I would be interested in understanding the inner workings of a laptop power supply. In particular how does it automatically "adapt" to world voltages and frequencies. I suppose there must active components to achieve this. In particular I would be interested in schematics explaining the principle behind this. Thanks!

Edit: By power supply I mean the components that are inside the "brick" with AC in-DC out


First, most (maybe all) laptop power adapters are offline flyback converters.

Here's a simplified flyback converter:


simulate this circuit – Schematic created using CircuitLab

When SW1 closes, D1 gets reverse biased so no secondary current flows. This results in an increase of primary current linearly and then energy storage in the primary of XFMR1 according to \$\frac{1}{2}LI^2\$ (\$I\$ is the peak value of that linear-ramp-shaped current waveform).

When SW1 opens, all the polarities on the transformer reverse, D1 conducts and the energy stored in the primary winding of XFMR1 is transferred to the secondary (i.e. load).

NOTE: Actually, XFMR1 acts as a coupled inductor, not a transformer.

The voltage across the load is sensed and stabilized by the feedback & control unit by controlling the on-time duration of SW1.

Let \$t_c\$ be the on-time duration of SW1 and \$f\$ be the switching frequency, so the duty cycle can be defined as \$D = t_c f\$ (Note that the switching frequency can be between, for example, 20kHz and 300kHz).

The output voltage can be calculated as \$V_o = D * V_{DC} \$.

So, if \$V_{DC}\$ gets too low then the FB&C unit increases \$D\$ (By the way, \$D \$ cannot be higher than 50%, theoretically. In practice, most designers limit it to around 45%). Likewise, if \$V_{DC}\$ gets too high then the FB&C unit decreases \$D\$. The transformer is designed according to minimum and maximum input voltages so that the circuit can work between those input voltages (Input voltage adaptation).

Since mains AC is rectified and filtered to obtain a DC voltage (because the flyback converter needs DC on its primary), the mains frequency does not matter a lot (Mains frequency adaptation).

  • \$\begingroup\$ Nit-pick: the energy is stored in a magnetic field by the primary winding. By the time SW1 opens, the energy is in a form that both coils share. The overall process transfers energy from primary to secondary, but I don't like the description of the energy still being in the primary and the secondary pulling it out. Your way might be easier to understand / think about though. \$\endgroup\$ – Peter Cordes Oct 3 '16 at 14:26
  • \$\begingroup\$ I assume the mains frequency does not matter as long as it is very small compared to the switching frequency? \$\endgroup\$ – njzk2 Oct 3 '16 at 18:21
  • \$\begingroup\$ @njzk2 The flyback converter requires a fine-filtered DC input voltage. Thus, every offline converter (doesn't matter flyback or any other topology) has a rectifier and filter in its input stage to rectify and filter the mains voltage. It should be noted that rectification process is not affected a lot by the mains frequency -this makes the mains frequency to be an unimportant parameter. Thus, a well-designed offline converter can work even with 120Hz mains voltage. \$\endgroup\$ – Rohat Kılıç Oct 3 '16 at 19:39

You might want to look into SMPS power supplies or offline switchers.

It works like this:

1) ac voltage (85-265 Vrms) is rectified using a bridge rectifier.

2) Ripples are smoothed out using a capacitor essentially giving you a high voltage DC. The DC voltage (120V to 375V) depends upon the input voltage. This is the point where you lose frequency information and hence the power supply can adapt to any frequency (50 Hz or 60 Hz).

3) Whatever DC voltage you have is switched at high frequency using a specialized circuitry to generate a fixed voltage. If you have a lower DC voltage to start with, the circuit will simply adapt to it by increasing duty cycle. Here power supply is adapting to multiple voltages.


One simple answer is that it keeps accepting input voltage - up to a maximum of 250V - and "builds up" to the desired output voltage, charging a capacitor for when the AC cycle doesn't have enough power to power things. As soon as the output voltage is reached, it stops doing the conversion. If the input voltage never gets to the maximum, it just uses more of the input voltage until it gets to the desired output.

Thus for a 240V it may use (say) 10% of the AC cycle, while for 120V it may use (say) 20% of the AC cycle. And this is why it is insensitive to the AC's duty cycle - it frankly doesn't care, since it's going to produce DC output.

There were some older devices that used the AC cycle rate to drive something else: maybe a clock, or a refresh rate, or something else. It was there, so why not? The answer was simply that it made it harder for worldwide acceptance. Now most devices (re)generate their own cycles as necessary from a pure DC source.


The powersupply of a laptop has a primary (mains) part and a secundary (laptop) part. The incomming voltage on the primary part is rectified and the dc is then fed to some kind of (flyback) inverter. The output side of the inverter is connected to a rectifier and filter to produce the output power for the laptop. The inverter is governed in such a way that the incomming voltage is adjusted for and the desired output is obtained.

For more information on the subject of switched mode power supplies you can look on the internet


Start from the output - say it is 12 V DC. This is produced by a process of voltage regulation. That voltage regulator might take an input dc voltage range of 14 V to 30 volts and still produce a regulated output voltage of 12 volts.

So, imagine instead that the voltage regulator could work with an input DC voltage range of 120 V to 375 V DC. It's the same principle but uses a few more components.

That DC voltage range arises when you rectify (and smooth) an incoming AC range 85 V AC to 265 V AC.


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