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LadyN

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Joined: 26/01/2019
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Posted: 07:22pm 01 Feb 2019

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How does a boost convertor have negative input impedance?

As I start to study boost convertors, I am having many questions. One of them being about their negative input impedance.

Information about this is all over the place.

I also saw an article that tries to explain why the equivalent resistance of the BOOST convertor is negative: https://www.element14.com/community/docs/DOC-65505/l/anp008-negative-input-resistance-of-switching-regulators

However, I did not understand their point although I understood the utexas document below:

http://users.ece.utexas.edu/~kwasinski/EE462LS14.html

Look for "Boost converter class notes (ppt file)" under "Week 6 documents:"

Click on it to get the file _7_EE462L_DC_DC_Boost_PPT.ppt

From page 16 onwards they talk about Impedance matching and very next slide about PV and BOOST

As you can see: the computed equivalent resistance of the BOOST convertor is positive unless Rload can go negative.

So my questions are:

1. Is their derivation of the equivalent resistance of the BOOST convertor wrong?
2. IF NOT, under what condition does Rload go negative?

Can you all explain or send me links to understand negative input impedance bits?

Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406

Posted: 09:57pm 01 Feb 2019

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Its not just boost converters that have this negative input impedance phenomena.

Take a plain ordinary resistive load, ohms law will tell you that as the applied voltage increases, the load current will increase in proportion. That is a normal expected "positive" resistive behavior if you like.

Now suppose you have some kind of voltage regulated switching power supply connected to the mains supply, and you load the thing up so it draws 50 watts from the grid.

What happens if the voltage on the grid suddenly drops to half ?
The switching power supply compensates for the reduced input voltage by working a bit harder to maintain the output voltage constant, and the voltage and current into the 50 watt load remain as before.

But what has happened at the input ?
The grid voltage has halved, but the input current has doubled !

Exactly the opposite of ohms law behavior. If the voltage halves, the current is "supposed" to halve according to Mr Ohm.

So the input to our switching power supply can be said to behave like a "negative" resistance where it behaves exactly opposite to a real resistor.

If the power source is the grid, or a battery, or some low impedance it does not matter. Solar panels act more like a current source of high impedance.

Many types of switching power supplies are completely incompatible with direct connection to solar panels.
If the load on the panel increases, the voltage from the panel falls.

That causes the switching power supply to draw higher current, which causes a further fall in voltage.
Unless you can build in some kinds of modified control system to solve this problem, an off the shelf switching power supply that tries to draw higher current with falling input voltage is not going to work well, if at all from a solar panel source.

Not all switching power supply typologies have this problem, but most do.

Buck converters work fine, but they can only reduce the voltage.
Boost converters (that have a voltage regulated output) definitely fall into the class of drawing a higher input current as the input voltage falls.Edited by Warpspeed 2019-02-03

Cheers, �Tony.

LadyN

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Joined: 26/01/2019
Location: United States
Posts: 408

Posted: 10:51pm 01 Feb 2019

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Warpspeed said
So the input to our switching power supply can be said to behave like a "negative" resistance where it behaves exactly opposite to a real resistor.

Warpspeed, THANK YOU!

Warpspeed said
If the power source is the grid, or a battery, or some low impedance it does not matter

Why not?

Let's consider a battery. If its voltage drop by half but we draw double the current, won't it affect the battery and make its voltage drop further?

We would, in this case, rather prefer a high impedance source?

Warpspeed said Buck regulators work fine, but that can only reduce the voltage

I don't follow. If the Buck regulator was to regulate the output power, and the input voltage dropped, it would STILL have to draw a higher current from the input to maintain regulation, just like a boost regulator would.

Obviously I am missing something: what?

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