USB PD PPS as general purpose power supply - Page 1
Forum section: electronics / testgear

author: dougg
author_rank: Regular Contributor
timestamp: April 02, 2020, 09:50:48 pm
content: To spell that a bit more: how suitable is a USB Power Delivery (PD) adapter supporting the Programmable Power Supply (PPS) part of the USB PD 3 spec as a general purpose power supply?There is at least one interesting and widely available example at a reasonable price: the Samsung EP-TA845 which is a 45 Watt wall wort with a single USB-C connector and is priced around $CAD51 currently. There are relatively cheap programmable "sinks" like these:https://www.tindie.com/products/pier42/usb-c-pd-sink/https://www.tindie.com/products/clarahobbs/pd-buddy-sink/that can be coupled (via an E-marked USB-C male to male cable) to make a bare-bones general purpose power supply.The "source" (power provider) capabilities of the EP-TA845 are:1: fixed PDO; vol:5000mV, current:3000mA2: fixed PDO; vol:9000mV, current:3000mA3: fixed PDO; vol:15000mV, current:3000mA4: fixed PDO; vol:20000mV, current:2250mA5: augmented PDO; vol:3300mV ~ 11000mV, current:5000mA, pow_lim=16: augmented PDO; vol:3300mV ~ 16000mV, current:3000mA, pow_lim=17: augmented PDO; vol:3300mV ~ 21000mV, current:2250mA, pow_lim=1The interesting ones are the last three, also known as APDOs [PDO is "Power Delivery Object"]. So that implies (with some software help) it can supply 3.3 to 9 volts @ 5 Amps when the power limiting cuts in stopping at 11 volts @ 45/11 Amps. Then from >11 Volts to 15 Volts at 3 Amps. You should see the pattern by now; topping off at 21 Volts and a bit less than 2.25 Amps.Where I would appreciate the insight of the forum members is to ask how reasonable or onerous are the USB PD 3 spec's requirement on a power sink? I am assuming that the "sink" will be used as a general purpose power supply? To help in that regard I have taken screen shots of 3 pages from the spec that itemize those requirements. If the names of the items aren't clear enough, the full spec can be freely obtained from usb.org with the pages leading up to 317 of interest in this case.That spec seems to dodge a few issues, for example ripple. It has a figure that shows that ripple may be larger than one LSB (by which I think they mean +- 20 mV). However it doesn't seem quantify that ripple (for example a maximum Vrms) like most power supplies. It is obviously a SMPS, an "isolated quasi-resonant flyback" topology if USB PD designs from chip manufacturers are a guide.With PPS the sink can select voltages with 20 mV precision and current with 50 mA precision. To send more than 3 Amps down a USB-C cable it must be "E-marked" (i.e. one or both of the plug connectors contains a chip) and its chip states the maximum current it can handle. Also USB-C has two rails: Vbus the one you know and love from USB-2; and a new one: Vconn that is optionally sent along either CC1 or CC2 to power the E-marked chip and possibly a retimer if it is an "active" cable. As Intel are finding out with PCIe4, when you try and send 10 Gbps and faster through a low quality cable, don't expect it to get more than 6 to 12 inch before it disappears into thermal noise, the solution is retimers. And in USB-C to send 20 Gbps down one full duplex digital pair (a USB-C connector has two such pairs) as required by USB4/TB3 then either very short cables or retimers are necessary. But the doesn't impact USB PD, arguably the only saving grace of USB :-)Doug Gilbert

author: dougg
author_rank: Regular Contributor
timestamp: April 04, 2020, 02:32:22 am
content: A few extra points. With reference to the "Output capacitance of higher end bench power supplies" topic, the USB PD spec has cSrcBulk(min) at 10 microFarads, no max is given. As can be seen in the first attachment cSnkBulkPd(min) is 1 and (max) is 100 microFarads. The source is also required to discharge the bulk capacitance whenever the Vbus voltage is changed including when it goes to "Vsafe0V" typically when the source has its input power removed.One curiosity is on the outside of the EP-TA845 is printed: "(PPS) 3.3-11.0V dc(symbol) 4.05A, or 3.3-16.0V dc 2.8A, or 3.3-21.0V dc 2.1A". Notice the discrepancy in current: lower values printed on the device and its documentation compared to APDOs shown in my first post. Which one is correct? Happily my Rigol DL3021 dissipates around 25 Watts when 6 volts at 5 Amps is requested. The other 5 watts disappear in the 1 metre USB-C male to male cable, the Pier42 sink device (linked above) and my cabling to the DL3021. So the APDOs are correct. Perhaps the Samsung lawyers felt trying to explain "power limit" @ 45 Watts a bit too much for the punters, so they scaled back the declared current such that a simple multiplication (max voltage times current) didn't exceed 45 Watts.Many chip manufacturers seem to be betting big money in the expectation that there are going to be a lot of USB PD PPS power supplies around in the future.Of the attachment abbreviations, "FRS" is an interesting one; it stands for "fast role swap". IOWs a sink can become a source in double quick time! An example is a USB-C battery pack with a single USB-C connector that needs to both act as a sink (when charging) or a source (when discharging). Assume the battery in a laptop is dead or removed and you have such a USB-C battery pack attached as a short term replacement. At some stage you will need to connect an AC charger (say with a proprietary DC connector) to charge up that battery pack (feeding power through the laptop). Now the dog comes in and manages to knock the AC cable out of the wall socket. FRS should be able to turn that battery pack around from being a sink to a source fast enough so the laptop doesn't power cycle. That is a pretty neat trick IMO.