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Droop voltage- dc dc converter

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kbmanick

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hi,

In our application, we need the load current of 20Amps. But the dc dc converter is able to supply 25Amp. In this case, will there be any droop in the output?

i.e. why droop occurs? what is the importance?. Please explain in detail.
 

enjunear

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If your circuit is capable of supplying 25 amps, and you only draw 20 amps, there should be no issue with voltage droop. This is similar to asking, my car can drive at speeds up to 90 mph, if I drive 60 mph, will there be any issues? The car can perform adequately at speeds above what you are asking of it... same applies to most any circuit (keep it within it's limits, and it should work fine).

Why does voltage droop occur.... that's a tougher question. All standard switch-mode power supplies work on the principal of building up a field within an inductive element, and then using that inductive element to either increase (boost) or decrease (buck) the output circuit voltage when the switching element in the circuit changes position (the switch is generally a transistor, either bipolar or FET).

Another analogy that works well for SMPS is the bucket of water concept. Take a flyback converter, for example. When the circuit starts up, the FET on the primary side of the transformer closes, drawing current through the primary side of the transformer. When the switch opens, the current through the primary (input) winding of the transformer must keep flowing, but can't go through the (now open) FET switch. The only place for the energy to go is across the transformer and into the secondary (output) winding. Once the energy has been "dumped" to the output winding, the primary-side switch closes again, charging up the primary winding, and the cycle repeats. The energy that is transferred to the output gets stored up in a large output capacitor. This smooths out the ripple from the switching action of "dumping" energy into the output many times every second. The charge in the output cap can be related to the level of water in a storage tank... more the water stored, the higher the voltage (potential to do work).

This charging of the primary and AC transfer to the secondary winding is akin to filling up a bucket of water and dumping it into another bucket on the other side of "the fence" (the isolation between the primary and secondary side circuits). To make the SMPS circuit work correctly, the water (energy) being filled up on the input side, must equal the water (energy) being drained by the output. Too much water (energy) being transferred over, compared to what's being drained off (energy used by load), and the output voltage begins to rise (level in the storage tank begins to rise). Too little water (energy) being transferred, compared to what's being drained off (energy used by load), the output voltage begins to fall... you start using up some of the water in the storage tank (the large output capacitor). Once you start pulling more energy (water) out of the storage tank (capacitor), than you put into it on the charging cycles, the level (voltage) starts to drop.... thereby, voltage droop on the output. That's what will happen when you try to pull too much power out of a SMPS circuit; the charging action through the inductive element can't move enough energy (water) to keep the cap (tank) filled up.

A similar analogy can be made for all types of SMPS circuits, just slightly different implementations given the different topologies.
 

kbmanick

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hi,


Thanks for your explanation in detail.

Please explain : Why droop occurs during load transient?
 

v_c

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A droop can occur during a load transient, if the demanded current from the load is not provided quickly enough by the converter. When this happens, the voltage drops until the converter recovers and is finally able to provide the demanded current. Consider what happens when you are in the shower with a correct value of temperature (your choice of hot to cold water). Suppose somebody decided to flush the toilet, then there will be a demand for cold water to fill the toilet tank. And while this happens, the amount of cold water demand is not met and this results in lower value of cold water mixed into the shower and you get burned :) This is when there is droop in cold water supply which increases the overall temperature.
 

enjunear

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hi,


Thanks for your explanation in detail.

Please explain : Why droop occurs during load transient?

v_c has a good example. Another way to think about the transient is to recall that a switching converter uses a feedback loop to increase and decrease the duty cycle on the switching element (transistor), depending on the load's current draw. When a fast transient occurs, the output voltage may fall as the storage capacitor is drained. The circuitry in the feedback loop may not be fast enough to "follow" the change in energy demand, so the output cap starves until the loop catches up and starts "pumping more water", to refill the output capacitor. The wider the bandwidth of your feedback loop, the faster it can respond to transient events, but you have to weigh that performance aspect against other measures of the feedback loop's capability.
 

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