns ratio. But in practice, the availability of standard 1:1 transformers with suitable inductance and peak current values makes this the most popular choice of turns ratio.

Note how the additional loading produces changes in the primary ripple current. Bold lines identify simplified changes to the main-inductor current shape with an active auxiliary output. The advantages of this technique are:

  • Positive or negative auxiliary output
  • Quasi-regulated auxiliary output
  • Isolated—may be referenced to ground or main positive output
  • Inductance value set by main step-down
  • Off-the-shelf magnetics (1:1 transformer ratio)
  • The disadvantages are:

  • Increased primary ripple current—increases onset of discontinuous current
  • Minimum load required on auxiliary output
  • Minimum load required on main positive output to maintain switching action at LX
  • Charge-Pump-Derived Negative Auxiliary Output
    The LX terminal voltage excursion may be used as a source for a charge pump used to generate an unregulated auxiliary negative output. The additional output is unregulated because the voltage at LX is not isolated from changes of VIN. Figure 5 shows the additional charge-pump components.

    When the power switch closes at the start of the power cycle, current flows into C7 via R6, and it also begins to ramp in inductor L1 (Fig. 6). When D1 conducts on the flyback cycle, the charge on C7 transfers to C8 and the load. R6 is an important addition, as it limits the peak current into C7. Without R6, the power switch's current limit will be exceeded, causing premature termination of the power cycle and even shutdown on protected step-down converters like the MAX5035. The source impedance of the unregulated charge pump due to R6 and C7 is:

    where the duty cycle D =

    and VOUT_MAIN = main step-down output.

    Therefore:

    Identifying the source impedance of the unregulated charge pump lets the designer estimate the charge-pump output voltage under variable load conditions. The open-circuit charge-pump auxiliary output voltage is approximately:

    assuming no spike storage on C8 (+20%).

    The loaded charge-pump auxiliary output voltage with a load resistor is:

    With capacitor values in the range of 1 to 10 µF, R1 will dominate the source impedance. Output ripple is due almost entirely to the ESR of C8. As the charge pump is unregulated, a linear regulator may be connected at the output to supply a regulated negative output. This scheme's advantages include small components and a lower cost than the 1:1 transformer architecture.

    However, the disadvantages are:

  • Unregulated output—needs an additional regulator at output
  • High peak currents for modest auxiliary load currents (approximately 4 × IOUT_AVE)
  • Negative auxiliary output only
  • Ground-referenced only
  • Minimum load required on auxiliary output to prevent spike-storage overvoltage
  • Minimum load required on main positive output to maintain switching action at LX
  • SEPIC Auxiliary Supply
    A negative output also may be obtained from the LX pin by employing a second inductor (L2) that shares the same core, and therefore the same value, as the main step-down inductor. In Figure 7, C5, D2, C6, and L2 form a single-ended primary-inductance converter (SEPIC) topology. The switching signal at LX driving the positive-output step down is also the same level for driving the negative output. During the switch ON period, the voltage across L1 is (VLX

    — V OUT). During the OFF period, it's (V OUT + V DIODE_1). By transformer action (1:1), this voltage is also impressed across L2 and generates —V OUT with D2 and C5. C5 creates the SEPIC connection and improves regulation of what would be a normal flyback auxiliary output with very modest regulation, thanks to the less than perfect coupling of the L1 and L2 windings.

    The coupling capacitor (C5) is chosen to produce a low-voltage ripple across it as a function of auxiliary-load-current duty cycle and clock period.

    For VIN = 15 V, 1% ripple, and a 200-mA output current, T = 8 µs (MAX5035), DMIN = 0.3, and C5MIN = 3.2 µF. (Choose 10 µF for this example). The advantages of this system are:

  • Quasi-regulated output
  • "Clean" inductor current waveform—less noise generation
  • Ripple reduction due to coupled inductors
  • Single magnetic component (off-the-shelf 1:1 transformer)
  • The disadvantages are:

  • Only —VOUT is available
  • Ground-referenced output
  • Although the MAX-5035 was chosen for these examples, the lower-output MAX5033 may also be employed in the same circuits, but at reduced outputs. The following is a summary of the three techniques:

    Flyback auxiliary: For complete independence of the auxiliary output reference, the flyback circuit, which adds a winding to the main step-down inductor along with a Schottky diode and capacitor, is very attractive and comes with modest regulation. With a 1:1 transformer (Cooper Bussmann DRQ125-101 for the MAX5035), the auxiliary output may ±VOUT with respect to ground or the main VOUT. Auxiliary output current may be up to 20% of the main output, but some distortion of the main inductor current is to be expected.

    Charge-pump inverter: This is the lowest-cost option (no additional inductor winding). The charge pump suits low-power outputs only because of the high peak currents and voltages associated with the topology. Open-circuit output is approximately —VIN, reducing as the loading is increased on the auxiliary output. Suggested maximum loading is 5% or less of the main positive output.

    Coupled-inductor SEPIC auxiliary: Not as versatile in grounding arrangements, the coupled-inductor SEPIC topology supplies a regulated —VOUT referenced to ground only. Regulation is better than the flyback approach, and inductor-current waveform distortion is small. Auxiliary output current may be up to 20% of the main output. The coupled inductor aids ripple reduction in the auxiliary output.

    The main positive output has to remain active at all times, and the main step-down inductor must remain continuous at all times. The auxiliary output will demand extra peak current, so this must be taken into account when minimum loading of the main output and maximum loading of the auxiliary output are considered. See the table for a list of component suppliers for the circuits shown in this article.

    COMPONENT SUPPLIERS
    Supplier Component Web Site
    AVX Ceramic capacitors www.avxcorp.com
    Coilcraft Power inductors www.coilcraft.com
    Coiltronics Power inductors www.cooperET.com
    Diodes Inc. Schottky diodes www.diodes.com
    Panasonic Ceramic/aluminum capacitors www.panasonic.com
    Sanyo Ceramic/aluminum capacitors www.sanyo.com
    TDK Ceramic capacitors www.component.tdk.com
    Vishay Diodes, resistors, capacitors www.vishay.com
    ON Semiconductor Schottky diodes www.onsemi.com
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