Power Balancing in DC-DC Converters - Electronic Circuits Projects Diagrams Free

dave_dj88 · 16th August 2008, 08:07 PM

hey all!

What happens if power input to a converter is greater than power output from the converter?
Where does the excess power go? Is it dangerous to the load and/or the converter?

We learned that in SMPS (PWM) converters, there is a need for the inductor to release the current it gained during the ON time of the duty cycle. Thus, if power input is greater than power output, does the current in the inductor keep rising, resulting in it blowing up?

Thanks in advance for your valuable help.

Diver300 · 16th August 2008, 11:43 PM

In any power supply, the power input will always be larger than the power output. The extra power becomes heat in the power supply.

Any power supply will have rated input voltage, and it will take whatever power from the supply that it needs. You can't force it to take more power than it needs just by connecting it up with bigger cable.

The mains sockets in your house can run big heaters that are up to 3 kW on 230 V ac supplies or about 1.5 kW on 120 V supplies, but it is fine to plug a cellphone charger that is rated at 5 - 10 W into the sockets, and it will only take a few mA from the mains.

The inductor in a SMPS has a maximum current. It is usual for the control circuit to limit the current to a bit less than the maximum.

The capacitors on SMPSs are large enough that the input current has to turn on and off many times to get the output voltage up from zero to the rated output. So when the output voltage gets up to the rated voltage, the input current will turn off, and the energy stored in the inductor will end up in the output capacitor. That will only be a very small voltage rise over the rated voltage. The voltage control circuit will then not turn the input current on again until the output voltage has fallen to less than the rated voltage.

audioguru · 17th August 2008, 12:07 AM

I think if the input power to a power supply exceeds its output power then it will make a Black Hole which will suck up all matter.

No, it will glow red hot and it might cook itself to death.

dave_dj88 · 17th August 2008, 12:15 PM

Quote:

Originally Posted by audioguru

I think if the input power to a power supply exceeds its output power then it will make a Black Hole which will suck up all matter.

No, it will glow red hot and it might cook itself to death.

Are you serious?

audioguru · 17th August 2008, 04:28 PM

Quote:

Originally Posted by dave_dj88

Are you serious?

Sometimes I am serious.

An electronic circuit draws only as much power as it needs. It doesn't take all the power that is available.

dave_dj88 · 17th August 2008, 05:43 PM

Hmm, ok.
I came up with this doubt when reading about a DC-DC converter with a PV panel as its input, supplying a load.

The PV panel is biased to operate at a certain point (its Maximum Power Point). But the load needs less than this maximum power.
The text said a regulator then changes the biasing point of the PV to make the power on either side of the converter equal.
So, what I understand is that for a very short while, power into > power out of converter. And the difference cannot be accounted for solely by heat in the conductors, can it?
I assumed that there is harm in keeping the input power too high, and that's why there is a control system to bring the power down.

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16th August 2008, 08:07 PM	(permalink)
dave_dj88	Power Balancing in DC-DC Converters hey all! What happens if power input to a converter is greater than power output from the converter? Where does the excess power go? Is it dangerous to the load and/or the converter? We learned that in SMPS (PWM) converters, there is a need for the inductor to release the current it gained during the ON time of the duty cycle. Thus, if power input is greater than power output, does the current in the inductor keep rising, resulting in it blowing up? Thanks in advance for your valuable help.

16th August 2008, 11:43 PM	(permalink)
Diver300	In any power supply, the power input will always be larger than the power output. The extra power becomes heat in the power supply. Any power supply will have rated input voltage, and it will take whatever power from the supply that it needs. You can't force it to take more power than it needs just by connecting it up with bigger cable. The mains sockets in your house can run big heaters that are up to 3 kW on 230 V ac supplies or about 1.5 kW on 120 V supplies, but it is fine to plug a cellphone charger that is rated at 5 - 10 W into the sockets, and it will only take a few mA from the mains. The inductor in a SMPS has a maximum current. It is usual for the control circuit to limit the current to a bit less than the maximum. The capacitors on SMPSs are large enough that the input current has to turn on and off many times to get the output voltage up from zero to the rated output. So when the output voltage gets up to the rated voltage, the input current will turn off, and the energy stored in the inductor will end up in the output capacitor. That will only be a very small voltage rise over the rated voltage. The voltage control circuit will then not turn the input current on again until the output voltage has fallen to less than the rated voltage.

17th August 2008, 12:07 AM	(permalink)
audioguru	I think if the input power to a power supply exceeds its output power then it will make a Black Hole which will suck up all matter. No, it will glow red hot and it might cook itself to death. __________________ Uncle $crooge

17th August 2008, 05:43 PM	(permalink)
dave_dj88	Hmm, ok. I came up with this doubt when reading about a DC-DC converter with a PV panel as its input, supplying a load. The PV panel is biased to operate at a certain point (its Maximum Power Point). But the load needs less than this maximum power. The text said a regulator then changes the biasing point of the PV to make the power on either side of the converter equal. So, what I understand is that for a very short while, power into > power out of converter. And the difference cannot be accounted for solely by heat in the conductors, can it? I assumed that there is harm in keeping the input power too high, and that's why there is a control system to bring the power down.