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Dual Output DC/DC Converter Softstart

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drkidd22

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Hello,

I'm working on a circuit that requires soft-start. The converter has a capacitive load limit of 200uF at each output, but my application requires energy storage caps of at least 1500uF. Any advice or circuit example on how to do a softstart on the output of the converter?
 

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ronsimpson

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It's required to be on the output.
A 1000uF cap on the 24V will look like a 1500uF on a 15V output.
A 2200uF cap on the 24V will look like 1500uF on each output.

Q42 will not work like that.

What are the loads like?
Current +19V to ground?
Current -11V to ground?
Current +19V to -11V?
 
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ronsimpson

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One MOSFET.
Make RC longer.
Diode across R238 and 2.7k resistor to make gate voltage discharge faster. (other wise the Gate voltage will not discharge)
Gate charges to 15V not 30V.
upload_2017-3-8_19-36-50.png
 

ronsimpson

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Different approach:
Forget time!!!!
Here is "current limit".
At start up M2 will hold the charge up current to 70mA. Then when the caps are charged M2 is turned on hard by 15 volts.
upload_2017-3-8_20-41-31.png
Red and Green are your two out puts. Blue is in put.
upload_2017-3-8_20-46-24.png
If the top of R16 goes to the center (15V) then remove D1. (The Gate to Source voltage must not get above 20V)
 

drkidd22

Member
Different approach:
Forget time!!!!
Here is "current limit".
At start up M2 will hold the charge up current to 70mA. Then when the caps are charged M2 is turned on hard by 15 volts.
How is the charge being held at 70mA? Can you please explain.
 

ronsimpson

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In your circuit #7; the charging current is limited by the R2 & R3. These resistors will kill any hope of voltage regulation. If the load varies the voltage across the resistors will change. This causes your output voltage to change with load. Also the RC time on the gate of the MOSFETs could limit the charging current for 0.1 second. Not useful.

I know this supply (isolator). It does not like over current. (Please plot the current in R2 or R3).

In circuit #6; The charging current goes through R11 which is 10 ohms. (much smaller than your R2+R3)
When the voltage across R11 reaches 0.6V then Q1 turns on and pulls down the voltage on M1. (turn off)
If the current is low R16 turns on M1. This way the charging current is held at 70mA which is close to the "max" of 66mA.
 

ronsimpson

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Current from isolator. During capacitor charging 70mA. After charging the current is from the load or 30mA in my case.
upload_2017-3-9_7-50-40.png
 

drkidd22

Member
Here are the current plots. The circuit will be used to provide gate voltages for an IGBT gate driver/buffer configuration.
It's 120mA. I have no load on it.

Screen Shot 2017-03-09 at 8.17.12 PM.png
 
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