A6727 design concern

DiogoPedrosa · ‎2025-05-24

I'm using E Design Suite to build a buck (step-down) converter circuit and would like to know if there's anything in the design that I should be concerned about. I've reviewed the schematics in the datasheets, and they seem to be about the same.

I plan to use the same MOSFET model for both the high-side and low-side switches to simplify component sourcing. Could this be a problem?

I intend to use this circuit to power several servos, so the output should provide at least 10 A. Should I add more capacitors at the output? Would it also be a good idea to include flyback diodes?

The link to the simulation: link

Peter BENSCH · ‎2025-05-26

Firstly, please note that A6727 is already set to NRND, you should use the A6727B, which has identical behavior.

The MOSFETs in such synchronous switching regulators usually have different parameters (Rdson, gate charge) at high output currents depending on the input and output voltage in order to optimise the unavoidable power losses. With your input voltage range, which is almost twice as high as the output voltage, the same type of MOSFET can certainly be used for high and low side because of ~50% duty cycle. If necessary, you should use the same gate resistor on LGATE as suggested for UGATE. In case of doubt, it can also be replaced by a zero-ohm resistor.

The Cin and Cout capacitors are a little small for 10A and should be increased by at least a factor of 10, of which at least 10% should be ceramic capacitors.

Where would you want to use a flyback diode?

Regards
/Peter

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Peter BENSCH · ‎2025-05-26

Firstly, please note that A6727 is already set to NRND, you should use the A6727B, which has identical behavior.

The MOSFETs in such synchronous switching regulators usually have different parameters (Rdson, gate charge) at high output currents depending on the input and output voltage in order to optimise the unavoidable power losses. With your input voltage range, which is almost twice as high as the output voltage, the same type of MOSFET can certainly be used for high and low side because of ~50% duty cycle. If necessary, you should use the same gate resistor on LGATE as suggested for UGATE. In case of doubt, it can also be replaced by a zero-ohm resistor.

The Cin and Cout capacitors are a little small for 10A and should be increased by at least a factor of 10, of which at least 10% should be ceramic capacitors.

Where would you want to use a flyback diode?

Regards
/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

RobK1 · ‎2025-05-26

May I suggest you look for a different type of MOSFET? The STP80NF55-06 is pretty old and has a horribly high total gate charge. There are many types available that have way better specifications. It will do wonders for your switching losses.

DiogoPedrosa · ‎2025-05-26

What would be your recommendations for MOSFETs?The one recommended in design suite is out of production and also many others are not recommended for new designs.
I’ve been browsing ST’s website and store and came across the STP95N4F3, which has a total gate charge of 40 nC — significantly lower than the STP80 or the STP100N6F7, which has 30 nC. My main criteria are a TO-220 package and availability for direct order from ST.

Should I prioritize a lower gate charge for my application?

RobK1 · ‎2025-05-26

The STP100N6F7 certainly looks very good.

DiogoPedrosa · ‎2025-05-26

Sorry but I was already using the A6727B — it was probably just a misselection in the software by me, but from what I’ve seen, it’s essentially the same.

I’ll also be adding the gate resistor, just in case. I noticed it in the datasheet listed as optional, but it doesn’t hurt to include an extra resistor for added protection.

Regarding the flyback diode, my initial plan was to block reverse current from the DC motor. However, I believe the low-side MOSFET already handles that. My main goal was to reduce the stress on the MOSFET during switching, but it might be unnecessary.

Peter BENSCH · ‎2025-05-26

The STP100N6F7 provides 30nC and typ. 4.7mOhms - at 10V gate drive.

For 5V gate drive you could consider:

Good luck!
/Peter

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