04 Jun 2014

Digital vs Analogue Power Supplies - Finding the right balance

Frank Bidwell Director of Engineering at XP looks at analogue & digital power supplies, explaining the differences, as well as monitoring & control techniques.

Today’s power supplies are starting to adopt digital technology to offer many different monitoring and control functions. However, finding the right balance between analogue and digital without adding too much cost and complexity may be tricky.

System effectiveness, status monitoring and energy efficiency require AC-DC power supplies to be monitored and controlled, and though the concept of doing this digitally has been around for some time, as end-designs are becoming more complex, many engineers are just starting to investigate the benefits that incorporating digital control into their system may bring.

Typical Electronic Power Supply - by XP Power

A design engineer looking into digital methods of monitoring and controlling an AC-DC power supply may be drawn by the prospect of access to a high level of information compared to an analogue system and the ability to control the output digitally. However, these functions come at the cost of system complexity.

Analogue vs Digital

Digital control undoubtedly offers higher levels of flexibility. Power supplies can be turned on and off remotely, limits can be set on output voltages and currents and alarms on these levels enabled. Digital power supplies can provide monitoring data, such as temperature and fan performance, to help predict failures.

Digital control can also ease calibration – parameters may be programmed instead of tweaking a pot in an analogue power supply. On the down side, full digital control of a power supply involves many operating parameters and can be extremely complex. It requires a DSP with sophisticated software, which can be difficult to troubleshoot.

By comparison, analogue control, having been used for 50+ years, is a tried and true method. Analogue power supplies are stable and less prone to glitches, though they have limited flexibility. Parameters are defined during the design phase and cannot be changed later. While status can be monitored, control is usually limited to one possible reaction per parameter; with a digital power supply, there is usually an extensive range of input scenarios with multiple possible actions based on those scenarios, albeit limited by the speed of the processor.

Analogue Digital balance

Finding a compromise between analogue and digital can be tricky. XP Power’s approach to finding a balance between the two domains is to use a tried and true analogue PWM controller with an addition of a digital interface, to create communication flexibility. In a typical XP design, an analogue PWM controls the output voltage in real time, and a microprocessor monitors and adjusts that.

The digital control board can artificially create multiple operating states by combining the single parameter control and status signals, allowing a higher level of flexibility than with the analogue controller alone. This level of control suits the majority of customers who prioritise a cost-effective solution.

The analogue PWM controller has the advantage of eliminating certain types of inaccuracies that are inherent to digital designs. For example, sampling signals with an ADC to feed to the DSP will introduce aliasing errors. Using a DSP to control the output will also introduce jitter into the PWM signal – this can cause sub-harmonics in the output leading to EMI problems in the application. Using an analogue PWM effectively avoids this. However, since an analogue PWM’s switching frequency is set by hardware, this can’t be adjusted. Also, a fixed control loop means this can’t be optimised for different conditions without changing components.

Communication & control

The most widely-used communications protocol for digital control of power supplies is PMBus, a clearly defined industry standard intended to make power supplies plug-and-play, so that the interface from the end equipment can be designed without even seeing the power supply.

While it’s perfectly possible to control a digital power supply using a CAN bus, or Ethernet for example, this requires much more time developing a protocol stack for use in each specific application. PMBus is much simpler. It features a relatively simple interface (two I/O lines) which can help make hardware smaller, and there are only three layers to the PMBus stack, compared to 10 or 12 for a comparable Ethernet implementation. For this reason, designers in a hurry, and those without deep networking expertise or resources often choose PMBus

Aside from monitoring and control functions, utilising the PMBus interface allows several additional applications. Battery charging is one example – since the output current and output power are monitored, a battery can be charged from the power supply without the need for an external interface. Power sequencing can also be implemented, using a multiple output power supply turned on or off at a particular time or in a particular sequence as determined by the system.

Power supply application examples

As an example, the GFR1K5, a 1U rack-mount 1500-W AC-DC front end from XP Power uses a PM Bus interface with a very simple command structure to give a practical amount of control, as shown in table 1.

The output can be turned on and off, and the overcurrent shut down point can be set; hardware based shut down occurs at 110-140% of Inom, but a firmware shut down point can be set for 105 to 0%. Response to a firmware shut down can be specified as well (latch off or a hiccup mode with an adjustable number of retries or continuous retry).

The PMBus interface also enables monitoring capabilities: signals are available for output voltage, output current and temperature, and the data bits for the alarms can be set easily using the interface. Information can be retrieved about the model number and serial number (useful in a system with many power supplies) and the runtime is also available.

Table 1. The PMBus command structure for the GFR1K5

Status Register CMD (hex) Function Protocol Type (R=Read / W=Write) Number of Bytes
01h On / Off command (OPERATION) Byte (R/W) 1 Read / Write
46h Current limit (in percent) (OUT_OC_FAULT_LIMIT_) Word (R/W) 2 Read / Write
47h Current Limit Fault Response (OUT_OC_FAULT_RESPONSE) Byte (R/W) 1 Read / Write
79h Alarm Data Bits (STATUS_WORD) Word (R Only) 2 Read Only
8Bh Output Voltage (READ_VOUT) Word (R Only) 2 Read Only)
8Ch Output Current (Read Only) Word (R Only) 2 Read Only
8Dh Power Supply Ambient Temp (READ_TEMPERATURE_1) Word (R Only) 2 Read Only
9Ah Unit Model Number (MFR_MODEL) Block (R/W) 10 Read / Write plus byte count
9Eh Unit Serial Number (MFR_MODEL) Block (R/W) 8 Read / Write plus byte count
99h Unit Manufacturer ID (MRF_ID) Block (R/W) 8 Read / Write plus byte count
D0h Unit Run Time Information (MFR_SPECIFIC_00) Block (R Only) 8 Read Only plus byte count

Taking digital control a step further is the EMH series of 250-350W AC-DC PSUs designed for IT and medical equipment. In addition to the control and monitoring functionality provided by the GFR1K5, this PSU has the ability to trim the output voltage to +/-10% of Vnom. The output current can also be trimmed (50-110% constant current mode). This device can also monitor the status of the device’s fan, for the top fan version of the power supply. The fan’s tachometer is monitored and if it appears to slow down or stop, a fault bit which represents the fan will be set high.

Table 2. The PMBus command structure of the EMH350, with a higher level of complexity compared to the GFR1K5.

Command Code Code Name Access Type Data Format Data Bytes
3h CLEAR_FAULTS Send Byte 1
20h VOUT_MODE Read Word Linear 1
22h VOUT_TRIM Write Word Linear 2
79h STATUS_WORD Read Word Binary 2
7Eh STATUS_CML Read Byte Binary 1
81h STATUS_FANS_1_2 Read Word Binary 1
8Bh READ VOUT Read Word Linear 2
8Ch READ_IOUT Read Word Linear 2
8Dh READ_TEMPERATURE_1 Read Word Linear 2
98h PMBUS_REVISION Read Byte ASCII 1
99h MFR_ID Block Read ASCII 8
9Ah MFR_MODEL Block Read ASCII 15
9Bh MFR_REVISION Block Read ASCII 2
9Eh MFR_SERIAL Block Read ASCII 9
D0h READ_VFAN Read Word Linear 2
D1h READ_RUN_TIME Block Word Linear 4
D2h IOUT_LIMIT_TRIM Write Word Linear 2

Summary

In summary, there are pros and cons to both the analogue and digital control of power supplies, but an analogue PWM controller with a PMBus interface picks the key benefits of both approaches, achieving a balance between functionality and cost. Different methods for accessing power supply functions are available, but the widely-used PMBus protocol is specially designed for power supplies and is therefore the easiest to implement, while offering full access to all the functionality of a modern AC-DC PSU.

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About the author

Frank Bidwell is an electrical engineer and a member of the Institute of Electrical and Electronics Engineers. He has worked with power and digital designs for over 25 years. He has been with XP Power for 10 years and is currently the Director of Engineering for the Engineering Services Group.

XP Power is committed to being a leading provider of power solutions, including AC-DC power supplies and DC-DC converters. With ISO9001:2008, XP Power offers total quality, from in-house design through to manufacturing facilities around the world. The company offers the widest range of power products available from one source and unrivalled technical and customer support, aiding both vendor consolidation and cost reduction programmes. XP has 27 sales offices throughout Europe, North America and Asia.

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