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# [SOLVED]DC DC converter output voltage rise time

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#### stark43

##### Member level 1
Hello, I have a question. Let's say we are trying to increase the voltage with a flyback converter, how long does it take to reach the desired voltage, is there a formula for this? For example, I want to see 600 volts at the output. How soon can I reach 600 volts?

Solution
[ apologies for the above comment from Barry - your question does indeed have an answer - but it appears Barry needs definite figures to compute same ]

how much energy can you get from your power source ?

Energy stored in a cap = 0.5 C . V^2, and power = energy over time ( J / sec )

so for minimum time to charge, power must be maximum.

Or to re-arrange the above: time = energy / power available = 0.5C.V^2 / power avail

so for 1000uF, 600V, and 20 secs, power in the flyback must be ( average ) = 0.5 1000u. 600^2 / 20

= 180 watts.

90 watts = 40 secs and so on ...
Hello, I have a question. Let's say we are trying to increase the voltage with a flyback converter, how long does it take to reach the desired voltage, is there a formula for this? For example, I want to see 600 volts at the output. How soon can I reach 600 volts
How long does it take to go from Detroit to St. Louis?

It‘s a pointless question. There are many, many variables, and you’ve identified none of them.

[ apologies for the above comment from Barry - your question does indeed have an answer - but it appears Barry needs definite figures to compute same ]

how much energy can you get from your power source ?

Energy stored in a cap = 0.5 C . V^2, and power = energy over time ( J / sec )

so for minimum time to charge, power must be maximum.

Or to re-arrange the above: time = energy / power available = 0.5C.V^2 / power avail

so for 1000uF, 600V, and 20 secs, power in the flyback must be ( average ) = 0.5 1000u. 600^2 / 20

= 180 watts.

90 watts = 40 secs and so on ...

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[ apologies for the above comment from Barry - your question does indeed have an answer - but it appears Barry needs definite figures to compute same ]

how much energy can you get from your power source ?

Energy stored in a cap = 0.5 C . V^2, and power = energy over time ( J / sec )

so for minimum time to charge, power must be maximum.

Or to re-arrange the above: time = energy / power available = 0.5C.V^2 / power avail

so for 1000uF, 600V, and 20 secs, power in the flyback must be ( average ) = 0.5 1000u. 600^2 / 20

= 180 watts.

90 watts = 40 secs and so on ...
You seem to be answering some other question.

Hi,

you mention a "flyback".
* Do you use your own design with an SMPS IC? --then read datasheet / application notes / design notes given by the semiconductor. They tell you how to calculate and optimize on this.

What time do you expect? (in values with units, not "as fast as possible" ;-) )

A solution may be to monitor the voltage and generate a "supply_OK" signal. Mind that on power up signals may oversoot and ring. Thus it´s a good idea to add some time for the voltage to stablilze, even if in a "good" range.

Klaus

@barry - the two questions were: 1) is there a formula for this? - answer yes - see above

2) How soon can I reach 600V ? depending on power available and cap size the answer in seconds is per the formula supplied above also - easy peasy.

[ apologies for the above comment from Barry - your question does indeed have an answer - but it appears Barry needs definite figures to compute same ]

how much energy can you get from your power source ?

Energy stored in a cap = 0.5 C . V^2, and power = energy over time ( J / sec )

so for minimum time to charge, power must be maximum.

Or to re-arrange the above: time = energy / power available = 0.5C.V^2 / power avail

so for 1000uF, 600V, and 20 secs, power in the flyback must be ( average ) = 0.5 1000u. 600^2 / 20

= 180 watts.

90 watts = 40 secs and so on ...
I think this might be what I'm looking for, thank you.

• you mention a "flyback".
Good to see you here too, sir. I can give an example for clarity. I looked through the datasheet but couldn't see a calculation or a chart.
Flyback IC Datasheet : https://www.analog.com/media/en/technical-documentation/data-sheets/8304fa.pdf
--- Updated ---

[ apologies for the above comment from Barry - your question does indeed have an answer - but it appears Barry needs definite figures to compute same ]

how much energy can you get from your power source ?

Energy stored in a cap = 0.5 C . V^2, and power = energy over time ( J / sec )

so for minimum time to charge, power must be maximum.

Or to re-arrange the above: time = energy / power available = 0.5C.V^2 / power avail

so for 1000uF, 600V, and 20 secs, power in the flyback must be ( average ) = 0.5 1000u. 600^2 / 20

= 180 watts.

90 watts = 40 secs and so on ...
Shouldn't the rise time also depend on the frequency?

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Hi,
Good to see you here too, sir. I can give an example for clarity. I looked through the datasheet but couldn't see a calculation or a chart.
Flyback IC Datasheet
What I did: I opened the datsheet and did a search for "time".
The first hit is "Soft start time" = 11ms typ.
I guess this is what you want. If not: please answer my question about the expected time...

Klaus

O.k., you are asking aout a specific flyback IC, you should have mentioned this from the start. At first sight, there are no performance data for current limited operation of the device.

You can use LTspice with provided test circuit for LT8304 to evaluate the start-up behaviour.

In practice, flyback primary inductance and winding ratio are the only parameters available for optimization, peak current and timing are fixed.

If you tell us why this is important in your application, we likely can give you better answers to your question.

I had heard from a friend that it had to make very fast (in US order) adjustable changes between 50-90 volts, so I was curious and wanted to research it, but couldn't find the answer I wanted or instead, I didn't quite understand what parameters were important. For example, if it used a step-up converter and wanted to change the voltage quickly, what would it need to change? Or, I started to think about how I can shorten the rise time in the flyback design I use. As you said, this is something that can change depending on the structure. It was something I was wondering about for now, if I need calculation, I will consult you on a separate issue, thank you. However, if anyone wants to give general information, I will close the issue as solved after 2 days so that it will be a source of information.

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the freq of the converter has no effect on the rise time, excepting if it is very low, e.g. below 1 Hz.

the power of the converter is the most salient aspect.

I had heard from a friend
You should ask him for details..

it had to make very fast
nanoseconds ... or seconds...? It makes a big difference for us to give a good answer.

You say "adjustable" ... so not just at power up?
And if "adjustable" .. is "falling" also important? Because this is a completele different problem/soluton.

and wanted to change the voltage quickly, what would it need to change?
Basically what Easy peasy writes is true. Charging a capacitor is quite good calculable. If you want to spped up: reduce the capacitance, or increase the current.
But there are a lot of parameters that influence the behaviour.
* You can´t guarantee that a xx Watt power supply for 600V can deliver xx Watts when output voltage is just 50V...90V.
* a power supply may be output current limited
* ... or input current limited
* ...or cycle by cycle inductor current limited
* go in hickup mode at overcurrent
* may have a soft start mode .. that is activated just at power up .. or when the difference between setpoint voltage and output voltage is too big.
* output filters
* loop compensation / slope complensation.
* and for sure the load current

You may optimize a power supply for fast rise time ... or low voltage ripple ... or any other parameter.

Klaus

You should ask him for details..

nanoseconds ... or seconds...? It makes a big difference for us to give a good answer.

You say "adjustable" ... so not just at power up?
And if "adjustable" .. is "falling" also important? Because this is a completele different problem/soluton.

Basically what Easy peasy writes is true. Charging a capacitor is quite good calculable. If you want to spped up: reduce the capacitance, or increase the current.
But there are a lot of parameters that influence the behaviour.
* You can´t guarantee that a xx Watt power supply for 600V can deliver xx Watts when output voltage is just 50V...90V.
* a power supply may be output current limited
* ... or input current limited
* ...or cycle by cycle inductor current limited
* go in hickup mode at overcurrent
* may have a soft start mode .. that is activated just at power up .. or when the difference between setpoint voltage and output voltage is too big.
* output filters
* loop compensation / slope complensation.
* and for sure the load current

You may optimize a power supply for fast rise time ... or low voltage ripple ... or any other parameter.

Klaus