RoadTest: Power Integrations Home Appliance and Industrial Power Reference Design Kit
Author: Jan Cumps
Creation date:
Evaluation Type: Power Supplies
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: a set of wall warts with similar input range and comparable output specifications
What were the biggest problems encountered?: It's a reference design. Focus is on showing a perfect design rather than on hackability. That makes it hard to experiment. But not impossible!
Detailed Review:
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TL;DR: I score the Power Integrations (P.I.) 12 V 6 W power supply reference design In my road test report, I'll show my hands-on experience while I use the design within its specs. |
The road test kit is a mains-to-12V isolated flyback design. It offers:
image: the PCB on the workbench.
The power supply is intended for devices that are mains connected at all times. Things like home appliances.
During stand-by, the supply should use minimal power. But still be able to run some functions (e.g. a display, remote control receiver, communication listener, ...).
Under full load conditions, it should have a decent efficiency.
These constraints are usually prescribed in consumer goods (and other) efficiency requirement standards.
Design characteristics:
The switch and regulation IC is part of P.I.'s LinkSwitch-TNZ Family. They integrate the power MOSFET and all control logic to build switch mode converters. P.I. shows example circuits for these configurations
image source: P.I. datasheet LNK33x2-7D. I coloured the safety related blocks
The IC provides soft start. During the initial cycles, the clock will run half speed, to limit the stress at startup. The system switches to 66 kHz when output level is reached, or after 256 cycles.
It's used to generate the timeouts and auto-restart for some fault conditions. And its main purpose: It generates the switch cycle start, and the maximum duty cycle signal.
The clock has intentional jitter. A range between 2 kHz under and 2 kHz above the core frequency.
image source: P.I. datasheet LNK33x2-7D. I highlighted the jitter.
This jitter with a modulation frequency of 1 kHz, lowers the EMI emission peaks at clock frequency by spreading out switching artifacts across that 4 kHz spectrum.
The IC supports over-voltage protection at input and output. Both are retried via the auto-restart block.
Over-current works differently. When that occurs. the FET is switched off for the remainder of one cycle. Over-temperature switches off the FET once kicking in, until the temperature drops below the threshold.
This functionality can be used to get a safe isolated pulse at each zero-crossing of the high-voltage AC input voltage. The signal can be used to synchronise your design, or as a utility-line-frequency based timer. In one of the ICs in the LinkSwitch-TNZ family (LNK3317D), this can also be used to discharge the X capacitor(s). To comply with some regulations (see X Capacitor Discharge Must Satisfy Both Safety and Energy Efficiency Rules), it can be a requirement to have X capacitor discharged to a safe level within a given time.
The output is regulated by sampling the output voltage, and suppressing switching cycles as needed. This is different to other topologies, that use concepts like variable duty cycles, variable on or off time, ... By default it switches the MOSFET on for max duty cycle time, +- 70%. The on/off control will cut off chunks of that on-time, as required to keep the output voltage at the desired level.
The switch FET and its driver are integrated in the package. It's an n-channel MOSFET. Some of the specs:
source: P.I. datasheet LNK33x2-7D.
Time to dig into the reference design. It's an isolated flyback, where the flyback transformer has four duties:
image source: Reference Design Report RDR-877
If we follow the design from top left to right: the power transfer part:
Then the feedback part, the middle part of the circuit from right to left:
The lower part of the circuit is the zero-cross detection. This runs from left to right.
What did I do during the road test?
What did I not do:
For the efficiency test, I attached a Programmable Electronic Load to the output. I used two DMMs to measure voltage and current. At the input, I also measured the power with two DMMs, just behind the rectifier bridge, where it connects to the bulk capacitor. I used the procedure from P.I. University's Course Notes: Techniques for Measuring Efficiency. This is fairly precise, but doesn't take in account the loss in the protection and full bridge rectifier.
Conditions: mains 230 VAC, 50Hz. Room temperature 20°C.
Board preparation: I cut the trace just after the bridge:
images: where I cut the PCB trace, and how DMMs were wired in place
image: test setup for efficiency measurement
Then I made the programmable load step through all settings, from standby to full load. I let it settle and recorded volts and amperes at input and output.
image: spreadsheet with the measurement
The results are in line with the reference design report:
image: graph of efficiency vs load. top: 1 - 10%, bottom 10 - 100% load. Note that the x-scale isn't fully linear. I increased the increment along the way
Let's compare with the graph from the design report:
image source: Reference Design Report RDR-877
The spreadsheet, with graphs, is attached to this review.
While I was running the exercise, I took some samples of the switch node. Where MOSFET connects to the primary winding. Below you can see my capture of that. Can you spot an occurrence of the on/off output voltage regulation?
image: comparing a capture at the drain of the MOSFET (bottom left) with the Operation at Moderately Heavy Loading (Flyback) signal (bottom right) from AN-98. Schema part taken from RDR-877.
The same data used for efficiency, can also be used to graph the output voltage. From stand-by to full load.
image: graph of output voltage (in V) vs load. The horizontal scale is not linear.
P.I. has provided a rich set of documents. I will elaborate here on the Reference Design Report RDR-877. It touches all steps of the design. That includes a full calculation of the flyback transformer. And how to correctly manufacture one. P.I. has a spreadsheet for these calculations. In the document they show the exact parameters that were used for the RDR-877 design.
image: snippet of the flyback transformer parameters, from Reference Design Report RDR-877
The spreadsheet calculated this transformer configuration:
image: snippet of the flyback transformer parameters, from Reference Design Report RDR-877
The winding instructions show step by step how to construct it:
image: snippet of the flyback transformer manufacture instructions, from Reference Design Report RDR-877
Other highlights: the principles are shown. How the P.I. LNK3306D regulates the output. What value and type of resistors and capacitors to use to set the output voltage, maximum current, ...
And a full set of characterisation graphs. A few of them I repeated in this road test. But they also show startup and shutdown behaviour. Load transient response. Ripple. Brown-in and -out. Thermal performance, And the waveforms at the interesting points in the circuit. Almost 50 pages of test data.
And that brings me back to the TL;DR section at the top of this road test review: the combination of a performant IC, a good reference design that reflects a real-world solution, and the rich documentation, resulted in a 5 star score.
