Powering an IR2155

Mar 14, 2011 3 Replies

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Figbash 15 years ago

Hello,

I'm having a little trouble understanding how to power an International Rectifier IR2155 half-bridge self-oscillating MOSFET driver. The datasheet shows a basic usage model where the device is powered by the high-voltage to be switched:

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However, the supply pin has a 15.6V Zener across it, which causes the supply voltage to drop to 15.6V. So my Vhi clamps to 15.6V, instead of 80~100V. Obviously defeating the point.

The footnotes on the device max table mentions that it must be powered by a current source, and I've only seen one example of that using an LM334Z, which I don't understand.

Any ideas how to supply this thing? FYI: the input voltage I'm trying to switch is 80~100V DC from a boost converter. I thought about just using a 555 with a simple mosfet driver, but would like to understand this odd device's power requirements, just for the sake of curiosity.

Thanks,

Peter

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Pimpom 15 years ago

It doesn't. Only Vcc, not VB is clamped to 15.6V. VB is bootstrapped by the diode and the cap between VB and VS and therefore can rise sufficiently above the output voltage to drive the high-side MOSFET's gate.

The footnote explains that clearly. The LM334 is a CONSTANT current device. The IR2155 needs a supply that is essentially current, not voltage. The combination of high voltage and high resistance provides that.

The basic application circuit "Typical Connection" already shows how to supply it. (They should really give serial numbers to the components). The resistor from the high voltage supply to VSS provides the current supply path. You just have to calculate the resistance. The IR2155 consumes up to 5mA. This amount of current should be available when the supply voltage is at its lowest - 80V in your case. So the maximum resistance is (80-15.6)V/5mA = 12.88k. A 10k resistor should be OK. It should be of sufficient wattage to keep from burning up at the highest exopected supply voltage. I'll leave it to you to work that out.

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Grant 15 years ago

Obviously you didn't notice the dropping resistor in the top left corner of the schematic ;)

Grant.

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Jamie 15 years ago

Yup. The Clamp is for the pump chip itself how ever, it does look like via the simple schematic there, if you need more than 15 volts between the source and gate to turn it on fully, putting in a voltage divider before the clamp that drives the diode needs to be installed, so that you can get more in the charge pump output might work.

Jamie

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