Digital control improves performance and efficiency

Today’s power supply designers must strive to meet increasingly tough environmental legislation governing standby power and average efficiency. On the other hand, they must also ensure designs comply with relevant technical requirements.

Wall adapters for charging smartphones, for example, must meet revision 1.2 of the USB Battery Charging specification (USB BC 1.2), which sets limits for DC load regulation and transient response. By setting out compliance criteria, this battery-charging specification allows chargers to deliver more power to portable devices thereby enabling faster charging than is possible simply by allowing the device to take power from the USB port without control.

Satisfying both sets of regulations within size, cost and performance limits that are acceptable to consumers can be difficult using traditional analogue control techniques. Digital control eliminates some of the toughest challenges such as compensating analogue feedback loops, and with careful component selection can ensure even better response times and voltage accuracy.

International efficiency standards

Mandatory regulations governing standby power and average efficiency of external power supplies have been in place since 2004, when the California Energy Commission introduced the first such rules in response to rapidly increasing use of devices such as laptop PCs and mobile phones equipped with external power adapters. Similar regulations have since come into force around the world.

Currently, all power supplies sold in the US must comply with the Level IV specifications of the International Energy Efficiency Marking Protocol, while the EU has mandated Level V compliance. In February 2014, the US Department of Energy (DoE) announced its intention to impose mandatory compliance with Level VI, to come into effect on February 10, 2016. Level VI imposes the toughest limits for standby power and average efficiency among any specification yet published.

Whereas the Level IV specification sets a maximum standby-power limit of 0.5W, Level IV reduces this limit to 0.3W for power supplies rated up to 49W. The level VI specification for AC/DC power supplies with output voltage below 6V, which includes USB wall adapters with 5V nominal output voltage, reduces allowable standby power to 0.1W.

Trading transient response

To achieve the extremely low standby power needed to gain product approval in important world markets such as the EU and US, power supply designers tend to restrict the current supply to the control circuitry. Although this approach is effective in saving energy, it can also compromise the power supply’s ability to transition quickly from no-load to full-load.

Technical requirements such as USB BC 1.2 limit the extent to which designers can trade transient response for no-load power savings. The end-user experience, also, must be taken into consideration: excessively slow wake-up can be unacceptable in today’s always-on, always-connected world.

To understand how reducing the current supplied to the controller compromises load-transient response, it is necessary to study the factors that determine the responsiveness of the circuit. Load transient response is, in reality, a measure of the large-signal response of the control loop and combines small-signal stability with large-signal factors such as the ability of the control circuit to slew the outputs of amplifiers and drivers quickly. When the slew-rate capability of the device is poor and the small-signal bandwidth is low, the output responds slowly to a load change.

In general, op-amps or comparators with very low operating currents cannot slew their outputs as fast as those with a larger amount of operating current. Propagation delays also tend to increase as current decreases, particularly when input stages are cascaded to reduce current. The cascaded stages increase the amount of time it takes for the signal to work its way through the circuit.  In the specific case of AC/DC converters, the added complication of the reflected impedance of the output transformer and the characteristics of the parasitic inductance create a complicated analysis of the large-signal response.

External components increase cost and size
The slow response time of low-power controllers can force the designer to rely on external components such as capacitors to take care of changes in load-current demand. This adds unwanted cost and increases size of the power supply.

By focusing on the performance of the control circuitry, independent of the passive components, it is possible to conceive either an analogue or digital solution that will deliver acceptable DC load regulation and transient performance while minimizing current demand.

Figure 1 shows the typical response of power supply output voltage when a step change in load current occurs. The initial change in current causes a sharp drop in the output voltage based on the actual slew rate change of the output current and the Equivalent Series Resistance (ESR) of the output capacitance. The Equivalent Series Inductance (ESL) determines the snap-back characteristic of the initial spike. Selecting a low-ESR/low-ESL bypass capacitor can reduce the spike to almost zero.

Once the output capacitance starts to supply current to the output, the voltage will decay as a function of the output current and total capacitance on the output (dV = (I/C)*dt).  The magnitude of the output voltage undershoot is dependent upon how fast the power supply responds (dt) to the stimulus and starts providing current to the output capacitor and load.  Once the output starts to supply current to the output, the output capacitor will charge up to the nominal output voltage, with a small amount of offset on the output.  That output offset is commonly referred to as load regulation and generally depends upon the gain characteristics of the control loop.  The more gain in the system, the better the output voltage accuracy under load.

The generic transient response analysis of Figure 1 can be compared with the requirements of USB BC 1.2 as described in the table of Figure 2. The USB charger specification sets maximum and minimum voltage limits for voltage undershoot and overshoot due to load transients, and also restricts the maximum duration of the undershoot. In addition, the output offset or load regulation must be within ±0.25V of the nominal 5V output voltage.

Transition to digital control

To ensure satisfactory transient response and load regulation, the flyback converter used in a USB wall charger or other external power supply may employ either analogue or digital control.

Analog control principles are well understood and rely on analog amplifiers that compare the feedback from the output with a reference voltage to generate an error signal that is used to modulate the output stage so as to regulate the output voltage. The analog error amplifier relies upon bias current to perform its duties. Low bias-current, high-performance error amplifiers do exist, but are relatively expensive owing to increased die size. The performance of a conventional analog amplifier degrades significantly if the current supply is restricted as necessary to meet the latest efficiency standards.

A simple digital controller based on a standard PID (Proportional-Integral-Derivative) filter can also suffer performance degradation under light-load conditions. Figure 3 shows the functional blocks of a generic digital controller. The ADC converts the feedback signal to a digital word, which is compared with a digital reference. The PID filter then determines how the digital PWM circuit will modulate the output to regulate the output voltage. Since a standard PID-based digital controller responds relatively slowly to load changes, response time can be poor, just as with an analog controller.

The latest digital controller designs incorporate additional analog or digital circuitry to boost performance of the overall circuit. Although it is possible to take a similar approach with a purely analog controller, complex compensation is required to stabilize multiple control loops. This is more difficult to achieve, and may force an unacceptable trade-off in terms of bandwidth so as to achieve stability.

Taking digital control one step further by monitoring the output voltage and current through an auxiliary winding on the main transformer, instead of using direct feedback from the output voltage, enables tight load regulation with fast transient response. The iW1760 digital controller from Dialog Semiconductor Power Conversion Business Group supports this approach, and uses proprietary digital real-time waveform analysis algorithms to perform cycle-by-cycle monitoring. The device’s digital control block also allows multiple operating modes and simplifies design by removing the need for external compensation components, since compensation can be done in the time domain instead of the frequency domain.

Figure 4 shows the response time of the iW1760 in a 10W USB charging application when a 2A load step is applied. The undershoot time of 6ms, and maximum voltage overshoot/undershoot of 5.3V/4.3V are significantly within the USB BC 1.2 specification. No-load power consumption is less than 50mW, which eases compliance with the latest EU and US DoE efficiency standards. The controller also simplifies compliance with the requirements for minimum average efficiency when the power supply is operating normally.

Design for enhanced response

A similar digital controller, the iW1786, is an example of a device that can be used with a secondary-side companion IC such as the iW671 as shown in Figure 5 to achieve even faster transient response. The companion IC accelerates the converter’s recovery from standby mode by integrating an adaptive voltage position monitor that detects the output voltage undershoot and sends the feedback instantly to the iW1786. This allows the controller output to be enabled more quickly than is possible using primary-side feedback alone, giving an even greater margin compared to the minimum requirements of the battery charging specification. Alternatively, the designer can choose to reduce the amount of bulk capacitance needed to hold up the output during bulk transitions.

This extra device added in the secondary side may appear to negate the advantage of using a smaller output capacitor. However, the iW671 incorporates a synchronous rectification circuit that allows the secondary-side Schottky rectifier diode used in conventional power supplies to be replaced with a more efficient MOSFET, and also implements proprietary digital control technology that eliminates the need for a parallel diode to prevent shoot-through.

Since the voltage drop across the MOSFET is less than the drop across a Schottky diode, power loss is reduced leading to higher overall efficiency. In practice, efficiency greater than 88% can be achieved thereby reducing demand for heatsinking. Overall, improved efficiency and reduced component count allows small form-factor units that are capable of delivering higher output power without overheating.

Figure 6 shows the transient response achieved by combining the iW1786 and iW671. There is a noticeable improvement over the response of the iW1760. In particular, the response time is approximately halved.  This improved speed accounts for the reduced undershoot of 200mV, which is less than half the 700mV undershoot measured with the first circuit.

Like the iW1760, the iW1786 implements multiple digital control loops that enable a combination of very fast response time with small external components while at the same time simplifying stabilization without requiring external compensation components.  An analog circuit capable of achieving comparable dynamic performance is typically larger, more costly and much more difficult to compensate.    

Figure 1. Transient Response Analysis

Figure 2. USB BC 1.2 Voltage Tolerances

Figure 3. Generic digital controller

Figure 4.  iW1760 - Load Transient Response Graph

Figure 5. Digital controller with secondary-side synchronous-rectifier/monitoring IC

Figure 6. iW1786+iW671 Load Transient Response Graph.