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[SOLVED] (Synchronous) Buck converter using SG3525

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It happens to have some SG3525 available and I need to quickly build a buck converter (48V -> 12V).

Reading the datasheet, seems like there's no way to control a synchronous buck converter (since the outputs are not complementary). It could be done by putting both outputs in parallel then using an CMOS inverter for the complementary output.

Anyway, I want to keep it simple so I wonder if I could use it with a high-side/low-side driver (IR2110 or something) to run a power MOSFET as a high-side switch.

The question is: could I use the freewheeling diode to charge the bootstrap capacitor?

It has an opposite current but it's voltage should be around 0.3V so theoretically the bootstrap capacitor could be charged to Vcc - 0.3V.

Still I can't see how the bootstrap capacitor charging current is flowing.. through the output inductor?
 

I'm going to ask a question: why don't you use a proper synchronous converter IC instead of attempting to figure out something with the 3525?

Dedicated synchronous controllers have the means to detect whether the current in your bottom Mosfet becomes negative, which occurs whenever the inductor current approaches the discontinuous conduction mode.
This does not happen with a diode, because the diode is naturally conmutated. But with a Mosfet, you must stop the gate drive when this occurs.
 

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Like I said, I just have a few spare SG3525 and I need(ed) a quick solution.
I guess I'll go with the "single ended" variant, with a bipolar PNP (as described in the SG3525 datasheet).

Btw, I've just checked for a synchronous buck driver but didn't find any "thru hole" variant (and I don't have tools/skills for TSSOP packages).

Do you know any?
 

Well, synchronous converters are used where the output voltage is so low that even a schottky's Vf is unacceptable. This usually means voltages below 5 volts.

Since your output is 12 volt, and you need a quick solution, and you already have the 3525, perhaps the most logical approach is to use a schottky.
 

But how to solve the high-side bootstrap charging problem?
 

A synchronous buck converter with two NMOSFETs is just a half bridge and can be driven by industry standard half-bridge drivers like IR2110 or any similar bootstrap driver design of your choice.

The only restriction is to keep the duty cycle below 100 percent to maintain bootstrap operation.
 

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Unfortunately, I cannot drive a IR2110 using a SG3525.

I guess I'll have to ged rid of SG3525 and use an Arduino that's just lying around (btw, half/full bridges are my favourite SMPS techniques).
 

I know, I wrote about that in the first post.

I have to sum up both outputs for driving the high-side switch then invert this signal for driving the low-side switch. Of course, I have to add a short delay first (dead time) before the rising edge of each of these signals.

I guess a single chip of NOR gates will be enough. If I'd be lucky to find one on my desk, I'll give it a try.
 

Hi! How about SG3525 summing up outputs with IR2110 drive or other drive? Could post the design/schematic...so I could try it also with my 3525 available. Thanks!
 

This is an example, using a single IC (74AHC02, QUAD 2-input Schmitt Trigger NOR gates):

SG3525-buck.gif

You may also use two diodes (and one resistor) to sum up the SG3525 outputs then use any available schmitt-trigger inverters (or NAND gates) instead of NOR gates.
 

Could you show a diagram implementation in that way (two diodes, one resistor with NAND gates)?
Others: best frequency and efficiency of this buck?
 

best frequency and efficiency of this buck?
Click to expand...

It depends on your MOSFETs (and your drivers). If they are fast enough and have low Rds(on), you could switch them at higher frequencies.

SG3525-buck-NAND.gif
 

Can use CD4049 instead of NANDs, for 10Khz?
The values of the R and C in between?
Could you post the 3525 circuit/components, including the feedback comp. net?
 
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