Linear Post Regulators for DC-DC Converters

The output ripple of a standard isolated dc-dc converter is between 0.5% and 1% of its nominal output voltage. For applications requiring a lower output ripple, linear voltage regulators can reduce a converter's output ripple or noise down to a few millivolts or even microvolts.

Fig. 1 shows a block diagram of a typical, isolated flyback dc-dc converter followed by a low-dropout linear regulator. At the output of the dc-dc converter, C_O1 provides coarse output filtering and energy storage, while L_O and C_O2 form a low-pass filter that performs the fine output filtering.

Clearly, the output of the linear regulator is lower than the output of the converter by at least the dropout voltage of the linear regulator. In Fig. 1, both the flyback converter and the linear regulator have an output adjust terminal. Either terminal can be used to set the required voltages of V_O1, V_O2 for proper operation of the system.

Under steady-state conditions (constant line and load), you can adjust V_O1 to be V_O1 = V_O2 + ΔV, where ΔV is the dropout voltage of the linear regulator. At room temperature, the system will operate fine, but it may run out of regulation over its operating temperature if the temperature coefficient (TC) of the reference voltage in the converter and that of the linear regulator do not track.

Under load transients, the system may also go out of regulation — especially when the output of the converter is set higher to provide the minimum dropout voltage for the regulator. Linear regulators offer very fast transient response to switching load, but the same is not true for all isolated dc-dc converters.

The transient response of a 50% to 100% to 50% load step can range from 100 µs to 300 µs for a typical isolated dc-dc converter, while a linear regulator with the same load transient ranges from 1 µs to 5 µs, even when the load switches from no load to full load. When the response time difference is greater than one order of magnitude between the two devices, try one of the following techniques:

Use a converter designed with a current-mode PWM that offers faster response times of 50 µs to 75 µs.
Use big capacitors at the output of the slow converter.
Increase the output voltage of the converter, which will increase the input voltage to the linear regulator, but reduce the overall efficiency.
Use a nonisolated switching regulator in place of a dc-dc converter if isolation is not required between input and output. Today, switching regulators can offer response times of a few nanoseconds even at a full load step.
Use a converter with an onboard linear regulator, such as Beta Dyne's series of low-noise 10-W and 15-W dc-dc converters. (For more information, refer to U.S. Patent 5,777,519: High-Efficiency Power Amplifier.)

As described before, the interface of a dc-dc converter and a linear regulator isn't as straightforward as it may first appear. Previous generation converters with linear regulators were designed with a dc chopper at the front end. The linear regulator was used at the output for line and load regulation, and it dissipated a lot of power at the high-line input level. The input voltage range was only ±10% of V_IN nominal, and efficiency was 60% to 70% at nominal line and much lower at high line, which lowered the converters' power density, operating temperature and reliability.

Converters designed with PWM or PFM at the front end offer higher efficiency, wide input-voltage range, relatively high output ripple and higher cost. In Beta Dyne's low-noise series of converters, a single voltage reference is employed for both the converter and regulator to eliminate the potential voltage reference TC mismatch. For the load transient mismatch, the loop compensation of the converter is optimized and the dropout voltage is set for worst-case over/undershoot of the converter (i.e., no load to full load step).

As seen in Fig. 2, the output of the linear regulator (waveform No. 4) is constant even under a no load-to-full load step. The output of the converter (Fig. 3, part B) takes 2 ms to return to within 1% of V_O1 and has ±160-mV overshoot. The dropout voltage is set for 220 mV or V_O1; in Fig. 1, it's set for 5.220 mV. Therefore, the actual dropout voltage for this regulator is 220 - 160 = 60 mV at 2 A (see Fig. 2, waveform No. 3). Even with 220 mV as ΔV, the power dissipated at the P-channel MOSFET is 0.44 W and the regulator's efficiency is (P_O/P_IN) * 100 = (10 / 10.44) * 100 = 95.8%.

Note that the transient response of V_O1 in Fig. 1 or waveform No. 2 in Fig. 2 becomes waveform No. 5 in Fig. 2 when a 50% load step is used with 50% constant load (50% full load to 100% full load to 50% full load). Under this loading condition, you could have set ΔV for 60 mV and improved the regulator's efficiency by 3% (10 / 10.12 * 100 = 98.8%).

To maintain a constant efficiency even when the customer needs to set the output of the converter at different voltages, the preset dropout voltage must be constant over the V_OUT adjust range while one terminal is used to adjust the outputs of both the converter and regulator.

Using the techniques previously described, Beta Dyne's low-noise series of converters eliminate all potential problems associated with the interfacing of a dc-dc converter with a linear regulator, while maximizing efficiency and minimizing noise. Nevertheless, the internal linear regulators in the low-noise 10-W series are designed with discrete components that reduce the power density of converters and increase their cost. (For more on regulator issues, see “Designing Efficient Linear Regulators” on page 33.)

As seen in Figs. 2 and 3, the linear regulator of a 10-W, 5-V at 2-A converter generates 2 mV to 5 mV of output ripple but also reduces the output noise by about 12 dB. The linear regulator acts as a low-pass filter but won't attenuate any common-mode noise between the converter's input and output. Notice the snubbers in the experimental converter were removed and that no high-frequency ceramic capacitors were installed across the converter's output terminals.

When a 6.8-µF capacitor is added across the output of the linear regulator, the noise floor drops by 10 dB (versus Fig. 3, part c, the spectra of the output without the 6.8-µF capacitor).

An Audio Application

The techniques used to interface a dc-dc converter with a linear regulator may be particularly beneficial in audio applications. For example, much greater power can be saved if a smart power supply is designed to power an audio power amplifier. Assume for a moment that a 60-W power amplifier is used to drive an 8-W speaker and a dual ±24-V power supply is needed for ±V_CC.

In Fig. 4, the volume control pot is used to adjust the power output. For 12-V_OUT, P_OUT = V²/R = 18 W. The power dissipated in the amplifier is 18 W; thus, 30% of the available power is wasted in the power amplifier, which would require cooling either through forced convection or a large heat sink.

The same amplifier will operate without a heat sink if the smart power supply was designed to deliver V_O + ΔV minimum. A smart power supply is designed to follow the needed output voltage as shown in Fig. 4. If we set the output of the power supply to be V_OUT + 2V, the dissipated power is 2 * I_O = 2 * 1.5 A = 3 W, a reduction in the power dissipation by a factor of six.

Some say it isn't possible to have an adjustable power source with 10-kHz to 20-kHz bandwidth. Actually, it's not even needed. A 10-Hz to 100-Hz bandwidth is sufficient because the smart power supply adjusts itself for a dc output. A peak detector or RMS converter can be used to set the required dc output. The feasibility of this adjustable power source is demonstrated by Beta Dyne's upcoming low-noise 35-W adjustable dc-dc converter series, which features linear regulators on the converter outputs. Fig. 5 shows the output of this converter. When the common output pin is used for a ground reference, a dual output supply is created. Because of the input-to-output isolation, any output terminals can be used as ground reference resulting in a 10-V to 100-V, ±50-V, or -10-V to -100-V adjustable output. The flat portion around 0 V indicates the output control circuit saturates at 2 V.

Designing Efficient Linear Regulators

Linear regulators employing a P-channel MOSFET as the pass transistor offer the best efficiency (see the figure). However, they cannot be used for high power or low-output voltages. MOSFETs with low R_DS(ON) have high parasitic capacitance, such as C_GS, C_DG and C_DS. High capacitance increases the transient response time of the regulator and requires a low-impedance source for gate drive.

The gate threshold voltage of the MOSFET sets the minimum output voltage of the regulator. Low-threshold MOSFETs, together with rail-to-rail op amps, will make it possible to have a 3.3-V output regulator with 5 A to 10 A of output current. The C_DS (drain to source) capacitance will couple high-frequency noise from input to output, but most linear regulators require large output capacitance for stability and improvement of transient response. The high capacitance output will eliminate any noise coupling to the output capacitor.

Meanwhile, the parasitic drain to source diode will pass any load dump charges from the output of the linear regulator to the output of the dc-dc converter, which may destroy both the output capacitors of the converter and the regulator. In this case, a high-power Zener for overvoltage protection at the output is highly recommended.

BiCMOS technology offers the advantages of both bipolar and CMOS technologies and can be applied to develop linear regulators to operate below 1 V at high output currents.

You can use the same techniques to design efficient negative linear regulators or a combination of a positive and negative regulator known as a dual tracking regulator, where the negative output tracks the positive output. These regulators are used with dual (or bipolar) ac-dc or dc-dc power supplies.

Low-output voltage regulators have their own unique design problems, and high-voltage linear regulators up to 1 kV present unique design challenges as well. High-voltage linear regulators not only require high-voltage transistors, but also op amps and voltage references that must operate in the micro power region. In addition, different design techniques for short-circuit protection must be used together with thermal protection, overvoltage protection, etc. Even though designing high-voltage dc-dc and linear regulators is challenging, it can be done.

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