Situation:
You have a battery pack with voltage of 300V. You have some appliances designed for 250V AC. These appliances have SMPS inside.
Claim:
Instead of using DC to AC inverter, you can use DC voltage to supply SMPS inside your devices.
Reasoning:
250V AC has peak voltage around 350V. First stage of SMPS rectifies 250V AC to maybe around 300V DC. This voltage is then used to supply another stage of SMPS, but that's not important for this question. So basically from 2 stage of SMPS onwards, SMPS knows no difference between AC and DC supply voltage.
Advantage:
0 cost of inverter, 0% losses on DC to AC inverter
Disadvantage:
You can use this method strictly just for appliances with SMPS in them.
Question:
Is there something I missed? Is there reason why this would not work?
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5\$\begingroup\$ Almost all smps will work perfectly when supplied with dc300V. But there are some very low power smps designed with series capacitors at front end. DC current will be blocked by this cap. \$\endgroup\$– soosai stevenOct 10, 2016 at 6:13
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1\$\begingroup\$ This is a valid point, that I overlooked. Thank you for input. \$\endgroup\$– ViliamOct 10, 2016 at 6:41
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2\$\begingroup\$ Active PFC (APFC) circuits may not work with DC input (If I'm wrong, please someone correct me). So, if some appliances have their own APFC + SMPS circuit inside, they may not work with DC input. \$\endgroup\$– Rohat KılıçOct 10, 2016 at 6:58
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2\$\begingroup\$ Actually, if your source is 250VDC, you can also power purely 250VAC resistive loads such as tungsten bulbs and heaters from it. And it should still power most SMPSes, as most have a wide input range (typically something like 100-250VAC). \$\endgroup\$– marcelmOct 10, 2016 at 7:57
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1\$\begingroup\$ Rohat kilic.... Active PFC circuit serve no purpose when the PSU is being supplied by DC. I believe it will remain idle. \$\endgroup\$– soosai stevenOct 10, 2016 at 13:50
5 Answers
I have practical experience with PC power supplies. Pretty much any modern ATX PSU contains a main section (that delivers full horsepower when the PSU is on) and a tiny section to deliver +5VSB. In a decent PSU, both are SMPS, i.e. do not contain a classic iron-core AC-only transformer (which would surely blow if fed by DC mains).
There are still some cheap ATX power supplies on the market, that do not contain a proper SMPS-based PFC stage, or use a so-called "passive" PFC, which is essentially an inductor in series... PSU's with a passive PFC typically have a switch to be configured for 120 vs. 240 V AC, or (here in Europe) are "240 V only", with a modest margin of tolerance. For the purposes of DC operation, I do not like the cheap variety very much - not exactly sure what to expect from the hefty inductor at the input on transient events (power on/off - probably not much risk but you'd better watch out) and also because my customers typically have a DC mains rail that's "somewhere inbetween", such as 250 V DC.
As you have mentioned, the direct-rectified peak voltages are about 340 V DC derived from 240V AC, or 170 V DC derived from 120 V AC (the ratio being a square root of 2). Thus, 250V DC is probably too low for a PSU specified at 240 V AC (= 340 V DC). The PSU would either refuse to run, or might risk heat overload of the primary (due to too much current flowing in, to compensate for the lower voltage). This latter risk depends a lot on actual load attached to the secondary side.
PSU's with active PFC typically have a wide input range specified, such as 100 - 240 V AC (140 to 340 V sinewave peak). This is because the SMPS PFC stage actually works as a boost converter, with a relatively small capacitor on its input, following the sinewave, and upconverting any voltage you give it to some 400 V DC in the PSU's main primary capacitor - for the main down-converting stage to have a "nice level playground".
The 140V DC minimum at the PFC stage's input is actually debatable. The PFC stage uses a special circuit, regulating its current draft to follow the input "rectified sinewave" waveform. I guess it's called a "current mirror", which sounds more like something from the "op-amp internals" nomenclature (see also "long-tailed pair"), rather than a term specific to PFC controllers... either way, this circuit modulates the PWM to make the current draft fluctuate in sync with input voltage, to mimick a resistive load... and if the input voltage is perfectly constant, this should result in a perfectly constant current draft, right? Back to the 140 V DC minimum (for 100 V AC): consider, that at 100 V AC, the PFC "current mirror" still strives to follow the sinewave, so it actually works from much lower voltages. And, at 100 V AC (140 V peak) = at the corresponding current (some sinusoidal Ief) it still has bearable losses = heat dissipation. Arguably, if fed by DC voltage, a similar level of loss and heat can be obtained at 100 V DC, rather than 140 V DC, agreed?
The one point you may be somewhat doubtful about, is the input Graetz bridge, which will be stressed in a half of the diodes only - but this is typically no problem.
One somewhat subtle issue remains: inrush limiting. AFAICT, even the modern PFC-equipped PSU's actually suffer from the power-on inrush when the big primary cap is getting charged (= with an ATX PSU, this happens when you plug the mains cable in, rather then when you push the PC's power-on button). The PFC stage's boost topology alone doesn't contain any inrush limiting device (except maybe for the choke, but that's relatively small). If the PSU is equipped with an NTC for inrush limit, it will probably work fine for DC as well. If the PSU contains a thyristor-based (PWM-based) inrush soft-starter, that would be a problem, as the PSU would either not work at all, or the thyristors would not switch off, and there would be a hell of an inrush :-) You are probably not likely to meet thyristors in the input Graetz in a PSU on the order of 400 W or thereabouts.
Inrush pulse limiting is a pain even at lower-volt (< 50 V) DC mains rails - a problem that hardly anyone cares to address, until it comes back to bite them...
As for overcurrent safety: people working with higher-volt DC in their automation gear (some railway and substation folks) typically use special models of circuit breakers that are specified for DC operation at the desired voltage. They look very much like your normal AC circuit breakers on the DIN rail, except that they have the special property that they can disconnect a DC current. Up to a certain level of current and voltage, anyway. See also "circuit breaker selectivity".
Overall, I myself would be very afraid of the DC voltage, should you ever touch a bare wire by mistake. Once you get caught, it won't let you go. Not very good for household use.
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\$\begingroup\$ “or might risk heat overload of the primary due to too much current flowing in” - Isn't there protective circuitry against this? \$\endgroup\$ Jul 12, 2019 at 19:09
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1\$\begingroup\$ @neverMind9 : there's no such general rule. Some under-voltage shutdown might be expected... then again, the PFC stage needs to "follow the sinewave", along instantaneous voltages as low towards zero as possible, to produce a nice sinusoidal current draft... so it would have to be a pretty intelligent protection. Try measuring any PSU at hand that you're considering for a particular application, at what DC voltage it shuts down. As for thermal protections, yes there may be some, though I cannot promise a thermistor coupled to the PFC-stage switching transistor... \$\endgroup\$– frrJul 13, 2019 at 21:07
Yes, it can be done.
- Your device must use a SMPS only (no hidden transformer or blocking capacitors.)
- Your device must be double insulated (has no external connections or exposed metal parts that are normally connected to ground.)
The German computer magazine c't tested several devices in a recent (early 2016 - link German only, not the full article) issue that tested this very thing. They found that many devices will run from a DC voltage well under 100VDC, which made it easy to build battery packs to power devices built for AC.
The minimum DC needed to operate an AC device varies depending on the design of the SMPS. I remember some operating on less than 40 VDC, though others needed higher voltages.
The thing to watch out for is devices that aren't double insulated. If the device expects to be grounded but isn't, then you can end up with dangerous voltages on exposed metal parts.
This last bit is especially important when using devices like oscilloscopes that have exposed ground connections.
There are some good explanations for why this is dangerous here on EESX.
As noted by frr, the devices may run hotter when running on the lower DC voltage. This may lead to the device failing earlier than it would if operated only on the specified AC.
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\$\begingroup\$ I have to say that c't have a nice unorthodox choice of topics. Almost biting the hands that feed the computing industry :-) They're certainly far from the mainstream advertising press covering the industry. \$\endgroup\$– frrOct 10, 2016 at 10:29
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\$\begingroup\$ @frr : Yes, I like c't for that very fact. I don't have much use for the typical computer magazines, but c't does get down to the nitty gritty on things. The product tests are real tests rather than gushing "OMG I love this gadget" so called "tests" in other magazines. \$\endgroup\$– JREOct 10, 2016 at 10:34
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\$\begingroup\$ Note that while small-wattage = cheap SMPS modules for mains AC may not have rigorous undervoltage protection, and may therefore run heavily under-volted, at the specified wattage throughput the primary side will get hotter than the designer intended. This is because the PSU will draw more current to compensate for the lower voltage. In datacenters, school PC labs etc, PSU failures typically occur when a whole room or a mains rail has to start up after a blackout = when the semi-worn PSU inputs are likely to face undervoltage for a couple (dozen) seconds. \$\endgroup\$– frrOct 10, 2016 at 10:34
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\$\begingroup\$ Why dp they run hotter on low DC voltage than high AC voltage? Isn't low DC voltage such as 40 V(DC) much closer to the 19V to 20V that a laptop SMPS is trying to reach? \$\endgroup\$ Mar 28, 2019 at 23:47
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1\$\begingroup\$ @neverMind: Say the device need 2A at 5V to operate. An SMPS designed to drop 110VAC to 5V at 2A will draw something like 100mA on the input side. If you now operate the SMPS from 20VDC, it will have to draw about 500mA on the input side. That increased current will cause more losses in the switching element and in the inductor. That's where the heat comes from. \$\endgroup\$– JREMar 29, 2019 at 6:39
Like it has been said in the comments. It is possible to use most smps units directly from a DC source. Therefore you can save the cost of an inverter. One situation should not be overlooked. Some appliances use the mains frequency for counting and/or timing. These situation are not always visible from the outside of the appliance.
This covers a subset case only, but deserves a minor answer in its own right (rather than a comment which can be deleted or missed.)
Some power supplies take advantage of the AC input variation to provide a smoother DC output than would occur with rectification and a given degree of smoothing.
Operation of these circuits on DC input will violate assumptions made by the circuit designer and may have implications for power dissipation or other functionality.
One example are "valley fill circuits" (wikipedia) which charge capacitors in series at high input voltage and then discharge them in parallel at lower voltages as Vin falls towards the AC zero crossing point. These circuits are more common n modest power applications such as LED lighting where avoidance of mains frequency induced flicker is important. These are similar to some power factor correction circuits mentioned below but the primary purpose may not be the same. (Wit LED drivers the arrangement may bot improve flicker performance and improve regulatory compliance.)
C1 & C2 charge in series and discharge in parallel - acting as a Vin_high halver to supply current during low Vin periods.
From here . They see this as a PFC correction circuit.
Related
What’s the Difference Between Passive and Active Power-Factor Correctors?
Some supplies use capacitor voltage doublers eg 'universal supplies' which voltage double when used on 100 VAC and do not double when used on 230 VAC. Operation on rectified 110 VAC would fail.
Power factor correction circuits have been mentioned. A class of these boost Vin to a voltage equal or higher than peak Vinmax. Whether these would function on DC input depends on design.
Yes it will work but there are concerns about the Breaking capacity of the internal fuses which are specified for say 250VAC but Not for 200VDC . What about any mains switches .Most mains switches are AC only .If you know what you are doing when it comes to fuses and switches then go ahead otherwise dont .Remember that DC power systems need larger switchgear . I have seen Off the shelf 12VDC SMPS used in electric vehicle conversions where the operating volts was about 300VDC .The 12V was used to supply standard auto electrics .When the AC Vs DC debate was alive this was postulated as one of the drawbacks of DC before the 3 phase induction motor clinched it for AC.
