Smart Power with CIP Hybrid Power Controllers

The need for Smart in Power

Electronic gadgetry continues to invade all aspects of our lives, but while most of the marketing messaging (advertising) is trying to focus our attention on a few expensive items, mostly mobile devices and large entertainment systems, like the tip of an iceberg they are actually eclipsed by the number of embedded devices (the submerged part) that make all things around us smarter.

Since all things electronics need, ahem ... electrons, be it extracted from a battery, harvested from a solar panel or more commonly milked from the grid, it follows that down inside the belly of every such object there has to be a power supply circuit.

The number is in the billions and all regulating agencies around the world have realised by now how the efficiency of these power supply circuits matters.

Moreover, the manufacturers have realised how a more efficient power supply design can help reduce an application size, weight, heat dissipation and eventually cost or simply make it more appealing to a public that is more and more sensitive to environmental issues.

Unfortunately, traditional (switch mode) power supply design rules for efficiency are not sufficient to meet the demands of the smart devices that increasingly surround us. They are in the vast majority digital objects, they change modes abruptly, shifting from consuming microAmperes in standby to several Amperes when awake, and possibly more states in between. In short, smart objects demand smart power.

Beyond the Analog vs. Digital Dichotomy

The answer to some of those challenges has been an increased effort to design (full) digital power supply circuits, where a Digital Signal Processor (DSP) is used to replace the traditional power supply (analog) control loop with an algorithm straight out of a control theory math book. DSPs have gotten cheaper, smaller and more integrated over the years, so that the cost and complexity of the resulting circuit has become manageable for smaller and smaller applications (today approaching the threshold of 300W and below in some circumstances). What has remained a struggle though is the software complexity incurred. The kind of expertise required for such applications is extremely rare and is almost completely disjoint from the traditional analog power domain.

For small, low cost designs (the vast majority) semiconductor manufacturers have been integrating some of the smarts in large portfolios of semi-custom controllers (ASICs) supporting efficiently switch mode topologies carefully trying to balance cost and complexity for the most common use cases.

Such ASICs are more easily understood by the traditional power supply designers but increasingly fall short of the demands for flexibility of modern embedded applications. In fact, it is increasingly common today to see smart power solutions implemented using a small microcontroller to bend one or more power ASICs to the needs of the application.

Hybrid Power Controllers

A small microcontroller can be effectively used to augment an otherwise analog switching regulator design without requiring the kind of performance and complexity of a full digital solution. This has been done for decades, you will find evidence of their use in most electronic ballasts, battery chargers and appliances in our homes. These are so called Hybrid Power solutions, in that they employ a traditional analog control loop but use the flexibility of a digital device to manipulate it. But the industry has, so far, struggled to make the next natural evolutionary step that is microcontroller integration.

While there is certainly a huge variety of ASICs for switch-mode power supply and a healthy market that supports even more customization, adding the complexity of even the smallest microcontroller to those designs increases the number of permutations/options astronomically. Adding a new switch (FET) or changing the parameters of a comparator in an ASIC is one thing, but selecting a microcontroller architecture, and its peripheral set, expands the field of choice by several orders of magnitude. Unfortunately, in the world of semiconductors volume is everything and, even if the power supply market is vast, specialization of microcontrollers into hybrid power controllers has proven very challenging.

Core Independent Peripherals for Power

An alternative approach comes from Microchip Technology as a result of their efforts developing the Core Independent Peripherals (CIP) technology ( http://microchip.com/cip). In fact, more than a technology this is a design philosophy that has been applied for the last eight years in successive waves of new small microcontrollers introductions and is today a feature found in several hundreds of individual PIC models.

The theory is simple: allow all peripheral in a microcontroller to interact directly with each other bypassing whenever possible the device core. This has been proven to produce more flexible systems, but most importantly also more responsive / reactive systems, while reducing software complexity and eventually power and cost. When before each peripheral required individual care and feed by means of some software polling or interrupt service, the same results can now be obtained automatically producing chains of events that in some cases can increase the system real-time performance by orders of magnitude.

The initial offering of such peripherals was quite traditional: timers, PWMs, Complementary Output Generators but also Analog Comparators, ADCs and DACs and some Voltage References. These were promising first steps sufficient to implement/augment only the simplest of hybrid power supply solutions. With time though the offering of peripherals has been expanded to include more CIP peripherals including: operational amplifiers, fast analog comparators, digital modulators, numerically controlled oscillators and even ramp generators.

In recent families like the PIC16F176x and PIC16F177x (link), not only these CIP modules were specified to meet the demands of power supply applications, but they were also provided in quantities sufficient to implement multiple hybrid power control loops simultaneously (up to 4 at present). Since the peripherals are working independently, the small 8-bit microcontroller is not involved in the cycle by cycle operation of the power circuit and takes instead a pure configuration and supervisory role. In other words, the micro takes care of the “smart” in the application, while all the heavy lifting is done efficiently by analog and digital peripherals.

An Automotive grade SEPIC LED Driver

As an example of use of CIPs for Hybrid Power, we can look at a recently published application note (and related reference design) AN1978 (link). It describes a “SEPIC LED driver for automotive applications”.

The design is based on the PIC16F1769 device and a Single-Ended Primary Inductance Converter (SEPIC). This hybrid DC/DC converter topology is an attractive LED driver solution for automotive applications because the SEPIC can provide a regulated output voltage or current even if the input supply voltage goes below or above the output voltage while providing a non-inverted output referring to the same ground potential as its input. When the automotive electrical supply voltage drops below the LED’s voltage during cold crank, or rises above the LED’s voltage during load dump, the SEPIC can maintain the LED current constant.

Thanks to the CIP approach, this can be implemented efficiently by interconnecting (on chip) a fraction of the peripherals available on an otherwise general-purpose microcontroller.

Figure 1 – SEPIC LED driver schematic

The reference design provides a compact solution using few external components (only a power FET, a coil, few resistors and small caps, see Figure 1) while delivering good efficiency over a large range of input and output parameters (see Table 1) but also provides great flexibility to implement a number of smart features (typical of this application) such as:

Fully compensated High Bandwidth Peak Current Control
PWM Dimming (16-bit)
Transient and Reversed Input Voltage Protection
Input Under- and Over- voltage Protection
Output Overvoltage Protection
Short-Circuit Protection
Over Temperature Protection
Fault Output Indicator
Automatic BIN (Brightness Index Number) Detection

At the same time, the microcontroller remains free to provide additional diagnostic and communication tasks that might be required. Contrary to a full digital approach none of the tasks, left to firmware implementation, is time critical and the application note describes a simple scheduling mechanism that can be easily expanded to include any number of additional smart features.

Table 1 – SEPIC performance specification

Not just as good as, but better

The core independent approach allows an otherwise general-purpose microcontroller to implement hybrid power supply functions (feedback, modulator, compensator, fault detection) without the level of specialization that an ASIC integration would impose. Any peripheral that is not used for power conversion is immediately available for any other imaginable purpose. For example, an operational amplifier not used in the power train, can be used to provide a gain stage in front of the ADC for a sensor input, as well as any of the digital peripheral can be employed to control motors and actuators of any sorts in other applications. In the past, similar hybrid design had been attempted in the hope to reach “as good as” (ASIC) performance. But in a series of recent application notes, Microchip has demonstrated how CIP Hybrid power solutions can be superior to traditional ASIC solutions.

The advantage of CIP integration, perhaps not obvious to the casual reader of a device datasheet come from a number of features that are characteristic of modern microcontroller designs but would be too expensive (therefore rarely found) if at all possible, in ASIC designs such as:

Oscillator jitter, which is extremely well controlled in PIC MCUs
On-chip Operational Amplifiers characteristics and connectivity
On-chip Programmable Ramp Generators for Slope Compensation
On-chip Re-configurability, trivial with CIP

High Performance Dimming

In the same application note (AN1978) we can see those features be put to best use when implementing a high-performance dimming engine.

The excellent on chip oscillator of the PIC16F1769 device is providing the system with a very low jitter reference clock, and the ramp generator (PRG) provides stability to the SEPIC topology without using any external components nor ever exposing any of the signals to an outside pin. More interestingly perhaps, dimming can be imposed on the constant current output driver by applying a modulation effect by means of a 16-bit PWM, controlling frequency and/or duty cycle.

But since the on-chip operation amplifier used to perform the current control loop is internally connect to the feedback circuit, it can be synchronized with the modulation signal so to eliminate the “integrator saturation” effect, and consequently the overshoot typical of such circuits. Similarly, a load-switch output can be easily produced to remove the “tail” effect (see Figure 2).

Oscilloscope capture of the modulated output

Figure 2 – Integrator Saturation and Tail effects

The modulated current output waveform (captured in Figure 3) shows clearly how well the CIP Hybrid power circuit behaves in this application. It will guarantee a longer LED string life and it enables very low dimming ratios while removing any undesirable LED color shift.

Figure 3 – Oscilloscope capture of the modulated output (I_LED)

In Summary

Core Independent Peripherals enable superior hybrid power solutions while maintaining the flexibility of general-purpose microcontrollers. This ensures that smart integrated applications can be achieved with reduced component count and lower system cost.

Bio

Lucio Di Jasio is the EMEA Business Development Manager for Microchip Technology Inc. He has been covering various technical and marketing roles within the company 8, 16 and 32-bit divisions for the past 18 years. As an opinionated and prolific technical author, Lucio has published numerous articles and several books on programming for embedded control applications. Following his passion for flying he has achieved both FAA and EASA private pilot license certifications.