There are several teardown videos of the LRS series on youtube, so I have not bothered to document the primary side. There is a flyback converter and a fully isolated transformer. Feedback is done via two optocouplers. One seems to be for regulating the output voltage, the other one is labelled O.V.P. (overvoltage protection?) in the block diagram of the data sheet: https://www.meanwell.com/Upload/PDF/LRS-150/LRS-150-SPEC.PDF
Anyway, the secondary side of the PCB seems to be designed for several versions, some of which seem to use active rectification. All those positions are unpopulated, so the secondary side, ingnoring the regulation and protectio part, consists of:
- two secondaries made of multi-strand HF litz wire which are already paralled on the transformer's solder pins
- another single wire secondary that is connected to the higher power secondaries on one side and simply left open on the other side
- a Schottky diode in TO-220 casing with the two diodes paralled with a wire bridge
- one snubber consisting of a 35 R resistor and an unknown capacitor parallel to each diode (strangely, one is very close and the other one has a long pcb trace --> why didn't they simply use one snubber?)
- a CLC filter consisting of 4x 470 µF in parallel, an inductor and another 470 µF
So what strikes me: why did they employ half wave rectification when then could have connected those two secondaries in series, creating a center tapped secondary? Then they could have used one of the diodes in the package on each side of the secondary, resulting in full wave rectification and hence much smaller ripple with exactly the same component count? They could even have economized some of those electrolytics!
The other thing that is not pretty: There is an isolating sheet made of FR4 of roughly 0.2 mm thickness between the pcb and the aluminum back side. Some of the wires of the through hole components press directly onto this isolation sheet, e.g. the blue input cap (or is it a surge protector?) right on the mains input. G-shocks and vibration could cause its quite sturdy wires to grind through the insulating sheet, or the solder joints could break over time. Yes, its a class I device, but simply clipping the leads would have eliminated this failure mode.Other than that, I am happy. No load power draw is about 0.3 W. Connecting a 24 V fan that draws about 0.8 W gives me an incrase to about 1.3 W, so excellent low power efficiency. I have not yet measured ripple.
How about stacking two of these to obtain +/- 24 V? There is a video on youtube where someone stacks three LRS 150 - 48 to obtain 144 V. The data sheet says nothing about not connecting the secondary sides. The only thing that has me uncomfortable is that the secondary side has three EMI caps to earth (you can see them better in the teardown videos).
Yes, you can wire these to make a bipolar supply. I've done it with good success. 🙂
In a flyback converter, the only useful part of the switching cycle is Toff. The conduction half-cycle is required to store energy in the primary inductance, but it cannot be regulated: it is strictly proportional to the input voltage.
That doesn't mean that it cannot be used: it can generate an auxiliary, unregulated supply, but joining them with a pair of diodes is ineffective at best, and probably dangerous for the switching element because it will have to drive the filter cap directly, with the current limited only by the parasitic impedances
Thanks, did not consider that as I am not familiar with flyback for galvanic isolation.
(btw, I did not verify that it is a flyback, this is what someone said in one of the teardown videos)
(btw, I did not verify that it is a flyback, this is what someone said in one of the teardown videos)
If you have a single primary switching device (could be composed of 2 or more paralleled), and no big filtering chokes at the secondary (smallish RFI filtering chokes don't count), it is almost certainly a flyback.
Single-ended, forward converters used to exist too, but they are never used anymore (and they also need a big secondary filter choke).
Some exotic configurations like Cuk might do away with these, but in practice manufacturers like Meanwell stick to more traditional options
Single-ended, forward converters used to exist too, but they are never used anymore (and they also need a big secondary filter choke).
Some exotic configurations like Cuk might do away with these, but in practice manufacturers like Meanwell stick to more traditional options
These things are so cheap. I'm thinking about this instead of building a regulated, capacitance multiplier, linear supply for a phono stage or a preamp. Is this the way to wire it up to stack? I guess you'd put a filter on the output it like the one in the DIY Store?
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Using a SMPS for a low-power, sensitive load seems counter-productive. Apart from cost, they have nothing going for them.
They inevitably generate HF hash, even if they comply with all of the EMI/RFI regulations, and to comply they need to use Y caps, even if they are class II, which creates ground loop issues which are particularly nasty and pervasive for phono and other preamps.
A 50Hz transformer generates inductive perturbations, and this can be very problematic too, but at least you can locate the transformer in a wall-wart or a remote supply linked by an umbilical.
They inevitably generate HF hash, even if they comply with all of the EMI/RFI regulations, and to comply they need to use Y caps, even if they are class II, which creates ground loop issues which are particularly nasty and pervasive for phono and other preamps.
A 50Hz transformer generates inductive perturbations, and this can be very problematic too, but at least you can locate the transformer in a wall-wart or a remote supply linked by an umbilical.