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How to turn on a buck transistor via a bootstrap circuit?

In my circuit, I use a AVR microcontroller to turn on and off a buck switch. What circuit should I use here?

Previously, I used optocoupler which does not have a fast response. I like to turn on and off the transistor for no more than 500 ns on/off transition.

There are some high side low side ICs which are designed for bridges and I am not sure if they would work for here.

Solutions with BJT/Capacitor/Diodes are very pleasant to me though I dont limit myself to only those solutions.

gate driver

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You're using an N-channel MOSFET, for it to come into an "on" (low-resistance) state Vgs must be large, let's say 5V. The highest voltage you have is 36 V (+ node of battery). Even this will not fully turn-on the NMOS. You would need at least 41 V on the gate. Instead I would use a PMOS, then you can just pull the gate low. But to switch it OFF the gate of the PMOS must be pulled to 36 V ! An AVR cannot handle this so you would need an additional transistor to pull the gate of the PMOS down.

A 1n4001 is not fast enough for these type of applications, better use a Schottky diode.

There is a fundamental flaw in your circuit! It has no current limiting ! This is an issue, especially when the battery pack has a low voltage and the inductor saturates. The current would only be limited by the series resistance of the batteries and this is BAD, avoid at all cost ! (Your wires could melt !)

Have a look at this document form TI and learn how it's done !

If you want to actually build this then I strongly advise you to use an IC for that but pay attention to the 36V battery voltage, not all ICs can handle that. Also there are a lot of ready-built modules for sale on eBay, that would be more convenient as the layout of a switched converter is not something for a beginner.

In general, you don't want an AVR or any micro Controller to control a DC-DC converter switch. They're too slow, software could get stuck with the switch turned on. The current will be uncontrolled. AVOID !!!

I do not understand what you mean by "designed for bridges" ?

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  • \$\begingroup\$ designed for bridges” -I think he means full bridge/half bridge MOSFET drivers. \$\endgroup\$ – Dzarda Jun 4 '15 at 13:51
  • \$\begingroup\$ @Dzarda yes, I mean haf/full bridge \$\endgroup\$ – barej Jun 4 '15 at 14:06
  • \$\begingroup\$ OK, I see. Bridge in the context of motor drivers. That would be highly unusual and not solve any of the other pitfalls. \$\endgroup\$ – Bimpelrekkie Jun 4 '15 at 14:11
  • \$\begingroup\$ I use PWM mode of microcontroller which is independent from code and is controlled by my microcontroller block. Though my circuit has not resistor to limit, it has an inductor which limits the current provided that the duty cycle is under control. In my circuit I need to do balancing between batteries too. So I cannot escape of bootstrap. And my voltage is high. each battery pack is 2x3.7 volt and there are four of them. So could you give solution for bootstrapping? \$\endgroup\$ – barej Jun 4 '15 at 14:18
  • \$\begingroup\$ Also, could you explain how P-Channel Mosfet should be bootstrapped in my case? \$\endgroup\$ – barej Jun 4 '15 at 14:22

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