Charger design

[elearn]

Hi,
I happened to open up a old nokia charger and tried to analyze. It looks like DC conversion is done at high voltage and is stepped down in the end.
I would have thought stepping down as a first step and then converting it into DC is much easier and efficient in terms of component requirements.
Any thoughts ?

el

 

[Les Jones]

Why do you think it better to step the voltage down first (presumably using a transformer at the mains frequency) rather than converting the input to DC first, chopping it at a few hundred KHZ and using a very small low cost transformer to step it down to the required output voltage ? The high frequency chopper will have PWM control so that the output will be stabilised without the use of a lossy linear regulator on the output. It is done this way to make the power supply lighter, cheaper and more efficient.

Les.

 

[MikeMl]

That is also why, as an inexperienced hobbyist, you shouldn't be opening these up and screwing around with them. The internal voltages are lethal, and since there is no AC line isolation on the high voltage side of the internal transformer, one slip with a scope ground clip can cause a direct short from the AC line to earth ground, causing an explosion that throws molten metal into your face and eyes...

 

[elearn]

Thanks Mike, i'll take care.
Les, iam not sure how big is the transformer at mains. I'm a new hobbyist in this area, i can think of problem areas like the diode bridge needs high wattage diodes, and the mosfet needs to sustain high voltage - am i wrong ?

 

[MikeMl]

Thanks Mike, i'll take care.
Les, iam not sure how big is the transformer at mains. I'm a new hobbyist in this area, i can think of problem areas like the diode bridge needs high wattage diodes, and the mosfet needs to sustain high voltage - am i wrong ?

Nope, the DC voltage at the output of the rectifier will be ~1.4*the LineVoltage. In the US, where our line voltage is 120Vac, you can expect ~170Vdc. At 240Vac in, expect about 340Vdc.

Like I said above, be damn careful screwing with these things...

 

[elearn]

ok, thanks for the warning mike, i'm not supplying power and doing anything with the scope right now. Just had a look at the circuit and was curious why it's done that way.

 

[bountyhunter]

Hi,
I happened to open up a old nokia charger and tried to analyze. It looks like DC conversion is done at high voltage and is stepped down in the end.
I would have thought stepping down as a first step and then converting it into DC is much easier and efficient in terms of component requirements.
Any thoughts ?

el

They all use an off line flyback design with some kind of cheap opto coupler for feedback. The flyback transformer winding provides the isolation necessary to make the UL safety rating. It's the cheapest way to do the design. The flyback FET does need to stand off the unregulated DC voltage (above 300V) but 400V and 600V FETs are readily available and pretty cheap. The current requirement is very low.

 

[AnalogKid]

They do it that way because of cost. The vast majority of older wall warts are what you expected, a small but standard offline transformer, a low voltage secondary, a rectifier, and a filter capacitor. The output had a voltage rating that was accurate only at its rated load current (when the output ripple voltage is at its worst), and could easily be 1.5 times that rated voltage at no or very low loads. This translated into extra work for the DC power processing/regulating circuits inside the calculator or whatever. Very few were truly regulated because a linear regulator makes heat, and that requires heatsinks and vents, and that makes the thing even larger and less safe.

After 50 years of development, today's small offline switchers are a marvel of integration. A 1000-to-1 increase in the frequency of what is going through the transformer has a huge impact on it's size, weight, efficiency, and cost. Power Integrations specializes in single-chip offline switcher chips that have the complete controller and power mosfet all included. You get a regulated output with no heatsink, no vents, and a cost (at 100,000 pieces) that is less than the old unregulated linear wall wart. Before USB ports took over charging everything, the world consumed B B Billions of wall warts each year. 100,000 units per day is less than 1% of that.

ak

 

[bountyhunter]

It is kind of astonishing we now can buy regulated off line switchers in a wall wart form factor for about $5..... not to mention 200W supplies that go in a PC sell for about $15. It's a new world.

 

[ronsimpson]

My wife got a Windows-10 PC. Needed new operating system.
Tower, but the power supply is a wall wart. There is almost noting in the box. No slots. The whole thing could have gone in a box one inch thick. There is some advantage to having all the high voltage outside the box. (UL, VDE, CSA) The mother board takes the 12 volts and makes all the very low voltages needed.

 

[bountyhunter]

My wife got a Windows-10 PC. Needed new operating system.
Tower, but the power supply is a wall wart. There is almost noting in the box. No slots. The whole thing could have gone in a box one inch thick. There is some advantage to having all the high voltage outside the box. (UL, VDE, CSA) The mother board takes the 12 volts and makes all the very low voltages needed.

My Sony 55" TV also has a "wall wart" external supply that puts out DC that plugs into the TV. The #1 advantage to having the supply outside is to easily be able to control the EMI that it spews and keep it from disrupting the circuitry it powers.

My wife got a Windows-10 PC. Needed new operating system.

They can't stop trying to give me Wndows 10 for free. Every time I turn my PC on, I have a half dozen offers to "upgrade" to Win 10 for free, but only for a limited time...... which has so far been FOREVER. They just can't understand I don't want it.

 

[ronsimpson]

They just can't understand I don't want it.

Windows-10: I have some programs that will not run on anything less. CAD
We are having problems with internet based programs (maps.google.com) that crash and run very slow. They are much better with Win-10.
Win-10 Mail is a problem. I like outlook express and know it well. So we have "Thunder Bird" doing mail. (actually both are running because I can't get Mail-10 to stop)
This is the last year TurboTax will run on XP.
I can see there will be no choice. Or you can wait for Windows-13. lol

 

[AnalogKid]

It is kind of astonishing we now can buy regulated off line switchers in a wall wart form factor for about $5..... not to mention 200W supplies that go in a PC sell for about $15. It's a new world.

Thanks to PC's, cameras, and phones, much of the new world has a completely skewed view of what stuff really costs. When the annual production of a gadget is a few hundred pieces a year, you can do the costing at Digi-Key online. When the worldwide consumption is 70 pieces per *second*, 24/7/365, things change.

Years ago I was at an Analog Devices seminar and the guy was talking about the device suggestions they get from customers and how he frequently has to explain the economics of semiconductor production. Much of their product line was subsidized by some high volume parts. His example - at that time Sony had standardized on one of their A/D converters for their cameras, and was buying 1,000,000 pieces per week.

ak

 

[elearn]

Am sorry, the discussion went off the topic, but no real simple explanation about my question! I would have expected the answer on the lines of - suppressing high harmonics is easy etc ... but there aren't really good technical answers apart from a few vague experiences.

 

[AnalogKid]

The reasons are a tight combination of technical and financial. See post #8.

ak

 

[Diver300]

Am sorry, the discussion went off the topic, but no real simple explanation about my question! I would have expected the answer on the lines of - suppressing high harmonics is easy etc ... but there aren't really good technical answers apart from a few vague experiences.

With a conventional power supply, the transformer has to run at 50 or 60 Hz, and the smoothing capacitor runs at near the output voltage.

With a switch-mode power supply, the transformer can be run at 20 - 100 kHz, and the smoothing capacitor runs at near the input voltage.

The amount of iron and the number of turns of wire on a transformer depend on the the voltage and the frequency. At 50 or 60 Hz, a small transformer will need maybe 2000 turns of wire and will weigh maybe 50 grammes. If the frequency is raised by 1000 times, there will be maybe 100 turns of wire, and the transformer will weigh 10 grammes.

So a switch mode transformer is much smaller, lighter and quicker to make, but is much more complicated to design and to design a circuit for.

The capacitor will be physically smaller and cheaper when smoothing the higher voltage, but lower current supply. On a normal power supply, the capacitor has to support the output for nearly 10 ms on 50 Hz (nearly 8.3 ms on 60 Hz) so the voltage will drop while it is supporting the output. On a switch-mode supply, where the regulator efficiency doesn't vary a lot as the supply varies, the efficiency stays fairly good for the whole cycle as the capacitor discharges and charges. On a conventional power supply, the regulator efficiency varies as the voltage changes, so in order to allow for the smoothing capacitor to discharge each cycle, the regulator has to be less efficient.

So the trade-off is complicated design, cheap manufacture and good efficiency or simple design, expensive manufacture and poor efficiency. Where millions of items are being made, cheap manufacture and good efficiency are important. Where a hobbyist wants to make one, simple design wins. Conventional transformers also have the advantage of trivial circuits on the high-voltage side, so far less risk of a shock when fault-finding.

 

[MikeMl]

And to add what Diver wrote, because SMPSs switch at 100s of kHz, they generate waveforms with rise/fall times in the ns range. This produces harmonics of their switching frequency that can be tuned on a radio receiver clear up to 100s of MHz. It is extremely difficult to suppress these harmonics by shielding the power supply and filtering the wires going into it. That reduces the radiated and conducted radio interference, but doesn't eliminate it.

I have been a Ham radio enthusiast for 53 years, during which I have seen the ambient noise level on the short wave bands go from natural atmospheric noise to the horrible mess it is today. All the problems started as SMPS started replacing linear power supplies.

In an urban setting, the average background HF noise level has increased by more than 40db ( a factor of 10,000) due to the widespread use of SMPS in CFL, florescent, and LED lighting, Plasma and LED TVs, chargers, heating and air-conditioning systems, all manner of consumer appliances, Solar systems, etc, etc.

About five years ago, I moved from typical US suburban neighborhood (1/4 acre lots) to a rural, farm-like setting, primarily because the RFI from my neighbor's SMPS made it impossible for me to enjoy my hobby. The noise level progressively got worse as more and more switchers came on line, to where I could no longer receive short wave signals (particularly 40meters, ~7.1MHz) from my regular friends.

My new area is sparsely populated; lots are 5 to 20 acres. We have underground power. I get to control what type of lighting I use (only incandescent, I stocked up before incandescent lamps were pulled off the marker) , and I test every appliance that uses an SMPS for RFI before deploying it on my property. I eliminate every SMPS I can by replacing it with an old-school linear power supply. I use the SMPS that I replace for target practice.

My ambient radio noise level at my own place is tenable, but if I point my rotatable beam antennas in different directions, I can detect the SMPS RFI noise that comes from my nearest neighbor's houses (about 1/4 mi away), and from the nearest town, about 15 miles away...

I can live with some SMPSs, but the problematic ones are typically the ones that come from China. The US market is flooded with Chinese products that are shoddily built, and where the Chinese manufacturers just plain lie about their Incidental Electromagnetic Radiation Compliance Testing...

 

[bountyhunter]

Am sorry, the discussion went off the topic, but no real simple explanation about my question! I would have expected the answer on the lines of - suppressing high harmonics is easy etc ... but there aren't really good technical answers apart from a few vague experiences.

Here is your question:

"I would have thought stepping down as a first step and then converting it into DC is much easier and efficient in terms of component requirements.
Any thoughts ?"

Stepping down first and then converting is the WORST possible design in terms of total cost and complexity. You are building two power converters when you could do the job with one. It doubles the component count and wastes energy because there are power losses associated with every conversion stage.

So, no, nobody does it that way.

 

[Diver300]

Here is your question:

"I would have thought stepping down as a first step and then converting it into DC is much easier and efficient in terms of component requirements.
Any thoughts ?"

Stepping down first and then converting is the WORST possible design in terms of total cost and complexity. You are building two power converters when you could do the job with one. It doubles the component count and wastes energy because there are power losses associated with every conversion stage.

So, no, nobody does it that way.

In terms of complexity, a switch-mode power supply has far more components than a mains frequency power supply. A switch-mode power supply has the additional stages of rectifying the supply voltage, smoothing it, and switching it on and off at high frequency. However, the additional components make the rest of the circuit cheaper and more efficient, so the extra complexity has big advantages.

 

[MikeMl]

... so the extra complexity has big advantages.

Except to users of the radio spectrum