Author Topic: Peak current through FETs given a duty cycle  (Read 1304 times)

Offline gregaryb

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Peak current through FETs given a duty cycle
« on: March 26, 2017, 10:28:33 AM »
Does anyone have a formula that will give me the peak current through the FETs primary coil given a duty cycle and voltage?

Offline Mads Barnkob

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Re: Peak current through FETs given a duty cycle
« Reply #1 on: March 26, 2017, 02:06:50 PM »
You can approximate the primary sinusoidal current from the DC bus voltage and reactance of the primary coil. To get a "RMS" value you need to find a time sum from frequency and on-time to get a duty cycle factor to multiply it with.

X(L) = 2 * pi * f * L

Primary peak current = DC bus voltage / X(L)
« Last Edit: May 08, 2017, 07:58:53 PM by Mads Barnkob »
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Offline gregaryb

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Re: Peak current through FETs given a duty cycle
« Reply #2 on: March 27, 2017, 05:54:54 PM »
So in other words just add a, for example, * 0.5 (for 50% duty cycle) factor to that AC formula?

Offline Mads Barnkob

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Re: Peak current through FETs given a duty cycle
« Reply #3 on: March 29, 2017, 10:09:23 AM »
So in other words just add a, for example, * 0.5 (for 50% duty cycle) factor to that AC formula?

To get the RMS value, multiply the peak current with square root(2) ~0.707. This value would be your RMS current if the coil is running continues wave (CW) mode. If you are running interrupted mode, at say 200 BPS, 200 us on-time, then you would have to multiply with a factor where you calculate the combined duty cycle from BPS and on-time.
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Offline E-Schmok

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Re: Peak current through FETs given a duty cycle
« Reply #4 on: May 08, 2017, 02:33:37 PM »
You can approximate the primary sinusoidal current from the DC bus voltage and reactance of the primary coil. To get a "RMS" value you need to find a time sum from frequency and on-time to get a duty cycle factor to multiply it with.

X(L) = 2 * pi * f * L

Primary peak current = DC bus voltage * X(L)

iam a litte bit confused Mads. I thought: Current is equal to DC bus voltage divided by X(L);not multiplied:o
Please clear up if i get it wrong.  ;)
« Last Edit: May 08, 2017, 03:02:37 PM by E-Schmok »

Offline Mads Barnkob

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Re: Peak current through FETs given a duty cycle
« Reply #5 on: May 08, 2017, 08:00:08 PM »
iam a litte bit confused Mads. I thought: Current is equal to DC bus voltage divided by X(L);not multiplied:o
Please clear up if i get it wrong.  ;)

Thank you for pointing out that error, it is of cause divided by, I am not trying to win a Nobel by reinventing Ohms law!  :-[
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Re: Peak current through FETs given a duty cycle
« Reply #5 on: May 08, 2017, 08:00:08 PM »

 


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