Buck Converter and Buck Boost Converter

Hi all,

Currently I need to build a buck converter and a buck boost converter for my project.

I am going to use IRF540N MOSFET as the switch of the converter and IR2110 mosfet driver to fire the pwm signal sent from a logic 5V microcontroller to drive the mosfet. From what I have read from the relevant threads in this forum, I know that I need to use high I/O drive pin instead of low I/O drive pin of IR2110 for buck & buck boost converters as the source of the mosfet in these circuit are not directly connected to ground.

So, I would like to ask if it is possible to use only high drive pin of IR2110 to achieve the switching of the converter? I have seen a lot of answers from the threads in different forums suggest the user to use both drive of the driver (half bridge) to switch the converters. But I would like to do the switching with only one pwm signal and one side (high sides) drive, is it possible to achieve this?? What should be the connection of the circuit then??

Another question here...what should be used as the load for the mentioned converters?? Can it be a normal a quarter watts resistor. I have burned several this type of resistors with my circuit (boost converter [another circuit]). Is it better to use higher watts resistor???

Please help. Thank you very much.

Regards,

The IR2110 has two problems with high side only operation. 1) The Low side needs to have VCC (low side output supply) hooked up, otherwise it inhibits both the high and low outputs. This is a very minor problem - just hook it up but don't use it. 2) The major problem: if you use a bootstrap supply on the high side (C between Vs and Vb, diode from Vcc to Vb to charge up C when buck regulator diode is conducting), how do you start the darn thing? You need to have current flowing in the buck inductor to make it drive the diode into conduction. But without a high side supply, there's no way to guarantee a start up.

(Tangent here: if you are *lucky* then, when you first turn on the power supplies, the Vb supply current flowing from Vcc through the diode and inductor to the load will be enough to get you started, if you hold off the input pwm signals to the IR2110 just the right amount of time. If you wait too long, then the capacitor voltage decays too much from the current drawn by the high side circuitry. If you turn it on too quickly, the IR2110 will ignore you. Also the Vcc supply plus diode drop plus resistor divider effect of Rload and high side static current has to produce enough Vbs voltage to overcome the undervoltage lockout of the highside supply.)

So, if you want to make the IR2110 work reliably, you need a second mosfet that you put across the inductor diode. At start-up, you *make sure* that the high side pwm signal is off , then turn on the second mosfet to pull Vs to ground and charge up the high side power supply (for 10-50us), then turn off the second mosfet, wait 50-500 us and turn on the high side pwm. Any time you shut down the pwm and restart it, you need to again "flash" the second mosfet for 10-50 us. The reason for the short time is that the second mosfet will be discharging the output capacitor though the inductor - no current limit - ouch!

OK, so are you sure you want to use the IR2110? P-channel MOSFETs are *much* easier to use, even with the voltage translation issue.

Hi,

Thanks for the reply.

Actually, it is not compulsory for me to use IR2110, but since I have bought several I just don't want to waste them...anyway, if I want to switch the mosfet in buck and buck boost converter, I must use high side drive of any mosfet driver right?? What type of driver will you recommend other than IR2110??

On the other hand, IR2110 should be suitable and easier to use for single low side drive right?? I have used it with my boost converter, it works fine but the only problem is the resistor which used as load is keep burning...Please advice, thank you very much.

Regards,

Dumb question I should have asked first, before spouting off. I was thinking about a particular circuit mistake I made with a buck regulator and laid out the lesson that I learned without approaching your circuit with an open mind. Sorry.

What's your DC Bus voltage?

IR2110 is great for voltages over 50V; you just have to provide a push pull output (use the low side to drive a mosfet that's across the inductor diode) to get it started.

If the voltage is less than 18V, I would use a TC4426 or similar device to drive a P-channel mosfet.

If the voltages is less than about 30V, you can voltage translate with a different circuit I can show you later.

---------- Post added at 17:40 ---------- Previous post was at 17:38 ----------

Also, re power dissipation: what is your output voltage and what kind of load current are you planning for?

I need to bow out of this thread for a few days while I am on travel. Hopefully some of the experienced designers here can help you.

Hi,

Thanks for the reply, how is your trip??

Actually, I am quite confused with one thing. I have referred to the single high side mosfet driver (IR2117), the circuit connnection after the Vs pin is connected to the RLC circuit, which is totally identical to the IR2110 if I am using that. But why IR2110 requires a second mosfet to charge the bootstrap yet IR2117 no need?? I mean we need a second mosfet to help pulling the Vs to ground to charge the bootstrap capacitor on IR2110, but why IR2117 still can work without this additional connection.

Please advice. Thank you very much.

*Anyone who know the answer are welcomed also, thanks.

Regards

The trip was fine, but it took longer to get the customer's stuff fixed than I expected. Thanks for asking.

I think you are looking at the IR2117 data sheet application circuits. In those applications, they never show *what* the load is. A PWM'ed incandescent light or LED for example, would work fine in their application circuits because those loads are "DC" loads - they pull Vs down to ground whenever the MOSFET is off. A buck converter using the IR2117 would not work right until there was enough current to make the buck diode conduct.

See also page 22 of the following app note. This is IR's way of starting the IR2117 in a buck converter.

http://www.irf.com/technical-info/appnotes/an-978.pdf

Hi,

Thanks for the reply. I have found the problem. The required minimum high input voltage for the chip is above 9.5V, my microcontroller pwm output only 5V, so the mosfet driver wont be able to drive my buck converter.

So, I have changed it to IR2101 which is a high side and low side mosfet driver with required minimum high input voltage of around 3V. I have tested it with my buck boost converter circuit, it is considered working below 50% duty cycle but I when I increase the pwm duty cycle to 50% or above, the output voltage of the buck boost converter will suddenly drop and my microcontroller will start getting warm and eventually get burned. Can I know what cause this happens???

Here are the spec of the components that I am using for the circuit, please have a look:

Frequncy: 20kHz, logic 5V pwm from microcontroller

1) Bootstrap capacitor (capacitor between Vb and Vs) - 22 microfarad, 50V
2) capacitor between Vcc and COM - 22 microfarad, 50V
3) Diode - 1N4148 fast switching diode
4) Resistor between gate and driver - 10-100 Ohms

For the converter circuit:
1) Inductor - 100 micro henry
2) Capacitor - 100 micro farad
3) Load - 27 Ohms, 10 Watts
4) Diode - 1N4148 fast switching diode

Please help. Hope to hear from you soon. Thank you very much.

Best Regards

I've never used 1N4148's successfully. I always use 1N5819 schottky diodes. Can you put up the schematic?

Hi
I want to build a buck boost converter with 5 to 12 input voltage. I want to fix output voltage at 12 volt.
I used this components :
f :20khz( use pwm pulse from avr microcontroller)
GH20N50 IGBT
120micro farad 400 v capacitor
Inductor 4.7 mh
6A10 diode
12 volt 20 watt halogen lamp as load
I drived IGBT with two BD139 transistor and it work properly.so I think driving circuit is true and igbt have no problem. but when I add diode ,inductor ,capacitor and load I don’t have any voltage at output.
I don’t know what is problem. Please help me. Thank you very much.