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Boost converter circuit

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j.azz

Fri Feb 06 2009, 02:51PM

Registered Member #888 Joined: Tue Jul 10 2007, 06:52PM
Location: Hannover, Germany
Posts: 40

I guess it's better to sense the current between the mosfet and ground?!
Then the booster can run even during the high switch-on current in the inductor.

latest simulation:
red - inductor current
turquoise - mosfet current
blue - gate voltage
green - cap voltage

KLH

Fri Feb 06 2009, 05:21PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

j.azz wrote ...
I guess it's better to sense the current between the mosfet and ground?!
Then the booster can run even during the high switch-on current in the inductor.

Putting the current sensing resistor between the MOSFET and ground only allows you to sense the current when the MOSFET turns on, and not when the inductor is discharging its stored currrent into the capacitor bank. The sensing resistor should be between the inductor and Vcc+. This somewhat surprised me too when I found this out, but this allows you to sense the current thourgh the inductor at all times.

As for determining the timing, you should have control over both the peak current and the "dI/dT near zero" sensing.

GeordieBoy

Fri Feb 06 2009, 11:18PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The current sense resistor goes in series with the MOSFET source lead and will likely be some fraction of an ohm. It only senses the current through the switch, but this is okay since exceeding the maximum safe current turns off the switch instantly. Once the switch is turned off the inductor current will then fall at least slightly before the next switching cycles starts. You can't see the falling current in the boost inductor because your shunt is in series with the MOSFET source, but that is fine - It is the MOSFET you are aiming to protect using peak current mode control.

Regarding the turn-on current surge through the boost inductor to charge the output capacitor: It is normally undesirable to turn on the MOSFET during the initial inrush current through the boost inductor. This current can be many tens of amps depending on the boost inductor value and the output capacitor value. If the MOSFET turns on and picks up this large inductor current it may be destroyed. The easiest way round this problem is to bypass the boost inductor and output diode with an "inrush diode". This inrush diode redirects the large current surge at power-up away from the boost inductor and the fast rectifier. Once the output capacitor is charged to the input voltage and the surge has passed, the operation of the boost converter can commence without worry of damage to the switch or fast diode.

-Richie,

KLH

Mon Feb 09 2009, 08:52PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

GeordieBoy wrote ...
It only senses the current through the switch, but this is okay since exceeding the maximum safe current turns off the switch instantly. Once the switch is turned off the inductor current will then fall at least slightly before the next switching cycles starts. You can't see the falling current in the boost inductor because your shunt is in series with the MOSFET source, but that is fine - It is the MOSFET you are aiming to protect using peak current mode control.

You could protect your MOSFET, but you don't want a fried inductor. The inductor current should be sensed, and it should be allowed to fall to zero (but don't let it stay at zero). If you use a fixed off time, the inductor current (depending on your inductance and frequency) can spiral out of control until the load impedance (the capacitor) is high enough to drain the inductor of its stored energy each cycle. This can lead to very high currents (possibly 100s of amps) that will kill your inductor and MOSFET.

MachineShop

Wed May 06 2009, 01:08AM

Registered Member #2107 Joined: Mon May 04 2009, 05:02AM
Location:
Posts: 12

i wanted tyo build one but then i realised its easyer to just use the camera circuit but it seems like they burn out rather quickly... it there anything i can do to the circuit to protect it from burning out so quickly? i imagine since these circuits were made for "disposable" cameras they made them just good enough to last for 30 some odd flashes. any ideas for reinforcing?

MachineShop

Wed May 06 2009, 07:43AM

Registered Member #2107 Joined: Mon May 04 2009, 05:02AM
Location:
Posts: 12

over the past 24 hours i have been trying to decide on an alternative circuit to use and it came to me. a taser?
would this even be plausable for use? i know it would esentialy be safer that this other stuff. low amps high volts but i cant fathom using 100,000+ volts for this there must be a way to govern the voltage down any ideas about how this could work if at all?

MachineShop

Fri May 08 2009, 12:29PM

Registered Member #2107 Joined: Mon May 04 2009, 05:02AM
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Posts: 12

bump

Steve Conner

Fri May 08 2009, 01:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

We don't do bumping!

(part II.g)
The conditions for double posting are: 48 hours elapsed AND useful new information to post

MachineShop

Sat May 09 2009, 09:40PM

Registered Member #2107 Joined: Mon May 04 2009, 05:02AM
Location:
Posts: 12

my apologies

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