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I was playing around disassembling an Asus laptop power brick (19V, 4.74A) and came across this strange arrangement in the circuit, where just after the bridge rectifier, there is a MOSFET (Q1, N channel) that can make a <1 Ohm connection from the + supply (330Vdc) to the negative. This seems to be its only function, and seems it can only result in a blown input fuse. Further down the line as you can see in my rough schematic is of course the switching MOSFET (Q2) feeding the SMPS transformer. Any of you engineers out there, what the heck is the purpose of this ludicrous circuit, if it is there to protect from overvoltage/spikes, wouldn't a MOV be much cheaper than the MOSFET, and wouldn't result in a bricked device after such an incident? I'm quite curious to hear your thoughts! Thanks in advance!

EDIT I kind of lied in the schematic, L4 is actually some kind of transformer, whose secondary goes towards the MCU, it is seen on the board as the yellow tape clad transformer with the wide copper foil strip over top. FYI

enter image description here

enter image description here

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    \$\begingroup\$ How do you know Asus made it?, there are hundreds of Chinese clones. \$\endgroup\$
    – Voltage Spike
    Aug 25, 2021 at 13:37
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    \$\begingroup\$ Impossible to be sure without having it in my hands, but Q1 seems to be a part of voltage boosting switch mode regulator which is in use with low (110 V)mains AC voltages. That makes the isolated voltage regulator simpler. \$\endgroup\$
    – user287001
    Aug 25, 2021 at 13:39
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    \$\begingroup\$ If it came with the laptop then it's genuine, if you purchased it then probably not, there are more clones than genuine supplies, they all have similar silkscreen \$\endgroup\$
    – Voltage Spike
    Aug 25, 2021 at 13:41
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    \$\begingroup\$ it's possibly for power factor correction with a sine wave controlled PWM dont. Assume ASUS is wrong \$\endgroup\$ Aug 25, 2021 at 13:43
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    \$\begingroup\$ There's inserted more info to the question after my guess. The circuit is more complex than it was originally said to be. The comment about voltage boosting if needed to make possible to simplify the actual PSU circuit should be considered only as a guess based on sparse information. The attempt to keep the current taken from the mains AC more sinusoidal (=PF correction for electronic loads) can as well be the original major goal and the possibility to make the actual PSU simpler due normalized input voltage can be only a lucky side product. \$\endgroup\$
    – user287001
    Aug 25, 2021 at 14:32

2 Answers 2

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As the other answer points out this looks like active power factor correction.

Power factor is the ratio of real power (short-term average rate of net energy transfer) divided by apparent power (RMS voltage times RMS current). The maximum possible power factor is 1 and is achived when the current waveform is exactly proportional to and in phase with the voltage waveform.

note that a lot of sources will talk about power factor in terms of phase, that worldview makes sense when you are discussing AC motors but doesn't really make sense when discussing rectifier-capacitor circuits.

Traditionally in a switched mode power supply you had a rectifier immediately followed by a large "primary capacitor". The problem with this setup is it's power factor is awful, current is drawn in small spikes close to the peak of the AC waveform and no current is drawn at other times.

The power factor can be improved somewhat by adding a large inductor in series with the input, this reduces the rate of change of current and hence spreads the spikes of current out a bit, it is known as "passive PFC".

Active PFC like your power supply has takes things a step further, it adds a boost converter between the rectifier and the main primary capacitor. In your schematic this boost converter is formed by Q1, L4 and the diode above Q1.

When Q1 is turned on charge builds in L4, when it is turned off L4 discharges into the capacitor. This allows current to be drawn from the mains in a controlled manner throughout the cycle and hence allows a very high power factor to be achieved. A power factor of 0.99 is often claimed for power supplies with active power factor correction.

It also allows a consistent voltage on the primary capacitor to be maintained regardless of line voltage changes. This is useful when implementing a universal-voltage power supply. The energy stored in a a capacitor depends on the square of voltage, so a 3x variation* in voltage translates to a 9x variation in stored energy.

* Universal voltage power bricks are designed for a nominal input voltage range of 100-240V, the actual voltage range will be wider to allow for supply voltage variation, so the full input voltage range is generally about 3x..

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  • \$\begingroup\$ Is that really power factor - it seems like you're describing harmonic distortion. When the current has a spike at the peak of the voltage waveform (and consequently the voltage waveform gets flattened at the peak) then the current and voltage are still technically in phase, but there's loads of harmonics at higher frequencies. It seems like the circuit is a low pass filter? \$\endgroup\$ Aug 25, 2021 at 22:23
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    \$\begingroup\$ And those harmonics reduce the power factor, because they contribute to the apparent power, but not the real power. \$\endgroup\$ Aug 25, 2021 at 22:24
  • \$\begingroup\$ Yeah I guess that's true, I've just never seen harmonic distortion described that way before. \$\endgroup\$ Aug 25, 2021 at 22:26
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    \$\begingroup\$ @DavidWaterworth Harmonic distortion is one of the mechanisms that lower the power factor (the other major mechanism being phase shift). A power factor of 1 means a resistive load where the current curve is just some fixed multiple of the voltage curve. In other words, the power factor measures how well the two curves match. Anything that makes the shape of the current curve deviate from the voltage curve lowers the power factor (no matter if the deviation is caused by shifting the current curve sideways or distorting it). \$\endgroup\$
    – TooTea
    Aug 26, 2021 at 8:15
  • \$\begingroup\$ Yeah I found a paper which explains the relationship between THD and PF - users.ece.utexas.edu/~grady/POWERFAC.pdf \$\endgroup\$ Aug 28, 2021 at 3:01
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That's the power factor correction circuitry.

The FET Q1 will be switching at high frequency so it won't short rectifed 300V to 0V as there are inductors in the path, most likely L4 is the boost inductor to convert the voltage higher and store it in the bulk capacitor C1.

But yes, obviously if FET Q1 gets permanently turned on, it will short the 300V to 0V when inductors saturate and it will blow the fuse.

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  • \$\begingroup\$ Ah I had a hankering it might be PFC but didn't recognize the inductor config. Could you just give me a quick rundown which cap and inductor charge/discharge to fix PF, I've numbered them in the schematic. Thanks! \$\endgroup\$
    – parkside
    Aug 25, 2021 at 13:37
  • \$\begingroup\$ @parkside what is PF? \$\endgroup\$
    – user253751
    Aug 25, 2021 at 13:56
  • \$\begingroup\$ @user253751 PF(C) power factor (correction) :) \$\endgroup\$
    – parkside
    Aug 25, 2021 at 14:02
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    \$\begingroup\$ @parkside oh, misread. Is it not C1? (L3/C4 looks like filtering, and it's before the diode anyway) \$\endgroup\$
    – user253751
    Aug 25, 2021 at 14:22
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    \$\begingroup\$ C1 buffers the voltage going into the SMPS transformer, L4 is the inductor participating in the boost converter circuit that acts as PFC. \$\endgroup\$
    – parkside
    Aug 25, 2021 at 15:28

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