Hi all
I'm very new to electronics design. i have designed a circuit with IR2011 High and Low side NMOS driver. Mosfet used is BSC0702LS
Intention is use High Side switch to turn on nmos to charge the DCDC converter with 50ohm resistor. and once the voltage reached desired level. Low side nmos will be used to switch on a contactor for DCDC converter.
However, i don't seem to be able to charge the capacitor to more than 5v.
also i see 5v on the high side output and gate when load is disconnected, even when HIN is pulled low, not sure if it's normal.

Below is the schematic.


i would really appreciate if anyone can help me out.
Thanks
Richard

 

I do not see any DC/DC converter with a 50 ohm resistor. I see Q1 apparently connected to pull down the low side of a contactor coil that should operate the contactor that I do not see to power the converter to charge a capacitor that I do not see.
In theory this is supposed to do something when the IR2011 is given some kind of signal.

So what I see is a portion of a system without an adequately detailed explanation of just how it is supposed to function.
In order to design any system that actually works, it is common to understand what each input to each element does, and what signal level results at the output of that segment, and where that output goes, and what it commands the next element to do. This is the normal system design process, and it offers a real benefit in that if I find that an output of one element is not what should make the next element do what is needed, I can stop and revise the string of elements. Quite often, finding an error in the string will allow a correction before time is even spent on an actual drawing. The results of this process usually perform as desired at first actual "power on."
Why is there a need to pre-charge a capacitor, anyway????

 

Hi MisterBill2
Thanks for the reply.
The reason for this design is. when I connect a DCDC converter(48V to 24V 120A), there is surge current which will weld my contactor on every time. the contactor is rated for 200A. so need to turn on the converter slowly then turn it on with contactor.
here is the similar design that works and he explained the need for such control circuit. https://www.hackster.io/CharlieO/solid-state-precharge-26b86e

CH1_L and CH2_H are the output of MCU(5V). CH1_PRECHARGER_OUT is going to converter input. CH1_Contactor is going to contactor coil.

Richard

 

What is U8? It looks like a resistor with three terminals.
it would be an awful lot simpler if you just switched the resistor with a P-channel MOSFET.

 

What voltage output/waveform does CH1_PRECHARGER provide?
Unless it is repetitively charging C13 the bootstrap arrangement won't work, so the HO output of the IR2011 won't be boosted above 5V.

 

I think @Alec_t got it. This type of driver must be switching constantly to operate. It is not appropriate for what you are trying to do.

And I agree with Ian, just use a P-channel. Dissipation will not be a problem since it is only on for a short time.

 

Hi MisterBill2
Thanks for the reply.
The reason for this design is. when I connect a DCDC converter(48V to 24V 120A), there is surge current which will weld my contactor on every time. the contactor is rated for 200A. so need to turn on the converter slowly then turn it on with contactor.
here is the similar design that works and he explained the need for such control circuit. https://www.hackster.io/CharlieO/solid-state-precharge-26b86e

CH1_L and CH2_H are the output of MCU(5V). CH1_PRECHARGER_OUT is going to converter input. CH1_Contactor is going to contactor coil.

Richard

OK, now the reason is clear. What I have seen in many large power supplies is a simpler scheme, that uses another contactor with a much lower current rating to connect a series resistor across the main contacts to charge the capacitors prior to closing the main contactor contacts. The resistor is selected to allow only a small inrush current, and sized based on the watt-seconds during the charging time, which is normally just a very few seconds. This method is much less complicated and a lot more reliable.

 

Thank you all for the help. now i can see the issue is boostrap circuit, it is not suitable for my application. my circuit works for switching motor, solenoids, relay, etc but not for charging capacitor. i will switch to P-Channel Mosfet.
By the way, the 3 pin resistor is TKH45P50R0FE-TR

cheers