How do power supplies that accept 100-240 VAC input work?

How do they produce a consistent voltage output for such a large range of input voltages?



What happens to their current draw as the input voltage changes? Does it change to keep the power input constant?How do power supplies that accept 100-240 VAC input work?
They use a full wave bridge and capacitor to generate 140 to 350 volts DC then apply that to a high voltage high frequency chopper to convert it to AC of a high frequency, then to a transformer for isolation. High frequency (usually in the range of 20kHz to 100 kHz) reduces the size of the transformer. The output of the (step down) transformer is rectified and filtered and is the output DC voltage. It is applied in a feedback loop (optically isolated) back to the chopper where the duty cycle is varied to adjust the output voltage to the value desired.



Yes, current input decreases as input AC voltage increases.



.How do power supplies that accept 100-240 VAC input work?
Variable input power supplies (simple ones) are based on bridge rectifiers. See here for an example:



http://www.electronickits.com/kit/comple



Basically, they allow voltage to pass up to a certain level, and block all voltage above that level. This is the reason that the output voltage is generally (but not always) lower than the input voltage. It's also the reason the output voltage is usually DC and the input is AC. A laptop power supply is a good example of this.



Once you have DC, you can use a transformer to lower the voltage level to the one you want. But they key is that the diode bridge will first lower the DC so that it is the same regardless of the input AC by capping the input AC.



For two equal devices (say two laptops) used at 120V and 240V, the input current should be half for the 240V system since the final power requirements should be the same for the two laptops.How do power supplies that accept 100-240 VAC input work?
in smps supplies it is done by pwm(pulse width modulation) technic. regulation is done by varying the width of the pulse. for lower voltages at the input we need to increase the width and for higher voltage at the input width is reduced.