Maximizing efficiency in synchronous boost converter

Question

^{simulate this circuit – Schematic created using CircuitLab}

The aim is to design a synchronous boost converter using a microcontroller ATMega 2560 switching at 31.25 kHz.

5V PWM signal is fed from the microcontroller to the high and low side driver (IR2101).

My current output is about 15A max and I read somewhere that ideally, diode current should be limited to 7A max to prevent high power dissipation.

I would like to minimize power dissipation through the diode and maximize efficiency.

I tried turning on the high side switch with NOT gate signal from the low side PWM signal.

High side mosfet turns on when the low side is turned off but the voltage remains at 4 volt and does not increase.

What could be the cause of this problem?

Answer 1

You're trying to use an N-channel MOSFET as a high-side switch. That isn't going to work without a special gate driver that includes a boost circuit.

Besides which, you've got it the wrong way around — think about which way the body diode is pointing!

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You should probably be using a P-channel device at M2, but even then, you have to make its gate drive relative to its source, which is the output voltage, not the input voltage.

Answer 2

All half bridges using dual NFETs must use PWM only needed on low side with series cap to Vdd diode clamp to create a boost voltage >2Vgs(th) above Vdd to drive the high side gate voltage.

The high side then can be controlled independently for say a Boost / Buck conversion with an inductive Load by storing + or -ve inductive Current energy.

• the clamp diode only conducts when the the low side turns off after charging the series capacitors then the high side pulls up the capacitor to forward bias the clamp diode. There must be some xxx ps or ~1us of deadtime depending on reactance if Load L/R=Tau decay time and turn off recovery time of switches which is often sped up with a shunt diode across Gate resistor between LO and Low side gate.

Answer 3

You can't use M2 like that. MOSFETs have an anti-parallel body diode and yours is upside down freely conducting. You also need a high side gate driver.

Answer 4

There’s nothing inherently wrong with using an n-FET on the high side (and it's the preferred solution since n-FETS typically have lower Rds(on) for a given size), but watch the body diode (you have it backwards in your design). Also, to drive the high-side FET you need a bootstrapped high-side gate driver to make a high enough Vgs to turn on FET M2. (Huh, I said 'high' three times. Must have something on my mind...) I see that you've done that with the IR2101 so you're halfway there.

Anyway, having addressed those issues, here’s an idea: have a look at buck-boost and boost controller circuits that use n-FETs that deliver currents in your target range. Yes, you’re implementing just the boost part of buck-boost, but at least you will get an idea of component selection and circuit topology.

Buck-boost example: https://www.analog.com/en/products/lt8708.html#product-overview

And... Sync-boost example which has an input disconnect too, something your circuit could use: https://www.monolithicpower.com/en/mp9184a.html

One more thing: Some of the designs show a Schottky diode in parallel with the high-side FET, probably for speed-up of the overall switching time.

One other, other thing: are you ensuring dead-time (non-overlap) in your PWM drive between high and low side to prevent shoot-through?