What is the effect of switching noise?

A long time ago I remember a series of adverts for domestic cookers with the tag line "what do pros use when the ingredients cost more than your cooker ?" .

Well, a phone or a laptop can easily cost more than £500 - so we are in that territory.

You need a power supply you can trust with a current limit that works and where you don't get overshoot  - this is the minimum you should contemplate.

But phones are susceptible to interference and switchers (especially the kind in the video) make lots of noise - I've measured 500mV pk-pk and more from Chinese 3780 based designs - expect odd results testing a phone with a very noisy supply.

Even very basic linear supplies tend to do better on current limiting than a 3780 based design (the 3780 was no designed for this purpose !).

I assume that you have an oscilloscope (can't fix phones and motherboards without) so you could test a power supply - some of the Farnell Roadtests show you how to do it. What goes wrong with the bad ones is that the the current limit can take a long time to start operating and that the output voltage will overshoot a lot if you remove some load so that the supply comes out of current limiting. This can happen easily with a mobile phone - the transmitter starts up and it draws a load of current, then it stops transmitting and the power supply overshoots by a volt or two and almost anything can happen next.

Good fast and precise current limiting is almost impossible with a pure switcher like the board in the video - good commercial switch mode bench supplies use either very fancy switchers or a linear regulator after the switching regulator.

If you need to save the money by far your best bet is to buy a second hand linear supply - I recently bought some  AimTTi PL155-PPL155-P 15V 5A supplies for about £60. You could find something cheaper if you look around.

MK

A long time ago I remember a series of adverts for domestic cookers with the tag line "what do pros use when the ingredients cost more than your cooker ?" .

Well, a phone or a laptop can easily cost more than £500 - so we are in that territory.

You need a power supply you can trust with a current limit that works and where you don't get overshoot  - this is the minimum you should contemplate.

But phones are susceptible to interference and switchers (especially the kind in the video) make lots of noise - I've measured 500mV pk-pk and more from Chinese 3780 based designs - expect odd results testing a phone with a very noisy supply.

Even very basic linear supplies tend to do better on current limiting than a 3780 based design (the 3780 was no designed for this purpose !).

I assume that you have an oscilloscope (can't fix phones and motherboards without) so you could test a power supply - some of the Farnell Roadtests show you how to do it. What goes wrong with the bad ones is that the the current limit can take a long time to start operating and that the output voltage will overshoot a lot if you remove some load so that the supply comes out of current limiting. This can happen easily with a mobile phone - the transmitter starts up and it draws a load of current, then it stops transmitting and the power supply overshoots by a volt or two and almost anything can happen next.

Good fast and precise current limiting is almost impossible with a pure switcher like the board in the video - good commercial switch mode bench supplies use either very fancy switchers or a linear regulator after the switching regulator.

If you need to save the money by far your best bet is to buy a second hand linear supply - I recently bought some  AimTTi PL155-PPL155-P 15V 5A supplies for about £60. You could find something cheaper if you look around.

MK